s an initial service request mess- powerful, feature rich system. age (ISRM) with information on who the caller is (OLI), category of the calling line (CLC), a service indicator code (SIC) and the destination address (DA). This is transmitted over timeslot 16 to the 34.4.7 Analogue private network signalling distant PABX which responds with a number acknowledgement message (NAM), the called line category (CLC) and the called line identity (CLI) if the called line is free. If the called line is busy the As the PABX private networks are places of high innovation and distant PABX responds with a clear request message and busy largely free from bureaucratic restrictions the users have found a indication. (See Figure 34.9.) need to re-use their existing analogue tie line private circuits, but Channel 16 is divided to permit virtual and real messages to be with enhanced features that came with DPNSS. Providing 2Mbit/s transferred, where the real messages are used to set up calls for links to all PABXs on a network with 30 voice or data channels was normal voice activity and the virtual channels can be used to pass an overkill for all applications, particularly if the network had a number of branch offices with a small number of tie lines connect- supplementary information to enhance the interconnection of the PABXs. Within the PABX network DPNSS has had to define ing them to the main network. If a user wanted the same features specific functions of a PABX. A transit function interconnects a call available at the small branch office as was available on the main between two DPNSS channels. The call may be the initial set up via network then a signalling system as comprehensive as DPNSS but a new channel or a supplementary service call over an existing which would operate over existing tie lines would be required. channel. This problem was solved by PABX manufacturers by re-using all An end function interconnects a DPNSS channel to an extension the software that had been developed for DPNSS and redirecting telephone, a no