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<TH align=middle colSpan=3>2.2. Protocols</TH></TR>
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<H2 class=title style="CLEAR: both"><A
id=d0e357>2.2. Protocols</H2></DIV></DIV>
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<DIV class=sect2 lang=en>
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<DIV>
<DIV>
<H3 class=title><A id=sec-h323>2.2.1. H.323</H3></DIV></DIV>
<DIV></DIV>
<P>The H.323 Series of Recommendations evolved out of the ITU-T's work on video
telephony and multimedia conferencing: after completing standardization on video
telephony and video conferencing for ISDN at up to 2 Mbit/s in the H.320 series,
the ITU-T took on work on similar multimedia communication over ATM networks
(H.310, H.321), over the analog Public Switched Telephone Network (PSTN) using
modem technology (H.324), and over the still-born Isochronous Ethernet (H.322).
The most widely adopted and hence most promising network infrastructure - and
the one bearing the largest difficulties to achieve well-defined Quality of
Service - was addressed in the beginning of 1995 in H.323: Local Area Networks,
with the focus on IP as network layer protocol. The primary goal was to
interface multimedia communication equipment on LANs to the reasonably
well-established base on circuit-switched networks.</P>
<P>The initial version of H.323 was approved by the ITU-T about one year later
in June 1996, thereby providing a basis on which the industry could converge.
The initial focus was clearly on local network environments, as QoS mechanisms
for IP-based wide area networks such as the Internet were not well established
at this point. In early 1996 Internet-wide deployment of H.323 was already
explicitly included in the scope as was the aim to support voice-only
applications and, thus, the foundations to use H.323 for IP Telephony were laid.
H.323 has continuously evolved towards becoming a technically sound and
functionally rich protocol platform for IP telephony applications, the first
major additions to this end being included in H.323 version 2 approved by the
ITU-T in January 1998. In September 1999, H.323 version 3 was approved by the
ITU-T, incorporating numerous further functional and conceptual extensions to
enable H.323 to serve as a basis for IP telephony on a global scale and to make
it meet requirements in enterprise environments as well. Moreover, many new
enhancements have been introduced into the H.323 protocol. Version 4 was
approved November 17, 2000 and contains enhancements in a number of important
areas, including reliability, scalability, and flexibility. New features help
facilitate more scalable Gateway and MCU solutions to meet the growing market
requirements. H.323 has been the undisputed leader in voice, video, and data
conferencing on packet networks, and Version 4 makes strides to keep H.323 ahead
of the competition.</P>
<DIV class=sect3 lang=en>
<DIV class=titlepage>
<DIV>
<DIV>
<H4 class=title><A id=d0e367>2.2.1.1. Scope</H4></DIV></DIV>
<DIV></DIV>
<P>As stated before, the scope of H.323 encompasses multimedia communication in
IP-based networks, with significant consideration given to gatewaying to
circuit-switched networks (particular to ISDN-based video telephony and to
PSTN/ISDN/GSM for voice communication).</P>
<DIV class=figure><A id=fig-h323scope>
<P class=title><B>Figure 2.1. Scope and Components defined in
H.323</B></P>
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<TR>
<TD align=middle><IMG
alt="Picture showing the scope and the components defined in 		H.323"
src="ch02s02.files/h323scope.png" width=496
align=middle></TD></TR></TBODY></TABLE></DIV></DIV>
<P>H.323 defines a number of functional / logical components as shown in figure
<A title="Figure 2.1. Scope and Components defined in H.323"
href="http://www.informatik.uni-bremen.de/~prelle/terena/cookbook/main/ch02s02.html#fig-h323scope">Figure 2.1</A>:
</P>
<DIV class=itemizedlist>
<UL type=disc compact>
<LI><SPAN class=emphasis><EM>Terminal</EM></SPAN> -- Terminals are
H.323-capable endpoints, which may be implemented in software on workstations
or as stand-alone devices (such as telephones). They are assigned to one or
more aliases (e.g. a user's name / URI) and/or telephone number(s).
<LI><SPAN class=emphasis><EM>Gateway</EM></SPAN> -- Gateways interconnect
H.323 entities (such as endpoints, MCUs, or other gateways) to other
network/protocol environments (such as the telephone network). They are also
assigned one or more aliases and/or telephone number(s). The H.323 series of
Recommendations provides detailed specifications for interfacing H.323 to
H.320, ISDN/PSTN, and ATM based networks. Recent work also addresses control
and media gateway specifications for telephony trunking networks such as
SS7/ISUP.
<LI><SPAN class=emphasis><EM>Gatekeeper</EM></SPAN> -- The gatekeeper is the
core management entity in an H.323 environment. It is, among other things,
responsible for access control, address resolution, and H.323 network (load)
management and provides the central hook to implement any kind of utilization
/ access policies. An H.323 environment is subdivided into zones (which may,
but need not, be congruent with the underlying network topology); each zone is
controlled by one primary gatekeeper (with optional backup gatekeepers).
Gatekeepers may also provide value-add, e.g. act as conferencing bridge or
offer supplementary call services. H.323 Gatekeeper can be also equipped with
the proxy feature. Such a feature enables the routing through the gatekeeper
of the RTP traffic (audio and video) and the T.120 traffic (data) so no
traffic is directly exchanged between endpoints. (it could be considered a
kind of IP to IP gateway that can be used for security and QoS purposes).
<LI><SPAN class=emphasis><EM>Multipoint Controller (MC)</EM></SPAN> -- A
multipoint controller is a logical entity that interconnects call signaling
and conference control channels of two or more H.323 entities in a star
topology. MCs coordinate the (control aspects of) media exchange between all
entities involved in a conference; they also provide the endpoints with
participant lists, exercise floor control, etc. MCs may be embedded in any
H.323 entity (terminals, gateways gatekeepers) or implemented as stand-alone
entities. They can be cascaded to allow conferences spanning multiple MCs.
<LI><SPAN class=emphasis><EM>Multipoint Processor (MP)</EM></SPAN> -- For
multipoint conferences with H.323, an optional Multipoint Processor may be
used that receives media streams from the individual endpoints, combines them
through some mixing/switching technique, and transmits the resulting media
streams back to the endpoints.
<LI><SPAN class=emphasis><EM>Multipoint Control Unit (MCU)</EM></SPAN> -- In
the H.323 world, an MCU simply is a combination of an MC and an MP in a single
device. The term originates in the ISDN videoconferencing world where MCUs
were needed to create multipoint conferences out of a set of point-to-point
connections.</LI></UL></DIV>
<P></P></DIV>
<DIV class=sect3 lang=en>
<DIV class=titlepage>
<DIV>
<DIV>
<H4 class=title><A id=d0e419>2.2.1.2. Signaling protocols</H4></DIV></DIV>
<DIV></DIV>
<P>H.323 resides - similar to the IETF protocols discussed in the next
subsection - on top of the basic Internet Protocols (IP, IP Multicast, TCP, UDP)
and can make use of integrated and differentiated services along with resource
reservation protocols. </P>
<DIV class=figure><A id=fig-h323portarch>
<P class=title><B>Figure 2.2. H.323 protocol architecture</B></P>
<DIV class=mediaobject align=center><IMG
alt="Picture showing the scope and the components defined in 		H.323"
src="ch02s02.files/h323protarch.png" align=middle></DIV></DIV>
<P>For basic call signaling and conference control interactions with H.323, the
aforementioned components communicate using three control protocols:</P>
<DIV class=itemizedlist>
<UL type=disc>
<LI><SPAN class=emphasis><EM>H.225.0 Registration, Admission, and Status
(RAS)</EM></SPAN> -- The RAS channel is used for communication between H.323
endpoints and their gatekeeper and for some inter-gatekeeper communication.
Endpoints use RAS to register with their gatekeeper, to request permission to
utilize system resources, to have addresses of remote endpoints resolved, etc.
Gatekeepers use RAS to keep track of the status of their associated endpoints
and to collect information about actual resource utilization after call
termination. RAS provides mechanisms for user / endpoint authentication and
call authorization.
<LI>
<P><SPAN class=emphasis><EM>H.225.0 Call Signaling</EM></SPAN> -- The call
signaling channel is used to signal call setup intention, success, failures,
etc. as well as to carry operations for supplementary services (see below).
Call signaling messages are derived from Q.931 (ISDN call signaling); however,
simplified procedures and only a subset of the messages are used in H.323. The
call signaling channel is used end-to-end between caller and callee and may
optionally run through one or more gatekeepers (the call signaling models are
later described in the <A title="2.2.1.4. Signaling models"
href="http://www.informatik.uni-bremen.de/~prelle/terena/cookbook/main/ch02s02.html#sec-signaling-models">Signaling
models section</A>).</P>
<P><SPAN class=emphasis><EM>Optimizations:</EM></SPAN> Since version 3 H.225.0
supports the following enhancements:</P>
<DIV class=itemizedlist>
<UL type=circle compact>
<LI><SPAN class=emphasis><EM>Multiple Calls</EM></SPAN> - To prevent using a
dedicated TCP connection for each call gateways can be built to handle
multiple calls on each connection.
<LI><SPAN class=emphasis><EM>Maintain Connection</EM></SPAN> - Similar to
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