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

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

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

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

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

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

7. Frame Relay Addresses

8. Frame Relay Operation • Switching • Two operations of a switch • checks a frame for errors using the FCS field: if an error, discard it • compares the DLCI to an entry in a switch table and find an outgoing port for the PVC identified by the DLCI

9. Frame Relay Operation • Congestion Control • does not solve the problem, but does provide ways to lessen the probability of its occurrence • A switch in a PVC warns its downstream switches and destination by turning on the FECN bit • The receiver, in turn, set BECN to warn upstream switches and the sender that the link is congested and to send frames more slowly. • 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

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