IP, SIP, URI and IP telephony

IP stands for Internet Protocol and is the standard protocol for transferring information over the Internet between computers and other devices. The same standard is normally used in local networks too. An IP address is the unique network address that a compu- ter has in the network.

An IP phone is a phone that, like a PC, is connected to the network/Internet. Like the PC, it has an IP address and the call is transmitted as data packets over the network. Calls can be made from an IP phone to other IP phones (without leaving the network) and to regular phones over the public telephone network.

SIP stands for Session Initiation Protocol and is a standardised protocol (communication regulations) for connecting phone calls via networks – in most cases via the Internet too.

To make and receive calls, the phone has to be registered to an SIP switch. The switch can be a company PBX or be located with an IP telephony service provider. The SIP switch ensures that the call is connected to the right address within the network or sends the call to the public telephone network if the recipient is not registered as an IP telephone in the same switch.

Making calls with IP phones

When you use an IP phone to make a call to another IP phone within the network, the call does not usually cost anything. When you use an IP phone to make a call to a regular analogue phone, you normally pay the standard rate from the point at which the SIP switch connects the call to the telephone network.

An IP phone can be moved and used all over the world. The only requirement is access to the Internet or, to be more accurate, access to the SIP switch where it is registered. Users must bear in mind that calls to analogue phones are connected to the telephone network where the switch is located.

A company can make free calls to its offices in different countries provided it uses IP phones connected to the same SIP switch. If the SIP switch being used is in another country, it may be necessary to have two SIP accounts in the phone. One for the company’s internal IP calls and one for local external calls via the analogue telephone network. Otherwise there is a risk of making overseas calls to order a pizza from down the road.

Phone numbers and network addresses

An IP phone has a normal phone number plus a network address. When someone calls an IP phone number, the SIP switch converts the call into data packets that are sent over the network to the phone’s network address.

You can make a call from an IP phone to another using the network address. When you send an email to someone, you don’t need to know the computer’s network address, and it’s the same when you make a call from an IP phone, you don’t need to know the