Standard Transmissio Duplex Media Standard Description n rate capability CCITT V.42bis Standard for data compression using 300bit/s Asynchronous 2 wire BTLZ for throughput up to 400% duplex Level 5 Data compression/Throughput up to 1200bit/s Asynchronous 2 wire duplex Level 6 Data compression with V.29 V.22 bis 2400bit/s Asynchronous/ 2 wire technology/Throughput up to 200% duplex synchronous Level 7 Predictive data compression/ 1200/ Asynchronous 4 wire Throughput up to 300% 75bit/s duplex leased Level 8 Predictive data compression with Asynchronous 2 wire V.29/Throughput up to 300% duplex Level 9 Predictive data compression with V.26 bis 2400bit/s Synchronous 2 wire V.32/Throughput up to 300% duplex leased Level 10 Enhanced Level 9/Throughput up to Synchronous 2 wire duplex V.27 ter 4800bit/s Synchronous 2 wire duplex nology. Previously, full duplex operation required the available 9600bit/s Synchronous 4 wire bandwidth to be split into two frequency bands SO that simultaneous duplex leased transmission in both directions was possible. Echo cancellation 9600bit/s made the full bandwidth available in both directions by allowing the Asynchronous/ 2 wire transmitting modem to detect its own signal mixed in with the duplex synchronous received signal of the distant modem and remove it. V.32 bis 14400bit/s Asynchronous/ 2 wire V.32 also uses a second technique called Trellis encoding which duplex synchronous uses an extra bit to help identify and correct errors. This extra bit, 14400bit/s Synchronous 4 wire added to those multiple bit per baud sequences for improving duplex leased throughput described earlier. do not carry information. Although this appears as a simple signal overhead, their presence can actually further increase throughput. A V.32 signal space diagram, or constellation, shows pictorially common transmission standard but a common error correction and all the 32 signalling combinations possible from a 5 bit digital data compression standard, connecting modems must undergo a sequence (Figures 39.6 and