TYPICAL LAYOUT

Figure 6-2 shows a typical layout for a data communications link. Several of the prominent physical layer (L_1) standards do not specify the complete physical interface between to user devices (called Data Terminal Equipment (DTE) in the ITU-T specifications). Specifically, the span between the modems may not be defined. Prominent examples are the EIA-232 and V.24 standards. Fortunately, this part of the physical interface is standardized through the ITU-T V Series modem specifications. Figure 6-2 shows the relationship of these specifications to the physical layer interface.

The cables and wires between the DTE ports and the modems are joined to connectors that take the shape of small metal pins. The pins serve as "plugs" into the computers and modems. These connectors are called interchange circuits. Figure 6-3 shows two choices for the physical interchange circuits, as well as the International Standards Organization (ISO) mechanical connectors. The connectors' interchange circuits are identified by numbers, by alphabetic identifiers (AB, AC, etc.), or by functional descriptions (transmit data circuit, request-to-send circuit, etc.). You may not see these connectors on your PC or workstation. Some manufacturers use a proprietary connector, and many have migrated to a conventional telephone jack.