 (ATM) (MAN) UNSTRUCTURED SERVICE: PRESENTATION (STRUCTURED) B-ISDN FRAME RELAY FRAME RELAY SERVICE SERVICE Figure 55.1 Fast Packet overview 55/4 Fast Packet Switching ADDRESS CONTROL ADDRESS ADDRESS CONTROL Figure 55.2 Frame Relay format: (a) standard LAPB frame; (b) LAPD frame as for Frame Relay most popular of the additional packet mode bearer services defined. Figure 55.3 shows the frame relay communication paths relative It also is the most simple. to the OSI seven layer model. It is only at layer 3 that these protocols The development of ISDN Frame Relay services is a packet mode begin to look substantially different. interface to narrowband and broadband ISDN networks, and is IBM originated what we now think of as the layer 2 or link layer designed to provide high speed packet transmission, minimal net- function with its SNA SDLC protocol, and submitted it to various work delay, and efficient use of network bandwidth. bodies for standardisation. SDLC emerged from ANSI as Advanced The basic aim of Frame Relay is to exploit the similarity of the Data Communications Control Protocol (ADCCP), from ISO as OSI layer 2 function (data link control) between X.25 packet HDLC and from CCITT as LAP (the link access procedure for switched networks, IBM's SNA signalling protocol (LAPD), the X.25). LAP evolved into LAPB for X.25 and into LAPD for ISDN. IEEE 802 LAN protocols, and MAP/TOP. The standard (LAPB) HDLC was adapted to the IEEE 802.2 Logical Link Control (LLC) frame and the LAPD frame are illustrated in Figure 55.2. for LANs, which also became the link layer for MAP/TOP. Layers 4 - 7 : APPLICATION User Specific DEFINED DEFINED Layer 3 : End to End NETWORK NETWORK Intelligence Layer 2 (Endpoint) : Congestion Control DATA LINK DATA LINK DATA LINK DATA LINK Error Detection Closed User Group Layer 1 PHYSICAL PHYSICAL PHYSICAL PHYSICAL Figure 55.3 Frame Relay communications path Frame Relay 55/5 In simpler terms, the object is to use a Frame Relay (level 2 in the switch and creating fixed packet sizes on the switch t