dths, an integrated service network was badly needed. Though SDH multiplexing hierarchies turned the dream of channel capacity in a few Gbps into a reality, it was still uncertain whether any service could practically use this much bandwidth and transmit almost every service known, using the same channel. This idea of an integrated service network led to the creation of an end-to-end digital network, which had the capability of providing various services like voice, video and data on the same channel and was called the integrated services digital network (ISDN). Original recommendations of ISDN were in ITI-U (CCITT) Recommendation I.120 (1984), which described some initial guidelines for implementing the ISDN. The ISDN is comprised of digital telephony and data transport services offered by regional telephone carriers. It involves the digitisation of the tele- phone network, which permits voice, data, text, graphics, music, video and other types of information to be transmitted over existing telephone wires. The emergence of the ISDN represents an effort to standard- ise subscriber services, user-to-network interfaces and network-to-network services. ISDN applications include high speed image applications, high speed file transfer and videoconferencing. Voice service is also an application for the ISDN, which has two types of services according to the number of channels Transmission Network Planning and Optimisation STM1#1 STM1#2 STM1#3 STM1#4 125 us 125 us 125 us 125 us Byte Interleaving STM - 4 Frame SOH (4 X 9) Payload (4 X 261) Figure 3.25 SDH multiplexing procedure and speeds it had to offer. The first one is the basic service having a speed of 192 kbps while the other one has rates of 1.544/2.048 Mbps and is known as the primary service. As the ISDN services were after not good enough, a new concept for integrated services came up an enhancement over the ISDN; it worked at rates greater than the primary rates (1.544/2.048 Mbps) and covered the broadband aspects of the present ISDN. This service was thus cal