105
Chapter 17. VoIP
17.1. What is VoIP?
Voice over IP (VoIP) is simply a means of carrying voice (telephone calls) over Internet Protocol (the Internet).
Instead of using pairs of wires to carry the signal electrically, the sound is sampled and converted to a sequence
of bytes. This is normally what is done in the telephone exchange before the data is sent over the telephone
network. The key difference with VoIP is that the bytes are placed in packets, typically 20ms long, and these
are sent via Internet Protocol. Unlike the telephone network, IP can cause packets to be delayed, lost or even
copied. It is the job of the receiving end to cope with this and produce the audio again for the recipient to hear.
The end result is that telephone calls can be made over the Internet. This can cause confusion as this is often
seen simply as free calls. Apart from costs for Internet traffic, this is indeed true where calls do not involve the
traditional telephone network and you control both ends, but typically you will need to subscribe to a carrier
who can route calls to and from the traditional telephone network.
The FB2700's role in this it to handle the IP packets used for VoIP. It does not get involved in converting
sound to, or from, packets of data, but in passing those packets of data between VoIP devices and carriers. The
protocol involves complex sequences of messages for control and authentication which the FB2700 handles.
17.2. Registration and Proxies
One of the common confusions with SIP/VoIP is the way registrations work.
17.2.1. Registrar
A SIP device can register with a service, e.g. with the FB2700, or with a SIP carrier. This is like logging in
and means that incoming calls are then sent to the device. The device will renew the registration periodically
to stay logged in, and if it fails to do this then incoming calls will fail.
This process uses a username and password for security. Obviously you also have to say where to register, the
proxy specifies IP address or host name of the SIP service with which you are registering, and the registrar
defines the hostname to use in the registration.
This process works well if the service does not have a fixed idea of where you are, which is normally the case
for SIP handsets. Even on a local network the IP of the handset will normally be dynamically allocated with
DHCP, and for a SIP carrier the IP could be anywhere in the world.
Note
It is possible to have a VoIP carrier that does have a pre-set idea of where calls are to be sent, and
sends the calls without registration. In this case there is no registration process but the handset/device
has to be able to accept calls from the carrier. Again, a username and password are used for security,
but this time it is the device checking the credentials of the carrier.
17.2.2. Proxy
To make an outgoing call via a SIP carrier you have to send the call details to a proxy. In the case of the FB2700
acting as the carrier, the same address is used for registrar and proxy.
The process uses a username and password in much the same way as registration, and they are usually the
same details. This checks that the device is allowed to make the calls, and allows the right person to be billed
for the call.