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rfc3525.txt
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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.
Groves, et al. Standards Track [Page 1]
RFC 3525 Gateway Control Protocol June 2003
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
Groves, et al. Standards Track [Page 2]
RFC 3525 Gateway Control Protocol June 2003
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
Groves, et al. Standards Track [Page 3]
RFC 3525 Gateway Control Protocol June 2003
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
Groves, et al. Standards Track [Page 4]
RFC 3525 Gateway Control Protocol June 2003
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.
Groves, et al. Standards Track [Page 5]
RFC 3525 Gateway Control Protocol June 2003
- 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.
Groves, et al. Standards Track [Page 6]
RFC 3525 Gateway Control Protocol June 2003
- 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).
Groves, et al. Standards Track [Page 7]
RFC 3525 Gateway Control Protocol June 2003
- 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.
Groves, et al. Standards Track [Page 8]
RFC 3525 Gateway Control Protocol June 2003
[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.
Groves, et al. Standards Track [Page 9]
RFC 3525 Gateway Control Protocol June 2003
[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.
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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
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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.
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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.
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+------------------------------------------------------+
|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
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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
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+------------------------------------------------------+
|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.
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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.
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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