's location to an cated transmitters and receivers have to be used. Office Channel Unit (OCU) located in the carrier's serving central office. The signalling structure used on DDS facilities is a modified Examples of digital network services bipolar signalling code. The modification to bipolar return to zero signalling results in the insertion of zero suppression codes to This section provides a brief description of three digital network maintain synchronisation whenever a string of six or more consecu- facilities that are based on T1 and PCM-30 time division Multi- tive zeros is encountered. DDS City Serving Central Office 300bit/s 64kbit/s 9.6kbit/s 5 X 9.6kbit/s 9.6kbit/s 1.544Mbit/s 64kbit/s 56kbit/s DSU = Data Service Unit TDM1 = Customer Time Division Multiplexer OCU = Office Channel Unit TDM2 = Bell Time Division Multiplexer TDM3 = Bell Time Division Multiplexer Figure 21.12 DDS carrier structure and multiplexing arrangement Examples of digital network services 21/15 Master timing N = Nodal Timing Supply L = Local Timing Supply S = Secondary Timing Supply Figure 21.13 DDS network timing It should be clear by now that precise synchronisation is the key a digital local line to the local exchange where it is fed into a CEPT to the success of an all digital network. Timing ensures that data bits 2.048 Mbit/s multiplexer, as illustrated in Figure 21.14. are generated at precise intervals, interleaved in time and read out The customer data rate can be 2400, 4800, 9600, 48000 or at the receiving end at the same interval to prevent the loss or 64000bit/s. The DTE/NTU interface depends on the data rate. garbling of data. To accomplish the necessary clock synchronisation Speeds of 2400, 4800, and 9600bit/s can be serviced through a on the AT&T digital network, a master clock is used to supply a CCITT X.21 or CCITT X.21 bis/V.24 interface. 48kbit/s can be hierarchy of timing in the network. Should a link to the master clock provided with a CCITT X.21 or CCITT X.21 bis/V.35 interface. A fail, the nodal timing supplies