dress sequence 16 bits k bits G (x) n bits M (x) Figure 15.2 A typical data frame. is set up by a signaling routine. The distant subscriber has a telephone address represented by a distinct telephone number consisting of 7 to 12 digits. The digit sequence of the number sets up a circuit route and connectivity for conversation. The circuit is maintained in place for the duration of the conversation, and it is terminated and taken down when one or the other party hangs up ("goes on hook"). The address sequence of dialed digits is sent just once, at the initiation of the connectivity. This whole process of setting up a circuit, holding the connectivity in place, and then taking down the circuit is called signaling. Signaling on data circuits is approached quite differently. There is the permanent virtual circuit (PVC), which has all the trappings similar to a voice circuit. The similarities stop here. Data transmission consists of frames or packets of data. A frame (or data packet) is made up of a header and payload. In some cases a portion of the "header" may be appended at the end or on the tail of the data frame (or packet). But every data frame (or packet) has a header consisting of a destination address (or addresses) and the originator's address. It nearly always will also contain some control information. This may be a word (or byte/octet) count of the payload, a CRC sequence for error detection and/or correction, message priority, or some other type of control sequence or sequences. Digital circuits on the PSTN have either 24-octet frames in the case of T1 (DS1) or 32-octet frames in the case of E1. Each 8-bit octet represents a voice circuit. Such a circuit may be set up using an Initial Address Message of CCITT Signaling System No. 7 or a sequence of DTMF tones where each frequency pair represents a digit in the range of 0 through 9. Once a circuit is set up, no more address messages or DTMF tones are required until the circuit is taken down. This is not the case on a data circuit. Such a circuit also uses