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Media Gateway Control Protocol (Megaco/H.248). Ömer Korçak. Introduction. Voice Over IP V iable alternative / addition to traditional circuit-switched telephony. Large companies (which have their own private global IP networks) already realize the benefits of VoIP networks.

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Media gateway control protocol megaco h 248 l.jpg

Media Gateway Control Protocol (Megaco/H.248)

Ömer Korçak

Megaco/H.248


Introduction l.jpg
Introduction

  • Voice Over IP

  • Viable alternative / addition to traditional circuit-switched telephony.

  • Large companies (which have their own private global IP networks) already realize the benefits of VoIP networks.

  • VoIP is now penetrating the wider population of small offices and residential Internet users.

Megaco/H.248


Voice over ip l.jpg
Voice Over IP

  • Poor quality characterized the first generation of VoIP products.

  • However, the opportunity for corporations to cut telephony costs and enhance voice communications with a range services has forced the technology to improve.

Megaco/H.248


Voice over ip4 l.jpg
Voice Over IP

  • Today's VoIP quality of service (QoS) has improved tremendously

  • However, the drawbacks are still apparent.

  • The architecture of a carrier-grade VoIP network that provides telephony service for a wider customer base cannot be completely based on the peer-to-peer architecture of current VoIP call-processing standards.

Megaco/H.248


Voice over ip5 l.jpg
Voice Over IP

  • Central management and call-routing functions are needed

  • Interoperable, easy-to-design, cost effective client is imperative.

Megaco/H.248


Megaco l.jpg
MEGACO

  • The newly-emerging media gateway control protocol (Megaco) attempts to bridge the gap between the first generation of VoIP networks and the VoIP networks of tomorrow.

Megaco/H.248


Megaco7 l.jpg
MEGACO

  • Megaco addresses the relationship between the Media Gateway (MG) and the Media Gateway Controller (MGC)

  • A Master/Slave protocol that removes intelligence from MGs

Megaco/H.248


Glossary l.jpg
Glossary

  • Decomposed Gateway Architectural model:

  • Gateway is decomposed into three functional components:

  • Signalling Gateway

  • Media Gateway

  • Media Gateway Controller

Megaco/H.248


Glossary9 l.jpg
Glossary

  • Media Gateway:

  • Central elements in the voice stream

  • Provides mapping of streams between two different networks.

  • Dumb Devices

Megaco/H.248


Glossary10 l.jpg
Glossary

  • Example Media Gateways

  • Residental Gateways:

  • Situated in the costumer premises and that provide an analog line where a traditional telephone set can be connected

  • On the other side, connected to a packet network providing VoIP.

Megaco/H.248


Glossary11 l.jpg
Glossary

  • Example Media Gateways

  • Trunking Gateways:

  • Interface between the telephone network and a VoIP network.

  • Manage large number of digital virtual circuits.

Megaco/H.248


Glossary12 l.jpg
Glossary

  • Signalling Gateway:

  • Intermediate element between IP network and SCN domain.

  • Provides the signalling mediation function between a voice over IP network and a switched circuit network.

Megaco/H.248


Glossary13 l.jpg
Glossary

  • Gatekeeper:

  • Controls terminals and gateways

  • Responsible for Registration, Admission and Status (RAS) messages.

  • A central element in the control of the calls through an IP system.

  • Contains the intelligence of the control part of IP network.

Megaco/H.248


Glossary14 l.jpg
Glossary

  • Media Gateway Controller:

  • Controls Media Gateways depending on the SCN signalling coming from the SG and IP signalling coming from the gatekeeper.

  • Intelligent Devices

Megaco/H.248


Slide15 l.jpg

GK

GK

Back-end

H.323

terminal

MGC

SG

MG

ESTI-TIPHON functional decomposition reference model

Megaco/H.248


Benefits of decomposed media gateway model l.jpg
Benefits of Decomposed Media Gateway Model

  • Removing the signaling to a fast server is more practical than trying to integrate it into the MG.

  • New services can be introduced without requiring any customer premises equipment (CPE) upgrades.

  • Handled by simply upgrading the centralized software that contains the intelligence for implementing services.

Megaco/H.248


Benefits of decomposed media gateway model17 l.jpg
Benefits of Decomposed Media Gateway Model

  • Central call control gains a strong power

  • Modularity satisfied

  • Interoperable, easy-to-design, cost effective client.

  • Any necessity to proprietary protocols is abandoned.

  • Better suited to implementing protocol stacks and large volumes of general-purpose code.

Megaco/H.248


Media gateway control vs call signalling l.jpg

PSTN

PSTN

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

signaling

Gateway

control

protocol

Gateway

control

protocol

H.323

Endpoint

MG

MG

Call signaling

Media gateway control signaling

Megaco/H.248

Media flows


Media gateway model l.jpg
Media Gateway Model

The MG is composed of two logical entitiesthatcan be controlled by the MGC:

  • Terminations: interfaces to the media streams.

  • Context: assotiation between a collection of terminations.

Megaco/H.248


Basic concepts l.jpg
Basic Concepts

  • Connection model: terminations, streams, and the context

  • Termination properties: descriptors

  • Message structure: transactions, actions, and commands

  • Event and signal processing

  • Packages

Megaco/H.248


Terminations l.jpg
Terminations

  • Source or sink of media flows

  • Available on both sides of the Media Gateway (SCN and IP-Network)

  • Media flows can be either one way or two way

  • Terminations can also sink/source multimedia streams that include several media streams.

Megaco/H.248


Terminations22 l.jpg
Terminations

Two types of terminations:

  • Persistent terminations:Instantiated by the MG when it boots and remain active all the time.

  • Ephemeral terminations: Created when they are needed.

  • ROOT termination:Represents the MG as an entity in itself.

Megaco/H.248


Context l.jpg
Context

  • Modeled as a mixing bridge between terminations.

  • Two or more terminations may placed into a context in order to mix and connect them.

  • Created and released by the MG under command of MGC.

  • Null context: holds the persistent terminations while they are not in use.

Megaco/H.248


Context example basic call l.jpg
Context Example: Basic call

Medium=audio,

Medium=audio,

Mode=sendReceive

Mode=sendReceive

T2

T1

Ordinary two-party conversation between terminations T1 and T2

Megaco/H.248


Context example conversation with passive listener l.jpg
Context Example:Conversation with passive listener

Medium=audio,

Medium=audio,

Mode=sendReceive

Mode=sendReceive

T2

T1

T3

  • Example:

  • Wiretap

  • Recording Machine

Medium=audio,

Mode=sendOnly

Megaco/H.248


Context example media conversion l.jpg
Context Example:Media Conversion

Medium=audio,

Medium=text,

Mode=sendReceive

Mode=sendReceive

T2

T1

Implicit conversion between media

Megaco/H.248


Context example multimedia l.jpg
Context Example:Multimedia

Stream=1,

medium=audio

T2

T1

T3

Stream=1,

medium=audio

Stream=2,

medium=video

Stream=2,

medium=video

Megaco/H.248


Descriptors l.jpg
Descriptors

  • Properties of terminations

  • Most important ones:

    • Media Descriptor : Describes thetransformations to be applied to media flows through the termination

    • Events Descriptor: Selects and reports events that are currently occurred and important for MGC

    • Signals Descriptor : Indicates which signals the MGC currently wishes the MG to play out the termination

Megaco/H.248


Descriptors29 l.jpg
Descriptors

  • Media Descriptors include some other descriptors:

    • Termination State Descriptor:Carries the state of the termination which is independent of any media flow

    • Local Control Descriptor: Provides Media-stream-related information relevant only between MGC and MG

    • Local and Remote Descriptor: Carries information describing media flows within a stream which must be coordinated with the remote entity

Megaco/H.248


Message structure l.jpg
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

Megaco/H.248


Commands l.jpg
Commands

  • Megaco uses some commands in order to manipulate terminations, contexts, signals and events.

  • For termination manipulation: Add, Subtract, Move, Modify

  • For event reporting: Notify

  • For management: AuditCapability, AuditValue, ServiceChange

Megaco/H.248


Commands32 l.jpg
Commands

  • From MGC to MC:

  • Add: adds a termination to a context

  • Subtract:removes a termination from a context

  • Move:moves a termination from a context to another

  • Modify:changes the characteristics of an existing termination, which can be in the null context

  • AuditValue & AuditCapabilities:return information about terminations, contexts and general state and capabilities of MG

Megaco/H.248


Commands33 l.jpg
Commands

  • From MG to MGC:

    • Notify: MG sends it to inform MGC that an event has occured.

  • Either from MG to MGC or from MGC to MG:

    • ServiceChange: creates a connection between MG and MGC.

  • Descriptors are parameters for all these commands & return values of some of them.

Megaco/H.248


Events l.jpg
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

  • Events can have side effects

    • stop playout of signals

    • start new signals

    • automatically update the set of events of interest

Megaco/H.248


Signals l.jpg
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.

Megaco/H.248


Packages l.jpg
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.

Megaco/H.248


Example call flows l.jpg
Example Call Flows

User A

RGW1

MGC

RGW2

User B

MODIFY to check off-hook

MODIFY to check off-hook

Response

Response

UserA offhook

NOTIFY offhook

Response

MODIFY SG:dialtone

Dial Tone

Response

User dials digits

NOTIFY digits

Response

Megaco/H.248


Example call flows 2 l.jpg
Example Call Flows (2)

User A

RGW1

MGC

RGW2

User B

ADD TermA SD:ringbacktone

ADD $, Local SDP

Ringback Tone

Response

ADD TermB SD:Ring

ADD $ Local, Remote SDP

User B phone ringing

Response

User goes offhook

NOTIFY offhook

Response

Megaco/H.248


Example call flows 3 l.jpg
Example Call Flows (3)

User A

RGW1

MGC

RGW2

User B

MODIFY TermA SendRecv

MODIFY EphA Remote SendRecv

Response

MODIFY TermB SendRecv

MODIFY EphB Remote SendRecv

Response

R T P M e d i a

Megaco/H.248


Sample message l.jpg
Sample Message

MGC to MG1:

MEGACO/1 [123.123.123.4]:55555

Transaction = 9999 {

Context = $ {

ADD=A4444 {Media= { LocalControl={mode=sendonly},Local={v=0c=IN IP4 $m=audio $ RTP/AVP 4 },

Megaco/H.248


Sample message cont l.jpg
Sample Message (cont.)

Remote={v=0c=IN IP4 124.124.124.222m=audio 2222 RTP/AVP 4 }}}}}

Megaco/H.248


What we are going to do l.jpg
What we are going to do

  • Megaco is the most complex and very extensive VoIP protocol.

  • In some companies hundreds of people are working for this.

  • No efficient way for the implementation.

  • We should work over a subset of Megaco implementation.

Megaco/H.248


What we are going to do43 l.jpg
What we are going to do

  • Megaco architecture can be considered as 3 basic components:

    • Protocol Stack

    • Media Gateway Application

    • Media Gateway Controller Application.

Megaco/H.248


What we are going to do44 l.jpg
What we are going to do

  • Both of the MG and MGC applications use the stack

  • They are in a higher level.

  • Therefore our aim is to implement the stack first.

  • Keep the scope of the stack implementation as small as possible.

Megaco/H.248


An example protocol stack net brahma technologies l.jpg
An Example Protocol StackNet Brahma Technologies

Megaco/H.248


What we are going to do46 l.jpg
What we are going to do

  • First we should do a careful definition of the interfaces to our stack.

    • An interface to the application state machines (control association, termination, context)

    • An interface to the Transport Layer.

  • Protocol specification provides an API to the application. We should decide how we are going to implement API.

Megaco/H.248


Protocol stack implementation l.jpg
Protocol Stack Implementation

Looking at the incoming message we should have do following operations:

  • receive it from the transport layer you are using

  • parse it at the message level, breaking it down into header and transactions 

  • parse transactions into actions 

  • parse actions into context properties and commands

  • parse commands into descriptors 

  • parse descriptors into individual parameters, including items defined in packages 

  • pass the results across our defined application interface.

Megaco/H.248


Protocol stack implementation48 l.jpg
Protocol Stack Implementation

  • Major Challenge :

    • To define the appropriate actions to take when we find syntactic errors.

    • They are generally could not be handled in stack level.

Megaco/H.248


Protocol stack implementation49 l.jpg
Protocol Stack Implementation

  • What our stack will support?

    • Media Gateways: Only residential gateways

    • Transport mechanism: Only TCP.

    • Encoding Mechanism: Only text encoding

    • Descriptors: All, except topology descriptor.

    • Packages: Only that are necessary (such as analog line supervision package, RTP package, generic package, DTMF detection package).

    • Commands: All commands except “Move” command.

Megaco/H.248


References l.jpg
References

  • RFC 3015: Megaco Protocol Version 1.0

  • T. Taylor, “Megaco/H248: A New Standard for Media Gateway Control”, IEEE Communications Magazine, Oct. 2000.

  • M. B. Brahmanapally, draft-madhu-megaco-callflow-00.txt

  • D. Allen, “Megaco and MGCP”, www.networkmagazine.com, May 2000.

  • www.netbrahma.com

  • S. Çabuk, E. Deveci, Ö. Köken, S. Üstün, “Issues in Softswitch Design”, Boğaziçi University, June 2001.

  • www.hssworld.com/products/protocolstacks/megaco/megaco_home.htm

  • H. Liu and P. Mouchtaris, “Voice over IP Signalling: H323 and Beyond”, IEEE Communications Magazine, Oct. 2000.

Megaco/H.248


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