1. An Introduction to Megaco/H.248
Tom Taylor

2. Media Gateway Control References for Megaco/H.248
RFC requirements
RFC 3015 (Megaco) or ITU-T Rec. H.248
H.248 Annexes F to K
History and tutorial:
T. Taylor, "Megaco/H.248: A New Standard For Media Gateway Control", IEEE Communications Magazine, October 2000.

3. Media gateway control vs. call signalling
SIP-T, ISUP in H.323, Q.BICC SG
MGC
MGC SG
SIP
SIP
User Agent
PSTN
PSTN
H.323 call
signalling
Gateway
control
protocol
Gateway
control
protocol
H.323
Endpoint MG MG
Call signalling
Media gateway control signalling
Media flows

4. Gateway control family tree
SGCP
Early 1998
IPDC
ASPEN
Late 1998
MGCP
(I-RFC 2705)
MDCP
1999
(Standard in 2000)
Megaco/H.248

5. Gateway control functions
Connection
Bearer Mediation
Media Transcoding
Events and Signals
Context
Terminations
The context supports
multiple media streams
if required for multimedia
service.

6. Basic concepts
Connection model: terminations, streams, and the context
Termination properties: descriptors
Context properties
Message structure: transactions, actions, and commands
Event and signal processing
Packages

7. Media flow connections
Connections achieved by placing two or more terminations into a common context.
Context viewed as mixing bridge
Termination = source or sink of media flows
provisioned vs. ephemeral terminations
provisioned terminations reside in “NULL context” when not active
Flows specified by stream
common streamID implies connected flows
default streamID = 1 to simplify audio-only operation
Transport, medium, encoding/decoding specified per stream at each termination
Flows are uni-directional. For a given termination, the convention is that the "send" direction is from the context outwards and the "receive" direction is from the exterior into the context.
Streams are bidirectional, thus support up to one active flow in each direction.

8. Context example: basic call
Medium=audio,
Medium=audio, T2 T1
Mode=sendReceive
Mode=sendReceive

9. Context example: multimedia
Stream=1,
medium=audio T2 T1 T3
Stream=1,
medium=audio
Stream=2,
medium=video
Stream=2,
medium=video

10. Descriptors
Properties of terminations are organized syntactically into descriptors
basic ones are Termination State, Media, Events, and Signals descriptors
Media descriptor actually composed of other descriptors: Stream descriptors, which in turn contain LocalControl, Local, and Remote descriptors
Default property values can be configured in the MG
The TerminationState descriptor describes termination properties which are independent of any media streams the termination supports at a given time.
The Media descriptor describes the streams currently supported by the termination. Streams can be added or deleted at any time. Each stream is described by a separate Stream descriptor. If only one stream is supported, the Stream level of syntax may optionally be omitted, and the StreamID is assumed to be 1.
Each Stream descriptor contains a LocalControl, a Local, and a Remote descriptor.
The LocalControl descriptor contains properties which relate to the stream as a whole rather than individual directions.
The Local descriptor specifies the media flow in the "send" direction, while the Remote descriptor does the same for the media flow in the "receive" direction. When text encoding of the Megaco/H.248 message is being used, the syntax of both descriptors is based on the Session Description Protocol (SDP).
Default property values apply to a termination between the time it is created and or it is returned to the NULL context through subtraction and the time the MGC specifies over-riding values. However, the MGC may modify these properties at any time, even in the same transaction

11. Context properties more could be added in the future
currently three properties can be specified for a context
more could be added in the future
topology descriptor allows detailed specification of connectivity between individual pairs of terminations
useful for legal interception
priority flag can guide MG's allocation of scarce resources
emergency flag can indicate contexts which must be maintained and restored in the event of failures
One possible future addition is a video bridging property, indicating the algorithm used for mixing video in multi-party contexts. Examples: current speaker only, mosaic of N most recent speakers, etc.

12. Null context and ROOT termination
Null context introduced as a convention
where persistent terminations are held when they are not in a real context
When terminations are returned to the null context, they take on their configured default property values.
ROOT termination represents the MG itself
useful for specifying properties of the MG as a device, and to refer to the MG in commands like AuditCapability and ServiceChange

13. Message structure Megaco/H.248 message
Header Transaction Transaction Transaction
Req or Reply Req or Reply Req or Reply
Trans Hdr Action Action
Ctx Hdr Ctx Properties Command Command
Cmd Hdr Descriptor Descriptor

14. Commands Megaco/H.248 provides the following commands
For termination manipulation: Add, Subtract, Move, Modify
For event reporting: Notify
For management: AuditCapability, AuditValue, ServiceChange

15. Events Events are detected at the MG and reported to the MGC
example: in-band signalling
MGC controls what events it wants to learn about at any given time
sets the termination Events descriptor
Events can have side effects
stop playout of signals
start new signals
automatically update the set of events of interest

16. Signals Signals cause things to happen on terminations
play a tone, display text, ...
Specified in the Signals descriptor for a termination
MGC can specify duration of signal ahead of time or signal can play until explicitly stopped
Signals stop playing when any event is detected unless MGC says otherwise.

17. Packages Add detailed content to the protocol
all events, signals, and statistics are specified in packages
can also specify additional properties
Package definition a continuing process
being created by multiple standards bodies
private packages also allowed
Packages can inherit from and extend other packages.

18. Example message flow PSTN MGC MG Destination 1. IAM
2. ADD (trunk, send only);ADD(RTP,both)
3. Reply
4. INVITE (local SDP)
Ringing (remote SDP)
6. MODIFY (RTP, remote SDP)
7. ACM
8. Reply

19. Example message flow (2)
PSTN MGC MG Destination OK
11. ANM
12. MODIFY (trunk, both)
13. Reply
14. ACK
15. BYE
16. REL
17. SUBTRACT (trunk), SUBTRACT (RTP)
18. Reply
20. RLC OK

20. Sample message (long text form)
MEGACO/1 <MGC1.gz.cn> Transaction=1432 { Context=$ { ADD=Ckt54/20 { Media= { LocalControl={mode=sendonly}, Local={ v=0 c=TDM NUL xxxx m=audio 0 31kHz/I230 basic },

21. Sample message (long text form) continued
Remote={ v=0 c=TDM NUL xxxx m=audio 0 UDI/I230 basic }} }}}

22. Sample message, compact text form
!/1 <MGC1.gz.cn> T=1432{C=${A=Ckt54/20{M={ O={MO=so},L={ v=0 c=TDM NUL xxxx m=audio 0 31kHz/I230 basic },R={ v=0 c=TDM NUL xxxx m=audio 0 31kHz/I230 basic }}}}}