Part III: IP Signaling Protocols
Chapter 10 H.323
Chapter 11
Session Initiation Protocol
Chapter 12
Gateway Control Protocols
Chapter 13
Virtual Switch Controller
Chapter 10. H.323
H.323 is an International Telecommunication Union Telecommunication Standardization Sector (ITU-T)
specification for transmitting audio, video, and data across an Internet Protocol (IP) network, including the
Internet. When compliant with H.323, vendors' products and applications can communicate and interoperate
with each other. The H.323 standard addresses call signaling and control, multimedia transport and control,
and bandwidth control for point-to-point and multipoint conferences. The H series of recommendations also
specifies H.320 for Integrated Services Digital Network (ISDN) and H.324 for plain old telephone service
(POTS) as transport mechanisms.
The H.323 standard consists of the following components and protocols:
The H.323 system is discussed in the following three sections:
• H.323 elements
• H.323 protocol suite
H.323 Elements
Figure 10-1 illustrates the elements of an H.323 system. These elements include terminals, gateways,
gatekeepers, and multipoint control units (MCU).

Feature Protocol
Call Signaling H.225
Media Control H.245
Audio Codecs G.711, G.722, G.723, G.728, G.729
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Figure 10-2. Relationships of H.323 Components

• System Control Unit—Provides H.225 and H.245 call control, capability exchange, messaging, and
signaling of commands for proper operation of the terminal.
• Media Transmission—Formats the transmitted audio, video, data, control streams, and messages onto
network interface. Media transmission also receives the audio, video, data, control streams, and
messages from the network interface.
• Audio Codec—Encodes the signal from the audio equipment for transmission and decodes the
incoming audio code. Required functions include encoding and decoding G.711 speech and
transmitting and receiving a-law and µ-law formats. Optionally, G.722, G.723.1, G.728, and G.729
encoding and decoding can be supported.
• Video Codec—Optional, but if provided, must be capable of encoding and decoding video according to
H.261 Quarter Comment Intermediate Format (QCIF).
• Data Channel—Supports applications such as database access, file transfer, and audiographics
conferencing (the capability to modify a common image over multiple users' computers
simultaneously), as specified in Recommendation T.120.
Gateway
Gateways are not needed unless interconnection with the SCN is required. Therefore, H.323 endpoints can
communicate directly over the packet network without connecting to a gateway. The gateway acts as an H.323
terminal or MCU on the network and an SCN terminal or MCU on the SCN, as illustrated in Figure 10-3
.
The following functions and capabilities are within the scope of the H.323 terminal:
• Network Interface—A packet-based interface capable of end-to-end Transmission Control Protocol
(TCP) and User Datagram Protocol (UDP) unicast and multicast services.
The H.323 gateway reflects the characteristics of a Switched Circuit Network (SCN) endpoint and H.323
endpoint. It translates between audio, video, and data transmission formats as well as communication systems
and protocols. This includes call setup and teardown on both the IP network and SCN.

An optional function, the gatekeeper provides pre-call and call-level control services to H.323 endpoints.
Gatekeepers are logically separated from the other network elements in H.323 environments. If more than one
gatekeeper is implemented, inter-communication is accomplished in an unspecified manner.
• Zone Management—Provided for registered terminals, gateways, and MCUs and discussed further in
the "RAS Signaling" section later in this chapter.
• Bandwidth Management—Enables the gatekeeper to reject admission if the required bandwidth is not
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The MCU and Elements
The multipoint processor (MP) receives audio, video, and/or data streams and distributes them to endpoints
participating in a multipoint conference.
The MCU is an endpoint that supports multipoint conferences and, at a minimum, consists of an MC and one
or more MPs. If it supports centralized multipoint conferences, a typical MCU consists of an MC and an audio,
video, and data MP.
H.323 Proxy Server
• Terminals that don't support Resource Reservation Protocol (RSVP) can connect through access or
local-area networks (LANs) with relatively good quality of service (QoS) to the proxy. Pairs of proxies
can then negotiate adequate QoSs to tunnel across the IP network. Proxies can manage QoS with
RSVP and/or IP precedence bits.
• Proxies support the routing of H.323 traffic separate from ordinary data traffic through application-
specific routing (ASR).
• A proxy is compatible with network address translation, enabling H.323 nodes to be deployed in
networks with private address space.
H.323 Protocol Suite
The H.323 protocol suite is based on several protocols, as illustrated in Figure 10-4. The protocol family
supports call admissions, setup, status, teardown, media streams, and messages in H.323 systems. These
protocols are supported by both reliable and unreliable packet delivery mechanisms over data networks.
Figure 10-4. Layers of the H.323 Protocol Suite

multicast message. Because endpoints do not have to be statically configured or reconfigured for gatekeepers,
this method has less administrative overhead. The gatekeeper discovery multicast address is 224.0.1.41, the
gatekeeper UDP discovery port is 1718, and the gatekeeper UDP registration and status port is 1719. The
following three RAS messages are used for H.323 gatekeeper auto discovery:
The H.323 protocol suite is split into three main areas of control:
The remainder of this section focuses on these three key signaling functions.
Gatekeeper discovery is a manual or automatic process endpoints use to identify which gatekeeper to register
with. In the manual method, endpoints are configured with the gate-keeper's IP address and, therefore, can
attempt registration immediately, but only with the predefined gatekeeper. The automatic method enables the
relationship between endpoints and gatekeepers to change over time and requires a mechanism known as
auto discovery.
• Gatekeeper Request (GRQ)—A multicast message sent by an endpoint looking for the gatekeeper.
Figure 10-5
illustrates the messaging and sequencing processes for auto discovery. • Gatekeeper Confirm (GCF)—The reply to an endpoint GRQ indicating the transport address of the
gatekeeper's RAS channel.
• Gatekeeper Reject (GRJ)—Advises the endpoint that the gatekeeper does not want to accept its
registration. This is usually due to a configuration on the gateway or gatekeeper.
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