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H.323. Chapter 4. Introduction. We have learned IP, UDP, RTP The set-up and tear-down of the sessions Signaling In traditional telephony networks ISUP, Integrated Services Digital Network User Part A component of the Signaling System 7 (SS7). H.323, ITU-T recommendation

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H 323


Chapter 4


  • We have learned

    • IP, UDP, RTP

  • The set-up and tear-down of the sessions

    • Signaling

  • In traditional telephony networks

    • ISUP, Integrated Services Digital Network User Part

      • A component of the Signaling System 7 (SS7)

  • H.323, ITU-T recommendation

    • The 1st version, 1996

      • Visual Telephone Systems and Equipment for Local Area Network which Provide A Non-Guaranteed Quality of Service

    • Version 2, 1998

      • Packet-based Multimedia Communications Systems

      • Widely implemented in VoIP solutions

The h 323 architecture
The H.323 Architecture

  • Entities

    • Terminals

    • Gateways

    • Gatekeepers

    • MCUs

  • Protocols

    • Parts of H.225.0 - RAS, Q.931

    • H.245

    • RTP/RTCP

    • Audio/video codecs

H 323 architecture
H.323 Architecture

H.323 Network Architecture and Components


  • Endpoint on a LAN

  • Supports real-time, 2-way communications with another H.323 entity

  • Must support:

    • Voice - audio codecs

    • Signaling and setup - Q.931, H.245, RAS

  • Optional support:

    • Video

    • Data


  • Interface between the LAN and the switched circuit network

  • Mandatory Functions

    • Transmission Format Translation

    • Communication Procedure Translation

    • Call Setup and Clearing On Both Sides

  • Optional Function

    • Media Format Translation

  • Example: IP/PSTN gateway


  • Optional, but must perform certain functions if present

    • e.g., Netmeeting does not use gatekeepers?

  • Manage a zone (a collection of H.323 devices)

  • Usually one gatekeeper per zone

    • alternate gatekeeper might exist for backup and load balancing

  • Typically a software application

    • implemented on a PC, but can be integrated in a gateway or terminal

  • Mandatory functions:

    • Address translation (routing)

    • Admission control

    • Minimal bandwidth control - request processing

    • Zone management

  • Optional functions:

    • Call control signaling - direct handling of Q.931 signaling between endpoints

    • Call authorization, bandwidth management, and call management using some policy

    • Gatekeeper management information (MIB)

    • Directory services


  • MCU - Multipoint Control Unit

    • Endpoint that supports conferences between 3 or more endpoints

    • Can be stand-alone device or integrated into a gateway, gatekeeper or terminal

  • Typically consists of multi-point controller (MC) and multi-point processor (MP)

    • MC - handles control and signaling for conference support

    • MP - receives streams from endpoints, processes them, and returns them to the endpoints in the conference

Centralized conference
Centralized Conference

  • MCU handles both signaling (MC) and stream processing (MP)



media stream (unicast)

control message




De centralized conference
De-centralized Conference

  • MCU handles only signaling

    • streams go directly between endpoints

    • In this case MCU functions without MP



media stream (multicast)

media stream (multicast)

control message

control message



media stream


Hybrid multipoint conference
Hybrid Multipoint Conference


audio stream (unicast)

control message



video stream


video stream


audio stream (unicast)

control message

audio stream (unicast)

control message



video stream


Mixed multipoint conference
Mixed Multipoint Conference

Decentralized side

Centralized side









multicast audio and video

unicast audio and video

Overview of h 323 singaling
Overview of H.323 Singaling

  • Audio codecs (G.711, G.723.1, G.728, etc.) and video codecs (H.261, H.263)

  • Media streams transported on RTP/RTCP

    • RTP carries actual media

    • RTCP carries status and control information

  • RTP/RTCP carried unreliably on UDP

  • Signaling is transported reliably over TCP

    • RAS - registration, admission, status (over UDP)

    • Q.931 - call setup and termination

    • H.245 - capabilities exchange

H 323 protocol stack1
H.323 Protocol Stack





A/V Cntl












H.225.0 H.245





Overview of h 323 protocols
Overview of H.323 Protocols

  • H.225.0, a two-part protocol

    • A variant of ITU-T recommendation Q.931, the ISDN layer 3 spec.

      • The set-up and tear-down of connections

      • Call signaling or Q.931 signaling

    • RAS signaling

      • Registration, Admissions, and Status

      • Between endpoints and gatekeepers

  • H.245, control protocols

    • Used between two or more endpoints

    • Manage the media streams of a session

      • Capability exchange

  • RAS, Q.931 and H.245

    • RAS to obtain permission from a gatekeeper

      • RAS channel

    • Q.931 to establish communication and set up the call

      • Call-signaling channel

    • H.245 to negotiate media parameters

      • H.245 control channel

    • Media streams over logical channels

H 323 addressing
H.323 Addressing

  • An entity in the H.323 network

    • A network address, an IP address

    • URL, Uniform Resource Locator

  • The TSAP, Transport Service Access Point

    • An id for a particular logical channel at a given entity

    • GK UDP Discovery Port = 1718

    • GK UDP Reg and Status Port = 1719

    • Call-signaling TCP Port =1720

  • Terminal and gateways

    • Have one or more aliases

    • Can take any number of forms

      • Must be unique within a zone

    • E.164 number

  • Codecs

    • Video codec is optional

    • G.711 (A-law and mu-law) is mandatory

Ras signaling
RAS Signaling

  • Used between a GK and endpoints in its zone

    • Since a GK is optional

  • Defined in H.225.0

    • GK Discovery

    • Registration

    • Unregistration

    • Admission

    • Bandwidth Change

    • Endpoint Location: GK (address translation)

    • Disengage

    • Status

    • Resource Availability: GW → GK

    • Non-standard

Gatekeeper discovery
Gatekeeper Discovery

  • Find a suitably accommodating GK

    • Several GKs, backup GK

  • GRQ – GK-request

    • Known addresses, multicast

    • GK id: if empty, soliciting GKs

  • GCF – GK-Confirm

    • Optionally, indicating one or more GKs to try

      • I cannot help you, but try the GK next door

    • Only one GK can be choosed

  • GRJ – GK-Reject

    • With a reason (e.g., a lack of resource)

Endpoint reg and reg cancellation
Endpoint Reg and Reg Cancellation

  • To become controlled by a GK

  • RegistrationRequest (RRQ)

    • RAS signaling port is 1719

    • Includes

      • An address for RAS messages

      • An address for call-signaling messages

      • An alias

      • One or more redundant addresses and aliases

      • Optional TTL, keepAlive parameters

  • RegistrationReject (RRJ)

  • RegistrationConfirm (RCF)

    • May assign an alias

  • UnregistrationRequest (URQ)

    • Cancel registration

    • By endpoints

    • By GKs

  • UnregistrationConfirm (UCF)

  • UnregistraionReject (URJ)

    • For an unregistered endpoint

Endpoint location
Endpoint Location

  • Request a real address of an alias

  • LocationRequest (LRQ)

    • To a GK or the GK discovery multicast address

    • A GK can also send an LRQ

  • LocationConfirm (LCF)

    • A call-signaling address

    • An RAS signaling address

  • LocationReject (LRJ)

    • The endpoint is not registered


  • Request permission from a GK to participate in a call

  • AdmissionRequest (ARQ)

    • The type of the call (e.g., two-party or multi-party)

    • The endpoint’s own id

    • A call identifier (a unique string)

    • A call-reference value (an integer)

    • Information of the other party

      • Aliases

      • Signaling address

    • Bandwidth (mandatory)

    • TransportQOS: endpoint or GK to reserve the resource

  • AdmissionConfirm (ACF)

    • Many of the same parameters as ARQ

      • A firm order from the GK

    • callModel

      • Optional in ARQ; mandtory in ACF

      • The endpoint sends call signaling directly or via the GK

  • AdmissionReject (ARJ)

    • With a reason

      • Bandwidth, address translation, unregistered endpoint

Bandwidth change
Bandwidth Change

  • Request an increase or decrease in allocated bandwidth

    • Can change without request if within the limit in ACF

  • BandwidthRequest (BRQ)

    • The new bandwidth, the call id

  • BCF

  • BRJ

  • The GK can also request an endpoint to change the bandwidth

    • The endpoint must comply

  • Closely tied to H.245 signaling (for logical channels)

    • A reduction in bandwidth

      • Require an existing logical channel to be closed and reopened


  • A GK be informed of the status of an endpoint

  • InformationRequestResponse (IRR)

    • Endpoint information

    • The active call information

      • Call id, call reference value, call type, the bandwidth

      • RTP session information (CNAME, RTP/RTCP address, etc.)

  • IRQ

  • or ACF with an irrFrequency parameter

    • willRespondToIRR parameter

  • IACK


    • An IRR message in error


  • The end of the call

  • DisengageRequest (DRQ)

    • Call id, call reference value, a disengage reason (e.g., normalDrop)

  • DCF

  • DRJ

    • For an unregistered endpoint

    • not a call party

  • The GK might issue DRQ to an endpoint

    • The endpoint must

      • Close the session

      • Respond to the GK with a DCF message

  • Resource Availability

    • ResourceAvailableIndicate (RAI)

      • An gateway sends to a GK

      • The available call capacity and bandwidth for each protocol

      • almostOutofResource parameter

    • RseourceAvailableConfirm (RAC)

  • Request in Progress

    • RIP

      • A given request takes longer the the timeout period

      • The expected delay and the reason

      • After timeout, the RAS message can be retransmitted

Call signaling
Call Signaling

  • For the establishment and tear-down of calls

  • Q.931 modified by rec. H.225.0

    • Reuse some messages with few modifications

    • Specify a number of rules regarding the usage of information elements defined in Q.931

      • E.g., no Transit Network Selection, …

      • Certain Q.931 mandatory information forbidden or optional

    • A clever use of User-to-User information element

      • Convey all of the extra information needed in H.323

      • E.g., H.245 addresses to be used for logical channel

  • Setup

    • The first call-signaling message

    • Q.931 Protocol Discriminator

    • A call reference

    • Bearer Capability

      • RTP information, such as payload type

      • A gateway needs to perform the mapping

    • User-to-User information element

      • Mandatory: Call id, call type, conference id, the caller information

      • Optional: source alias, destination alias, H.245 address, destination call-signaling address

  • Call Proceeding

    • Optional

    • call-establishment procedures are underway

    • Mandatory

      • Protocol discriminator, call reference, and message type, user-to-user

      • User-to-user: similar to the setup message

  • Alerting

    • The called user is being alerted

    • The same parameters as Call Proceeding

  • Connect

    • The called party has accepted the call

    • Must be sent if the call is to be completed

      • Call Proceeding and Alerting are optional

    • User-to-User information

      • The same as Call Proceeding

      • Plus Conference Identifier

        • The same as Setup

  • Release Complete

    • Terminate a call

    • No Release message

      • In ISDN, Release and Release Complete

    • Cause information element, optional

      • Otherwise, a Release reason in User-to-User

  • Facility (Q.932)

    • A call should be redirected

    • Also be used for supplementary services

    • User-to-User contains reason parameter

      • E.g., routeCallToGatekeeper

  • Interaction between Call Signaling and H.245

    • Call signaling: call establishment and tear-down

    • H.245: the negotiation and establishment of media streams

    • When to begin the exchange of H.245 messages?

      • Between the Setup and Connect messages

      • Equipment dependent

Call scenarios
Call Scenarios

  • Basic Call without GKs

    • Reliable transport

  • Gatekeeper-routed call signaling

    • EP <-> GK <-> GK <-> EP

    • ARJ with a cause code of routeCallToGatekeeper

    • A Facility with a reason indicating the call be rerouted

H 245 control signaling
H.245 Control Signaling

  • To establish and control media streams

    • Agree on the media formats and bandwidth

    • Multiplexing multiple media streams

    • No actual media

    • A generic protocol for the control of media streams

    • How it works in an H.323?

H 245 message groupings
H.245 Message Groupings

  • Requests

    • Require the recipient to perform some action and response

  • Response

    • In reply to Requests

  • Commands

    • Require the recipient to perform some action, but no response is necessary

  • Indications

    • Of an informational nature only

The concept of logical channels
The Concept of Logical Channels

  • A Logical channel

    • A unidirectional media path

    • An IP address and port number

    • Has a number

  • A two-party conversation

    • Two logical channels exist

    • Potentially in different formats

    • Or a bidirectional channel consists of two logical channels

  • An endpoint issues Open Logical Channel

    • Logical channel number and media information (RTP payload type)

  • Far endpoint responds with Open Logical Channel Ack

    • An RTP port

  • Messages over H.245 Control Channel (channel number 0)

    • Permanently open

H 245 procedures
H.245 Procedures

  • Capabilities Exchange

    • Share information regarding receive and transmit capabilities

    • Indicate a preference

    • TerminalCabilitySet message

      • A request message

      • A sequence number plus the types of audio and video formats

    • TerminalCapabilitySetAck

      • Empty with a sequence number

    • TerminalCapabilitySetReject

      • With a reason for rejection

  • TerminalCapabilitySetRelease

    • If no response within a timeout period

  • SendTerminalCapabilitySet

    • Request terminal Capability information

    • Request to indicate all of its capabilities

    • Or request confirmation about specific capabilities

    • A command message that is replied

Master slave determination
Master-Slave Determination

  • One of the endpoints needs to be the master

    • Of particular importance for the setup of a multi-party conference

  • Compare two pieces of information at each entity

    • A terminal type value

      • A terminal without an MC: 50

      • A gateway without an MC: 60

      • An MCU for audio, vedio: 190

      • An MCU managing a conference: 240 (the highest)

    • A random number (1..16,777,215)

  • Master-Slave Determination message

  • Master-Slave Determination Ack

    • A “master” or “slave” indication

  • Open Bidirectional Logicl Channels

    • Fig. 4-15

  • Closing Logic Channels and Ending a session

    • CloseLogicalChannel, CloseLogicalChannelAck

    • Only the initiator can issue

    • Or the receiving end can humbly request

    • A bidirectional channel can be closed by either end

    • Once all logical channels are closed

      • EndSession, EndSession commands

A slow start
A Slow Start


Capability exchange

Master-slave determination

Fast connect procedure
Fast-connect Procedure

  • The Setup

    • A faststart element

      • OpenLogicalChannel requests

    • No H.245 control channel

Conference calls
Conference Calls

  • MC manage multi-point conference

  • A Pre-Arranged Conference

    • Establish a call with the MCU

  • The MCU specifies the conference mode

    • Communication Mode command (H.245)

      • Specify all the sessions in the conference

      • The transmit requirements of each endpoint, not the receive requirements

An ad hoc conference
An Ad-Hoc Conference

  • Expand an existing two-party call

    • One the the endpoints must contain an MC

New Features in H.323 Version 2

  • H.235 - security and authentication, i.e. passwords for registration with gatekeeper

  • H.450.x - supplementary services such as call transfer and forwarding

  • Fast call setup:

    • Bypasses some setup messages

    • Triggered by Q.931 Fast Start message that contains basic capabilities

New Features in H.323 Version 2 (cont.)

  • Mechanism to specify alternative gatekeepers to endpoints

  • Gatekeeper can request forwarding of Q.931 information on direct routed calls; only RADCOM can play back H.323 streams off a network: a true differentiation

  • Smoother integration of T.120 (optional standard for data)

    • T.120 channel opened like any H.323 channel

T.120 Data Channel

  • Defined in T.120

  • A Channel Between Endpoints

  • A Reliable Channel

  • Functions

    • Transport Data

    • Multipoint Delivery

    • Internetworking

      • PSTN, ISDN

      • LAN (TCP/IP, IPX)

The Future of H.323

Inter-Gatekeeper Communication:

  • Current H.323 standards do not provide an inter-zone model that scales well for large networks

  • Inter-gatekeeper protocols being discussed to enable gatekeepers to efficiently locate one another to route calls to non-local address

  • Hierarchical arrangements with “clearing house” gatekeepers have been proposed

  • This is critical for widespread interoperability between VoIP service providers

H 323 version 3
H.323 Version 3

  • Modest improvements to H.323 version 2

  • Maintaining and Reusing Connections

  • Conference out of Consultation

  • Caller ID

  • Language Preference

  • Usage of Annex E/H.323

  • Remote Device Control

  • Generic Capabilities

  • Annex G/H.225.0 - Communication between Administrative Domains

H 323 version 4
H.323 Version 4 Transport

  • Many new enhancements

  • Gateway Decomposition

  • Multiplexed Stream Transmission

  • Supplementary Services

  • Annex K/H.323

  • Annex L/H.323

  • H.450.8 - Name Identification Service

  • H.450.9 - Call Completion

  • H.450.10 - Call Offer

  • H.450.11 - Call Intrusion

  • Additive Registrations Transport

  • Alternate Gatekeepers

  • Usage Information Reporting

  • Endpoint Capacity

  • Caller Identification Service

  • Tones and Announcements

  • Mapping Aliases

  • Indicating Desired Protocols

  • Bandwidth Management

  • Reporting Call Status

  • Enhancements to Annex D (Real-Time Fax) Transport

  • Call Linkage

  • Tunneling

  • QoS

  • H.245 in Parallel with Fast Connect

  • Generic Extensibility Framework

  • H.323 URL

  • Call Credit-Related Capabilities

  • DTMF Relay via RTP