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The H.323 protocol suite

How works one of the protocol architectures forVoIP

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Basic characteristics

Standard for LAN communicationPacket switched networks without guaranteed quality

Extended to operate over a WAN

Support forAudio (mandatory)

Video

Data (shared backboard, etc)

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Components of H.323 specification

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Multipoint Control Unit (MCU)

Two components:Multipoint Controller (mandatory): it is used to negotiate common features

Multipoint Processor (optional): stream mixing/switching, bandwidth adaptation, ...

It is used only in case of:Conference between 3 or more parties, with unicast transmission

Conference between 3 or more parties in mixed mode, unicast and multicast

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Zone

Set of H.323 components managed by the same gatekeeper

It requires at least one terminal

It should not include more than 1 gatekeeper

No fault-tolerance mechanisms allowed

No assuption on the topology of the underlying network

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Message format

Messages encoded using ASN.1It includes advanced features (byte ordering, ...)

Complex

Difficult to encode/decode and to debug

One of the major criticisms to H.323

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Protocol architecture

G.711G.722G.723G.728G.729

H.261H.263

RTCP RAS Control

H.225

CallControl

H.225

Control

H.245Q.931

Dati

T.120

RTP

UDP TCP /UDP

IP

Audio Video Medium control

Call control

RAS: Registration, Admission and Status

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Block diagram of an H.323 terminal

Video hardware

Audio hardware

User data applications

User interface for the control

system

Video CodecH.261H.263

Audio CodecG.711 G.722G.724 G.728

G.729

ReceivePathDelay

Control system

H.245 controller

Callcontroller

RAS controller

H.225layer

LAN interface

Covered by H.323 specificationsUser interface

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H.225

Main operationsCreating and formatting logical channels

Transmission and reception of control packets

Sequence numbering

Assigning a logical channel ID (0-65525)

0 reserved to H.245

Error detection and/or correction

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RAS controller

It is active only if a gatekeeper is present

It uses H.225 messagesTerminal registration

Terminal admission

Chyanges in the bandwidth (even when a call is in progress)

Status control

Closing procedure between terminal and gatekeeper

RAS channelOpen within H.225

Indipendent from H.245 signaling and control messages

Open before any other channel

Communication channel between user-agent and gatekeeper

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Call controller

It uses H.225 signaling

It establishes a connection between 2 endpoints

Activated after a RAS channel is open

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H.245 controller

End-to-end logical channel control

AimsTerminal capacity

Requesting operating modes

General commands and information

It is normally established between endpoint and gatekeeper (this acts like a “proxy”)

It may exist also directed to the endpoint

One channel for each callGatekeeper may have several open channels

Physical implementationSeparate channel

Channel 0 within H.225

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Gateway

It is seen as an H.323 terminal in the IP network and as a telephone terminal in the PSTN

It translatesData channels (e.g. G.729 / RTP in telephone sampling)

Control channel (e.g. H.225 to H.221)

Signaling procedures (e.g. H.245 to H.242)

UsageInterface between different technologies

Adaptation device in homogeneous networks (e.g. compression)

Backup of the IP network over PSTN

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Gatekeeper

It is responsible of one zoneIt transaltes addresses, from alias H.323 address to a transport layer address

Admission control, by means of RAS messages

Management of the H.323 zone

Optional functionalitiesCall authorization

Bandwidth management

E.g. by limiting the number of H.323 terminals connected

Call control

It keeps track of the terminal currently involved in a call

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Addressing

Need for a unique identifierNetwork address (IP address)

TSAP identifier (Transport layer Service Access Point)It corresponds to a TCP/UDP port

Well known: signaling channel, RAS channel

Decided at run-time: data channels

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Addressing: alias

It is possible to have different formsname@domain.com

E-164 phone numebr

nickname

...

They are available only if a gatekeeper exists

They are mapped onto an address/port pair

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Main phases of an H.323 call

Search for a gatekeeper

Registration with the gatekeeper

Call setup

Negotiation of terminal functionalities

Negotiation and opening of data channels

Data transfer

Call tear down

H.225 RAS

H.225 RAS

Q.931 modifed

H.245

H.245

Closing data channel H.245

H.245 + H.225 RAS

RTP

Only if a gatekeeper exists

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Serch for and registration with a gatekeeper

Gatekeeper Request (GRQ), in multicast

Gatekeeper Confirmation / Reject (GCF / GRJ)

Registration Request (RRQ)

Registration Confirmation / Reject (RCF / RRJ)

The client shouldselect only 1 Gatekeeper, in case several responses are received

Alternatively: staticconfiguration of the gatekeeperaddress

Unregister Request (URQ)

Unregister Confirmation / Reject (UCF / URJ)

The gatekeeper maystart the de-registrationwith URQ

The GCF may include a list alternative gatekeepers to be usedin case of failure

Gatekeeper Discovery IP address / port 224.0.1.41/1718 (UDP)

Gatekeeper Registration and Status port 1719 (UDP)

This includes IP address, list of alias forthe terminal, and a “TTL” field defining the validity of the registration

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Call admission and bandwidth change

Admission Request (ARQ)

Admission Confirmation / Reject (ACF / ARJ)

Bandwidth Change Request (BRQ)

Bandwidth Change Confirmation / Reject (BCF / BRJ)

Request of authorizing the call and for a certain

bandwidth

Modification of the bandwidth requested: allowed while the

call is in progress

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Direct call, without gatekeeper (Q.931)

Setup

Call Proceeding

Alerting

Connect

Optional, provided thatl Connect(oppure Release Complete) is

received in 4 sec.

It is possible to open a control channel withH.245

The user terminal isringing

The request has beentaken into consideration

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Gatekeeper Direct Endpoint (1)

Admission Request (ARQ)

Admission Confirmation / Reject(ARQ / ARJ)

Setup

Call Proceeding

Admission Request (ARQ)

Admission Confirmation / Reject(ARQ / ARJ)

Alerting

Connect

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Segnalazione

Gatekeeper Direct Endpoint (2)

Media Stream

Canale di controllo RAS

DCF/DRJ

DRQ

DCF/DRJ

DRQ

Segnalazione

Call in progress

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Gatekeeper Routed Call

Admission Request (ARQ)

Admission Confirmation / Reject(ARQ / ARJ)

SetupSetup

Admission Request (ARQ)

Admission Confirmation / Reject(ARQ / ARJ)

AlertAlert

ConnectConnect

Routed Call: media streams are sent end-to-

end

Gatekeeper Proxy: eventhe media streams pass through the gatekeeper

(not specified in the standard documents)

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Routing control messages

Two alternativesDirect channel between the endpoints

It is sued in case of direct call

Experimental in case of call through a Gatekeeper

Channel directed to the Gatekeeper

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beginning of the call

Terminal Cability Set

Terminal Cability Set Ack

Terminal Cability Set

Terminal Cability Set Ack

Open Logical Channel

Open Logical Channel Ack

Open Logical Channel Ack

Open Logical Channel

H.245 messages

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Call termination

End Session Command

End Session Command

Release Complete

Bandwidth Change Request (BRQ)

Bandwidth Change Confirmation (BCF)

Bandwidth Change Request (BRQ)

Bandwidth Change Confirmation (BCF)

H.225 RAS

H.245

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Conclusions

Good initial diffusionBoth in corporate and in telecom provider environments

Mature implementations

Complicated

There is an ever growing interest in SIP