Internet RFC/STD/FYI/BCP Archives
Alternate Formats: rfc3525.txt
Network Working Group C. Groves
Request for Comments: 3525 M. Pantaleo
Obsoletes: 3015 LM Ericsson
Category: Standards Track T. Anderson
Consultant
T. Taylor
Nortel Networks
Editors
June 2003
Gateway Control Protocol Version 1
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines the protocol used between elements of a
physically decomposed multimedia gateway, i.e., a Media Gateway and a
Media Gateway Controller. The protocol presented in this document
meets the requirements for a media gateway control protocol as
presented in RFC 2805.
This document replaces RFC 3015. It is the result of continued
cooperation between the IETF Megaco Working Group and ITU-T Study
Group 16. It incorporates the original text of RFC 3015, modified by
corrections and clarifications discussed on the Megaco
E-mail list and incorporated into the Study Group 16 Implementor's
Guide for Recommendation H.248. The present version of this document
underwent ITU-T Last Call as Recommendation H.248 Amendment 1.
Because of ITU-T renumbering, it was published by the ITU-T as
Recommendation H.248.1 (03/2002), Gateway Control Protocol Version 1.
Users of this specification are advised to consult the H.248 Sub-
series Implementors' Guide at http://www.itu.int/itudoc/itu-
t/com16/implgd for additional corrections and clarifications.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents
1 Scope.........................................................5
1.1 Changes From RFC 3015.....................................5
1.2 Differences From ITU-T Recommendation H.248.1 (03/2002)...5
2 References....................................................6
2.1 Normative references......................................6
2.2 Informative references....................................9
3 Definitions..................................................10
4 Abbreviations................................................11
5 Conventions..................................................12
6 Connection model.............................................13
6.1 Contexts.................................................16
6.2 Terminations.............................................17
6.2.1 Termination dynamics.................................21
6.2.2 TerminationIDs.......................................21
6.2.3 Packages.............................................22
6.2.4 Termination properties and descriptors...............23
6.2.5 Root Termination.....................................25
7 Commands.....................................................26
7.1 Descriptors..............................................27
7.1.1 Specifying parameters................................27
7.1.2 Modem descriptor.....................................28
7.1.3 Multiplex descriptor.................................28
7.1.4 Media descriptor.....................................29
7.1.5 TerminationState descriptor..........................29
7.1.6 Stream descriptor....................................30
7.1.7 LocalControl descriptor..............................31
7.1.8 Local and Remote descriptors.........................32
7.1.9 Events descriptor....................................35
7.1.10 EventBuffer descriptor..............................38
7.1.11 Signals descriptor..................................38
7.1.12 Audit descriptor....................................40
7.1.13 ServiceChange descriptor............................41
7.1.14 DigitMap descriptor.................................41
7.1.15 Statistics descriptor...............................46
7.1.16 Packages descriptor.................................47
7.1.17 ObservedEvents descriptor...........................47
7.1.18 Topology descriptor.................................47
7.1.19 Error Descriptor....................................50
7.2 Command Application Programming Interface................50
7.2.1 Add..................................................51
7.2.2 Modify...............................................52
7.2.3 Subtract.............................................53
7.2.4 Move.................................................55
7.2.5 AuditValue...........................................56
7.2.6 AuditCapabilities....................................59
7.2.7 Notify...............................................60
7.2.8 ServiceChange........................................61
7.2.9 Manipulating and Auditing Context Attributes.........65
7.2.10 Generic Command Syntax..............................66
7.3 Command Error Codes......................................66
8 Transactions.................................................66
8.1 Common parameters........................................68
8.1.1 Transaction Identifiers..............................68
8.1.2 Context Identifiers..................................68
8.2 Transaction Application Programming Interface............69
8.2.1 TransactionRequest...................................69
8.2.2 TransactionReply.....................................69
8.2.3 TransactionPending...................................71
8.3 Messages.................................................72
9 Transport....................................................72
9.1 Ordering of Commands.....................................73
9.2 Protection against Restart Avalanche.....................74
10 Security Considerations.....................................75
10.1 Protection of Protocol Connections......................75
10.2 Interim AH scheme.......................................76
10.3 Protection of Media Connections.........................77
11 MG-MGC Control Interface....................................78
11.1 Multiple Virtual MGs....................................78
11.2 Cold start..............................................79
11.3 Negotiation of protocol version.........................79
11.4 Failure of a MG.........................................80
11.5 Failure of an MGC.......................................81
12 Package definition..........................................82
12.1 Guidelines for defining packages........................82
12.1.1 Package.............................................83
12.1.2 Properties..........................................84
12.1.3 Events..............................................85
12.1.4 Signals.............................................85
12.1.5 Statistics..........................................86
12.1.6 Procedures..........................................86
12.2 Guidelines to defining Parameters to Events and Signals.86
12.3 Lists...................................................87
12.4 Identifiers.............................................87
12.5 Package registration....................................88
13 IANA Considerations.........................................88
13.1 Packages................................................88
13.2 Error codes.............................................89
13.3 ServiceChange reasons...................................89
ANNEX A Binary encoding of the protocol.......................90
A.1 Coding of wildcards......................................90
A.2 ASN.1 syntax specification...............................92
A.3 Digit maps and path names...............................111
ANNEX B Text encoding of the protocol.........................113
B.1 Coding of wildcards.....................................113
B.2 ABNF specification......................................113
B.3 Hexadecimal octet coding................................127
B.4 Hexadecimal octet sequence..............................127
ANNEX C Tags for media stream properties......................128
C.1 General media attributes................................128
C.2 Mux properties..........................................130
C.3 General bearer properties...............................130
C.4 General ATM properties..................................130
C.5 Frame Relay.............................................134
C.6 IP......................................................134
C.7 ATM AAL2................................................134
C.8 ATM AAL1................................................136
C.9 Bearer capabilities.....................................137
C.10 AAL5 properties........................................147
C.11 SDP equivalents........................................148
C.12 H.245..................................................149
ANNEX D Transport over IP.....................................150
D.1 Transport over IP/UDP using Application Level Framing ..150
D.1.1 Providing At-Most-Once functionality................150
D.1.2 Transaction identifiers and three-way handshake.....151
D.1.3 Computing retransmission timers.....................152
D.1.4 Provisional responses...............................153
D.1.5 Repeating Requests, Responses and Acknowledgements..153
D.2 Using TCP...............................................155
D.2.1 Providing the At-Most-Once functionality............155
D.2.2 Transaction identifiers and three-way handshake.....155
D.2.3 Computing retransmission timers.....................156
D.2.4 Provisional responses...............................156
D.2.5 Ordering of commands................................156
ANNEX E Basic packages.......................................157
E.1 Generic.................................................157
E.2 Base Root Package.......................................159
E.3 Tone Generator Package..................................161
E.4 Tone Detection Package..................................163
E.5 Basic DTMF Generator Package............................166
E.6 DTMF detection Package..................................167
E.7 Call Progress Tones Generator Package...................169
E.8 Call Progress Tones Detection Package...................171
E.9 Analog Line Supervision Package.........................172
E.10 Basic Continuity Package...............................175
E.11 Network Package........................................178
E.12 RTP Package............................................180
E.13 TDM Circuit Package....................................182
APPENDIX I EXAMPLE CALL FLOWS (INFORMATIVE)...................184
A.1 Residential Gateway to Residential Gateway Call.........184
A.1.1 Programming Residential GW Analog Line Terminations
for Idle Behavior...................................184
A.1.2 Collecting Originator Digits and Initiating
Termination.........................................186
APPENDIX II Changes From RFC 3015............................195
Intellectual Property Rights..................................210
Acknowledgments...............................................211
Authors' Addresses............................................212
Full Copyright Statement......................................213
1 Scope
The present document, which is identical to the published version of
ITU-T Recommendation H.248.1 (03/2002) except as noted below, defines
the protocols used between elements of a physically decomposed
multimedia gateway. There are no functional differences from a
system view between a decomposed gateway, with distributed sub-
components potentially on more than one physical device, and a
monolithic gateway such as described in ITU-T Recommendation H.246.
This document does not define how gateways, multipoint control units
or interactive voice response units (IVRs) work. Instead it creates
a general framework that is suitable for these applications.
Packet network interfaces may include IP, ATM or possibly others.
The interfaces will support a variety of Switched Circuit Network
(SCN) signalling systems, including tone signalling, ISDN, ISUP, QSIG
and GSM. National variants of these signalling systems will be
supported where applicable.
1.1 Changes From RFC 3015
The differences between this document and RFC 3015 are documented in
Appendix II.
1.2 Differences From ITU-T Recommendation H.248.1 (03/2002)
This document differs from the corresponding ITU-T publication in the
following respects:
- Added IETF front matter in place of the corresponding ITU-T
material.
- The ITU-T summary is too H.323-specific and has been omitted.
- The IETF conventions have been stated as governing this document.
As discussed in section 5 below, this gives slightly greater
strength to "should" requirements.
- The Scope section (just above) has been edited slightly to suit
its IETF context.
- Added normative references to RFCs 2026 and 2119.
- Figures 4, 5, and 6 show the centre of the context for greater
clarity. Also added Figure 6a showing an important additional
example.
- Added a paragraph in section 7.1.18 which was approved in the
Implementor's Guide but lost inadvertently in the ITU-T approved
version.
- This document incorporates corrections to the informative examples
in Appendix I which also appear in H.248.1 version 2, but which
were not picked up in H.248.1 (03/2002).
- This document includes a new Appendix II listing all the changes
from RFC 3015.
- This document includes an Acknowledgements section listing the
authors of RFC 3015 but also many other people who contributed to
the development of the Megaco/H.248.x protocol.
- Moved the Intellectual Property declaration to its usual place in
an IETF document and added a reference to declarations on the IETF
web site.
2 References
The following ITU-T Recommendations and other references contain
provisions which, through reference in this text, constitute
provisions of this RFC. At the time of publication, the editions
indicated were valid. All Recommendations and other references are
subject to revision; all users of this RFC are therefore encouraged
to investigate the possibility of applying the most recent edition of
the Recommendations and other references listed below. A list of the
currently valid ITU-T Recommendations is regularly published.
2.1 Normative references
- ITU-T Recommendation H.225.0 (1999), Call signalling protocols and
media stream packetization for packet-based multimedia
communication systems.
- ITU-T Recommendation H.235 (1998), Security and encryption for
H-Series (H.323 and other H.245-based) multimedia terminals.
- ITU-T Recommendation H.245 (1998), Control protocol for multimedia
communication.
- ITU-T Recommendation H.246 (1998), Interworking of H-series
multimedia terminals with H-series multimedia terminals and
voice/voiceband terminals on GSTN and ISDN.
- ITU-T Recommendation H.248.8 (2002), H.248 Error Codes and Service
Change Reasons.
- ITU-T Recommendation H.323 (1999), Packet-based multimedia
communication systems.
- ITU-T Recommendation I.363.1 (1996), B-ISDN ATM adaptation layer
(AAL) specification: Type 1 AAL.
- ITU-T Recommendation I.363.2 (1997), B-ISDN ATM adaptation layer
(AAL) specification: Type 2 AAL.
- ITU-T Recommendation I.363.5 (1996), B-ISDN ATM adaptation layer
(AAL) specification: Type 5 AAL.
- ITU-T Recommendation I.366.1 (1998), Segmentation and Reassembly
Service Specific Convergence Sublayer for the AAL type 2.
- ITU-T Recommendation I.366.2 (1999), AAL type 2 service specific
convergence sublayer for trunking.
- ITU-T Recommendation I.371 (2000), Traffic control and congestion
control in B-ISDN.
- ITU-T Recommendation Q.763 (1999), Signalling System No. 7 - ISDN
user part formats and codes.
- ITU-T Recommendation Q.765.5 (2001), Application transport
mechanism - Bearer independent call control (BICC).
- ITU-T Recommendation Q.931 (1998), ISDN user-network interface
layer 3 specification for basic call control.
- ITU-T Recommendation Q.2630.1 (1999), AAL type 2 signalling
protocol (Capability Set 1).
- ITU-T Recommendation Q.2931 (1995), Digital Subscriber Signalling
System No. 2 (DSS2) - User-Network Interface (UNI) - Layer 3
specification for basic call/connection control.
- ITU-T Recommendation Q.2941.1 (1997), Digital Subscriber
Signalling System No. 2 - Generic identifier transport.
- ITU-T Recommendation Q.2961.1 (1995), Additional signalling
capabilities to support traffic parameters for the tagging option
and the sustainable call rate parameter set.
- ITU-T Recommendation Q.2961.2 (1997), Additional traffic
parameters: Support of ATM transfer capability in the broadband
bearer capability information element.
- ITU-T Recommendation Q.2965.1 (1999), Digital subscriber
signalling system No. 2 - Support of Quality of Service classes.
- ITU-T Recommendation Q.2965.2 (1999), Digital subscriber
signalling system No. 2 - Signalling of individual Quality of
Service parameters.
- ITU-T Recommendation V.76 (1996), Generic multiplexer using V.42
LAPM-based procedures.
- ITU-T Recommendation X.213 (1995), Information technology - Open
Systems Interconnection - Network service definition plus
Amendment 1 (1997), Addition of the Internet protocol address
format identifier.
- ITU-T Recommendation X.680 (1997), Information technology -
Abstract Syntax Notation One (ASN.1): Specification of basic
notation.
- ITU-T Recommendation X.690 (1997), Information Technology - ASN.1
Encoding Rules: Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
- ATM Forum (1996), ATM User-Network Interface (UNI) Signalling
Specification - Version 4.0.
[RFC 1006] Rose, M. and D. Cass, "ISO Transport Service on top of the
TCP, Version 3", STD 35, RFC 1006, May 1987.
[RFC 2026] Brander, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC 2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[RFC 2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[RFC 2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
2.2 Informative references
- ITU-T Recommendation E.180/Q.35 (1998), Technical characteristics
of tones for the telephone service.
- CCITT Recommendation G.711 (1988), Pulse Code Modulation (PCM) of
voice frequencies.
- ITU-T Recommendation H.221 (1999), Frame structure for a 64 to
1920 kbit/s channel in audiovisual teleservices.
- ITU T Recommendation H.223 (1996), Multiplexing protocol for low
bit rate multimedia communication.
- ITU-T Recommendation H.226 (1998), Channel aggregation protocol
for multilink operation on circuit-switched networks
- ITU-T Recommendation Q.724 (1998), Signalling procedures.
- ITU-T Recommendation Q.764 (1999), Signalling system No. 7 - ISDN
user part signalling procedures.
- ITU-T Recommendation Q.1902.4 (2001), Bearer independent call
control protocol - Basic call procedures.
[RFC 768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC 1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC 1889] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
[RFC 1890] Schulzrinne, H. and G. Fokus, "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 1890,
January 1996.
[RFC 2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 2543] Handley, M., Schulzrinne, H., Schooler, E. and J.
Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
March 1999.
[RFC 2805] Greene, N., Ramalho, M. and B. Rosen, "Media Gateway
Control Protocol Architecture and Requirements", RFC 2805,
April 2000.
3 Definitions
This document defines the following terms:
Access gateway:
A type of gateway that provides a User-Network Interface (UNI) such
as ISDN.
Descriptor:
A syntactic element of the protocol that groups related properties.
For instance, the properties of a media flow on the MG can be set by
the MGC by including the appropriate descriptor in a command.
Media Gateway (MG):
The media gateway converts media provided in one type of network to
the format required in another type of network. For example, a MG
could terminate bearer channels from a switched circuit network
(e.g., DS0s) and media streams from a packet network (e.g., RTP
streams in an IP network). This gateway may be capable of processing
audio, video and T.120 alone or in any combination, and will be
capable of full duplex media translations. The MG may also play
audio/video messages and perform other IVR functions, or may perform
media conferencing.
Media Gateway Controller (MGC):
Controls the parts of the call state that pertain to connection
control for media channels in a MG.
Multipoint Control Unit (MCU):
An entity that controls the setup and coordination of a multi-user
conference that typically includes processing of audio, video and
data.
Residential gateway:
A gateway that interworks an analogue line to a packet network. A
residential gateway typically contains one or two analogue lines and
is located at the customer premises.
SCN FAS signalling gateway:
This function contains the SCN Signalling Interface that terminates
SS7, ISDN or other signalling links where the call control channel
and bearer channels are collocated in the same physical span.
SCN NFAS signalling gateway:
This function contains the SCN Signalling Interface that terminates
SS7 or other signalling links where the call control channels are
separated from bearer channels.
Stream:
Bidirectional media or control flow received/sent by a media gateway
as part of a call or conference.
Trunk:
A communication channel between two switching systems such as a DS0
on a T1 or E1 line.
Trunking gateway:
A gateway between SCN network and packet network that typically
terminates a large number of digital circuits.
4 Abbreviations
This RFC document uses the following abbreviations:
ALF Application Layer Framing
ATM Asynchronous Transfer Mode
CAS Channel Associated Signalling
DTMF Dual Tone Multi-Frequency
FAS Facility Associated Signalling
GSM Global System for Mobile communications
GW GateWay
IANA Internet Assigned Numbers Authority (superseded by Internet
Corporation for Assigned Names and Numbers - ICANN)
IP Internet Protocol
ISUP ISDN User Part
IVR Interactive Voice Response
MG Media Gateway
MGC Media Gateway Controller
NFAS Non-Facility Associated Signalling
PRI Primary Rate Interface
PSTN Public Switched Telephone Network
QoS Quality of Service
RTP Real-time Transport Protocol
SCN Switched Circuit Network
SG Signalling Gateway
SS7 Signalling System No. 7
5 Conventions
In the H.248.1 Recommendation, "SHALL" refers to a mandatory
requirement, while "SHOULD" refers to a suggested but optional
feature or procedure. The term "MAY" refers to an optional course of
action without expressing a preference. Note that these definition
are overridden in the present document by the RFC 2119 conventions
stated at the beginning of this document. RFC 2119 has a more
precise definition of "should" than is provided by the ITU-T.
6 Connection model
The connection model for the protocol describes the logical entities,
or objects, within the Media Gateway that can be controlled by the
Media Gateway Controller. The main abstractions used in the
connection model are Terminations and Contexts.
A Termination sources and/or sinks one or more streams. In a
multimedia conference, a Termination can be multimedia and sources or
sinks multiple media streams. The media stream parameters, as well
as modem, and bearer parameters are encapsulated within the
Termination.
A Context is an association between a collection of Terminations.
There is a special type of Context, the null Context, which contains
all Terminations that are not associated to any other Termination.
For instance, in a decomposed access gateway, all idle lines are
represented by Terminations in the null Context.
Following is a graphical depiction of these concepts. The diagram of
Figure 1 gives several examples and is not meant to be an
all-inclusive illustration. The asterisk box in each of the Contexts
represents the logical association of Terminations implied by the
Context.
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context +-------------+ | |
| | | Termination | | |
| | |-------------| | |
| | +-------------+ +->| SCN Bearer |<---+->
| | | Termination | +-----+ | | Channel | | |
| | |-------------| | |---+ +-------------+ | |
<-+--->| RTP Stream |---| * | | |
| | | | | |---+ +-------------+ | |
| | +-------------+ +-----+ | | Termination | | |
| | | |-------------| | |
| | +->| SCN Bearer |<---+->
| | | Channel | | |
| | +-------------+ | |
| +-------------------------------------------------+ |
| |
| |
| +------------------------------+ |
| (NULL Context) |Context | |
| +-------------+ | +-------------+ | |
| | Termination | | +-----+ | Termination | | |
| |-------------| | | | |-------------| | |
| | SCN Bearer | | | * |------| SCN Bearer |<---+->
| | Channel | | | | | Channel | | |
| +-------------+ | +-----+ +-------------+ | |
| +------------------------------+ |
| |
| |
| +-------------------------------------------------+ |
| |Context | |
| | +-------------+ +-------------+ | |
| | | Termination | +-----+ | Termination | | |
| | |-------------| | | |-------------| | |
<-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+->
| | | Channel | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
| ___________________________________________________ |
+------------------------------------------------------+
Figure 1: Examples of Megaco/H.248 Connection Model
The example in Figure 2 shows an example of one way to accomplish a
call-waiting scenario in a decomposed access gateway, illustrating
the relocation of a Termination between Contexts. Terminations T1
and T2 belong to Context C1 in a two-way audio call. A second audio
call is waiting for T1 from Termination T3. T3 is alone in Context
C2. T1 accepts the call from T3, placing T2 on hold. This action
results in T1 moving into Context C2, as shown in Figure 3.
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context C1 | |
| | +-------------+ +-------------+ | |
| | | Term. T2 | +-----+ | Term. T1 | | |
| | |-------------| | | |-------------| | |
<-+--->| RTP Stream |---| * |------| SCN Bearer |<---+->
| | | | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
| |
| +-------------------------------------------------+ |
| |Context C2 | |
| | +-------------+ | |
| | +-----+ | Term. T3 | | |
| | | | |-------------| | |
| | | * |------| SCN Bearer |<---+->
| | | | | Channel | | |
| | +-----+ +-------------+ | |
| +-------------------------------------------------+ |
+------------------------------------------------------+
Figure 2: Example Call Waiting Scenario / Alerting Applied to T1
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context C1 | |
| | +-------------+ | |
| | | Term. T2 | +-----+ | |
| | |-------------| | | | |
<-+--->| RTP Stream |---| * | | |
| | | | | | | |
| | +-------------+ +-----+ | |
| +-------------------------------------------------+ |
| |
| +-------------------------------------------------+ |
| |Context C2 | |
| | +-------------+ +-------------+ | |
| | | Term. T1 | +-----+ | Term. T3 | | |
| | |-------------| | | |-------------| | |
<-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+->
| | | Channel | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
+------------------------------------------------------+
Figure 3. Example Call Waiting Scenario / Answer by T1
6.1 Contexts
A Context is an association between a number of Terminations. The
Context describes the topology (who hears/sees whom) and the media
mixing and/or switching parameters if more than two Terminations are
involved in the association.
There is a special Context called the null Context. It contains
Terminations that are not associated to any other Termination.
Terminations in the null Context can have their parameters examined
or modified, and may have events detected on them.
In general, an Add command is used to add Terminations to Contexts.
If the MGC does not specify an existing Context to which the
Termination is to be added, the MG creates a new Context. A
Termination may be removed from a Context with a Subtract command,
and a Termination may be moved from one Context to another with a
Move command. A Termination SHALL exist in only one Context at a
time.
The maximum number of Terminations in a Context is a MG property.
Media gateways that offer only point-to-point connectivity might
allow at most two Terminations per Context. Media gateways that
support multipoint conferences might allow three or more Terminations
per Context.
6.1.1 Context attributes and descriptors
The attributes of Contexts are:
- ContextID.
- The topology (who hears/sees whom).
The topology of a Context describes the flow of media between the
Terminations within a Context. In contrast, the mode of a
Termination (send/receive/...) describes the flow of the media at
the ingress/egress of the media gateway.
- The priority is used for a Context in order to provide the MG with
information about a certain precedence handling for a Context.
The MGC can also use the priority to control autonomously the
traffic precedence in the MG in a smooth way in certain
situations (e.g., restart), when a lot of Contexts must be handled
simultaneously. Priority 0 is the lowest priority and a priority
of 15 is the highest priority.
- An indicator for an emergency call is also provided to allow a
preference handling in the MG.
6.1.2 Creating, deleting and modifying Contexts
The protocol can be used to (implicitly) create Contexts and modify
the parameter values of existing Contexts. The protocol has commands
to add Terminations to Contexts, subtract them from Contexts, and to
move Terminations between Contexts. Contexts are deleted implicitly
when the last remaining Termination is subtracted or moved out.
6.2 Terminations
A Termination is a logical entity on a MG that sources and/or sinks
media and/or control streams. A Termination is described by a number
of characterizing Properties, which are grouped in a set of
Descriptors that are included in commands. Terminations have unique
identities (TerminationIDs), assigned by the MG at the time of their
creation.
Terminations representing physical entities have a semi-permanent
existence. For example, a Termination representing a TDM channel
might exist for as long as it is provisioned in the gateway.
Terminations representing ephemeral information flows, such as RTP
flows, would usually exist only for the duration of their use.
Ephemeral Terminations are created by means of an Add command. They
are destroyed by means of a Subtract command. In contrast, when a
physical Termination is Added to or Subtracted from a Context, it is
taken from or to the null Context, respectively.
Terminations may have signals applied to them (see 7.1.11).
Terminations may be programmed to detect Events, the occurrence of
which can trigger notification messages to the MGC, or action by the
MG. Statistics may be accumulated on a Termination. Statistics are
reported to the MGC upon request (by means of the AuditValue command,
see 7.2.5) and when the Termination is taken out of the call it is
in.
Multimedia gateways may process multiplexed media streams. For
example, Recommendation H.221 describes a frame structure for
multiple media streams multiplexed on a number of digital 64 kbit/s
channels. Such a case is handled in the connection model in the
following way. For every bearer channel that carries part of the
multiplexed streams, there is a physical or ephemeral "bearer
Termination". The bearer Terminations that source/sink the digital
channels are connected to a separate Termination called the
"multiplexing Termination". The multiplexing termination is an
ephemeral termination representing a frame-oriented session. The
MultiplexDescriptor for this Termination describes the multiplex used
(e.g., H.221 for an H.320 session) and indicates the order in which
the contained digital channels are assembled into a frame.
Multiplexing terminations may be cascades (e.g., H.226 multiplex of
digital channels feeding into a H.223 multiplex supporting an H.324
session).
The individual media streams carried in the session are described by
StreamDescriptors on the multiplexing Termination. These media
streams can be associated with streams sourced/sunk by Terminations
in the Context other than the bearer Terminations supporting the
multiplexing Termination. Each bearer Termination supports only a
single data stream. These data streams do not appear explicitly as
streams on the multiplexing Termination and they are hidden from the
rest of the context.
Figures 4, 5, 6, and 6a illustrate typical applications of the
multiplexing termination and Multiplex Descriptor.
+-----------------------------------+
| Context +-------+ |
+----+ | | |
Circuit 1 -|--| TC1|---------+ Tmux | |
| +----+ (Str 1) | | Audio +-----+
| | | +-----*-----+ |-----
| +----+ | H.22x | Stream 1 | |
Circuit 2 -|--| TC2|---------+ multi-| | TR1 |
| +----+ (Str 1) | plex | |(RTP)|
| | | | Video | |
| +----+ | +-----*-----+ |-----
Circuit 3 -|--| TC3|---------+ | Stream 2 | |
/ +----+ (Str 1) | | +-----+
/ | +-------+ |
/ +-----------------\-----------------+
Audio, video, and control \
signals are carried in frames Tmux is an ephemeral with two
spanning the circuits. explicit Stream Descriptors
and a Multiplex Descriptor.
Figure 4: Multiplexed Termination Scenario - Circuit to Packet
(Asterisks * denote the centre of the context)
Context
+--------------------------------------+
| +-------+ +-------+ |
+----+ | | | | +----+
Circuit 1 ----| TC1|---+ Tmux1 | Audio | Tmux2 +---| TC4|---
+----+ | +---*----+ | +----+
| | | Str 1 | | |
+----+ | H.22x | | H.22x | +----+
Circuit 2 ----| TC2|---+ multi-| | multi-+---| TC5|---
+----+ | plex | | plex | +----+
| | | Video | | |
+----+ | +---*----+ | +----+
Circuit 3 ----| TC3|---+ | Str 2 | +---| TC6|---
+----+ | | | | +----+
| +-------+ +-------+ |
+-----------------\-----/--------------+
\ /
Tmux1 and Tmux2 are ephemerals each with two
explicit Stream Descriptors and a Multiplex Descriptor.
Figure 5: Multiplexed Termination Scenario - Circuit to Circuit
(Asterisks * denote the centre of the context)
+-----------------------------------+
| Context +-------+ |
+----+ | | |
Circuit 1 -|--| TC1|---------+ Tmux | |
| +----+ (Str 1) | | Audio +-----+
| | | +-----*-----+ TR1 |-----
| +----+ | H.22x | Stream 1 |(RTP)|
Circuit 2 -|--| TC2|---------+ multi-| +-----+
| +----+ (Str 1) | plex | |
| | | | Video +-----+
| +----+ | +-----*-----+ TR2 |-----
Circuit 3 -|--| TC3|---------+ | Stream 2 |(RTP)|
/ +----+ (Str 1) | | +-----+
/ | +-------+ |
/ +-----------------\-----------------+
Audio, video, and control \ Tmux is an ephemeral with two
signals are carried in frames explicit Stream Descriptors and
spanning the circuits. and a Multiplex Descriptor.
Figure 6: Multiplexed Termination Scenario - Single to Multiple
Terminations
(Asterisks * denote the centre of the context)
Context
+---------------------------------------------+
| +-------+ +-------+ |
Cct 1 +----+ | | | | Audio +-----+
----| TC1|---+ Tmux1 | | Tmux2 +-----*-----| TR1 |-----
+----+ | | | | Stream 1 |(RTP)|
| | | Data | | +-----+
Cct 2 +----+ | H.226 +-------+ H.223 | |
----| TC2|---+ multi-|(Str 1)| multi-| Control +-----+
+----+ | plex | | plex +-----*-----+ Tctl|-----
| | | | | Stream 3 +-----+
Cct 3 +----+ | | | | |
----| TC3|---+ | | | +-----+
+----+ | | | +-----*-----+ TR2 |-----
| +-------+ | | Video |(RTP)|
| +-------+ Stream 2 +-----+
| |
+---------------------------------------------+
Tmux1 has a Multiplex Descriptor and a single data stream.
Tmux2 has a Multiplex Descriptor with a single bearer and
three explicit Stream Descriptors.
Figure 6a: Multiplexed Termination Scenario - Cascaded Multiplexes
(Asterisks * denote the centre of the context)
Note: this figure does not appear in Rec. H.248.1
Terminations may be created which represent multiplexed bearers, such
as an ATM AAL Type 2 bearer. When a new multiplexed bearer is to be
created, an ephemeral Termination is created in a Context established
for this purpose. When the Termination is subtracted, the
multiplexed bearer is destroyed.
6.2.1 Termination dynamics
The protocol can be used to create new Terminations and to modify
property values of existing Terminations. These modifications
include the possibility of adding or removing events and/or signals.
The Termination properties, and events and signals are described in
the ensuing subclauses. An MGC can only release/modify Terminations
and the resources that the Termination represents which it has
previously seized via, e.g., the Add command.
6.2.2 TerminationIDs
Terminations are referenced by a TerminationID, which is an arbitrary
schema chosen by the MG.
TerminationIDs of physical Terminations are provisioned in the Media
Gateway. The TerminationIDs may be chosen to have structure. For
instance, a TerminationID may consist of trunk group and a trunk
within the group.
A wildcarding mechanism using two types of wildcards can be used with
TerminationIDs. The two wildcards are ALL and CHOOSE. The former is
used to address multiple Terminations at once, while the latter is
used to indicate to a media gateway that it must select a Termination
satisfying the partially specified TerminationID. This allows, for
instance, that a MGC instructs a MG to choose a circuit within a
trunk group.
When ALL is used in the TerminationID of a command, the effect is
identical to repeating the command with each of the matching
TerminationIDs. The use of ALL does not address the ROOT
termination. Since each of these commands may generate a response,
the size of the entire response may be large. If individual
responses are not required, a wildcard response may be requested. In
such a case, a single response is generated, which contains the UNION
of all of the individual responses which otherwise would have been
generated, with duplicate values suppressed. For instance, given a
Termination Ta with properties p1=a, p2=b and Termination Tb with
properties p2=c, p3=d, a UNION response would consist of a wildcarded
TerminationId and the sequence of properties p1=a, p2=b,c and p3=d.
Wildcard response may be particularly useful in the Audit commands.
The encoding of the wildcarding mechanism is detailed in Annexes A
and B.
6.2.3 Packages
Different types of gateways may implement Terminations that have
widely differing characteristics. Variations in Terminations are
accommodated in the protocol by allowing Terminations to have
optional Properties, Events, Signals and Statistics implemented by
MGs.
In order to achieve MG/MGC interoperability, such options are grouped
into Packages, and typically a Termination realizes a set of such
Packages. More information on definition of packages can be found in
clause 12. An MGC can audit a Termination to determine which
Packages it realizes.
Properties, Events, Signals and Statistics defined in Packages, as
well as parameters to them, are referenced by identifiers (Ids).
Identifiers are scoped. For each package, PropertyIds, EventIds,
SignalIds, StatisticsIds and ParameterIds have unique name spaces and
the same identifier may be used in each of them. Two PropertyIds in
different packages may also have the same identifier, etc.
To support a particular package the MG must support all properties,
signals, events and statistics defined in a package. It must also
support all Signal and Event parameters. The MG may support a subset
of the values listed in a package for a particular Property or
Parameter.
When packages are extended, the properties, events, signals and
statistics defined in the base package can be referred to using
either the extended package name or the base package name. For
example, if Package A defines event e1, and Package B extends Package
A, then B/e1 is an event for a termination implementing Package B. By
definition, the MG MUST also implement the base Package, but it is
optional to publish the base package as an allowed interface. If it
does publish A, then A would be reported on the Package Descriptor
in AuditValue as well as B, and event A/e1 would be available on a
termination. If the MG does not publish A, then only B/e1 would be
available. If published through AuditValue, A/e1 and B/e1 are the
same event.
For improved interoperability and backward compatibility, an MG MAY
publish all Packages supported by its Terminations, including base
Packages from which extended Packages are derived. An exception to
this is in cases where the base packages are expressly "Designed to
be extended only".
6.2.4 Termination properties and descriptors
Terminations have properties. The properties have unique
PropertyIDs. Most properties have default values, which are
explicitly defined in this protocol specification or in a package
(see clause 12) or set by provisioning. If not provisioned
otherwise, the properties in all descriptors except TerminationState
and LocalControl default to empty/"no value" when a Termination is
first created or returned to the null Context. The default contents
of the two exceptions are described in 7.1.5 and 7.1.7.
The provisioning of a property value in the MG will override any
default value, be it supplied in this protocol specification or in a
package. Therefore if it is essential for the MGC to have full
control over the property values of a Termination, it should supply
explicit values when ADDing the Termination to a Context.
Alternatively, for a physical Termination the MGC can determine any
provisioned property values by auditing the Termination while it is
in the NULL Context.
There are a number of common properties for Terminations and
properties specific to media streams. The common properties are also
called the Termination state properties. For each media stream,
there are local properties and properties of the received and
transmitted flows.
Properties not included in the base protocol are defined in Packages.
These properties are referred to by a name consisting of the
PackageName and a PropertyId. Most properties have default values
described in the Package description. Properties may be read-only or
read/write. The possible values of a property may be audited, as can
their current values. For properties that are read/write, the MGC
can set their values. A property may be declared as "Global" which
has a single value shared by all Terminations realizing the package.
Related properties are grouped into descriptors for convenience.
When a Termination is added to a Context, the value of its read/write
properties can be set by including the appropriate descriptors as
parameters to the Add command. Similarly, a property of a
Termination in a Context may have its value changed by the Modify
command.
Properties may also have their values changed when a Termination is
moved from one Context to another as a result of a Move command. In
some cases, descriptors are returned as output from a command.
In general, if a Descriptor is completely omitted from one of the
aforementioned Commands, the properties in that Descriptor retain
their prior values for the Termination(s) upon which the Command
acts. On the other hand, if some read/write properties are omitted
from a Descriptor in a Command (i.e., the Descriptor is only
partially specified), those properties will be reset to their default
values for the Termination(s) upon which the Command acts, unless the
package specifies other behavior. For more details, see clause 7.1
dealing with the individual Descriptors.
The following table lists all of the possible descriptors and their
use. Not all descriptors are legal as input or output parameters to
every command.
Descriptor name Description
Modem Identifies modem type and properties when
applicable
Mux Describes multiplex type for multimedia
Terminations (e.g., H.221, H.223, H.225.0) and
Terminations forming the input mux
Media A list of media stream specifications (see 7.1.4)
TerminationState Properties of a Termination (which can be defined
in Packages) that are not stream specific
Stream A list of remote/local/localControl descriptors for
a single stream
Local Contains properties that specify the media flows
that the MG receives from the remote entity.
Remote Contains properties that specify the media flows
that the MG sends to the remote entity.
LocalControl Contains properties (which can be defined in
packages) that are of interest between the MG and
the MGC.
Events Describes events to be detected by the MG and what
to do when an event is detected.
EventBuffer Describes events to be detected by the MG when
Event Buffering is active.
Signals Describes signals (see 7.1.11) applied to
Terminations.
Audit In Audit commands, identifies which information is
desired.
Packages In AuditValue, returns a list of Packages realized
by Termination.
DigitMap Defines patterns against which sequences of a
specified set of events are to be matched so they
can be reported as a group rather than singly.
ServiceChange In ServiceChange, what, why service change
occurred, etc.
ObservedEvents In Notify or AuditValue, report of events observed.
Statistics In Subtract and Audit, report of Statistics kept on
a Termination.
Topology Specifies flow directions between Terminations in a
Context.
Error Contains an error code and optionally error text;
it may occur in command replies and in Notify
requests.
6.2.5 Root Termination
Occasionally, a command must refer to the entire gateway, rather than
a Termination within it. A special TerminationID, "Root" is reserved
for this purpose. Packages may be defined on Root. Root thus may
have properties, events and statistics (signals are not appropriate
for root). Accordingly, the root TerminationID may appear in:
- a Modify command - to change a property or set an event
- a Notify command - to report an event
- an AuditValue return - to examine the values of properties and
statistics implemented on root
- an AuditCapability - to determine what properties of root are
implemented
- a ServiceChange - to declare the gateway in or out of service.
Any other use of the root TerminationID is an error. Error code
410 - Incorrect identifier shall be returned in these cases.
7 Commands
The protocol provides commands for manipulating the logical entities
of the protocol connection model, Contexts and Terminations.
Commands provide control at the finest level of granularity supported
by the protocol. For example, Commands exist to add Terminations to
a Context, modify Terminations, subtract Terminations from a Context,
and audit properties of Contexts or Terminations. Commands provide
for complete control of the properties of Contexts and Terminations.
This includes specifying which events a Termination is to report,
which signals/actions are to be applied to a Termination and
specifying the topology of a Context (who hears/sees whom).
Most commands are for the specific use of the Media Gateway
Controller as command initiator in controlling Media Gateways as
command responders. The exceptions are the Notify and ServiceChange
commands: Notify is sent from Media Gateway to Media Gateway
Controller, and ServiceChange may be sent by either entity. Below is
an overview of the commands; they are explained in more detail in
7.2.
1) Add - The Add command adds a Termination to a Context. The Add
command on the first Termination in a Context is used to create a
Context.
2) Modify - The Modify command modifies the properties, events and
signals of a Termination.
3) Subtract - The Subtract command disconnects a Termination from its
Context and returns statistics on the Termination's participation
in the Context. The Subtract command on the last Termination in a
Context deletes the Context.
4) Move - The Move command atomically moves a Termination to another
Context.
5) AuditValue - The AuditValue command returns the current state of
properties, events, signals and statistics of Terminations.
6) AuditCapabilities - The AuditCapabilities command returns all the
possible values for Termination properties, events and signals
allowed by the Media Gateway.
7) Notify - The Notify command allows the Media Gateway to inform the
Media Gateway Controller of the occurrence of events in the Media
Gateway.
8) ServiceChange - The ServiceChange command allows the Media Gateway
to notify the Media Gateway Controller that a Termination or group
of Terminations is about to be taken out of service or has just
been returned to service. ServiceChange is also used by the MG to
announce its availability to a MGC (registration), and to notify
the MGC of impending or completed restart of the MG. The MGC may
announce a handover to the MG by sending it a ServiceChange
command. The MGC may also use ServiceChange to instruct the MG to
take a Termination or group of Terminations in or out of service.
These commands are detailed in 7.2.1 through 7.2.8.
7.1 Descriptors
The parameters to a command are termed Descriptors. A descriptor
consists of a name and a list of items. Some items may have values.
Many Commands share common descriptors. This subclause enumerates
these descriptors. Descriptors may be returned as output from a
command. In any such return of descriptor contents, an empty
descriptor is represented by its name unaccompanied by any list.
Parameters and parameter usage specific to a given Command type are
described in the subclause that describes the Command.
7.1.1 Specifying parameters
Command parameters are structured into a number of descriptors. In
general, the text format of descriptors is
DescriptorName=<someID>{parm=value, parm=value, ...}.
Parameters may be fully specified, overspecified or underspecified:
1) Fully specified parameters have a single, unambiguous value that
the command initiator is instructing the command responder to use
for the specified parameter.
2) Underspecified parameters, using the CHOOSE value, allow the
command responder to choose any value it can support.
3) Overspecified parameters have a list of potential values. The
list order specifies the command initiator's order of preference
of selection. The command responder chooses one value from
the offered list and returns that value to the command initiator.
If a required descriptor other than the Audit descriptor is
unspecified (i.e., entirely absent) from a command, the previous
values set in that descriptor for that Termination, if any, are
retained. In commands other than Subtract, a missing Audit
descriptor is equivalent to an empty Audit descriptor. The Behaviour
of the MG with respect to unspecified parameters within a descriptor
varies with the descriptor concerned, as indicated in succeeding
subclauses. Whenever a parameter is underspecified or overspecified,
the descriptor containing the value chosen by the responder is
included as output from the command.
Each command specifies the TerminationId the command operates on.
This TerminationId may be "wildcarded". When the TerminationId of a
command is wildcarded, the effect shall be as if the command was
repeated with each of the TerminationIds matched.
7.1.2 Modem descriptor
The Modem descriptor specifies the modem type and parameters, if any,
required for use in e.g., H.324 and text conversation. The
descriptor includes the following modem types: V.18, V.22, V.22 bis,
V.32, V.32 bis, V.34, V.90, V.91, Synchronous ISDN, and allows for
extensions. By default, no Modem descriptor is present in a
Termination.
7.1.3 Multiplex descriptor
In multimedia calls, a number of media streams are carried on a
(possibly different) number of bearers. The multiplex descriptor
associates the media and the bearers. The descriptor includes the
multiplex type:
- H.221;
- H.223;
- H.226;
- V.76;
- possible extensions,
and a set of TerminationIDs representing the multiplexed bearers, in
order. For example:
Mux = H.221{ MyT3/1/2, MyT3/2/13, MyT3/3/6, MyT3/21/22}
7.1.4 Media descriptor
The Media descriptor specifies the parameters for all the media
streams. These parameters are structured into two descriptors: a
TerminationState descriptor, which specifies the properties of a
Termination that are not stream dependent, and one or more Stream
descriptors each of which describes a single media stream.
A stream is identified by a StreamID. The StreamID is used to link
the streams in a Context that belong together. Multiple streams
exiting a Termination shall be synchronized with each other. Within
the Stream descriptor, there are up to three subsidiary descriptors:
LocalControl, Local, and Remote. The relationship between these
descriptors is thus:
Media descriptor
TerminationState Descriptor
Stream descriptor
LocalControl descriptor
Local descriptor
Remote descriptor
As a convenience, LocalControl, Local, or Remote descriptors may be
included in the Media descriptor without an enclosing Stream
descriptor. In this case, the StreamID is assumed to be 1.
7.1.5 TerminationState descriptor
The TerminationState descriptor contains the ServiceStates property,
the EventBufferControl property and properties of a Termination
(defined in Packages) that are not stream specific.
The ServiceStates property describes the overall state of the
Termination (not stream specific). A Termination can be in one of
the following states: "test", "out of service", or "in service". The
"test" state indicates that the Termination is being tested. The
state "out of service" indicates that the Termination cannot be used
for traffic. The state "in service" indicates that a Termination can
be used or is being used for normal traffic. "in service" is the
default state.
Values assigned to Properties may be simple values
(integer/string/enumeration) or may be underspecified, where more
than one value is supplied and the MG may make a choice:
- Alternative Values - multiple values in a list, one of which must
be selected
- Ranges - minimum and maximum values, any value between min and max
must be selected, boundary values included
- Greater Than/Less Than - value must be greater/less than specified
value
- CHOOSE Wildcard - the MG chooses from the allowed values for the
property
The EventBufferControl property specifies whether events are buffered
following detection of an event in the Events descriptor, or
processed immediately. See 7.1.9 for details.
7.1.6 Stream descriptor
A Stream descriptor specifies the parameters of a single
bidirectional stream. These parameters are structured into three
descriptors: one that contains Termination properties specific to a
stream and one each for local and remote flows. The Stream
Descriptor includes a StreamID which identifies the stream. Streams
are created by specifying a new StreamID on one of the Terminations
in a Context. A stream is deleted by setting empty Local and Remote
descriptors for the stream with ReserveGroup and ReserveValue in
LocalControl set to "false" on all Terminations in the Context that
previously supported that stream.
StreamIDs are of local significance between MGC and MG and they are
assigned by the MGC. Within a Context, StreamID is a means by which
to indicate which media flows are interconnected: streams with the
same StreamID are connected.
If a Termination is moved from one Context to another, the effect on
the Context to which the Termination is moved is the same as in the
case that a new Termination were added with the same StreamIDs as the
moved Termination.
7.1.7 LocalControl descriptor
The LocalControl descriptor contains the Mode property, the
ReserveGroup and ReserveValue properties and properties of a
Termination (defined in Packages) that are stream specific, and are
of interest between the MG and the MGC. Values of properties may be
underspecified as in 7.1.1.
The allowed values for the mode property are send-only, receive-only,
send/receive, inactive and loop-back. "Send" and "receive" are with
respect to the exterior of the Context, so that, for example, a
stream set to mode=sendOnly does not pass received media into the
Context. The default value for the mode property is "Inactive".
Signals and Events are not affected by mode.
The boolean-valued Reserve properties, ReserveValue and ReserveGroup,
of a Termination indicate what the MG is expected to do when it
receives a Local and/or Remote descriptor.
If the value of a Reserve property is True, the MG SHALL reserve
resources for all alternatives specified in the Local and/or Remote
descriptors for which it currently has resources available. It SHALL
respond with the alternatives for which it reserves resources. If it
cannot not support any of the alternatives, it SHALL respond with a
reply to the MGC that contains empty Local and/or Remote descriptors.
If media begins to flow while more than a single alternative is
reserved, media packets may be sent/received on any of the
alternatives and must be processed, although only a single
alternative may be active at any given time.
If the value of a Reserve property is False, the MG SHALL choose one
of the alternatives specified in the Local descriptor (if present)
and one of the alternatives specified in the Remote descriptor (if
present). If the MG has not yet reserved resources to support the
selected alternative, it SHALL reserve the resources. If, on the
other hand, it already reserved resources for the Termination
addressed (because of a prior exchange with ReserveValue and/or
ReserveGroup equal to True), it SHALL release any excess resources it
reserved previously. Finally, the MG shall send a reply to the MGC
containing the alternatives for the Local and/or Remote descriptor
that it selected. If the MG does not have sufficient resources to
support any of the alternatives specified, it SHALL respond with
error 510 (insufficient resources).
The default value of ReserveValue and ReserveGroup is False. More
information on the use of the two Reserve properties is provided in
7.1.8.
A new setting of the LocalControl Descriptor completely replaces the
previous setting of that descriptor in the MG. Thus, to retain
information from the previous setting, the MGC must include that
information in the new setting. If the MGC wishes to delete some
information from the existing descriptor, it merely resends the
descriptor (in a Modify command) with the unwanted information
stripped out.
7.1.8 Local and Remote descriptors
The MGC uses Local and Remote descriptors to reserve and commit MG
resources for media decoding and encoding for the given Stream(s) and
Termination to which they apply. The MG includes these descriptors
in its response to indicate what it is actually prepared to support.
The MG SHALL include additional properties and their values in its
response if these properties are mandatory yet not present in the
requests made by the MGC (e.g., by specifying detailed video encoding
parameters where the MGC only specified the payload type).
Local refers to the media received by the MG and Remote refers to the
media sent by the MG.
When text encoding the protocol, the descriptors consist of session
descriptions as defined in SDP (RFC 2327). In session descriptions
sent from the MGC to the MG, the following exceptions to the syntax
of RFC 2327 are allowed:
- the "s=", "t=" and "o=" lines are optional;
- the use of CHOOSE is allowed in place of a single parameter value;
and
- the use of alternatives is allowed in place of a single parameter
value.
A Stream Descriptor specifies a single bi-directional media stream
and so a single session description MUST NOT include more than one
media description ("m=" line). A Stream Descriptor may contain
additional session descriptions as alternatives. Each media stream
for a termination must appear in distinct Stream Descriptors. When
multiple session descriptions are provided in one descriptor, the
"v=" lines are required as delimiters; otherwise they are optional in
session descriptions sent to the MG. Implementations shall accept
session descriptions that are fully conformant to RFC 2327. When
binary encoding the protocol the descriptor consists of groups of
properties (tag-value pairs) as specified in Annex C. Each such
group may contain the parameters of a session description.
Below, the semantics of the Local and Remote descriptors are
specified in detail. The specification consists of two parts. The
first part specifies the interpretation of the contents of the
descriptor. The second part specifies the actions the MG must take
upon receiving the Local and Remote descriptors. The actions to be
taken by the MG depend on the values of the ReserveValue and
ReserveGroup properties of the LocalControl descriptor.
Either the Local or the Remote descriptor or both may be:
1) unspecified (i.e., absent);
2) empty;
3) underspecified through use of CHOOSE in a property value;
4) fully specified; or
5) overspecified through presentation of multiple groups of
properties and possibly multiple property values in one or more of
these groups.
Where the descriptors have been passed from the MGC to the MG, they
are interpreted according to the rules given in 7.1.1, with the
following additional comments for clarification:
a) An unspecified Local or Remote descriptor is considered to be a
missing mandatory parameter. It requires the MG to use whatever
was last specified for that descriptor. It is possible that there
was no previously specified value, in which case the descriptor
concerned is ignored in further processing of the command.
b) An empty Local (Remote) descriptor in a message from the MGC
signifies a request to release any resources reserved for the
media flow received (sent).
c) If multiple groups of properties are present in a Local or Remote
descriptor or multiple values within a group, the order of
preference is descending.
d) Underspecified or overspecified properties within a group of
properties sent by the MGC are requests for the MG to choose one
or more values which it can support for each of those properties.
In case of an overspecified property, the list of values is in
descending order of preference.
Subject to the above rules, subsequent action depends on the values
of the ReserveValue and ReserveGroup properties in LocalControl.
If ReserveGroup is True, the MG reserves the resources required to
support any of the requested property group alternatives that it can
currently support. If ReserveValue is True, the MG reserves the
resources required to support any of the requested property value
alternatives that it can currently support.
NOTE - If a Local or Remote descriptor contains multiple groups of
properties, and ReserveGroup is True, then the MG is requested to
reserve resources so that it can decode or encode the media stream
according to any of the alternatives. For instance, if the Local
descriptor contains two groups of properties, one specifying
packetized G.711 A-law audio and the other G.723.1 audio, the MG
reserves resources so that it can decode one audio stream encoded in
either G.711 A-law format or G.723.1 format. The MG does not have to
reserve resources to decode two audio streams simultaneously, one
encoded in G.711 A-law and one in G.723.1. The intention for the use
of ReserveValue is analogous.
If ReserveGroup is true or ReserveValue is True, then the following
rules apply:
- If the MG has insufficient resources to support all alternatives
requested by the MGC and the MGC requested resources in both Local
and Remote, the MG should reserve resources to support at least
one alternative each within Local and Remote.
- If the MG has insufficient resources to support at least one
alternative within a Local (Remote) descriptor received from the
MGC, it shall return an empty Local (Remote) in response.
- In its response to the MGC, when the MGC included Local and Remote
descriptors, the MG SHALL include Local and Remote descriptors for
all groups of properties and property values it reserved resources
for. If the MG is incapable of supporting at least one of the
alternatives within the Local (Remote) descriptor received from
the MGC, it SHALL return an empty Local (Remote) descriptor.
- If the Mode property of the LocalControl descriptor is RecvOnly,
SendRecv, or LoopBack, the MG must be prepared to receive media
encoded according to any of the alternatives included in its
response to the MGC.
If ReserveGroup is False and ReserveValue is False, then the MG
SHOULD apply the following rules to resolve Local and Remote to a
single alternative each:
- The MG chooses the first alternative in Local for which it is able
to support at least one alternative in Remote.
- If the MG is unable to support at least one Local and one Remote
alternative, it returns Error 510 (Insufficient Resources).
- The MG returns its selected alternative in each of Local and
Remote.
A new setting of a Local or Remote descriptor completely replaces the
previous setting of that descriptor in the MG. Thus, to retain
information from the previous setting, the MGC must include that
information in the new setting. If the MGC wishes to delete some
information from the existing descriptor, it merely resends the
descriptor (in a Modify command) with the unwanted information
stripped out.
7.1.9 Events descriptor
The EventsDescriptor parameter contains a RequestIdentifier and a
list of events that the Media Gateway is requested to detect and
report. The RequestIdentifier is used to correlate the request with
the notifications that it may trigger. Requested events include, for
example, fax tones, continuity test results, and on-hook and off-hook
transitions. The RequestIdentifier is omitted if the
EventsDescriptor is empty (i.e., no events are specified).
Each event in the descriptor contains the Event name, an optional
streamID, an optional KeepActive flag, and optional parameters. The
Event name consists of a Package Name (where the event is defined)
and an EventID. The ALL wildcard may be used for the EventID,
indicating that all events from the specified package have to be
detected. The default streamID is 0, indicating that the event to be
detected is not related to a particular media stream. Events can
have parameters. This allows a single event description to have some
variation in meaning without creating large numbers of individual
events. Further event parameters are defined in the package.
If a digit map completion event is present or implied in the
EventsDescriptor, the EventDM parameter is used to carry either the
name or the value of the associated digit map. See 7.1.14 for
further details.
When an event is processed against the contents of an active Events
Descriptor and found to be present in that descriptor ("recognized"),
the default action of the MG is to send a Notify command to the MGC.
Notification may be deferred if the event is absorbed into the
current dial string of an active digit map (see 7.1.14). Any other
action is for further study. Moreover, event recognition may cause
currently active signals to stop, or may cause the current Events
and/or Signals descriptor to be replaced, as described at the end of
this subclause. Unless the Events Descriptor is replaced by another
Events Descriptor, it remains active after an event has been
recognized.
If the value of the EventBufferControl property equals LockStep,
following detection of such an event, normal handling of events is
suspended. Any event which is subsequently detected and occurs in
the EventBuffer descriptor is added to the end of the EventBuffer (a
FIFO queue), along with the time that it was detected. The MG SHALL
wait for a new EventsDescriptor to be loaded. A new EventsDescriptor
can be loaded either as the result of receiving a command with a new
EventsDescriptor, or by activating an embedded EventsDescriptor.
If EventBufferControl equals Off, the MG continues processing based
on the active EventsDescriptor.
In the case of an embedded EventsDescriptor being activated, the MG
continues event processing based on the newly activated
EventsDescriptor.
NOTE 1 - For purposes of EventBuffer handling, activation of an
embedded EventsDescriptor is equivalent to receipt of a new
EventsDescriptor.
When the MG receives a command with a new EventsDescriptor, one or
more events may have been buffered in the EventBuffer in the MG. The
value of EventBufferControl then determines how the MG treats such
buffered events.
Case 1
If EventBufferControl equals LockStep and the MG receives a new
EventsDescriptor, it will check the FIFO EventBuffer and take the
following actions:
1) If the EventBuffer is empty, the MG waits for detection of events
based on the new EventsDescriptor.
2) If the EventBuffer is non-empty, the MG processes the FIFO queue
starting with the first event:
a) If the event in the queue is in the events listed in the new
EventsDescriptor, the MG acts on the event and removes the
event from the EventBuffer. The time stamp of the Notify shall
be the time the event was actually detected. The MG then waits
for a new EventsDescriptor. While waiting for a new
EventsDescriptor, any events detected that appear in the
EventsBufferDescriptor will be placed in the EventBuffer. When
a new EventsDescriptor is received, the event processing will
repeat from step 1.
b) If the event is not in the new EventsDescriptor, the MG SHALL
discard the event and repeat from step 1.
Case 2
If EventBufferControl equals Off and the MG receives a new
EventsDescriptor, it processes new events with the new
EventsDescriptor.
If the MG receives a command instructing it to set the value of
EventBufferControl to Off, all events in the EventBuffer SHALL be
discarded.
The MG may report several events in a single Transaction as long as
this does not unnecessarily delay the reporting of individual events.
For procedures regarding transmitting the Notify command, refer to
the appropriate annex or Recommendation of the H.248 sub-series for
specific transport considerations.
The default value of EventBufferControl is Off.
NOTE 2 - Since the EventBufferControl property is in the
TerminationStateDescriptor, the MG might receive a command that
changes the EventBufferControl property and does not include an
EventsDescriptor.
Normally, recognition of an event shall cause any active signals to
stop. When KeepActive is specified in the event, the MG shall not
interrupt any signals active on the Termination on which the event is
detected.
An event can include an Embedded Signals descriptor and/or an
Embedded Events descriptor which, if present, replaces the current
Signals/Events descriptor when the event is recognized. It is
possible, for example, to specify that the dial-tone Signal be
generated when an off-hook Event is recognized, or that the dial-tone
Signal be stopped when a digit is recognized. A media gateway
controller shall not send EventsDescriptors with an event both marked
KeepActive and containing an embedded SignalsDescriptor.
Only one level of embedding is permitted. An embedded
EventsDescriptor SHALL NOT contain another embedded EventsDescriptor;
an embedded EventsDescriptor MAY contain an embedded
SignalsDescriptor.
An EventsDescriptor received by a media gateway replaces any previous
Events descriptor. Event notification in process shall complete, and
events detected after the command containing the new EventsDescriptor
executes, shall be processed according to the new EventsDescriptor.
An empty Events Descriptor disables all event recognition and
reporting. An empty EventBuffer Descriptor clears the EventBuffer
and disables all event accumulation in LockStep mode: the only events
reported will be those occurring while an Events Descriptor is
active. If an empty Events Descriptor is activated while the
Termination is operating in LockStep mode, the events buffer is
immediately cleared.
7.1.10 EventBuffer descriptor
The EventBuffer descriptor contains a list of events, with their
parameters if any, that the MG is requested to detect and buffer when
EventBufferControl equals LockStep (see 7.1.9).
7.1.11 Signals descriptor
Signals are MG generated media such as tones and announcements as
well as bearer-related signals such as hookswitch. More complex
signals may include a sequence of such simple signals interspersed
with and conditioned upon the receipt and analysis of media or
bearer-related signals. Examples include echoing of received data as
in Continuity Test package. Signals may also request preparation of
media content for future signals.
A SignalsDescriptor is a parameter that contains the set of signals
that the Media Gateway is asked to apply to a Termination. A
SignalsDescriptor contains a number of signals and/or sequential
signal lists. A SignalsDescriptor may contain zero signals and
sequential signal lists. Support of sequential signal lists is
optional.
Signals are defined in packages. Signals shall be named with a
Package name (in which the signal is defined) and a SignalID. No
wildcard shall be used in the SignalID. Signals that occur in a
SignalsDescriptor have an optional StreamID parameter (default is 0,
to indicate that the signal is not related to a particular media
stream), an optional signal type (see below), an optional duration
and possibly parameters defined in the package that defines the
signal. This allows a single signal to have some variation in
meaning, obviating the need to create large numbers of individual
signals.
Finally, the optional parameter "notifyCompletion" allows a MGC to
indicate that it wishes to be notified when the signal finishes
playout. The possible cases are that the signal timed out (or
otherwise completed on its own), that it was interrupted by an event,
that it was halted when a Signals descriptor was replaced, or that it
stopped or never started for other reasons. If the notifyCompletion
parameter is not included in a Signals descriptor, notification is
generated only if the signal stopped or was never started for other
reasons. For reporting to occur, the signal completion event (see
E.1.2) must be enabled in the currently active Events descriptor.
The duration is an integer value that is expressed in hundredths of a
second.
There are three types of signals:
- on/off - the signal lasts until it is turned off;
- timeout - the signal lasts until it is turned off or a specific
period of time elapses;
- brief - the signal will stop on its own unless a new Signals
descriptor is applied that causes it to stop; no timeout value is
needed.
If a signal of default type other than TO has its type overridden to
type TO in the Signals descriptor, the duration parameter must be
present.
If the signal type is specified in a SignalsDescriptor, it overrides
the default signal type (see 12.1.4). If duration is specified for
an on/off signal, it SHALL be ignored.
A sequential signal list consists of a signal list identifier and a
sequence of signals to be played sequentially. Only the trailing
element of the sequence of signals in a sequential signal list may be
an on/off signal. The duration of a sequential signal list is the
sum of the durations of the signals it contains.
Multiple signals and sequential signal lists in the same
SignalsDescriptor shall be played simultaneously.
Signals are defined as proceeding from the Termination towards the
exterior of the Context unless otherwise specified in a package.
When the same Signal is applied to multiple Terminations within one
Transaction, the MG should consider using the same resource to
generate these Signals.
Production of a Signal on a Termination is stopped by application of
a new SignalsDescriptor, or detection of an Event on the Termination
(see 7.1.9).
A new SignalsDescriptor replaces any existing SignalsDescriptor. Any
signals applied to the Termination not in the replacement descriptor
shall be stopped, and new signals are applied, except as follows.
Signals present in the replacement descriptor and containing the
KeepActive flag shall be continued if they are currently playing and
have not already completed. If a replacement signal descriptor
contains a signal that is not currently playing and contains the
KeepActive flag, that signal SHALL be ignored. If the replacement
descriptor contains a sequential signal list with the same identifier
as the existing descriptor, then
- the signal type and sequence of signals in the sequential signal
list in the replacement descriptor shall be ignored; and
- the playing of the signals in the sequential signal list in the
existing descriptor shall not be interrupted.
7.1.12 Audit descriptor
The Audit descriptor specifies what information is to be audited.
The Audit descriptor specifies the list of descriptors to be
returned. Audit may be used in any command to force the return of
any descriptor containing the current values of its properties,
events, signals and statistics even if that descriptor was not
present in the command, or had no underspecified parameters.
Possible items in the Audit descriptor are:
Modem
Mux
Events
Media
Signals
ObservedEvents
DigitMap
Statistics
Packages
EventBuffer
Audit may be empty, in which case, no descriptors are returned. This
is useful in Subtract, to inhibit return of statistics, especially
when using wildcard.
7.1.13 ServiceChange descriptor
The ServiceChangeDescriptor contains the following parameters:
. ServiceChangeMethod
. ServiceChangeReason
. ServiceChangeAddress
. ServiceChangeDelay
. ServiceChangeProfile
. ServiceChangeVersion
. ServiceChangeMGCId
. TimeStamp
. Extension
See 7.2.8.
7.1.14 DigitMap descriptor
7.1.14.1 DigitMap definition, creation, modification and deletion
A DigitMap is a dialing plan resident in the Media Gateway used for
detecting and reporting digit events received on a Termination. The
DigitMap descriptor contains a DigitMap name and the DigitMap to be
assigned. A digit map may be preloaded into the MG by management
action and referenced by name in an EventsDescriptor, may be defined
dynamically and subsequently referenced by name, or the actual
digitmap itself may be specified in the EventsDescriptor. It is
permissible for a digit map completion event within an Events
descriptor to refer by name to a DigitMap which is defined by a
DigitMap descriptor within the same command, regardless of the
transmitted order of the respective descriptors.
DigitMaps defined in a DigitMapDescriptor can occur in any of the
standard Termination manipulation Commands of the protocol. A
DigitMap, once defined, can be used on all Terminations specified by
the (possibly wildcarded) TerminationID in such a command. DigitMaps
defined on the root Termination are global and can be used on every
Termination in the MG, provided that a DigitMap with the same name
has not been defined on the given Termination. When a DigitMap is
defined dynamically in a DigitMap descriptor:
- A new DigitMap is created by specifying a name that is not yet
defined. The value shall be present.
- A DigitMap value is updated by supplying a new value for a name
that is already defined. Terminations presently using the
digitmap shall continue to use the old definition; subsequent
EventsDescriptors specifying the name, including any
EventsDescriptor in the command containing the DigitMap
descriptor, shall use the new one.
- A DigitMap is deleted by supplying an empty value for a name that
is already defined. Terminations presently using the digitmap
shall continue to use the old definition.
7.1.14.2 DigitMap Timers
The collection of digits according to a DigitMap may be protected by
three timers, viz. a start timer (T), short timer (S), and long timer
(L).
1) The start timer (T) is used prior to any digits having been
dialed. If the start timer is overridden with the value set to
zero (T=0), then the start timer shall be disabled. This implies
that the MG will wait indefinitely for digits.
2) If the Media Gateway can determine that at least one more digit is
needed for a digit string to match any of the allowed patterns in
the digit map, then the interdigit timer value should be set to a
long (L) duration (e.g., 16 seconds).
3) If the digit string has matched one of the patterns in a digit
map, but it is possible that more digits could be received which
would cause a match with a different pattern, then instead of
reporting the match immediately, the MG must apply the short timer
(S) and wait for more digits.
The timers are configurable parameters to a DigitMap. Default values
of these timers should be provisioned on the MG, but can be
overridden by values specified within the DigitMap.
7.1.14.3 DigitMap Syntax
The formal syntax of the digit map is described by the DigitMap rule
in the formal syntax description of the protocol (see Annex A and
Annex B). A DigitMap, according to this syntax, is defined either by
a string or by a list of strings. Each string in the list is an
alternative event sequence, specified either as a sequence of digit
map symbols or as a regular expression of digit map symbols. These
digit map symbols, the digits "0" through "9" and letters "A" through
a maximum value depending on the signalling system concerned, but
never exceeding "K", correspond to specified events within a package
which has been designated in the Events descriptor on the Termination
to which the digit map is being applied. (The mapping between events
and digit map symbols is defined in the documentation for packages
associated with channel-associated signalling systems such as DTMF,
MF, or R2. Digits "0" through "9" MUST be mapped to the
corresponding digit events within the signalling system concerned.
Letters should be allocated in logical fashion, facilitating the use
of range notation for alternative events.)
The letter "x" is used as a wildcard, designating any event
corresponding to symbols in the range "0"-"9". The string may also
contain explicit ranges and, more generally, explicit sets of
symbols, designating alternative events any one of which satisfies
that position of the digit map. Finally, the dot symbol "." stands
for zero or more repetitions of the event selector (event, range of
events, set of alternative events, or wildcard) that precedes it. As
a consequence of the third timing rule above, inter-event timing
while matching a terminal dot symbol uses the short timer by default.
In addition to these event symbols, the string may contain "S" and
"L" inter-event timing specifiers and the "Z" duration modifier. "S"
and "L" respectively indicate that the MG should use the short (S)
timer or the long (L) timer for subsequent events, overriding the
timing rules described above. If an explicit timing specifier is in
effect in one alternative event sequence, but none is given in any
other candidate alternative, the timer value set by the explicit
timing specifier must be used. If all sequences with explicit timing
controls are dropped from the candidate set, timing reverts to the
default rules given above. Finally, if conflicting timing specifiers
are in effect in different alternative sequences, the long timer
shall be used.
A "Z" designates a long duration event: placed in front of the
symbol(s) designating the event(s) which satisfy a given digit
position, it indicates that that position is satisfied only if the
duration of the event exceeds the long-duration threshold. The value
of this threshold is assumed to be provisioned in the MG.
7.1.14.4 DigitMap Completion Event
A digit map is active while the Events descriptor which invoked it is
active and it has not completed. A digit map completes when:
- a timer has expired; or
- an alternative event sequence has been matched and no other
alternative event sequence in the digit map could be matched
through detection of an additional event (unambiguous match); or
- an event has been detected such that a match to a complete
alternative event sequence of the digit map will be impossible no
matter what additional events are received.
Upon completion, a digit map completion event as defined in the
package providing the events being mapped into the digit map shall be
generated. At that point the digit map is deactivated. Subsequent
events in the package are processed as per the currently active event
processing mechanisms.
7.1.14.5 DigitMap Procedures
Pending completion, successive events shall be processed according to
the following rules:
1) The "current dial string", an internal variable, is initially
empty. The set of candidate alternative event sequences includes
all of the alternatives specified in the digit map.
2) At each step, a timer is set to wait for the next event, based
either on the default timing rules given above or on explicit
timing specified in one or more alternative event sequences. If
the timer expires and a member of the candidate set of
alternatives is fully satisfied, a timeout completion with full
match is reported. If the timer expires and part or none of any
candidate alternative is satisfied, a timeout completion with
partial match is reported.
3) If an event is detected before the timer expires, it is mapped to
a digit string symbol and provisionally added to the end of the
current dial string. The duration of the event (long or not long)
is noted if and only if this is relevant in the current symbol
position (because at least one of the candidate alternative event
sequences includes the "Z" modifier at this position in the
sequence).
4) The current dial string is compared to the candidate alternative
event sequences. If and only if a sequence expecting a
long-duration event at this position is matched (i.e., the event
had long duration and met the specification for this position),
then any alternative event sequences not specifying a long
duration event at this position are discarded, and the current
dial string is modified by inserting a "Z" in front of the symbol
representing the latest event. Any sequence expecting a long-
duration event at this position but not matching the observed
event is discarded from the candidate set. If alternative event
sequences not specifying a long duration event in the given
position remain in the candidate set after application of the
above rules, the observed event duration is treated as irrelevant
in assessing matches to them.
5) If exactly one candidate remains and it has been fully matched, a
completion event is generated indicating an unambiguous match. If
no candidates remain, the latest event is removed from the current
dial string and a completion event is generated indicating full
match if one of the candidates from the previous step was fully
satisfied before the latest event was detected, or partial match
otherwise. The event removed from the current dial string will
then be reported as per the currently active event processing
mechanisms.
6) If no completion event is reported out of step 5, processing
returns to step 2.
7.1.14.6 DigitMap Activation
A digit map is activated whenever a new Event descriptor is applied
to the Termination or embedded Event descriptor is activated, and
that Event descriptor contains a digit map completion event. The
digit map completion event contains an eventDM field in the requested
actions field. Each new activation of a digit map begins at step 1
of the above procedure, with a clear current dial string. Any
previous contents of the current dial string from an earlier
activation are lost.
A digit map completion event that does not contain an eventDM field
in its requested actions field is considered an error. Upon receipt
of such an event in an EventsDescriptor, a MG shall respond with an
error response, including Error 457 - Missing parameter in signal or
event.
7.1.14.7 Interaction Of DigitMap and Event Processing
While the digit map is activated, detection is enabled for all events
defined in the package containing the specified digit map completion
event. Normal event behaviour (e.g., stopping of signals unless the
digit completion event has the KeepActive flag enabled) continues to
apply for each such event detected, except that:
- the events in the package containing the specified digit map
completion event other than the completion event itself are not
individually notified and have no side-effects unless separately
enabled; and
- an event that triggers a partial match completion event is not
recognized and therefore has no side effects until reprocessed
following the recognition of the digit map completion event.
7.1.14.8 Wildcards
Note that if a package contains a digit map completion event, then an
event specification consisting of the package name with a wildcarded
ItemID (Property Name) will activate a digit map; to that end, the
event specification must include an eventDM field according to
section 7.1.14.6. If the package also contains the digit events
themselves, this form of event specification will cause the
individual events to be reported to the MGC as they are detected.
7.1.14.9 Example
As an example, consider the following dial plan:
0 Local operator
00 Long-distance operator
xxxx Local extension number (starts with 1-7)
8xxxxxxx Local number
#xxxxxxx Off-site extension
*xx Star services
91xxxxxxxxxx Long-distance number
9011 + up to 15 digits International number
If the DTMF detection package described in E.6 is used to collect the
dialed digits, then the dialing plan shown above results in the
following digit map:
(0| 00|[1-7]xxx|8xxxxxxx|Fxxxxxxx|Exx|91xxxxxxxxxx|9011x.)
7.1.15 Statistics descriptor
The Statistics Descriptor provides information describing the status
and usage of a Termination during its existence within a specific
Context. There is a set of standard statistics kept for each
Termination where appropriate (number of octets sent and received for
example). The particular statistical properties that are reported
for a given Termination are determined by the Packages realized by
the Termination. By default, statistics are reported when the
Termination is Subtracted from the Context. This behaviour can be
overridden by including an empty AuditDescriptor in the Subtract
command. Statistics may also be returned from the AuditValue
command, or any Add/Move/Modify command using the Audit descriptor.
Statistics are cumulative; reporting Statistics does not reset them.
Statistics are reset when a Termination is Subtracted from a Context.
7.1.16 Packages descriptor
Used only with the AuditValue command, the PackageDescriptor returns
a list of Packages realized by the Termination.
7.1.17 ObservedEvents descriptor
ObservedEvents is supplied with the Notify command to inform the MGC
of which event(s) were detected. Used with the AuditValue command,
the ObservedEventsDescriptor returns events in the event buffer which
have not been Notified. ObservedEvents contains the
RequestIdentifier of the EventsDescriptor that triggered the
notification, the event(s) detected, optionally the detection time(s)
and any parameters of the observed event. Detection times are
reported with a precision of hundredths of a second.
7.1.18 Topology descriptor
A Topology descriptor is used to specify flow directions between
Terminations in a Context. Contrary to the descriptors in previous
subclauses, the Topology descriptor applies to a Context instead of a
Termination. The default topology of a Context is that each
Termination's transmission is received by all other Terminations.
The Topology descriptor is optional to implement. An MG that does
not support Topology descriptors, but receives a command containing
one, returns Error 444 Unsupported or unknown descriptor, and
optionally includes a string containing the name of the unsupported
Descriptor ("Topology") in the error text in the error descriptor.
The Topology descriptor occurs before the commands in an action. It
is possible to have an action containing only a Topology descriptor,
provided that the Context to which the action applies already exists.
A Topology descriptor consists of a sequence of triples of the form
(T1, T2, association). T1 and T2 specify Terminations within the
Context, possibly using the ALL or CHOOSE wildcard. The association
specifies how media flows between these two Terminations as follows.
- (T1, T2, isolate) means that the Terminations matching T2 do not
receive media from the Terminations matching T1, nor vice versa.
- (T1, T2, oneway) means that the Terminations that match T2 receive
media from the Terminations matching T1, but not vice versa. In
this case use of the ALL wildcard such that there are Terminations
that match both T1 and T2 is not allowed.
- (T1, T2, bothway) means that the Terminations matching T2 receive
media from the Terminations matching T1, and vice versa. In this
case it is allowed to use wildcards such that there are
Terminations that match both T1 and T2. However, if there is a
Termination that matches both, no loopback is introduced.
CHOOSE wildcards may be used in T1 and T2 as well, under the
following restrictions:
- the action (see clause 8) of which the topology descriptor is part
contains an Add command in which a CHOOSE wildcard is used;
- if a CHOOSE wildcard occurs in T1 or T2, then a partial name SHALL
NOT be specified.
The CHOOSE wildcard in a Topology descriptor matches the
TerminationID that the MG assigns in the first Add command that uses
a CHOOSE wildcard in the same action. An existing Termination that
matches T1 or T2 in the Context to which a Termination is added, is
connected to the newly added Termination as specified by the Topology
descriptor.
If a termination is not mentioned within a Topology Descriptor, any
topology associated with it remains unchanged. If, however, a new
termination is added into a context its association with the other
terminations within the context defaults to bothway, unless a
Topology Descriptor is given to change this (e.g., if T3 is added to
a context with T1 and T2 with topology (T3, T1, oneway) it will be
connected bothway to T2).
Figure 7 and the table following it show some examples of the effect
of including topology descriptors in actions. In these examples it
is assumed that the topology descriptors are applied in sequence.
+------------------+ +------------------+ +------------------+
| +----+ | | +----+ | | +----+ |
| | T2 | | | | T2 | | | | T2 | |
| +----+ | | +----+ | | +----+ |
| ^ ^ | | ^ | | ^ |
| | | | | | | | | |
| +--+ +--+ | | +---+ | | +--+ |
| | | | | | | | | |
| v v | | v | | | |
| +----+ +----+ | | +----+ +----+ | | +----+ +----+ |
| | T1 |<-->| T3 | | | | T1 |<-->| T3 | | | | T1 |<-->| T3 | |
| +----+ +----+ | | +----+ +----+ | | +----+ +----+ |
+------------------+ +------------------+ +------------------+
1. No Topology Desc. 2. T1, T2, Isolate 3. T3, T2, Oneway
+------------------+ +------------------+ +------------------+
| +----+ | | +----+ | | +----+ |
| | T2 | | | | T2 | | | | T2 | |
| +----+ | | +----+ | | +----+ |
| | | | ^ | | ^ ^ |
| | | | | | | | | |
| +--+ | | +---+ | | +--+ +--+ |
| | | | | | | | | |
| v | | v | | v v |
| +----+ +----+ | | +----+ +----+ | | +----+ +----+ |
| | T1 |<-->| T3 | | | | T1 |<-->| T3 | | | | T1 |<-->| T3 | |
| +----+ +----+ | | +----+ +----+ | | +----+ +----+ |
+------------------+ +------------------+ +------------------+
4. T2, T3 oneway 5. T2, T3 bothway 6. T1, T2 bothway
Note: the direction of the arrow indicates the direction of flow.
Figure 7: Example topologies
Topology Description
1 No topology descriptors When no topology descriptors are
included, all Terminations have a
bothway connection to all other
Terminations.
2 T1, T2 Isolate Removes the connection between T1 and
T2. T3 has a bothway connection with
both T1 and T2. T1 and T2 have bothway
connection to T3.
3 T3, T2 oneway A oneway connection from T3 to T2 (i.e.,
T2 receives media flow from T3). A
bothway connection between T1 and T3.
4 T2, T3 oneway A oneway connection between T2 to T3.
T1 and T3 remain bothway connected.
5 T2, T3 bothway T2 is bothway connected to T3. This
results in the same as 2.
6 T1, T2 bothway (T2, T3 All Terminations have a bothway
bothway and T1, T3 connection to all other Terminations.
bothway may be implied or
explicit).
A oneway connection must be implemented in such a way that the other
Terminations in the Context are not aware of the change in topology.
7.1.19 Error Descriptor
If a responder encounters an error when processing a transaction
request, it must include an error descriptor in its response. A
Notify request may contain an error descriptor as well.
An error descriptor consists of an IANA-registered error code,
optionally accompanied by an error text. H.248.8 contains a list of
valid error codes and error descriptions.
An error descriptor shall be specified at the "deepest level" that is
semantically appropriate for the error being described and that is
possible given any parsing problems with the original request. An
error descriptor may refer to a syntactical construct other than
where it appears. For example, Error descriptor 422 - Syntax Error
in Action, could appear within a command even though it refers to the
larger construct - the action - and not the particular command within
which it appears.
7.2 Command Application Programming Interface
Following is an Application Programming Interface (API) describing
the Commands of the protocol. This API is shown to illustrate the
Commands and their parameters and is not intended to specify
implementation (e.g., via use of blocking function calls). It
describes the input parameters in parentheses after the command name
and the return values in front of the Command. This is only for
descriptive purposes; the actual Command syntax and encoding are
specified in later subclauses. The order of parameters to commands
is not fixed. Descriptors may appear as parameters to commands in
any order. The descriptors SHALL be processed in the order in which
they appear.
Any reply to a command may contain an error descriptor; the API does
not specifically show this.
All parameters enclosed by square brackets ([. . .]) are considered
optional.
7.2.1 Add
The Add Command adds a Termination to a Context.
TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
Add( TerminationID
[, MediaDescriptor]
[, ModemDescriptor]
[, MuxDescriptor]
[, EventsDescriptor]
[, EventBufferDescriptor]
[, SignalsDescriptor]
[, DigitMapDescriptor]
[, AuditDescriptor]
)
The TerminationID specifies the Termination to be added to the
Context. The Termination is either created, or taken from the null
Context. If a CHOOSE wildcard is used in the TerminationID, the
selected TerminationID will be returned. Wildcards may be used in an
Add, but such usage would be unusual. If the wildcard matches more
than one TerminationID, all possible matches are attempted, with
results reported for each one. The order of attempts when multiple
TerminationIDs match is not specified.
The optional MediaDescriptor describes all media streams.
The optional ModemDescriptor and MuxDescriptor specify a modem and
multiplexer if applicable. For convenience, if a Multiplex
descriptor is present in an Add command and lists any Terminations
that are not currently in the Context, such Terminations are added to
the Context as if individual Add commands listing the Terminations
were invoked. If an error occurs on such an implied Add, error 471 -
Implied Add for Multiplex failure shall be returned and further
processing of the command shall cease.
The EventsDescriptor parameter is optional. If present, it provides
the list of events that should be detected on the Termination.
The EventBufferDescriptor parameter is optional. If present, it
provides the list of events that the MG is requested to detect and
buffer when EventBufferControl equals LockStep.
The SignalsDescriptor parameter is optional. If present, it provides
the list of signals that should be applied to the Termination.
The DigitMapDescriptor parameter is optional. If present, it defines
a DigitMap definition that may be used in an EventsDescriptor.
The AuditDescriptor is optional. If present, the command will return
descriptors as specified in the AuditDescriptor.
All descriptors that can be modified could be returned by MG if a
parameter was underspecified or overspecified. ObservedEvents,
Statistics, and Packages, and the EventBuffer descriptors are
returned only if requested in the AuditDescriptor.
Add SHALL NOT be used on a Termination with a serviceState of
"OutofService".
7.2.2 Modify
The Modify Command modifies the properties of a Termination.
TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
Modify( TerminationID
[, MediaDescriptor]
[, ModemDescriptor]
[, MuxDescriptor]
[, EventsDescriptor]
[, EventBufferDescriptor]
[, SignalsDescriptor]
[, DigitMapDescriptor]
[, AuditDescriptor]
)
The TerminationID may be specific if a single Termination in the
Context is to be modified. Use of wildcards in the TerminationID may
be appropriate for some operations. If the wildcard matches more
than one TerminationID, all possible matches are attempted, with
results reported for each one. The order of attempts when multiple
TerminationIDs match is not specified. The CHOOSE option is an
error, as the Modify command may only be used on existing
Terminations.
For convenience, if a Multiplex Descriptor is present in a Modify
command, then:
- if the new Multiplex Descriptor lists any Terminations that are
not currently in the Context, such Terminations are added to the
context as if individual commands listing the Terminations were
invoked.
- if any Terminations listed previously in the Multiplex Descriptor
are no longer present in the new Multiplex Descriptor, they are
subtracted from the context as if individual Subtract commands
listing the Terminations were invoked.
The remaining parameters to Modify are the same as those to Add.
Possible return values are the same as those to Add.
7.2.3 Subtract
The Subtract Command disconnects a Termination from its Context and
returns statistics on the Termination's participation in the Context.
TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
Subtract(TerminationID
[, AuditDescriptor]
)
TerminationID in the input parameters represents the Termination that
is being subtracted. The TerminationID may be specific or may be a
wildcard value indicating that all (or a set of related) Terminations
in the Context of the Subtract Command are to be subtracted. If the
wildcard matches more than one TerminationID, all possible matches
are attempted, with results reported for each one. The order of
attempts when multiple TerminationIDs match is not specified.
The use of CHOOSE in the TerminationID is an error, as the Subtract
command may only be used on existing Terminations.
ALL may be used as the ContextID as well as the TerminationId in a
Subtract, which would have the effect of deleting all Contexts,
deleting all ephemeral Terminations, and returning all physical
Terminations to Null Context. Subtract of a termination from the
Null Context is not allowed.
For convenience, if a multiplexing Termination is the object of a
Subtract command, then any bearer Terminations listed in its
Multiplex Descriptor are subtracted from the context as if individual
Subtract commands listing the Terminations were invoked.
By default, the Statistics parameter is returned to report
information collected on the Termination or Terminations specified in
the Command. The information reported applies to the Termination's
or Terminations' existence in the Context from which it or they are
being subtracted.
The AuditDescriptor is optional. If present, the command will return
only those descriptors as specified in the AuditDescriptor, which may
be empty. If omitted, the Statistics descriptor is returned, by
default. Possible return values are the same as those to Add.
When a provisioned Termination is Subtracted from a Context, its
property values shall revert to:
- the default value, if specified for the property and not
overridden by provisioning;
- otherwise, the provisioned value.
7.2.4 Move
The Move Command moves a Termination to another Context from its
current Context in one atomic operation. The Move command is the
only command that refers to a Termination in a Context different from
that to which the command is applied. The Move command shall not be
used to move Terminations to or from the null Context.
TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
Move( TerminationID
[, MediaDescriptor]
[, ModemDescriptor]
[, MuxDescriptor]
[, EventsDescriptor]
[, EventBufferDescriptor]
[, SignalsDescriptor]
[, DigitMapDescriptor]
[, AuditDescriptor]
)
The TerminationID specifies the Termination to be moved. It may be
wildcarded, but CHOOSE shall not be used in the TerminationID. If
the wildcard matches more than one TerminationID, all possible
matches are attempted, with results reported for each one. The order
of attempts when multiple TerminationIDs match is not specified. The
Context to which the Termination is moved is indicated by the target
ContextId in the Action. If the last remaining Termination is moved
out of a Context, the Context is deleted.
The Move command does not affect the properties of the Termination on
which it operates, except those properties explicitly modified by
descriptors included in the Move command. The AuditDescriptor with
the Statistics option, for example, would return statistics on the
Termination just prior to the Move. Possible descriptors returned
from Move are the same as for Add.
For convenience, if a multiplexing Termination is the object of a
Move command, then any bearer Terminations listed in its Multiplex
Descriptor are also moved as if individual Move commands listing the
Terminations were invoked.
Move SHALL NOT be used on a Termination with a serviceState of
"OutofService".
7.2.5 AuditValue
The AuditValue Command returns the current values of properties,
events, signals and statistics associated with Terminations.
TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
AuditValue(TerminationID,
AuditDescriptor
)
TerminationID may be specific or wildcarded. If the wildcard matches
more than one TerminationID, all possible matches are attempted, with
results reported for each one. The order of attempts when multiple
TerminationIDs match is not specified. If a wildcarded response is
requested, only one command return is generated, with the contents
containing the union of the values of all Terminations matching the
wildcard. This convention may reduce the volume of data required to
audit a group of Terminations. Use of CHOOSE is an error.
The appropriate descriptors, with the current values for the
Termination, are returned from AuditValue. Values appearing in
multiple instances of a descriptor are defined to be alternate values
supported, with each parameter in a descriptor considered
independent.
ObservedEvents returns a list of events in the EventBuffer. If the
ObservedEventsDescriptor is audited while a DigitMap is active, the
returned ObservedEvents descriptor also includes a digit map
completion event that shows the current dial string but does not show
a Termination method.
EventBuffer returns the set of events and associated parameter values
currently enabled in the EventBufferDescriptor. PackagesDescriptor
returns a list of packages realized by the Termination.
DigitMapDescriptor returns the name or value of the current DigitMap
for the Termination. DigitMap requested in an AuditValue command
with TerminationID ALL returns all DigitMaps in the gateway.
Statistics returns the current values of all statistics being kept on
the Termination. Specifying an empty Audit descriptor results in
only the TerminationID being returned. This may be useful to get a
list of TerminationIDs when used with wildcard. Annexes A and B
provide a special syntax for presenting such a list in condensed
form, such that the AuditValue command tag does not have to be
repeated for each TerminationID.
AuditValue results depend on the Context, viz. specific, null, or
wildcarded. (Note that ContextID ALL does not include the null
Context.) The TerminationID may be specific, or wildcarded.
The following are examples of what is returned in case the context
and/or the termination is wildcarded and a wi