ks in use today are virtual in nature rather than physical trunks be- cause they are digitized and multiplexed onto some form of digital facility capable of handling multiple simultaneous trunks. (In telephony, we are using the term virtual for these trunks because they are not separate wires but multiplexed into a larger fa- cility. These trunks have a well-defined presence on the specified facility, with specific bits reserved for them. In data communications, when we refer to a virtual circuit the connection is defined only by its end points and not by any specific facility or route.) The basic digital building block in North America is the DS-1, capable of supporting 24 trunks. These trunks may traverse many other types of equipment in order to get from one switch to another, such as multiplexers capable of routing the trunk over both cop- per and optical fiber, cross-connects, and equipment designed to cancel any echo from the other end of the call. The most common form of digital encoding used by these digital facilities is referred to as pulse code modulation (PCM) and has been standardized to be a 64-kbps bit stream consisting of 8-bit samples taken at 8000 samples per second. This data encod- ing technique (described in Sec. 18.1) is assumed for the design of the various digital transmission facilities and equipment used to interconnect telephone switches and as- sumed in the design of the switches themselves. The DS-1 facility, capable of carrying 24 simultaneous calls, is formed by concate- nating 24 eight-bit samples into a frame and then adding a framing bit used for frame synchronization. Therefore a DS-1 frame is 193 bits long, carrying 192 bits of informa- tion. The repetition rate is 8000 frames per second, as required by the PCM coding method, resulting in a bit rate of 1.544 Mbps, of which 1.536 Mbps is information, that is, the 24 samples. The position of the sample within the frame is commonly referred to as a time slot. A DS-1 facility actually consists of two DS-1 framed transmission paths,