th directions and is accomplished by using four wire lines or (Tables 39.1 to 39.3). much more commonly by dividing the bandwidth of two wire lines This is particularly important in the dial environment where the using different transmit signals frequencies. CCITT V.32 echo can- modem model operating at the distant end may not even be known. cellation techniques broke new ground by providing full duplex At one time, high speed modems would only operate at peak operation over two wires with the full bandwidth available in each performance when talking to another modem of the same type, direction. typically operating a proprietary transmission standard. When con- The patterns of 1's and 0's of course have to be arranged in a way nected to a different modem, they would have to accept the lowest that makes sense to the digital equipment. Modems operating asyn- common level of operation, often significantly less than their indi- chronously do this by sending a start bit. The modem can then count vidual optimum performance. off the required number of bits that make up a byte or character, The main force for standards in Europe and increasingly more in including a parity and stop bit before sending the next start bit. the United States is the CCITT. Over the years they have developed Synchronous modems on the other hand, solve this timing problem standards from the 300bit/s V.21 operation of the early 1970s to the by synchronising their clocks. Instead of having to re-synchronise recently agreed 14400bit/s V.32bis standard and beyond. Digital after each character, synchronous modems send timing information signal processing technology now delivers chip sets covering along with the data in a continuous sequence. A protocol is used to multiple standards in an attempt to provide universal connectivity. define these synchronising sequences. block sizes and formatting Since optimum performance of links may depend not only on a 39/6 Modems Table 39.1 CCITT V Series transmission standards Table 39.3 Data compression standards 