SIP devices can communicate directly if they know each
other’s URI or IP address, but in practice SIP servers are
often used in the network to provide an infrastructure for
routing, registration, and authentication/authorization
services.
IP-based devices can identify and communicate with one
another using IP addressing alone. However, in most
cases, your network uses the Domain Name Service (DNS)
to establish sessions with device names, which DNS trans-
lates into IP addresses. Similarly, SIP devices frequently
consult directory servers (often by name), which provide
endpoint addresses that the devices then contact to set
up a call.
Serving up SIP servers
SIP servers provide centralized information and enablement
services in a SIP ecosystem. The core SIP servers and their
functions are summarized here.
ߜ Registrar Server. When users come online, they need to
make sure that others are aware that they’re available
to take and make calls. The Registrar authenticates and
registers users when they come online, and then stores
information on the users’ logical identities and the devices
that they can use for communications. The devices are
identified by their URIs.
ߜ Location Service. As users roam, the network needs to
be continually aware of their locations. The location
service is a database that keeps track of users and their
locations. The location service gets its input from the
registrar server and provides key information to the
proxy and redirect servers. A SIP proxy or redirect
server uses this information to obtain the mapping from
logical SIP addresses to physical SIP addresses, so that
communication sessions can be properly established
and maintained.
Part 2: SIP at a Glance
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