#### In-depth

## Signalling protocols

### What is a protocol?

#### In order for information to be correctly and efficiently routed from source to destination within a network, it is necessary for the path through the network (that is, the switch settings) to be set appropriately.
The signalling between source, switching centres and destination used to set up the route must be understood by all parties if it is to operate successfully. In other words, it must conform to a pre-established protocol. This protocol usually takes the form at the lowest level of sending information, (data words, or perhaps tones (DTMF) alongside the message signal which could convey information about the destination address, the type of message, the message length, the occurrence of transmission errors, and so on, to be picked up by the switching centres or terminal equipment as appropriate.
There are many different protocols used within large communications networks, each hopefully optimized for a given control task. For example, the protocol used to control information flow over a wireless portion of a network often needs to be more robust at detecting and correcting errors than a protocol employed on a good quality wired link. A well-known protocol for computer interconnection is the RS232 (see in-depth) protocol. A commonly used packet switch protocol, is the X.25 standard.