What's SIP

SIP - Session Initiation Protocol

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

Why Sip

Open Standard - SIP based equipment is compatible with all other SIP equipment. Because SIP is flexible, it is in the manufacturers interest to stick to the standard. With h.323 (the existing VoIP standard), many vendors have broken the standard to suit their needs, and in doing so produced non standard solutions. The flexibility of SIP should stop this happening.

You can save money on phonecalls by making VOIP SIP calls over the internet, rather than over the standard phone network.

Much more flexible than a traditional PABX system. Thinking in terms of a traditional PABX severely limits the flexibility that your company needs. Start to think of an IP telephony solution for your business.

Your office phone system is not restricted to where you can run phone cables to. People can work from home or be anywhere in the world on the office phone systems using an ADSL line.

It dissolves the concept of having a phone number for a phone, more an identifier for a person or workgroup. Where necessary this SIP address will also be reachable from the PSTN using a standard telephone number.

SIP is not like having a PABX system where the PABX and endpoints are linked together by IP. It is a true internet phone system. While it can be be used to replicate the functionality in an PABX system it can potentially do much more.

We believe that in the future, you will just quote one address to people. This is because SIP addresses have the same format as email addresses. For most people they will be the same. No need to quote your email address, mobile phone, work phone and home phone numbers separately. Your SIP address will provide one telephony identity where ever you are.

Things you can do with a SIP Phone System

	Have the same phone identifier follow you around. Just log on to whichever phone you are nearest to.
	Work from home and be on the office phone system. You would need a suitable ADSL line at home. Breakout onto the PSTN using the office gateway, thus eliminating the need for a second phoneline or to claim for work calls.
	If you are a multisite organisation, then have the phone system bridged across all of your sites. Have free calls between offices. Many companies already have data links between offices.

What you need

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

	SIP Proxy/Registrar software
	SIP telephones
	A gateway
	SIP Media Server software (if voice mail or auto attendant are required)

We expect that most people will 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 know as a SIP proxy. The software forms a soft directory of users on the system.

The phones register with the proxy to allow dialing 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. For example, a user could register either at home or at work. A user could even log on from home and work at the same time.

PSTN Gateway

This 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.

ISDN gateways work a lot better than analogue FXO gateways.

Media Server

Although SIP phones have significant intelligence themselves, sometimes a server is required to help out. A media server is a software product that provides features such as Voice Mail, music on hold, conference calling, fault messages and auto attendant.