H.323: An Introduction
In order to communicate effectively, a common language between
the two (or more) participants must be chosen. Without a common
language, there will be little or no understanding, resulting in
a passing of garbage between the parties. This doesn't apply only
to people; computers particularly must have common ground when talking,
notably because of the context-free world in which they operate.
Networks of computers, therefore, are built on standards and protocols,
decided upon so that the networks can exist and operate at their
fullest capabilities. These recommendations are made by a body
called the ITU (the International Telecommunications Union), a group
that makes recommendations on standardization for computers, radios,
and other machines that communicate with each other.
The H.32x family of standards handles multimedia communications.
This family includes H.320 (communication over ISDN [integrated
services digital networks]) and H.324 (communication over SCN [switched
circuit network], better known as traditional phone services).
The H.323 standard came about in late 1996, aimed at the emerging
multimedia communications over LANs (local area networks). The
standard has since been revised to include voice-over IP and IP
telephony, as well as gatekeeper-to-gatekeeper communications and
other data communications that involve packet-based networks (these
networks include IP-based networks like the Internet, Internet Packet
Exchange (IPX) LANs, and WANs).
The standard specifies a lot of information about the properties
and components of the H.323 network. It specifies the pieces that
combine to provide the communication service:
º terminals, either PC or stand alone devices, these are
the endpoints of the communication lines
º gatekeepers, the brains of the network; providing services like
addressing, authorization, and bandwidth management
º gateways, which serve as translators when connecting to a dissimilar
network (such as an H.324, for example)
º MCUs (multipoint control units) which allow multipoint conferencing,
or communication between more than two parties at once (much like
a traditional conference call on a telephone)
Besides equipment, there is more information provided by the H.323
standard. Included also are protocol standards, controlling the
audio and video codecs used, RAS (registration, admission, and status),
call signaling, and control signaling. H.323 also specifies that
all terminals on the network must provide support for the above
specifications.
For a much more detailed overview of the H.323 specifications,
see http://www.webproforum.com/h323/topic01.html.
H.323: Tutorials and specific papers worth reading
------------------------------------------------------------------
http://www.uwex.edu/ics/support/video/H323/tutorials/links.html
http://www.aarnet.edu.au/engineering/projects/voip/
overview.html (very
gentle overview of VoIP)
http://www.packetizer.com/iptel/h323/papers/primer/ (single
long web page, some structural detail)
http://www.cis.ohio-state.edu/~jain/cis788-99/h323/ (set
of web pages, a PDF file with some PDF
bugs in it. Some structural detail, a little message level detail, links to other resources)
http://www.iec.org/online/tutorials/h323/index.html (10
web pages, some structural detail, some message level detail)
http://www.iec.org/online/tutorials/gatekeep/ (6
web pages, some structural detail of gatekeepers, very little message
level detail)
http://www.intel.com/technology/itj/q21998/pdf/h323.pdf (11
pages, good structural detail, some message level detail)
http://www.switch.ch/vconf/ws2003/h323_basics_handout.pdf (22
slides, good structural overview, some message level detail)
http://www.video.ja.net/323intro.pdf (55
pages, good structural detail, good message level detail)
http://www.h323forum.org/papers/polycom/DeployingSecure
IPVideoNetworks.pdf (vendor-specific, approach
to handling security between endpoints by
tunneling over VPNs)
http://www.h323forum.org/papers/basic_h323_von_99.zip (ppt
slide show, some structural overview, some (excessive?) message
level detail. Not many words)
http://www.h323forum.org/papers/H.323_Protocol_Overview.ppt (ppt
slide show, 130 pages, lots of structural detail, message
detail and some packet/ASN detail)
H.323: Summary sites with lots of content to drill into
----------------------------------------------------------------------
http://www.packetizer.com (mainly links to other sites, the
standards,
plus the above primer)
http://www.h323forum.org (links
to other sites, the standards, and a
very nice list of papers locally stored and elsewhere)
http://www.openh323.org (source
code for h323 implementations,
clients, mcu's, gatekeepers, etc. Limited technology
implemented, but a
fair bit of the basic signalling is there)
|
