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788 WIDEAREA ANDMETROPOLITANAREANETWORKSIP networkgatewaygatekeeperterminalsanalog
phonePSTN(^123456789)
*8#
Figure 14: H.323 network elements.
encoding rules, error codes, and authentication methods
to exchange text messages. Like H.323, SIP assumes that
the end-point devices (i.e., terminals) are intelligent, run-
ning software known as the user agent. The agent has two
components:User Agent Client, which is responsible for
initiating all outgoing calls, and theUser Agent Server,
which answers incoming calls. In the network itself, SIP
provides support with three types of server:
Registrationservers keep track of where all users are lo-
cated.
Proxyservers receive requests and forward them along to
the next appropriate hop in the network.
Redirectservers also receive requests and determine the
next hop, but rather than forwarding the request, they
return the next-hop server address to the requester.
An alternative approach to multimedia communica-
tion control developed by the IETF is called the media
gateway control protocol (MGCP). It is quite different
from H.323 and SIP because it assumes that the end-
user devices are not very intelligent. Consequently MGCP
takes a central server approach to communication coor-
dination and control. Two elements are defined: theMe-
dia Gateway Controller(also known as the call agent),
which provides the central intelligence and controls all
of theMedia Gateways, which perform a variety of in-
terface functions such as with the PSTN, residential de-
vices, and business private branch exchanges (PBXs).
MGCP defines the communication that takes place be-
tween the call agent and the Gateways that execute its
commands.
In practice H.323, SIP, and MGCP will likely coexist to
support multimedia communication in the Internet en-
vironment because each has advantages for specific ap-
plications or coverage. MGCP is particularly useful to
MAN/WAN service providers with large installed bases of
unintelligent end-point devices, and its gateway approach
allows for tailored interfaces to each different underly-
ing technology. The simplicity of SIP is more attractive
to enterprise networks designed primarily for data traffic
with smaller requirements for supporting voice and video.
Finally, H.323 is the most mature and most comprehen-
sive. As usual in the telecommunication industry, vendor
support and suitability to customer business models are
likely to determine which, if any, one approach becomes
dominant.
PROVIDERS AND SERVICES
Carriers and Service Providers
The public provision of telecommunication services to
subscribers for a fee has a history of being government-
regulated in most parts of the world (the term “common
carrier,” for example, dates back to public transportation
for people, first by stagecoach, then by trains, buses, etc.).
Regulation was required because access to telecommuni-
cation services depended on cabling that was run from
subscriber premises (residential or business) across pub-
lic property (e.g., along roads) to a provider’s central of-
fice as a service point. Governments could also impose
standards to ensure that services offered by providers in
different locations would be compatible enough to inter-
operate. In some countries, infrastructure was built and
services operated by the government itself (e.g., PTTs that
provided postal, telegraph, and telephone services nation-
wide). In the United States, telephone industry regulation
was divided between LECs whose cabling and local ser-
vices go to individual premises, and IXCs who provided
the interconnection (i.e., long-distance services) between
LECs.
The Internet as a means of public data communica-
tion has grown up rather differently, driven largely by the
U.S. regulatory environment, where telecommunication
companies were prohibited from providing data services.
Consequently, a new type of company called an Internet
service provider (ISP) was born. Data would move from a
subscriber’s premises, across cables belonging to an LEC,
to ISP equipment in a point of presence, where it was
transferred onto Internet resources. The subscriber thus
had to be a customer of both the LEC and the ISP unless
a private link could be installed directly to the ISP’s POP.
The Internet connections from one ISP location to an-
other are most often lines leased from an IXC. As telecom-
munication services have been increasingly deregulated
world-wide, the distinctions among voice and data service
providers have become blurred.