Data Communications Networking Devices - 4th Ed.pdf

4.10 DIGITAL SERVICE UNITS ______________________________________________________ 507Figure 4.82 Bipolar violations. Two successive negative or positive pulses represents abipolar violation of a bipolar return-to-zero signaling technique: a) bipolar coding of data; b)bipolar violationwhere 1s are alternately encoded as positive and negative voltages for de®nedperiods. In Section 4.11, we will examine several methods used to develop bipolarviolations that are used to maintain synchronization when a string of consecutivezeros is encountered. These methods are commonly referred to as zero suppressioncodes.DDS structureDDS facilities are routed from a subscriber's location to an Of®ce Channel UnitOCU) located in the carrier's serving central of®ce. Since there is a variety ofmultiplexing methods that can be employed by a communications carrier tocombine DDS facilities onto a T-carrier, let us focus our attention upon twomethods that will illustrate the relationship between DDS and a T1 circuit. Figure4.83 illustrates the multiplexing arrangement within an AT&T serving centralof®ce that supports DDS transmission at 9.6 kbps and 56 kbps.The data service units DSUs) at the subscriber's location can be viewed as`digital modems' since they modulate the unipolar signal received from dataterminal equipment, including computer ports, multiplexer ports and terminalports, into a modi®ed bipolar signal suitable for transmission on the DDS network.Originally, separate channel service units CSUs) and data service units wererequired to interface equipment to the DDS network. Today, most vendorsmanufacture DSUs that, in effect, combine the functions performed by separateDSUs and CSUs.The user TDMs shown in Figure 4.83 illustrate two methods by which enduserscan transmit data to maximize the data handling capacity of different DDSfacilities. The user TDM shown in the upper left corner of Figure 4.83 illustrates