70 | Section 5
server in the network. Proxy servers can provide functions such as authentication, autho-
rization, network access control, routing, reliable request retransmission and security.
Redirect server: Provides the client with information about the next hop or hops that a
message should take, and then the client contacts the next-hop server or client directly.
Registrar server: Processes requests from clients for registration of their current loca-
tion. Registrar servers are often co-located with a redirect or proxy server.
While SIP-enabled endpoints are able to connect directly to each other, SIP servers provide a
number of valuable services, including the following:
♦ Register SIP client devices
♦ Register individual human users for access to their services
♦ Perform authentication, authorization, and accounting (when needed)
♦ Look up the address of the far-endpoint
♦ Support user mobility across networks and devices
♦ Support for multipoint conferencing
♦ Support presence information
♦ Inform users as to call progress
♦ Communicate requests for QoS to various network elements, such as IP routers
While the various servers above could run on individual machines and could even be physi-
cally separated by thousands of kilometers, in usual practice they are often just software
elements of an application running on a single machine. As we’ll see, many small IP PBXs
include the Gateway as well, making a one-box solution that includes everything needed for a
small office installation.
An example a SIP Server being used in the broadcast world comes from the Telos Z/IP codec
family, which uses a Telos-developed, enhanced SIP Server, called (naturally) Z/IP Server. The
server is provided as a service on the Internet, but may also be installed by users who prefer
to maintain their own. In addition to basic SIP functions (registration, address look-up), the Z/IP
server offers additional services:
♦ Allows display and dialing by simple text name. Keeps a database of names and per-
forms DNS/IP look-up upon a dialing request from an endpoint codec.
♦ Maintains group lists created by users. Upon entering a group name and password, the
list is displayed on endpoint codecs so that users aren’t burdened with having to enter
or upload lists manually.
♦ Provides geolocation services by associating IP numbers with physical location. Allows
display of a routing map on the codec LCD display.
♦ Upon request, keeps a record of network performance in order to assist in troubleshoot-
ing problems caused by QoS impairments.
Many products that support SIP for its standards-based interconnection capability do not have
an internal architecture corresponding to the SIP specifications, so you would not see these SIP
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