tched services and interconnec- layers may be inactive in certain applications. Nevertheless, keep- tion of multiple subnetworks en route. Both connection-mode and ing the basic OSI structure intact will ensure a consistent evolution connectionless-mode operations have been widely applied in the in the future. network layer. The Network Service Definition is provided in ISO 8348 and a number of standards are available for the various protocols and configurations that can be employed. 12.8 Appendix 12.1 12.5.6 Data link layer The following references used in this chapter are International Standards published by the International Organization for Stand- ardization, 1 rue Varembe, Geneva, Switzerland. They are also The data link layer provides the means for synchronising the bit available from national standards bodies such as the British Stand- stream flowing to and from the physical layer and for detection of ards Institution in the UK. errors due to transmission problems. Typically, Local Area Net- For brevity, the following abbreviations have been used in the works (LANs) operate the data link in connectionless-mode, whilst titles below: wide area network applications operate in the connection-mode. In the connection-mode, the additional functions of flow control and IPS - Information Processing Systems error recovery are included. The Data Link Layer Service Definition OSI - Open Systems Interconnection is specified in ISO 8886. IT - Information Technology TC - Text Communication 12.5.7 Physical layer ISO 7498, IPS - OSI - Basic Reference Model ISO 7498 AD1, IPS - OSI - Basic Reference Model - Addendum The physical layer provides the functions for transparent trans- 1: Connectionless-Mode Transmission mission of the bit stream between data link entities. When the ISO 7498-2, IPS - OSI - Basic Reference Model - Part 2: Security physical layer is active, the bits flow through to the transmission Architecture medium. The electrical characteristics and physical connector serve ISO 7498-4, IPS - OSI - Basic Referenc