h.323

H.323

Ng Tai Hung

Internet Telephony 4-2

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 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

Version 4


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 Network Architecture and Components



Terminals

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



Gateways

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



Gatekeepers

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


MCUs

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


MC and MP

T1521250-96

MC

MC MC MP MC

MC

Gateway 1 MCU 1

LAN

MCU 2

Gatekeeper 1 Terminal 1 Terminal 2

NOTE – Gateway, Gatekeeper and MCU can be a single device.

Gatekeeper 2

Gateway 2 Gateway 3

Gatekeeper 3

MC MP

MC MP


MCU(MC+MP)

MCU(MC+MP)

TerminalTerminal TerminalTerminal TerminalTerminal

media stream (unicast)control message

Centralized Conference

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


De-centralized Conference

MCU handles only signaling streams go directly between endpoints In this case MCU functions without MP

MCU(MC)

MCU(MC)

TerminalTerminal TerminalTerminal

media stream (multicast)control message

media stream (multicast)control message

media stream(multicast)


Hybrid Multipoint Conference

MCU(MC+MP)

MCU(MC+MP)

TerminalTerminal TerminalTerminal

video stream(multicast)



TerminalTerminal

control message

audio stream (unicast)

control messageaudio stream (unicast)

control messageaudio stream (unicast)


Mixed Multipoint Conference

TerminalTerminal

TerminalTerminal

TerminalTerminal

TerminalTerminal

TerminalTerminal

TerminalTerminal

MCU(MC+MP)

MCU(MC+MP)

multicast audio and video unicast audio and video

Decentralized side Centralized side


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 Stack


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

An entity in the H.323 network A network address, an IP address URL, Uniform Resource Locator

E.g., ras://GK1@somedomain A port number appended RAS, a default port number = 1719

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

H.261 QCIF G.711 (A-law and -law) is mandatory


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

Find a suitably accommodating GK Several GKs, backup GK

GRQ – GK-request Known addresses, multicast 224.0.1.41:1718 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)


GK Discovery


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

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


Admission

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


Direct call signaling


GK-routed call signaling


P regranted Admission to minimize call setup delay A GK provide an endpoint with admission in advance the RCF includes the parameter preGrantedARQ


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


An example bandwidth change


Status

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 INACK

An IRR message in error


Disengage

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)


Service Control SCI (ServiceControlIndication) SCR (ServiceControlResponse) to enable advanced features

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

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


Progress sent by a called gateway the Cause information – in-band tones the User-to-User info. – same 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

Basic Call without GKs Reliable transport


Call with GKs and Direct-Endpoint Call Signaling


Gatekeeper routed/direct-endpoint call signaling


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



Optional called-endpoint 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

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

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

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

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


Establishing and Releasing Media Streams For media exchange


Open Unidirectional Logical Channel


Open Bidirectional Logicl Channels


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


Close a Logical Channel


A Slow Start

PlusCapability exchangeMaster-slave determination


Fast-connect Procedure

The Setup A faststart element

OpenLogicalChannel requests

No H.245 control channel


H.245 Message Encapsulation is encapsulated with Q.931 messages Set the element h245Tunneling true in the first

Q.931 message The h245Control element contains the

encapsulated data Conflict with the faststart element Use Facility

when no Q.931 message can be used to start a separate H.245 control channel


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

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

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


Annex E/H.323 - Protocol for Multiplexed Call Signaling Transport

Annex F/H.323 - Simple Endpoint Type H.341 - H.323 Series MIB Supplementary Services


H.323 Version 4

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 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) 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


The Decomposed Gateway

Media gateway Media gateway controller

h.323

Documents

routed call

user information

endpoint call

slave determination

faststart

media streams

logical channels

media stream