SIP Enabled Network
Figure 6.1 shows the elements of a SIP-enabled IP communication network.
The network is composed of the following:
■■ SIP endpoints, such as phones, gateways, and various types of computers—
SIP endpoints are fully qualified Internet hosts, as defined in RFCs 1122
and 1123. Internet hosts are very different from telecommunication
devices, such as phones, fax machines, or mobile phones in the sense
that (1) they may use the services of any other host on the IP network
and (2) they may run any and all applications the user may desire. The
user can direct communications via any service provider and can load
any application, similar to other services on the Internet. Depending on
the service and user preferences, most communication services can also
be controlled from end to end without support from the network for
call setup. There are two types of SIP endpoints:
■■ User devices, such as phones and personal computers.
■■ Gateways to other networks, especially IP telephony gateways that
use CAS, Q.931 or SS7 signaling. Other gateways can connect to
H.323 or other legacy VoIP networks and device control networks,
such as found in certain IP PBXs and so-called softswitches, using
MGCP, MEGACO, or H.248 master-slave protocols.
■■ SIP servers—Most users have no desire to understand and manage the
services they use, and there are also security and technical reasons to
place services on dedicated servers in the network, where they can be
accessed from anywhere and used with various communication
devices. SIP servers accomplish the functions found in the telecom AIN,
in e-mail systems, and in web servers, as well as new functions, specific
to SIP. SIP servers can be stateless, similar to other Internet devices. SIP
servers can be deployed in geographically distributed clusters to avoid
service failures. All this ensures very fast response time and avoids fail-
ures in the network to disable calls, since the call state is kept at the
periphery of the network and not in the core. Users do not depend on
any potential central points of failure in the network and can communi-
cate as long as they have working end devices.A caller can send an INVITEmessage to establish a session to the called
party, without knowing exactly where the other endpoint may be, and the SIP
servers will route the call to the destination. The route to the destination can be
forked in the network so as to find the other endpoint. The same infrastructure
can also serve for an instant message and presence service. A watcher can sub-
scribe to a presentity and receive NOTIFYmessages from the presentity. The
watcher and presentity can exchange short text messages using SIP itself, or
RTP packets for any other communication media: audio, various data applica-
tions, video, or games for instant communications.98 Chapter 6