Basic Telecommunications Signalling

The demands put on telecommunications signalling system keep changing. Signalling is a very vital part of telecommunications and has a number of characteristics that it needs to exhibit. A signalling system must be able to cope with large numbers of calls or requests at one time without becoming noticeably slower. At the same time, the cost of running the signalling system must not detract from a carrier’s ability to make money.

The system must be reliable and always be available for use. As components or individual messages can fail, the system as a whole must contain devices and procedures for recovering from problem areas.

One deficiency of early signalling systems based on tones was that it was very hard to adapt them to new requirements. This problem ran on into Signalling System #6 which was also a computer protocol but designed for absolute bandwidth efficiency; it was not easily capable of extension. Thus Signalling System #7 was born.

Between PABXs we often use DPNSS which stands for Digital Private Network Signalling System. This protocol is used on digital trunk lines for connecting two PABXs. It supports a limited set of inter-networking facilities based on the ISDN protocol. This protocol has been defined by BT

A subscriber and telephone company notify each other of call status with audible tones and an exchange of electrical current. This exchange of information is called supervisory signaling. There are three different types of supervisory signaling:

On Hook – When the handset rests on the cradle, the circuit is on hook. The switch prevents current from flowing through the telephone. Regardless of the signaling type, a circuit goes on hook when the handset is placed on the telephone cradle and the switch hook is toggled to an open state. This prevents the current from flowing through the telephone. Only the ringer is active when the telephone is in this position.

On Hook – When the handset is removed from the telephone cradle, the circuit is off hook. The switch hook toggles to a closed state, causing circuit current to flow through the electrical loop. The current notifies the telephone company equipment that someone is requesting to place a telephone call. When the telephone network senses the off-hook connection by the flow of current, it provides a signal in the form of a dial tone to indicate that it is ready.

Ringing – When a subscriber makes a call, the telephone sends voltage to the ringer to notify the other subscriber of an inbound call. The telephone company also sends a ringback tone to the caller alerting the caller that it is sending ringing voltage to the recipient telephone. Although the ringback tone sounds similar to ringing, it is a call-progress tone and not part of supervisory signaling.

The Digital Subscriber Signalling System No.1 (DSS1) is a suite of protocols that provides the means for users to invoke the full range of services and capabilities available from the Integrated Services Digital Network (ISDN).

The Q.931 standard defines a protocol that takes care of these services. This protocol is located at the third, that is, at the network layer of the OSI protocol stack. All the above mentioned signalling is achieved through the D-channel defined by the ISDN standard. Q.931 will use this channel for setting up the wanted connection. As stated in the OSI reference model, higher protocol layers depend on the services provided by lower layers. Thus Q.931 is requesting services from the data-link layer which resides at layer two. The Q.921 standard defines a protocol that can be called a Data-Link Service Provider (DLSP). Q.931 is thus the data-link service user (DLSU). Q.921 is also better known as LAPD, as in Link Access Protocol D-channel. Service requests that come from the above layers are packed in LAPD frames and are sent through the network. All Q.931 messages are built up by a certain message body.

Signalling System Number 7 (SS7) is a set of signalling protocols developed for telephony in 1975, which is used to set up, maintain and tear down telephone calls. It is also used for a number of other functions and features such as:

Local Number Portability

Number Translation

SMS (Short Message Service)

Prepaid Billing

It is referred to by a number of abbreviations such as CCSS7 (Common Channel Signalling System 7) and C7 (CCITT number 7), The International Telegraph and Telephone Consultative Committee.

The SS#7 model uses the concept of layers to separate the different functions, however, the structure predates the adoption of the OSI model and there are only 4 layers. Circuit and Non-circuit components are shown, including the overlap where ISUP uses both. There are other non-circuit related top protocols such as MAP (Mobile application Part) By being open ended, SS#7 can be extended for new requirements.