SIP Overview

The objective of this document is to explain briefly how Session Initiation Protocol (SIP) standards work and to describe the components required to make a working system.

SIP - Good Way to do Voice over IP

SIP is a set of open standards managed by the IETF. SIP devices are generally interoperable between different vendors. In a SIP based VoIP system, most intelligence is in the phones rather than centrally in the PABX. This results in a much more scalable system. Installing additional phones only requires a suitable network connection, apart from licensing issues, the system cannot become "full" unlike a PABX.


  • SIP based equipment is compatible with all other SIP equipment.
  • Save money by making VoIP SIP calls over the internet, not over standard phone network.
  • More flexible than a traditional PABX system.
  • People can work from home or be anywhere in the world on the office phone systems using an ADSL line.
  • Dissolves the concept of having a phone number for a phone, more an identifier for a person or workgroup.
  • True internet phone system.
  • SIP address provides one telephony identity wherever you are.

Things you can do with a SIP Phone System

  • One phone identifier, log on to whichever phone you are nearest to.
  • Work remotely on the office phone system. Eliminating the need for a second phone line or to claim for work calls.
  • Multi-site organisations can have the phone system bridged across all sites- free calls between offices.


If you wanted to completely replace your existing PABX system you would need:

  • SIP Proxy/Registrar software
  • SIP telephones (such as Snom Phones)
  • A gateway (eg the vegastream)
  • SIP Media Server software (if voice mail or auto attendant are required)

Most people already have an existing LAN. This may need extra ports or capacity. With some IP phones, the phone contains an inbuilt switch, so the phone can plug into the wall, and the computer plug into the phone.

Simple Glossary

SIP Registrar

Also known as a SIP proxy. Software forms a soft directory of users on the system.

Phones register with the proxy to allow dialling by methods other than IP address. The proxy provides hold and transfer services.

Ideally your proxy would be connected to a globally routable IP address which (with suitable firewall holes) would enable calls to phones registered on other proxies (to make IP calls outside your organisation.)

A registration can be roamed from one phone to another. User could register either at home or at work, could even log on from home and work at the same time.

The software licenses for the Snom SIP registrar are sold based on the number of registrations that they will accept. You need one registration license per phone per registration.

PSTN Gateway

Provides a gateway between the IP phone system and the traditional telephone system or an existing PABX system. Physically it is a box with Ethernet on one side and either ISDN or analogue phone connections on the other. Most gateways also contain some logging and billing software so you can extract log information to produce usage reports or billing information.

ProVu has found that ISDN gateways work a lot better than analogue FXO gateways.

Media Server

A media server may sometimes be required, it is a software product that provides features such as Voice Mail, music on hold, conference calling, fault messages and auto attendant.