ISDN and Q.931

The ISDN protocol is where telephone calls to the outside world get started. ISDN is the digital telephone line standard, and is what the phone company provides to organizations that ask for digital lines. By itself, ISDN is not exactly a voice mobility protocol, but because a great number of voice calls from voice mobility devices must go over the public telephone network at some point, ISDN is important to understand.

With ISDN, however, we leave the world of packet-based voice, and look at tightly timed serial lines, divided into digital circuits. These circuits extend from the local public exchange—where analog phone lines sprout from before they run to the houses—over the same types of copper wires as for analog phones. The typical ISDN line that an enterprise uses starts from the designation T1, referring to a digital line with 24 voice circuits multiplexed onto it, for 1536kbps. The concept of the T1 (also known, somewhat more correctly, as a DS1, with each of the 24 digital circuits known as DS0s) is rather simple. The T1 line acts as a constant source or sink for these 1536kbps, divided up into the 24 channels of 64kbps each. With a few extra bits for overhead, to make sure both sides agree on which channel is which, the T1 simply goes in round-robin order, dedicating an eight-bit chunk (the actual byte) for the first circuit (channel), then the second, and so on. The vast majority of traffic is bearer traffic, encoded as standard 64kbps audio. The 23 channels dedicated for bearer traffic are called B channels.

As for signaling, an ISDN line that is running a signaling protocol uses the 24th line, called the D channel. This runs as a 64kbps network link, and standards define how this continuous serial line is broken up into messages. The signaling that goes over this channel usually falls into the ITU Q.931 protocol.

Q.931's job is to coordinate the setting up and tearing down of the independent bearer channels. To do this, Q.931 uses a particular structure for their messages. Because Q.931can run over any number of different protocols besides ISDN, with H.323 being the other major one, the descriptions provided here will steer clear of describing how the Q.931 messages are packaged.

Table 1 shows the basic format of the Q.931 message. The protocol discriminator is always the number 8. The call reference refers to the call that is being referred to, and is determined by the endpoints. The information elements contain the message body, stored in an extensible yet compact format.

Table 1: Q.931 Basic Format 

Protocol Discriminator	Length of Call Reference	Call Reference	Message Type	Information Elements
1 byte	1 byte	1-15 bytes	1 byte	variable

The message type is encompasses the activities of the protocol itself. To get a better sense for Q.931, the message types and meanings are:

• SETUP: this message starts the call. Included in the setup message is the dialed number, the number of the caller, and the type of bearer to use.
• CALL PROCEEDING: this message is returned by the other side, to inform the caller that the call is underway, and specifies which specific bearer channel can be used.
• ALERTING: informs the caller that the other party is ringing.
• CONNECT: the call has been answered, and the bearer channel is in use.
• DISCONNECT: the phone call is hanging up.
• RELEASE: releases the phone call and frees up the bearer.
• RELEASE COMPLETE: acknowledges the release.

There are a few more messages, but it is pretty clear to see that Q.931 might be the simplest protocol we have seen yet! There is a good reason for this: the public telephone system is remarkably uniform and homogenous. There is no reason for there to be flexible or complicated protocols, when the only action underway is to inform one side or the other of a call coming in, or choosing which companion bearer lines need to be used. Because Q.931 is designed from the point of view of the subscriber, network management issues do not need to be addressed by the protocol. In any event, a T1 line is limited to only 64kbps for the entire call signaling protocol, and that needs to be shared across the other 23 lines.

Digital PBXs use IDSN lines with Q.931 to communicate with each other and with the public telephone networks. IP PBXs, with IP links, will use one of the packet-based signaling protocols mentioned earlier.