1Data Communication

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

2Data Communication

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

3Data Communication

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.

4Data Communication

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.

5Data Communication

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

6Data Communication

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

7Data Communication

Frame Relay Addresses

8Data Communication

Frame Relay Operation Switching

Two operations of a switchchecks a frame for errors using the FCS field: if an error, discard

itcompares the DLCI to an entry in a switch table and find an

outgoing port for the PVC identified by the DLCI

9Data Communication

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

10Data Communication

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