ly restricted to data and proprietary in technique. A third generation of multiplexers then appeared in the late 1980s and early 1990s, these were characterised by: The ability to integrate any form of service, be it voice, data, image, video and LAN. Connectivity would be the issue for the 1990s. PTT Service Information Signalling 2. Robust, evolving, flexible architecture. Adoption of new tech- (Alarm format Channel nology would be required in the 1990s. CRC information) 3. Totally software driven. The ability to offer complete manage- ment capability. 4. International in design using a balance of standard and proprie- Figure 56.7 E1 transmission frame tary technology, i.e. Global networking. 5. A greater reach. The ability to network the smallest to the largest office from a 3 node network to a 10,000 node network. In due course PTTs offered lower speed digital services. In North 6. Hybrid in design. The ability to operate in the PTT network or America this was because of Digital Data Services (DDS) which private network or both, as a single product. offered 64Kbit and below connectivity. This was followed in the late 1990s by Fractional T1 services, which simply is a full T1 Today's multiplexers adopt these basic principles and have delivered to the user but only a fraction of it, as desired, is utilised. become the main solution for wide area networking. 56/6 Local and Wide Area Networking The features and benefits realised by such networking capabilities working link), channel priority (nominating in order of priority the together with the growing reduction of digital service tariffs and most important channels to remain in operation when limited ser- their availability means their continual adoption by corporate vices are available), channel down speeding (maintain a channel clients into their networking connection solutions. connection even at lower speed when resources become limited), Today multiplexers incorporate a rich set of features to allow parameter routing (to define the manner in which channels