cunet T1.5 are typical examples of T carrier multiplexer networks, preceding the all zero byte is positive, the inserted B8ZS code is 000 while BT's KiloStream and MegaStream networks are based on the +- 0 -+. If the pulse preceding the all zero byte is negative, the European standards. inserted B8ZS code is 000 Both examples result in bipolar violations occurring in the fourth and seventh bit positions. This 21.8.1 Bell's DDS subrate facility special pattern is unique because of the embedded code violations. When detected at the receive side it is removed and not seen as a code violation. Bell's Dataphone Digital Service (DDS) was approved by the U.S. If a string of nine logic Os were to be transmitted, then the first Federal Communications Commission in December 1974. Curren- eight logic Os would be replaced by the special pattern while the tly, there are over 100 cities connected to the DDS network in the ninth 0 would be transmitted normally. At the receive end, the United States as well as international connections to other digital special pattern, which is expected by the receiver, is decoded back networks. to the eight zeros removed. The data output by the receiver is still DDS is an all synchronous facility. Currently supported trans- the nine logic Os that were to be transferred. Two things can be seen mission rates and services include 2400, 4800, 9600 and 56000bit/s from the example. The addition of the extra, special pattern gave the leased circuits. Recently a 56kbit/s switched service has been added. receiver the clocking data it required to allow the PLL to maintain For transmission at different data rates, specialised equipment to synchronisation. Secondly, to transmit the sequence of nine binary include multiplexers and/or converters must be employed. digits still requires only eight pulses on the line. Although the The carrier structure of the DDS network is illustrated in Figure receiver will be less likely to lose synchronisation, more compli- 21.12. DDS facilities are routed from a subscriber