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336 From GSM to LTE-Advanced Pro and 5G

5.2 The Session Initiation Protocol (SIP)

A telephony service has to fulfill two basic tasks independently of its implementation as
a circuit‐switched or IP‐based service. The first task of the service when a user makes a
call is to locate the destination party and to signal the incoming call. The second task is
to establish a direct or indirect connection, also referred to as a session. In the case of
voice telephony the session is then used to transport a voice data stream in both directions.
In practice, Internet‐based voice services such as Skype, WhatsApp and others have
become popular. Also, systems that use the standardized Session Initiation Protocol
(SIP) are in wide use, especially as PBX systems in companies. An open‐source PBX
implementation that uses SIP and has become quite popular is the Asterisk platform
(https://www.asterisk.org).
SIP is a generic protocol and can therefore be used for the establishment of a connection
between two or more parties for many different session types. In this chapter, SIP is
mainly described as a protocol for establishing a voice session. Details can be found in
the IETF RFC 3261 specification [2] as well as in various 3GPP documents that are
freely available on the Internet.
The core of a SIP‐based telephony system is the SIP Registrar and the SIP proxy as
shown in Figure 5.1. When powered on, a device has to register with the SIP system to be
reachable by others and also to establish outgoing calls. The SIP software on a user’s
device is referred to as a SIP User Agent (UA). On the network side the SIP Registrar is
responsible for the authentication and registration of devices, i.e. the UAs. Figure 5.2
shows how registration is performed in practice. At the beginning, the device sends a
request to the Domain Name System (DNS) server to retrieve the IP address of the SIP
Registrar server, whose domain name, together with the user’s identity and authentica-
tion information, has been configured in the user’s device. Afterward the UA sends a

Subscriber
Database

SIP Proxy
Internet and
Mobile Device SIP Registrar
with a SIP
User Agent (UA)
Media Signaling
Gateway Gateway

Classic voice
switching center

Figure 5.1 The basic SIP infrastructure.