 circuit between Interchange System (MOTIS). the Network Layers, and it has facilities for detecting and correcting errors, and for retransmissions. The Physical Layer interfaces to the Directory services physical transmission media and converts the data into signals which are compatible with it, e.g. optical, electrical and radio. The These are a set of standards for the electronic supply and update services in these last three layers is what is normally provided by of information covering, for example, applications or people. It can the public X.25 network. consist of application addresses, telephone numbers, telex numbers, As stated earlier the OSI reference model describes a general etc. The directory may be located on distributed databases, in which structure only, whilst other standards define the services and proto- case the standard also specifies the interconnection between these cols which need to be provided at the various layers. Some of these and the user. 16/10 Network management MOTIS DIRECTORY CMISE ISO 9595 ISO 8831 CCITT CCITT X.500 ISO 9040 ISO 8571 ISO 10026 (SERVICE) (SERVICE) X.400 10021 ISO 9594 ISO 9596 ISO 8832 (SERVICE) (PROTOCOL) (PROTOCOL) ISO 9041 (PROTOCOL) CCITT X.228 CCITT X.229 ISO 9804 (SERVICE) Application (PROTOCOL) layer CCITT X.227 ISO 8649 (SERVICE) ISO 8650 (PROTOCOL) CCITT X 226 ASN 1 CCITT X. 208 (LANGUAGE) Presentation ISO 8822 (SERVICE) CCITT X .209 (Basic encoding rules) layer ISO 8823 (PROTOCOL - CONNECTION MODE) (LANGUAGE) ISO 9576 (PROTOCOL - CONNECTIONLESS MODE) (Basic encoding rules) CCITT X 225 Session ISO 8327 (SERVICE) layer ISO 8327 (PROTOCOL - CONNECTION MODE) ISO 9548 (PROTOCOL - CONNECTIONLESS MODE) CCITT X 224 Transport ISO 8072 (SERVICE) layer ISO 8073 (PROTOCOL-CONNECTION - MODE) ISO 8602 (PROTOCOL-CONNECTIONLESS - MODE) INTERNETWORK PROTOCOL PACKET LEVEL PROTOCOL Network CCITT X 25 (Connection mode) CCITT Q.931 layer ISO 8473 (Connectionless mode) ISO 8208 LOGICAL LINK CONTROL ISO 8802.2 CCITT X .25 LAPB CCITT LAPD CSMA / CD TOKEN TOKEN SLOTTED ISO 7776