Multiservice Networking, IPX Networking, and Security - CCIE Routing and Switching Exam Certification Guide - page 553

SIP supports user mobility by using proxy and redirect servers to redirect requests to the user's
current location. Users can register their current location and SIP location services, which
provide the location of user agents.

SIP uses a modular architecture that includes the following components:
•

SIP user agent —Endpoints that create and terminate sessions, SIP phones, SIP PC
clients, or gateways

•

SIP proxy server —Routes messages between SIP user agents

•

SIP redirect server —Call control device that provides routing information to user agents

•

SIP registrar server —Stores the location of all user agents in the domain or subdomain

•

SIP location services —Provides the logical location of user agents; used by proxy,
redirect, and registrar servers

•

Back-to-back user agent —Call-control device that allows centralized control of network
call flows

Signaling System 7 (SS7)
SS7 is a global ITU standard for telecommunications that allows voice network calls to be
routed and controlled by central call-control centers. SS7 implements call setup, routing, and
control, which ensure that intermediate and far-end switches are available when a call is placed.
With SS7, telephone companies can implement modern consumer telephone services, such as
caller ID, toll-free numbers, call forwarding, and so on.

SS7 provides mechanisms for exchanging control, status, and routing messages on public tele-
phone networks. SS7 messages pass over a separate channel that is reserved for voice commu-
nication. This technique is commonly referred to as out of band signaling . SS7 is the network
that controls call signaling, routing, and connections between the CO, inter-exchange carrier,
and competitive local exchange carrier (LEC) switches. Figure 12-4 shows the connectivity
between SS7 components, which are described in this section.

As shown in Figure 12-4, SS7 has the following system components:
•

SCP (Signaling Control Point) —Databases that provide the necessary information for
special call processing and routing, including 800 and 900 call services, credit card calls,
local number portability, cellular roaming services, and advanced call center applications.

•

STP (Signaling Transfer Point) —Receives and routes incoming signaling messages
toward their destinations. STPs are deployed in mated pairs and share the traffic between
them.

•

SSP (Signaling Switching Point) —Telephone switches equipped with SS7 software and
signaling links. Each SSP is connected to both STPs in a mated pair.