System interfaces
34
System functions and features as of R4.1
syd-0570/1.2 – R4.1 – 08.2016
2. 1. 3 SIP
2. 1. 3. 1 What is SIP?
The Session Initiation Protocol (SIP) is a network protocol used for setting up, con-
trolling and clearing down a communication session between two or more subscribers
(source: Wikipedia). SIP is an open standard and was developed by an IETF (Internet
Engineering Task Force) working group. While the text-based protocol has a great deal
in common with HTTP (Hypertext Transfer Protocol) in terms of both structure and se-
quence, it is not compatible with it.
SIP is now widely used in IP telephony. However SIP alone cannot enable VoIP con-
nections. With the aid of the Session Description Protocol (SDP), SIP merely negoti-
ates the communication modalities between the SIP subscribers. The actual audio
data stream is exchanged via other, more suitable protocols, such as the Real-Time
Transport Protocol (RTP) or the encrypted Secure Real-Time Transport Protocol
(SRTP). For this, the coded and compressed data is packed into packets and sent via
the User Datagram Protocol (UDP) or the Transmission Control Protocol (TCP).
The SIP connection is used to transmit not just voice but other multimedia data, too
(video, fax, text, etc.).
SIP subscribers have an address whose structure is similar to that of an e-mail ad-
dress (e.g. URL: "sip:12345@sip-server.com"). SIP subscribers can be reached via
this address, regardless of their location. However, this is only possible if they register
with an SIP provider and regularly update their IP address.
Gateways at the SIP providers enable the transition into the public telephone network,
for example the leased-line network or the mobile phone network.
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