H.323 Standard
The H.323 standard has been developed by the ITU-T for equipment manufacturers and
vendors who provide Voice over IP service. It provides technical recommendations for voice
communication over LANs assuming that no Quality of Service (QoS) is being provided by
LANs. It was originally developed for multimedia conferencing on LANs, but was later
extended to Voice over IP. The first and second versions of H.323 were released in 1996 and
1998 respectively. Currently, version 4 of H.323 is under consideration.
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Components of H.323
The H.323 standard proposes an architecture that is composed of four logical components –
Terminal, Gateways, Gatekeepers and Multipoint Control Units (MCUs). The architecture
schematic is depicted in the following diagram. The various components are described
subsequently.
Terminals
These are LAN client endpoints that provide real-time, two-way communications. All H.323
terminals are required to support H.245, H.225, Q.931, Registration Admission Status (RAS)
and real-time transport (RTP) protocols. H.245 is used for controlling channel usage, while
H.225 or Q.931 are used for call signaling, call setup and teardown. RTP is used as a media
transport protocol that carries the voice traffic. RAS is used by the endpoint for interacting
with the gatekeeper. H.323 terminals may also use T.120 data conferencing protocols, video
codecs and support for MCU. An H.323 terminal can communicate with either another H.323
terminal, a H.323 gateway or a MCU.
Gateways
An H.323 gateway is an endpoint on the network that provides for real-time, two-way
communications between H.323 terminals on the IP network with other ITU terminals on a
switch-based network like PSTN or to another H.323 gateway. The gateways handle different
transmission formats. Gateways are optional devices in the H.323 architecture because
terminals in a single LAN can communicate directly with each other without using a gateway.
Only if the communication needs to span to other networks such as the PSTN, will a gateway
be required. In such cases, H.245 and Q.931 protocols are used, by the participating
endpoints and the intermediate gateway.
Gatekeepers
This is an important component of the H.323 architecture and functions as its “manager”. It
is the central point for all calls within its zone and provides services to the registered
endpoints. A zone is the aggregation of the gatekeeper and the registered endpoints.
A gatekeeper performs functions such as address translation, admissions control, call
signaling, call authorization, call management and bandwidth management.
Multipoint Control Units (MCU)
The MCU acts as an endpoint on the network for providing capability for three or more
terminals and gateways to participate in a multipoint conference. The MCU consists of a
mandatory Multipoint Controller (MC) and an optional Multipoint Processor (MP). The MC’s
functions are to determine the common capabilities of conferencing terminals, using the
H.245 protocol. It however does not perform multiplexing of audio, video and data streams.
The multiplexing of these media streams is handled by the MP under control of the MC.
H.323 Protocol Stack
A schematic description of the H.323 protocol stack is given in the following diagram. The
unreliable but low latency UDP is used to transport audio, video and registration packets.
Whereas the reliable but slow TCP is used for data and control packets in call signaling, the
T.120 protocol is used for data conferencing.
Call Control and signalinData g Audio/Video Registration
T.120
H.225 Call
signaling
H.245
Conference control
RTP/RTCP H.225 RAS
TCP UDP
Physical Layer
Link Layer
Network Layer (IP)
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Control and signaling in H.323 system
H.323 provides three control protocols – H.225/Q.931 call signaling, H.225/RAS call signaling
and H.245 Media control. The H.225/Q.931 is used for call signaling control. The H.225/RAS
channel is used for establishing a call from the source to the receiving host. After the call is
established, H.245 is finally used to negotiate the media streams.
H.225/RAS
This RAS (Registration, Admission and Signaling) channel is used between the endpoints and
the gatekeeper. RAS uses unreliable UDP and hence also implements timeouts and retry
count mechanisms for incorporating reliability.
RAS procedures used by endpoints encompass Gateway discovery, Endpoint registration,
Endpoint location, admission, bandwidth negotiation and status change.
H.225/Call Signaling
This channel is used to carry H.225 control messages. In networks that do not contain a
gatekeeper, call signaling messages are exchanged directly between endpoints using the Call
Signaling Transport Address. In this case, it is assumed that the calling endpoint knows the
called endpoints. However, in networks containing gatekeepers, the initial admission
message can take place between the calling endpoint and the gatekeeper, using the
gatekeeper’s RAS channel Transport Address. This call signaling is done over TCP.
H.245 Media and Conference Control
A fter establishment of a call, the H.323 systems use the H.245 media control protocol to
negotiate and establish all the media channels to be carried by RTP/RTCP.
This protocol is used to perform functions such as determination of master and slave in a
multi-party conference, capability exchange, media channel control and conference control.
H.323 Call Setup
Given below is a set of steps required for setting up a H.323 call.
! Discover a gatekeeper that would manage the endpoint
! Register the endpoint with the gatekeeper
! Endpoint enters the call setup phase
! Capability exchange between the endpoints
! Call is established
! After calling, the call can be terminated by either party
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H.323 Implementations
One of the most popular H.323 implementation available in the market is from Radvision. Its
H.323 stack is widely used by service providers. Some of the other H.323 implementations
available in the market are from Elemedia, Cisco, Micom, Nortel, Vocaltec, Neura Solutions
and Ericsson. A description of their products is given in the Industry products section of this
document.