Data Communications Networking Devices - 4th Ed.pdf

4.10 DIGITAL SERVICE UNITS ______________________________________________________ 513®rst six pins, where pin 1 is signal ground, pin 2 is status indicator, pins 3 and 4 arethe receive signal pair, while pins 5 and 6 are the transmit signal pair. Today mostcommunications carriers and third-party vendors manufacture combined DSU/CSU devices, integrating the functions of both devices into a common housingwhich is powered by a common power supply. The lower portion of Figure 4.87illustrates the connection of end-user terminal equipment to DDS using acombined DSU/CSU unit.DSU/CSU tests and indicatorsThrough the use of intentional bipolar violations, the DSU/CSU can generate arequest to the OCU for the loop-back of the received signal onto the transmitcircuit or it can interpret DDS network codes and illuminate relevant indicators onthe device. When the loop-back button on the DSU/CSU is pressed, the devicewill generate four successive repetitions of the sequence 0B0X0V when operatingat data rates up to 19.2 kbps or N0B0X0V at 56 kbps, whereB denotes ‡ or A V, with the polarity determined by bipolar coding for abinary 1X denotes a 0 V for coding of binary 0 or B, depending upon the requiredpolarity of a bipolar violationV denotes ‡ or A V, with the polarity determined by the coding of a bipolarviolationN denotes a do not care condition where the coding for a binary 0 or binary 1 isacceptableIn Figure 4.88 the DDS loop-back 6-bit sequence is illustrated for data rates ator under 19.2 kbps. Note that the sequence transmitted is dependent upon whetherthe previous binary 1 was transmitted as a positive or negative voltage.Other bipolar violation sequences used by DDS include an idle sequence whichindicates that a DTE does not have data to transmit, an out-of-service sequence andan out-of-frame sequence. The idle sequence is generated by the DSU while theout-of-service and out-of-frame sequences indicate a problem in the DDS networkFigure 4.88 DDSloop-back sequence data rates up to 19.2 kbps). DDSloop-back codesare intentional bipolar violations