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A resource for packet-switchedconversational protocols

PacketizerTM

Copyright © 2004

Overview of H.323http://www.packetizer.com/voip/h323/papers/

Paul E. JonesRapporteur, ITU-T Q2/SG16

[email protected] 2004

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A resource for packet-switched conversational protocols

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Executive Summary• H.323 was first approved in February 1996, the same

month that the first SIP draft was published• Designed to operate over complex networks, such as the

Internet• Although base-level functionality required only voice,

video dominated early implementations remains a majorstrength of H.323

• First standards-based “Voice over IP”• Today, H.323 is the most widely deployed standards-

based voice and videoconferencing standard for packet-switched networks, with literally billions of minutes ofbillable traffic every month

• ITU-T now considering work on H.323v6

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The Basics of H.323

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What is H.323?

• H.323* is a multimedia conferencingprotocol, which includes voice, video, anddata conferencing, for use over packet-switched networks

* H.323 is “ITU-T Recommendation H.323: Packet-based multimediacommunications systems”

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Elements of an H.323 System

• Terminals• Multipoint Control Units (MCUs)• Gateways• Gatekeeper• Border Elements

Referred to as“endpoints”

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Terminals

• Telephones• Video phones• IVR devices• Voicemail Systems• “Soft phones” (e.g., NetMeeting®)

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MCUs

• Responsible for managing multipointconferences (two or more endpointsengaged in a conference)

• The MCU contains a Multipoint Controller(MC) that manages the call signaling andmay optionally have Multipoint Processors(MPs) to handle media mixing, switching,or other media processing

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Gateways

• The Gateway is composed of a “Media GatewayController” (MGC) and a “Media Gateway”(MG), which may co-exist or exist separately

• The MGC handles call signaling and other non-media-related functions

• The MG handles the media• Gateways interface H.323 to other networks,

including the PSTN, H.320 systems, and otherH.323 networks (proxy)

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Gatekeeper

• The Gatekeeper is an optional componentin the H.323 system which is primarilyused for admission control and addressresolution

• The gatekeeper may allow calls to beplaced directly between endpoints or it mayroute the call signaling through itself toperform functions such as follow-me/find-me and forward on busy

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Border Elementsand Peer Elements

• Peer Elements, which are often co-located with aGatekeeper, exchange addressing information andparticipate in call authorization within and betweenadministrative domains

• Peer Elements may aggregate address information toreduce the volume of routing information passed throughthe network

• Border Elements are a special type of Peer Element thatexists between two administrative domains

• Border Elements may assist in callauthorization/authentication directly between twoadministrative domains or via a clearinghouse

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The Protocols

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The Protocols (cont)• H.323 is a “framework” document that describes how the

various pieces fit together• H.225.0 defines the call signaling between endpoints and

the Gatekeeper• RTP/RTCP (RFC 3550) is used to transmit media such as

audio and video over IP networks• H.225.0 Annex G and H.501 define the procedures and

protocol for communication within and between PeerElements

• H.245 is the protocol used to control establishment andclosure of media channels within the context of a call andto perform conference control

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The Protocols (cont)• H.450.x is a series of supplementary service protocols• H.460.x is a series of version-independent extensions to

the base H.323 protocol• T.120 specifies how to do data conferencing• T.38 defines how to relay fax signals• V.150.1 defines how to relay modem signals• H.235 defines security within H.323 systems• X.680 defines the ASN.1 syntax used by the

Recommendations• X.691 defines the Packed Encoding Rules (PER) used to

encode messages for transmission on the network

Slide added by C. Pham for educational purposes

H323 Physical entities


H323 & ISO model


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Registration, Admission, andStatus - RAS

• Defined in H.225.0• Allows an endpoint to request authorization to

place or accept a call• Allows a Gatekeeper to control access to and

from devices under its control• Allows a Gatekeeper to communicate the address

of other endpoints• Allows two Gatekeepers to easily exchange

addressing information

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Registration, Admission, andStatus – RAS (cont)

T GKRRQRCF

ARQ

(endpoint is registered)

ACF(endpoint may place call)

DRQ

DCF(call has terminated) T Terminal

GK Gatekeeper

GW Gateway

Symbol Key:

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H.225.0 Call Signaling

• Allows an endpoint to initiate andterminate a call with another endpoint

GW GWSetupAlerting

Connect(call is established)

Release Complete(call is terminated)

H.245Signalingmay takeplace atany point

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Open achannel in

eachdirection

H.245 Signaling

• H.245 is used to negotiate capabilities andto control aspects of the conferencebetween two or more endpoints

GW GWTCS + MSDTCS + TCS Ack + MSD Ack

TCS Ack + MSD Ack + OLCOLC Ack + OLC

OLC Ack

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Fast Connect and H.245

• Some H.323 calls do not utilize the richcapabilities offered by H.245 and simplymedia channels using the “Fast Connect”procedures

• In this mode, a call may be established withas few as two messages (Setup / Connect)

GW GWSetup

Connect

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An H.323 Stack

RAS RTP / RTCP

Packet Network

H.323 Application

H.245

H.225.0Call Signaling

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Resolving Addresses• A Gatekeeper may resolve addresses in a number of ways

– Sending a Location Request (LRQ) message to another Gatekeeper– Accessing a Peer Element– Accessing a back-end database (e.g., LDAP)

• Gatekeepers and Peer Elements may query otherGatekeepers and Peer Elements and may exchangeaddress information outside the context of a call

• Since a Gatekeeper is not required, endpoints may resolveaddresses themselves using, for example, DNS, LDAP, ora local “phonebook” containing static IP addresses

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Using LRQs

TGK

LRQ

GK

GK

ARQLRQ

• A Gatekeeper may send anLRQ to one ore moreGatekeepers

• It may accept any LCFresponse and utilize thatinformation to satisfy theoriginal ARQ

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Using LRQs with HierarchicalGatekeepers (cont)

• A Gatekeeper mayforward an LRQ receivedon to another Gatekeeperin order to resolve theaddress

• The response may bedirected back to theoriginating Gatekeeperor the intermediateGatekeeper

TGK

LRQ

GK

ARQ

GK

LRQ

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Using a Border Element• As with hierarchical

Gatekeepers, BorderElements may sendAccessRequestmessages to otherBorder Elements andindicate where to send areply

• Border Elements mayalso reply directly to arequest by utilizingaddress informationcached from previousexchanges with otherBorder Elements

TGK

LRQ

GK/BE

ARQ

GK/BE

AccessRequest

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H.323 Features

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Advanced Videoconferencing

• Supports advanced videoconferencingfeatures, including– Cascading MCUs– MCU control over audio and video mixing– Chair control– Far-end camera control

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Supplementary Services• Standard mechanisms to provide a variety of services,

including– Call transfer– Call forward– Call park/pick-up– Call Hold– Call Waiting– Message Waiting Indication– Call Completion on Busy / No-Answer– Call Intrusion

• Support for HTTP-based service control (H.323 Annex K)

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Dynamic Routing and Re-Routing

• Gatekeeper may provide multiple “routes”to a destination, including information suchas:– Multiple destination addresses– Alternate “source” alias information– Source and destination circuit information

• H.460.8 allows an endpoint to re-query thegatekeeper for an alternate route in theevent that the primary route is unavailable

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Addressing

• Supports URLs, including the “h323” URLand “tel” URLs

• Supports dialed digits from a traditionaltelephone

• Supports various numbering types used inthe PSTN

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QoS

• H.460.9 allows an endpoint to reportQuality of Service information to theGatekeeper, aiding in determine how toroute calls

• H.323 devices may utilize IETF standardsfor providing quality of service, includingDiffServ and RSVP

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Miscellaneous Capabilities• Device Diagnostics: H.323 allows testing

equipment to place “test calls” to a device andestablish “media loops” in order to measurepacket loss and delay

• Emergency Services– H.323 devices may indicate the priority of a call– Emergency call centers have the wherewithal to

control how and when a call is released• Support for user, terminal, and service mobility

(H.510)• Ability to tunnel any legacy protocol

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On to Next Generation Network

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Scalability

• H.323 allows calls to be routed directlybetween endpoints without the need for anintermediate entity that maintains call state

• Ability to utilize network services foraddress resolution, including ENUM,LDAP, and DNS

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Gateway Decomposition

• H.248.1 defines how to decompose anH.323 gateway into a Media GatewayController and one or more MediaGateways MGC

MGMGMGMGMG

MGMGMGMGMG

MGMGMGMGMG

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Robustness

• Alternate gatekeepers• Mechanisms for “failing over” to an

alternate softswitch or other routing entity(H.323 Annex R)

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Re-Routing Active Calls (H.460.15)

• Useful to allow “session bordercontrollers” to participate in initial callsetup (e.g., manipulate source ordestination addresses) and then exit the callsignaling path

• Allows devices to participate in signalingonly when there is a need to exchangemessages during a call (often only thebeginning and the end of a call)

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Flexibility

• Voice over IP (or any packet-basednetwork)

• Videoconferencing• Support for T.120 data conferencing• Support for real-time text communication• Support for fax and modem relay

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Multimedia

• The most important aspect of the Next GenerationNetwork is the ability to communicate in newways

• Video will be a very important component• H.323 has very strong support for video• H.323 and T.120 allow users to work side by side

on a document using voice, video, text, andapplication sharing technologies

• H.323 is “multimedia over IP”, ushering in theNext Generation Network that users are seeking

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