RFC 2885
Megaco Protocol version 0.8
Network Working Group F. Cuervo Request for Comments: 2885 N. Greene Category: Standards Track Nortel Networks C. Huitema Microsoft Corporation A. Rayhan Nortel Networks B. Rosen Marconi J. Segers Lucent Technologies August 2000 Megaco Protocol version 0.8 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 (2000). All Rights Reserved.Abstract
This document is common text with Recommendation H.248 as redetermined in Geneva, February 2000. It must be read in conjunction with the Megaco Errata, RFC 2886. A merged document presenting the Megaco protocol with the Errata incorporated will be available shortly. The protocol presented in this document meets the requirements for a media gateway control protocol as presented in RFC 2805.
TABLE OF CONTENTS
1. SCOPE..........................................................6 2. REFERENCES.....................................................6 2.1 Normative references..........................................6 2.2 Informative references........................................8 3. DEFINITIONS....................................................9 4. ABBREVIATIONS.................................................10 5. CONVENTIONS...................................................11 6. CONNECTION MODEL..............................................11 6.1 Contexts.....................................................14 6.1.1 Context Attributes and Descriptors....................15 6.1.2 Creating, Deleting and Modifying Contexts.............15 6.2 Terminations.................................................15 6.2.1 Termination Dynamics..................................16 6.2.2 TerminationIDs........................................17 6.2.3 Packages..............................................17 6.2.4 Termination Properties and Descriptors................18 6.2.5 Root Termination......................................20 7. COMMANDS......................................................20 7.1 Descriptors..................................................21 7.1.1 Specifying Parameters.................................21 7.1.2 Modem Descriptor......................................22 7.1.3 Multiplex Descriptor..................................22 7.1.4 Media Descriptor......................................23 7.1.5 Termination State Descriptor..........................23 7.1.6 Stream Descriptor.....................................24 7.1.7 LocalControl Descriptor...............................24 7.1.8 Local and Remote Descriptors..........................25 7.1.9 Events Descriptor.....................................28 7.1.10 EventBuffer Descriptor...............................31 7.1.11 Signals Descriptor...................................31 7.1.12 Audit Descriptor.....................................32 7.1.13 ServiceChange Descriptor.............................33 7.1.14 DigitMap Descriptor..................................33 7.1.15 Statistics Descriptor................................38 7.1.16 Packages Descriptor..................................39 7.1.17 ObservedEvents Descriptor............................39 7.1.18 Topology Descriptor.................................39 7.2 Command Application Programming Interface....................42 7.2.1 Add...................................................43 7.2.2 Modify................................................44 7.2.3 Subtract..............................................45 7.2.4 Move..................................................46 7.2.5 AuditValue............................................47 7.2.6 AuditCapabilities.....................................48 7.2.7 Notify................................................49 7.2.8 ServiceChange.........................................50
7.2.9 Manipulating and Auditing Context Attributes..........54
7.2.10 Generic Command Syntax...............................54
7.3 Command Error Codes..........................................55
8. TRANSACTIONS..................................................56
8.1 Common Parameters............................................58
8.1.1 Transaction Identifiers...............................58
8.1.2 Context Identifiers...................................58
8.2 Transaction Application Programming Interface................58
8.2.1 TransactionRequest....................................59
8.2.2 TransactionReply......................................59
8.2.3 TransactionPending....................................60
8.3 Messages.....................................................61
9. TRANSPORT.....................................................61
9.1 Ordering of Commands.........................................62
9.2 Protection against Restart Avalanche.........................63
10. SECURITY CONSIDERATIONS......................................64
10.1 Protection of Protocol Connections..........................64
10.2 Interim AH scheme...........................................65
10.3 Protection of Media Connections.............................66
11. MG-MGC CONTROL INTERFACE....................................66
11.1 Multiple Virtual MGs........................................67
11.2 Cold Start..................................................68
11.3 Negotiation of Protocol Version.............................68
11.4 Failure of an MG............................................69
11.5 Failure of an MGC...........................................69
12. PACKAGE DEFINITION...........................................70
12.1 Guidelines for defining packages............................71
12.1.1 Package..............................................71
12.1.2 Properties...........................................72
12.1.3 Events...............................................72
12.1.4 Signals..............................................73
12.1.5 Statistics...........................................73
12.1.6 Procedures...........................................73
12.2 Guidelines to defining Properties, Statistics and Parameters
to Events and Signals.......................................73
12.3 Lists.......................................................74
12.4 Identifiers.................................................74
12.5 Package Registration........................................74
13. IANA CONSIDERATIONS.........................................74
13.1 Packages....................................................74
13.2 Error Codes.................................................75
13.3 ServiceChange Reasons.......................................76
ANNEX A: BINARY ENCODING OF THE PROTOCOL (NORMATIVE).............77
A.1 Coding of wildcards..........................................77
A.2 ASN.1 syntax specification...................................78
A.3 Digit maps and path names....................................94
ANNEX B TEXT ENCODING OF THE PROTOCOL (NORMATIVE)................95
B.1 Coding of wildcards..........................................95
B.2 ABNF specification...........................................95 ANNEX C TAGS FOR MEDIA STREAM PROPERTIES (NORMATIVE)............107 C.1 General Media Attributes....................................107 C.2 Mux Properties..............................................108 C.3 General bearer properties...................................109 C.4 General ATM properties......................................109 C.5 Frame Relay.................................................112 C.6 IP..........................................................113 C.7 ATM AAL2....................................................113 C.8 ATM AAL1....................................................114 C.9 Bearer Capabilities.........................................116 C.10 AAL5 Properties............................................123 C.11 SDP Equivalents............................................124 C.12 H.245......................................................124 ANNEX D TRANSPORT OVER IP (NORMATIVE)...........................125 D.1 Transport over IP/UDP using Application Level Framing.......125 D.1.1 Providing At-Most-Once Functionality.................125 D.1.2 Transaction identifiers and three-way handshake......126 D.1.2.1 Transaction identifiers....................126 D.1.2.2 Three-way handshake........................126 D.1.3 Computing retransmission timers......................127 D.1.4 Provisional responses................................128 D.1.5 Repeating Requests, Responses and Acknowledgements...128 D.2 using TCP..................................................130 D.2.1 Providing the At-Most-Once functionality..........130 D.2.2 Transaction identifiers and three way handshake...130 D.2.3 Computing retransmission timers...................131 D.2.4 Provisional responses.............................131 D.2.5 Ordering of commands..............................131 ANNEX E BASIC PACKAGES..........................................131 E.1 Generic.....................................................131 E.1.1 Properties...........................................132 E.1.2 Events...............................................132 E.1.3 Signals..............................................133 E.1.4 Statistics...........................................133 E.2 Base Root Package...........................................133 E.2.1 Properties...........................................134 E.2.2 Events...............................................135 E.2.3 Signals..............................................135 E.2.4 Statistics...........................................135 E.2.5 Procedures...........................................135 E.3 Tone Generator Package......................................135 E.3.1 Properties...........................................135 E.3.2 Events...............................................136 E.3.3 Signals..............................................136 E.3.4 Statistics...........................................136 E.3.5 Procedures...........................................136 E.4 Tone Detection Package......................................137
E.4.1 Properties...........................................137
E.4.2 Events...............................................137
E.4.3 Signals..............................................139
E.4.4 Statistics...........................................139
E.4.5 Procedures...........................................139
E.5 Basic DTMF Generator Package................................140
E.5.1 Properties...........................................140
E.5.2 Events...............................................140
E.5.3 Signals..............................................140
E.5.4 Statistics...........................................141
E.5.5 Procedures...........................................141
E.6 DTMF detection Package......................................141
E.6.1 Properties...........................................142
E.6.2 Events...............................................142
E.6.3 Signals..............................................143
E.6.4 Statistics...........................................143
E.6.5 Procedures...........................................143
E.7 Call Progress Tones Generator Package.......................143
E.7.1 Properties...........................................144
E.7.2 Events...............................................144
E.7.3 Signals..............................................144
E.7.4 Statistics...........................................145
E.7.5 Procedures...........................................145
E.8 Call Progress Tones Detection Package.......................145
E.8.1 Properties...........................................145
E.8.2 Events...............................................145
E.8.3 Signals..............................................145
E.8.4 Statistics...........................................145
E.8.5 Procedures...........................................146
E.9 Analog Line Supervision Package.............................146
E.9.1 Properties...........................................146
E.9.2 Events...............................................146
E.9.3 Signals..............................................147
E.9.4 Statistics...........................................148
E.9.5 Procedures...........................................148
E.10 Basic Continuity Package...................................148
E.10.1 Properties..........................................148
E.10.2 Events..............................................148
E.10.3 Signals.............................................149
E.10.4 Statistics..........................................150
E.10.5 Procedures..........................................150
E.11 Network Package............................................150
E.11.1 Properties..........................................150
E.11.2 Events..............................................151
E.11.3 Signals.............................................152
E.11.4 Statistics..........................................152
E.11.5 Procedures..........................................153
E.12 RTP Package...............................................153
E.12.1 Properties..........................................153
E.12.2 Events..............................................153
E.12.3 Signals.............................................153
E.12.4 Statistics..........................................153
E.12.5 Procedures..........................................154
E.13 TDM Circuit Package........................................154
E.13.1 Properties..........................................155
E.13.2 Events..............................................155
E.13.3 Signals.............................................155
E.13.4 Statistics..........................................156
E.13.5 Procedures..........................................156
APPENDIX A EXAMPLE CALL FLOWS (INFORMATIVE).....................157
A.1 Residential Gateway to Residential Gateway Call.............157
A.1.1 Programming Residential GW Analog Line Terminations for
Idle Behavior..............................................157
A.1.2 Collecting Originator Digits and Initiating Termination
...........................................................159
Authors' Addresses..............................................168
Full Copyright Statement........................................170
1. SCOPE
This document defines the protocol 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 H.246. This
recommendation does not define how gateways, multipoint control units
or integrated 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 SCN signalling systems,
including tone signalling, ISDN, ISUP, QSIG, and GSM. National
variants of these signalling systems will be supported where
applicable.
The protocol definition in this document is common text with ITU-T
Recommendation H.248. It meets the requirements documented in RFC
2805.
2. REFERENCES
2.1 Normative references
ITU-T Recommendation H.225.0 (1998): "Call Signalling Protocols and
Media Stream Packetization for Packet Based Multimedia Communications
Systems".
ITU-T Recommendation H.235 (02/98): "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.323 (1998): "Packet Based Multimedia Communication Systems". ITU-T Recommendation I.363.1 (08/96), "B-ISDN ATM Adaptation Layer specification: Type 1 AAL". ITU-T Recommendation I.363.2 (09/97), "B-ISDN ATM Adaptation Layer specification: Type 2 AAL". ITU-T Recommendation I.363.5 (08/96), "B-ISDN ATM Adaptation Layer specification: Type 5 AAL". ITU-T Recommendation I.366.1 (06/98), "Segmentation and Reassembly Service Specific Convergence Sublayer for the AAL type 2". ITU-T Recommendation I.366.2 (02/99), "AAL type 2 service specific convergence sublayer for trunking". ITU-T Recommendation I.371 (08/96), "Traffic control and congestion control in B-ISDN". ITU-T Recommendation Q.763 (09/97), "Signalling System No. 7 - ISDN user part formats and codes". ITU-T Recommendation Q.765, "Signalling System No. 7 - Application transport mechanism". ITU-T Recommendation Q.931 (05/98): "Digital Subscriber Signalling System No. 1 (DSS 1) - 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 (10/95), "Broadband Integrated Services Digital Network (B-ISDN) - Digital Subscriber Signalling System No. 2 (DSS 2) - User-Network Interface (UNI) - Layer 3 specification for basic call/connection control". ITU-T Recommendation Q.2941.1 (09/97), "Digital Subscriber Signalling System No. 2 - Generic Identifier Transport".
ITU-T Recommendation Q.2961 (10/95), "Broadband integrated services digital network (B-ISDN) - Digital subscriber signalling system no.2 (DSS 2) - additional traffic parameters". ITU-T Recommendation Q.2961.2 (06/97), "Digital subscriber signalling system No. 2 - Additional traffic parameters: Support of ATM transfer capability in the broadband bearer capability information element." ITU-T Recommendation X.213 (11/1995), "Information technology - Open System Interconnection - Network service definition plus Amendment 1 (08/1997), Addition of the Internet protocol address format identifier". ITU-T Recommendation V.76 (08/96), "Generic multiplexer using V.42 LAPM-based procedures". ITU-T Recommendation X.680 (1997): "Information technology-Abstract Syntax Notation One (ASN.1): Specification of basic notation". 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". Rose, M. and D. Cass, "ISO Transport Service on top of the TCP, Version 3", RFC 1006, May 1987. Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998. 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 (05/99),"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 Q.724 (1988): "Signalling procedures". Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. Postel, J., "Internet protocol", STD 5, RFC 791, September 1981. Postel, J., "TRANSMISSION CONTROL PROTOCOL", STD 7, RFC 793, September 1981. Simpson, W., "The Point-to-Point Protocol", STD 51, RFC 1661, July 1994. Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. Schulzrinne, H., "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 1890, January 1996. Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, March 1999. Greene, N., Ramalho, M. and B. Rosen, "Media Gateway control protocol architecture and requirements", RFC 2805, April 1999.3. DEFINITIONS
Access Gateway: A type of gateway that provides a User to 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 performs 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 recommendation defines the following terms. ATM Asynchronous Transfer Mode BRI Basic Rate Interface CAS Channel Associated Signalling DTMF Dual Tone Multi-Frequency FAS Facility Associated Signalling GW GateWay IANA Internet Assigned Numbers Authority IP Internet Protocol ISUP ISDN User Part
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. 75. CONVENTIONS
In this 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.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.
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context +-------------+ | |
| | | Termination | | |
| | |-------------| | |
| | +-------------+ +->| SCN Bearer |<---+->
| | | Termination | +-----+ | | Channel | | |
| | |-------------| | |---+ +-------------+ | |
<-+--->| RTP Stream |---| * | | |
| | | | | |---+ +-------------+ | |
| | +-------------+ +-----+ | | Termination | | |
| | | |-------------| | |
| | +->| SCN Bearer |<---+->
| | | Channel | | |
| | +-------------+ | |
| +-------------------------------------------------+ |
| |
| |
| +------------------------------+ |
| |Context | |
| +-------------+ | +-------------+ | |
| | Termination | | +-----+ | Termination | | |
| |-------------| | | | |-------------| | |
<-+->| SCN Bearer | | | * |------| SCN Bearer |<---+->
| | Channel | | | | | Channel | | |
| +-------------+ | +-----+ +-------------+ | |
| +------------------------------+ |
| |
| |
| +-------------------------------------------------+ |
| |Context | |
| | +-------------+ +-------------+ | |
| | | Termination | +-----+ | Termination | | |
| | |-------------| | | |-------------| | |
<-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+->
| | | Channel | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
| ___________________________________________________ |
+------------------------------------------------------+
Figure 1: Example of H.248 Connection Model
Figure 1 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.
The example below 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 below.
+------------------------------------------------------+
|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. . 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. Signals are MG generated media streams such as tones and announcements as well as line signals such as hookswitch. 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 section 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 Termination. The Terminations that source/sink the digital channels are connected to a separate Termination called the multiplexing Termination. This Termination describes the multiplex used (e.g. how the H.221 frames are carried over the digital channels used). The MuxDescriptor is used to this end. If multiple media are carried, this Termination contains multiple StreamDescriptors. The media streams can be associated with streams sourced/sunk by other Terminations in the Context. Terminations may be created which represent multiplexed bearers, such as an ATM AAL2. 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 sections. 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. 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. 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 a Termination realizes a set of such Packages. More information on definition of packages can be found in section 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.6.2.4 Termination Properties and Descriptors
Terminations have properties. The properties have unique PropertyIDs. Most properties have default values. When a Termination is created, properties get their default values, unless the controller specifically sets a different value. The default value of a property of a physical Termination can be changed by setting it to a different value when the Termination is in the null Context. Every time such a Termination returns to the null Context, the values of its properties are reset to this default value. 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. Properties not mentioned in the command retain their prior values. Similarly, a property of a Termination in a Context may have its value changed by the Modify command. Properties not mentioned in the Modify command retain their prior values. 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. 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 and/or actions to be
applied (e.g. Busy Tone) to the
Terminations.
Audit In Audit commands, identifies which
information is desired.
Packages In AuditValue, returns a list of Packages
realized by Termination.
DigitMap Instructions for handling DTMF tones at
the MG.
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.
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 and events (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 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.
(page 20 continued on part 2)
