Computer networks

FRAME RELAYFRAME RELAY CONTENTSCONTENTS DEFINITIONDEFINITION ADVANTAGESADVANTAGES DISADVANTAGESDISADVANTAGES VIRTUAL CIRCUITSVIRTUAL CIRCUITS FRAME RELAY SWITCHFRAME RELAY SWITCH FRAME RELAY STRUCTUREFRAME RELAY STRUCTURE USE OF FRAME RELAYUSE OF FRAME RELAY CONGESTION CONTROLCONGESTION CONTROL

introductionintroduction

Frame relay is a relatively high-Frame relay is a relatively high-speed protocol that can provide speed protocol that can provide some services not available in some services not available in other WAN technologies such as other WAN technologies such as DSL, cable TV, T-lines. It is quite DSL, cable TV, T-lines. It is quite popular. popular.

Frame relay provides:Frame relay provides:

Higher data rate at lower costHigher data rate at lower cost

Bursty dataBursty data

Less overheadLess overhead

LANLAN

LANLAN

LANLAN

15 T-LINES

a.) Using T-lines

HIGHER DATA RATE AT LOWER COST

LAN

LAN

LAN

LAN

LANLAN

Frame relay

network

6 T-lines

b). Using frame relay

Frame relay is a way to transmit data across a leased line

Data rate

0 1 2 3 4 5 6 7 8 9 10

Total data sent:15.44mbps

1.54

4 M

bp

s

seconds

a. Fixed-rate data

BURSTY DATA V/S FIXED RATE DATA

b. Bursty data

6 M

bp

s0 1 2 3 4 5 6 7 8 9 10

Total data sent:15:44mbps

12 MB

seconds

Data rate

3.44MB

3.44

mb

ps

LESS OVERHEAD

Data from the network layer sent in data link layer

source

data data

Frame relay traffic

destination

Routers

Routers

Routers Bridges

Advantages of Frame RelayAdvantages of Frame Relay

It operates at a It operates at a higher speedhigher speed (1.544 (1.544 Mbps & 44.376Mbps)Mbps & 44.376Mbps)

It operates in physical & data link It operates in physical & data link layerlayer

It allows It allows bursty databursty data

It allows It allows frame sizeframe size of 9000 bytes, of 9000 bytes, which can accommodate all LAN which can accommodate all LAN framesframes

Less expensiveLess expensive than other WANs than other WANs

Disadvantages of Frame Disadvantages of Frame RelayRelay

Not highNot high enough for protocols with enough for protocols with even even higher data rateshigher data rates

It allows variable-length frames. This It allows variable-length frames. This may may create delayscreate delays for different users for different users

Frame relay is not suitable for Frame relay is not suitable for sending sensitive datasending sensitive data

Virtual CircuitsVirtual Circuits

The Frame Relay frame is transmitted to The Frame Relay frame is transmitted to its destination by way of virtual circuits to a its destination by way of virtual circuits to a destination point.destination point.

A VC in frame relay is identified by data A VC in frame relay is identified by data link connection identifier (DLCI)link connection identifier (DLCI)

When a VC is established, a DTE is When a VC is established, a DTE is given DLCIgiven DLCI to send frames to remote DTE.to send frames to remote DTE.

Permanent Virtual Circuit Permanent Virtual Circuit (PVC)(PVC)

I

II

III

IV

A PVC connection is established between two DTEs.

The two DTEs are permanently connected through a virtual connection.

The same virtual circuit is provided between two users

Switched virtual circuit Switched virtual circuit (SVC)(SVC)

A virtual circuit is created whenever it is A virtual circuit is created whenever it is needed & exists only for duration of the needed & exists only for duration of the specific exchangespecific exchange

Single route exists for the duration of Single route exists for the duration of transmission, although the network transmission, although the network could pick an alternate routecould pick an alternate route

Switched virtual circuitSwitched virtual circuit

I

II

III

IV

a. Connection establishment I

II

III

IV

c. Connection release

I

II

III

IV

b. Data transfer

Frame relay switchFrame relay switch

Each switch in a frame relay network has a table to route

The table matches an incoming interface –DLCI combination with an outgoing interface-DLCI combination 1.

2.

3.

DLCI:121DLCI:124

DLCI:041

DLCI:112

Incoming

Interface DLCI DLCIInterface

11

121124

23

041112

Outgoing

Frame structureFrame structure

Frame relay layersFrame relay layers

Frame relay has only physical and data link layers

CongestionCongestion

When the Frame Relay switch recognizes congestion, it sends a FECN message to the destination router.

This message tells the router that congestion was experienced on the virtual circuit.

In addition, the switch sends BECN message to the transmitting, or source, router.

The router's reaction to the BECN should be to reduce the amount of traffic it is sending.

Thus, during times of congestion, the router can discard DE frames in order to provide a higher, more reliable, service to those frames that are not discard eligible.