User guide

Unfortunately, there are many ways to generate these kind of signals. Each mechanism is known as a
“signalling method”. Signalling methods vary from one place to another, so you need to know which
type is used in your telephony lines. Two common access signalling methods are known as loop start
and ground start. If you do not know which signalling method is used between your phones and your
phone lines you can start by testing with “loop start.” A common effect of choosing the wrong signalling
method is unexpected “hangs” in the phone line.
4.3.2 Telephone Exchange Signalling
SS7 is a set of standards developed by AT&T and the ITU that among other things deal with the
establishment of calls and call routing between telephone exchanges in the PSTN. The important thing
to understand here is that in traditional phone networks the voice and the signalling are separated. This
means that there is one “circuit” for the voice (the conversation) and another circuit to exchange
additional (supervisory) information necessary for the call to be established. This “additional” but very
necessary information is exchanged using the SS7 protocol.
The fact that “signalling” and “voice” are separated means that they do not necessarily use the same
physical path for their transmissions. The “phone conversations” can travel by one cable while the
numbers of the caller and callee go by another cable. This concept is important to understand for the
next concept: signalling in IP Telephony.
4.4 Signalling in IP Telephony
Voice over IP signalling follows a philosophy similar to traditional PSTN signalling. Signals and
conversations are clearly differentiated. In this section we introduce two VoIP protocols that we are
going to integrate into our PBX: SIP and IAX2.
4.4.1 Session Initiation Protocol (SIP)
The Session Initiation Protocol (SIP) is an Internet protocol developed by
IETF to provide some of the functionality of SS7, but in IP-based
communication networks over the Internet. The SIP protocol is responsible
for setting up the calls and signalling.
Remember that when we talk about signalling in the context of voice calls, we are talking about
indicating a busy line, ring tones, or that someone has answered on the other end of the line.
Page 12 TRICALCAR | www.wilac.net/tricalcar – Version: February 2008