Depending on the electrical signalling, devices may be connected directly, otherwise line drivers or modems are necessary to carry the signals over long distances. 54.4.2 Packet switching at the Data Link Layer Up to about one million bits per second over distances below about one kilometre, twisted pair (ordinary telephone wire) is nor- The aim of the Data Link Layer is to ensure that data passing mally adequate. Beyond this fibre optic cable, which is insensitive between devices is error free and in the correct order. Given the to electrical noise, may be required to ensure the error rate on the global influence of the X.25 standard, it is no surprise that the most link is not so high that the error detection and correction at layer 3 important PSN link layer in use is X.25 layer 2, although there are is rendered ineffective. other variations of layer 2 implementation that achieve the same The CCITT chose the X.21 specification as the physical interface objective. between a DTE and DCE for X.25, despite the prevalence of the Link layer information is passed between the two end point Electrical Industries Association's RS-232-C standard for com- devices in frames. The most common protocol used for encapsulat- puter-modem connections. There are only minor differences, how- ing data into frames is the ISO's High-Level Data Link Control ever, with X.21 geared eventually to delivering a digital signal to (HDLC) dating back to the mid-1970s. Its origins lie in work done customer premises, replacing the analogue circuit switched network by IBM seeking to replace older Binary Synchronous Communica- with ISDN. tions with its Synchronous Data Link Control protocol. Prior to ISDN, the CCITT approved an interim standard, X.21bis, Unlike previous protocols, HDLC, SDLC and their peers, such as which is practically identical to the ubiquitous RS-232-C. It spe- ANSI's Advanced Data Communications Control Procedure, are bit cifies a leased line connection, but dial-up is feasible. oriented rather than character oriented. Frame 