MEGACO/H.248

Introduction

A Context is an association between a number of Terminations. Null Context is a special context that contains Terminations that are not associated to any other Termination.

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.

Commands:

Add
The Add Command adds a Termination to a Context.

Modify
The Modify Command modifies the properties of a Termination.

Subtract
The Subtract Command disconnects a Termination from its Context and
returns statistics on the Termination's participation in the Context.

Move
The Move Command moves a Termination to another Context from its
current Context in one atomic operation.

AuditValue
The AuditValue Command returns the current values of properties,
events, signals and statistics associated with Terminations.

AuditCapabilities
The AuditCapabilities Command returns the possible values of
properties, events, signals and statistics associated with
Terminations.

Notify
The Notify Command allows the Media Gateway to notify the Media
Gateway Controller of events occurring within the Media Gateway.

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.













































Example Call Flow: Initiating a telephone call
Power point slide of the pictures of this calls flow: MEGACO-Call-Flow.ppt


Initial assumptions:
One MGC controlling both ends
Both gateways have associated with the MGC
MG1 and MG2 have TDM terminations programmed to listen to events









STEP1: MG1 detects an offhook event and notifies MGC

STEP2: MGC modifies MG1 to collect digits

STEP3: MG1 notifies when digits are collected

STEP4: MGC analyzes the digits and begins to connect MG1 and MG2

STEP5: MGC adds a TDM and RTP termination to context in MG1


















STEP6: MG1 replies with local IP address, UDP port and choice of codec

















STEP7: MGC adds a TDM(recvonly) and RTP termination to context in MG2





















STEP8: MG2 replies with local IP address, UDP port and choice of codec











STEP9: MGC modifies the RTP termination at MG1 to sendrecv.











STEP10: User 1 hears ringback and User 2 hears phone ringing

STEP11: User 2 lifts up the phone

STEP12: The telephone call is initiated


Abbreviations/Terms
Package :

The primary mechanism for extension is by means of Packages.Packages define additional Properties, Events, Signals and Statistics that may occur on Terminations.

Examples of packages -

Tone Generator Package
Tone Detection Package
Basic DTMF Generator Package
DTMF detection Package
Network Package
RTP Package

Events: Events are detected at MG and reported to MGC. (example: inband signaling). MGC controls what events it wants to learn about at any given time sets the termination Events descriptor

Signals: Signals cause things to happen on terminations. For e.g. play a tone, display text, ...

DTMF: Dual Tone Multi Frequency

RTP: Real Time Protocol

Packetization Time (ptime):