Data Communication 1 Frame Relay n X.25 l Provides extensive error checking and flow control l station-to-station checking at the data link layer l Error checking from source to receiver at network layer l It consumes network bandwidth and requires storage at each station

Data Communication 2 Frame Relay l No error checking and acknowledgment at the data link layer l All error checking is left to the protocols at the network and transport layers l operates at only the physical and data link layers

Data Communication 3 Frame Relay Layers l Eliminates all of the network layer functions and a portion of conventional data link layer functions. l Physical Layers: no specific protocol is defined for the physical layer. It is left to the implementers.

Data Communication 4 Frame Relay Layers n Data Link Layer l employ a simplified version of HDLC - HDLC fields for extensive error checking and flow control not needed. l Addressing and control fields combined into one field.

Data Communication 5 Frame Relay Layers n Frame fields l Addressing(DLCI) fields: two parts (6bits, 4bits). A part of the 16-bit data link connection identifier. l Command/Response(C/R): allows upper layers to identify a frame as either a command or a response. l Extended address (EA):EA=0 another address byte follows, EA=1 the current byte is the final one. l Forward explicit congestion notification(FECN): indicate that traffic is congested in the direction in which the frame is traveling. l Backward explicit congestion notification (BECN) l Discard eligibility (DE) : priority level of the frame

Data Communication 6 Frame Relay Operation l Transmission is based on permanent virtual circuit l DLCI identifies a permanent virtual circuit that is set up when the system is put in place n Replay l Routing information is included in the destination information. l The path form point A to point D always passes through the same node. l The functions of routing and switching can be handled by the data link layer. l Frame relay (frame switching) occurs at the date link layer where the transmission unit is the frame l Packet switching occurs at the network layer where the transmission unit is the packet

Data Communication 7 Frame Relay Addresses

Data Communication 8 Frame Relay Operation n Switching l Two operations of a switch ächecks a frame for errors using the FCS field: if an error, discard it äcompares the DLCI to an entry in a switch table and find an outgoing port for the PVC identified by the DLCI

Data Communication 9 Frame Relay Operation n Congestion Control l does not solve the problem, but does provide ways to lessen the probability of its occurrence l A switch in a PVC warns its downstream switches and destination by turning on the FECN bit l The receiver, in turn, set BECN to warn upstream switches and the sender that the link is congested and to send frames more slowly. l This option can not be used unless the channel is either full- or half-duplex and the receiver is sending its own data or acknowledgments to the sender

Data Communication 10 Frame Relay Implementation n The most likely implementation: Used as a WAN backbone to connect a number of LANs using T-1 links l Frame relay assembler/disassembler(FRADs): assembles and disassembles packets coming from other protocols to allow them to be carried by frame relay frames