In the evolution of telecommunications networks, Asynchronous Transfer Mode (ATM) technology is rapidly becoming the universal networking standard as an approach to integrating all different kinds of communication traffic. Typically, ATM networks operate in connection oriented mode in which a pre-established virtual circuit is required for every connection. Signalling, as an essential component, has great impact on the efficient use of network resources, and the services that a network can offer. It is important that the details of a connection can be specified and conveyed to the network, otherwise the user will not be able to receive the required services in a dynamic manner. Essentially, signalling allows the transfer of service-related information in real time between the user and the network, and among network entities to establish, maintain and release end-to-end virtual connections.

The standardisation activity on signalling protocols is being undertaken by the ITU-T and the ATM Forum to facilitate a widespread deployment of ATM services. Two ATM interfaces are defined for signalling purposes. The user-network interface (UNI) is the interface between an endpoint equipment and the network, whereas the network-network interface (NNI) is the interface between either two private networks or two public networks. The separation of the private and public domains is a consequence of the significant differences in the administrative responsibility. For each particular signalling function, the corresponding procedures are defined such that the sequence and message format are specific to the network interface across which the exchange of signalling information takes place.