ultiplexers interleave the output of 12 codecs Leased lines could be "conditioned" by the tele- into a multiplex frame. Three standards prevail for phone company to make them usable at higher rates of multiplexing-North American, European, and Japa- transmission up to 19.2 bps. That basic limit was nese. All three are based on a DSO channel that is the imposed by the previously mentioned 4000-Hz band- pulse code modulation (PCM) output of a codec, and width of voice channels, although recent advances in define five or six levels of successive multiplexing. They error correction and data compression techniques have differ in the number of voice channels multiplexed onto made effective rates significantly beyond that possible. a given level. A leased line would usually be composed, not of an The circuit that combines signals at the source actual dedicated set of copper wires, but of a dedi- (transmitting) end of a communications link is known as cated FDM channel between area offices of the phone a multiplexer. It accepts the input from each individual company. To obtain effective data transfer rates end user, breaks each signal into segments, and assigns greater than 19.2 bps, it was then necessary for the the segments to the composite signal in a rotating, subscriber to lease multiple lines and transmit data in repeating sequence. The composite signal thus contains parallel. data from all the end users. At the other end of the Leased lines have an additional characteristic that long-distance cable, the individual signals are separated makes them desirable for certain uses: The length of out by means of a circuit called a demultiplexer, and the circuit is known (the telephone company usually is routed to the proper end users. A two-way communi- able to provide this information). The length does not cations circuit requires a multiplexer/demultiplexer at usually vary from day to day. Fixed length is important each end of the long-distance, high-bandwidth cable. If to certain applications that have timing considerat