Data Communications Networking Devices - 4th Ed.pdf

140 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSThe wait-before-transmit af®rmative acknowledgement WACK) code is usedby a receiving station to inform a transmitting station that the former is atemporary not-ready-to-receive condition. In addition to denoting the previouslydescribed condition,the WACK also functions as an af®rmative acknowledgementACK) of the previously received data block. Once a WACK is received by thesending station,that station will normally transmit ENQs at periodic intervals tothe receiving station. The receiving station will continue to respond to each ENQwith a WACK until it is ready to receive data.The reverse interrupt RVI) code is used by a receiving station to request thetermination of a current session to enable a higher priority message to be sent.Similar to a WACK,the RVI also functions as a positive acknowledgement to themost recently received block.The temporary text delay TTD) is used by a sending station to keep control of aline. TTD is normally transmitted within 2 s of a previously transmitted block andindicates that the sender cannot transmit the next block within a prede®nedtimeout period.Although BISYNC usage has considerably declined during the past few years,itcontinues to retain a degree of popularity. Unfortunately,there are actually threeversions of BISYNC you must consider,with each version slightly different withrespect to the character composition supported,use of control codes,and themethod error control. Table 1.31 summarizes the major differences between thethree versions of the BISYNC protocol.Table 1.31Comparing BISYNC versionsCharacter codesFunction/control code; SBT ASCII EBCDICCompositionNumber of bits per character 6 7 8Number of de®ned characters 64 128 144256 possible)Control codeDLEO code DLE- DLEO DLE70DLEI code DLET DLE1 DLE/WACK DLEW DLE; DLE,RVI DLE2 DLE DLE@TTD STX ENQ STX ENQ STX ENQError controlNo transparency CRC-12 VRC/LRC CRC-16Tranparent mode installedand operating CRC-12 CRC-16 CRC-16Transparent mode installedbut not operating CRC-12 VRC/CRC-16 CRC-16TimeoutsTimeouts are incorporated into most communications protocols to preclude thein®nite seizure of a facility due to an undetected or detected but not corrected error