An Introduction to Megaco/H.248

1
An Introduction to Megaco/H.248

• Tom Taylor
• taylor_at_nortelnetworks.com

2
Media Gateway Control

• References for Megaco/H.248
• RFC 2805 -- 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.

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Media gateway control vs. call signalling
SIP-T, ISUP in H.323, Q.BICC
SG
MGC
SG
MGC
SIP
SIP User Agent
H.323 call signalling
Gateway control protocol
Gateway control protocol
H.323 Endpoint
MG
MG
Call signalling
Media gateway control signalling
Media flows
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Gateway control family tree
SGCP
Early 1998
IPDC
ASPEN
Late 1998
MGCP (I-RFC 2705)
MDCP
1999 (Standard in 2000)
Megaco/H.248
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Gateway control functions
Connection Bearer Mediation Media Transcoding
Events and Signals
Context
Terminations
The context supports multiple media streams if
required for multimedia service.
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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

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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

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Context example basic call
Mediumaudio,
Mediumaudio,
T2
T1
ModesendReceive
ModesendReceive
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Context example multimedia
Stream1, mediumaudio
T2
T1
T3
Stream1, mediumaudio
Stream2, mediumvideo
Stream2, mediumvideo
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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

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Context properties

• 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

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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

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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
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Commands

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

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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

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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.

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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.

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Example message flow
PSTN MGC MG
Destination
1. IAM
2. ADD (trunk, send only)ADD(RTP,both)
3. Reply
4. INVITE (local SDP)
5. 180 Ringing (remote SDP)
6. MODIFY (RTP, remote SDP)
7. ACM
8. Reply
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Example message flow (2)
PSTN MGC MG
Destination
10. 200 OK
11. ANM
12. MODIFY (trunk, both)
13. Reply
14. ACK
15. BYE
16. REL
17. SUBTRACT (trunk), SUBTRACT (RTP)
18. Reply
19. 200 OK
20. RLC
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Sample message (long text form)
MEGACO/1 ltMGC1.gz.cngtTransaction1432
Context ADDCkt54/20 Media
LocalControlmodesendonly, Localv0cTDM
NUL xxxxmaudio 0 31kHz/I230 basic
,
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Sample message (long text form) continued
Remotev0cTDM NUL xxxxmaudio 0 UDI/I230
basic
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Sample message, compact text form
!/1 ltMGC1.gz.cngt T1432CACkt54/20MOMOs
o,Lv0cTDM NUL xxxxmaudio 0 31kHz/I230
basic,R v0cTDM NUL xxxxmaudio 0
31kHz/I230 basic