updated 12/2001 1

Frame Relay

Nirmala Shenoy

Information Technology Department

Rochester Institute of Technology

updated 12/2001 2

Frame Relay

• Purpose– Faster transmission

• Virtual circuit technology

• Data rates up to 2.048 Kbps to the end user

• Network Data rates – 44.376 Mbps – T3 lines

• Assumes smart end systems & reliable media

• No error & flow control in the network

– High data rates at low costs

updated 12/2001 3

Frame Relay

• Purpose– Replacement for a number of leased T1 lines

• Virtual private networks

• Lower costs for similar resources

– Wide Area coverage – interconnect LANs– Ideally suited to bursty traffic

updated 12/2001 4

Frame Relay

• Purpose– Bursty traffic

10 secs

Dat

a ra

te

1.544mbps

Data at a constant rate

10 secs

Dat

a ra

te

Bursty Data

Total data 15.44 Mbits

6 Mbps for 2 secs 3.44 Mbps

for 1 secs Total data 15.44 Mbits

updated 12/2001 5

Frame Relay

• Advantages– High data rates & access rates– Lower 2 layers- ideal for backbone networks– Support bursty data– Maximum frame size – 9000 bytes– Less expensive technology– Can be used on lease or on requirement basis

updated 12/2001 6

Frame Relay

• Disadvantages– Not high enough data rates– Variable length frames– Not suited to time sensitive applications

updated 12/2001 7

Frame Relay

• Topology

SW

SW

SW

SW

router

router

router

Main frame

LAN1

LAN2 LAN3

Frame Relay Network

UNI

updated 12/2001 8

Frame Relay

• Topology – virtual circuits

SW

SW

SW

SW

A

B

C

Frame Relay Network

D

DLCI =21

updated 12/2001 9

Frame Relay

• Topology – virtual circuits– At the Data link layer– DLCI – Data Link Connection Identifier– PVCs and SVCs– Different connections get different DLCI– Local significance– DLCIs within the network

updated 12/2001 10

Frame Relay

• Route table at FR switch

SW

Incoming Outgoing Interface DLCI interface DLCI 1 121 2 041 1 124 3 112 2 167 3 367 3 167 1 192

1

2

3 DLCI 124 DLCI 121

DLCI 041

DLCI 112

updated 12/2001 11

• Layers

• Core data link function – LAPF– Simplified HDLC

Frame Relay

Physical layer ANSI

Core data link layer

updated 12/2001 12

• LAPF – PDU– Flag– Address field

• DLCI• C/R (command response bit) not used • EA – extended address bit• FECN• BECN• DE - Drop eligibility bit

Frame Relay

updated 12/2001 13

• LAPF – PDU– Information– FCS – Frame check sequence– Flag

Frame Relay

flag flag Address Information FCS

DLCI C/R EA DLCI FECN BECN DE EA

6 1 1 4 1 1 1 1

updated 12/2001 14

• LAPF – PDU

• EA bit – useful for extended address capability– 0 signifies – another address byte to follow

• FECN – Forward explicit congestion notification

• Warning to receiver of message that there is congestion along the direction of flow

Frame Relay

updated 12/2001 15

• LAPF – PDU• BECN –

– Notify sender that there is congestion in a direction opposite to the information flow

– Use response frames going in reverse direction– Use predefined DLCI connection 1023

• FECN & BECN inform end systems of network congestion

Frame Relay

updated 12/2001 16

• FECN & BECN

Frame Relay

A B

FECN BECN 0 0

FECN BECN 0 0

A B

FECN BECN 1 0

FECN BECN 0 1

No congestion

Congestion B->A

A B

FECN BECN 0 1

FECN BECN ! 0

Congestion A->B

A B

FECN BECN 1 1

FECN BECN 1 1

Congestion –both directions

updated 12/2001 17

• Discard Eligibility bit– This frame can be dropped during congestion– A congestion control mechanism

Frame Relay

updated 12/2001 18

• Traffic Control - Attributes– Committed burst size – Bc

• Eg 400kbs for 4 seconds

• During the 4 second period – max traffic 400kbs

– Committed information rate• Average rate

• Bc/T (T –predefined period for burst)

• CIR = 400/4 = 100kbs/sec

Frame Relay

updated 12/2001 19

• Traffic Control - Attributes– Excess burst size – Be– Bits in excess of Bc that can be sent in T– May not be transferred under congestion

Frame Relay

seconds

rate

CIR

Access rate

T

Bc

Be

updated 12/2001 20

• Traffic Control

Frame Relay

seconds

rate

CIR

Access rate

T

Actual rate

Area = total bits sent in T seconds

Area < Bc DE=0 Bc+Be > Area >Bc DE =1 Area > Bc+Be discard

updated 12/2001 21

• Traffic Control

• Forwarding of traffic– Fast forward– Leaking CIR

• Traffic control via leaky bucket

Frame Relay

updated 12/2001 22

• Traffic Control – Control of output from leaky bucket – leak rate– Timer (T) and counter for counting bits sent

Frame Relay

3200 bytes

3400 bytes

2800 bytes

3000 bytes

2900 bytes

3000 bytes

3000 bytes

2900 bytes

3000 bytes

3200 bytes

3200 bytes

3200 bytes

Can not be sent

Output rate = 10,000 bytes

Counter =10,000

Counter = 6,800

Counter =3,400

Counter = 600

First time slot – allocated bw = 10,000 bytes

Next time slot – waiting packet sent, new arrivals

New arrivals

updated 12/2001 23

• Traffic Control – leaky bucket

Frame Relay

Bc

Be

Input rate

Output rate

updated 12/2001 24

• Traffic Control

• Use of the DE bit– User setting– Network setting, based on Bc, Be– May not be used

• Policing & Traffic shaping

Frame Relay

updated 12/2001 25

• Service class categorization– Be only – all data can be dropped on congestion– CIR and Bc,

• User sets DE flag, network will discard these frames under severe network congestion if exceeding Bc

– CIR, Bc and Be• Network will tag Be traffic and drop if severe

congestion

Frame Relay

updated 12/2001 26

• UNI NNI inter-working– ANSI T1.617

Frame Relay

Router Router

PVC segment PVC segment

Multi network PVC

Frame relay network

Frame relay network

SE

S

SE

SE

SE S

S

S

UNI NNI UNI

updated 12/2001 27

• UNI NNI inter-working– Messages– SE - Status Enquiry– S – Status– FS – full status on all PVCs– Use of Unnumbered Information frames of

HDLC – DLCI = 0

Frame Relay

updated 12/2001 28

• NNI operations– Adding a PVC notification– Detection of PVC deletion– UNI, NNI failures– PVC segment availability– Link verification– Node verification

Frame Relay

updated 12/2001 29

• Typical Bellcore PVC service

• Exchange Access Frame Relay XA-FR

• Defined between LEC (Local Exchange Carrier) and IC(Interchange Carrier)– FR-ICI interface

• Consistent service – end-to-end PVC, UNI to UNI

Frame Relay

updated 12/2001 30

• XA-FR topology

Frame Relay

LEC IC LEC

CPE

CPE

CPE

CPE

FR-UNI FR-UNI FR-ICI FR-ICI

updated 12/2001 31

• XA-FR

• Service parameters at FR-ICI –compliant frames

• Performance objective– Delay– Accuracy– Availability

Frame Relay

updated 12/2001 32

• XA-FR – delay– FR-UNI access rate– FR-ICI access rate– Frame size– Time the first bit placed on the UNI, till the last

bit received at ICI < specified value for 95% of the frames

Frame Relay

updated 12/2001 33

• XA-FR – accuracy– Number of errored frames– Number of lost frames– Number of extra frames– Compute

• Frames not delivered ratio

• Errored frames ratio

• Extra frames ratio

Frame Relay

updated 12/2001 34

• XA-FR – availability – Scheduled hours of service– Service availability– MTTSR – mean time to service restoration– MTBSO – mean time between service outages– Fraction of time in non-congestion notification

state– Mean time between congestion notification st

Frame Relay

updated 12/2001 35

• DLCI Values – two octet field– 0 – in channel signaling– 1-15 reserved– 16- 991 – assigned by FR connections– 992-1007 – layer management for bearer

service– 1008-1022 – reserved– 1023 – in-channel layer management

Frame Relay

updated 12/2001 36

• DLCI Values – – Global addressing – unique destination address– Semi-broadcast – copied to multiple routers

Frame Relay

updated 12/2001 37

• Frame Relay SVC operation

Frame Relay

Router Router Frame relay network

Ingress Node

Egress Node

Set up message

connect

Call proceeding Call proceeding

Connect ack Connect ack

updated 12/2001 38

• Frame Relay SVC operation– Set up information

– DLCI

– An explicit address

– Requested end-to-end delay

– Max frame size

– Requested throughput (incoming & outgoing)

– Requested Bc (incoming & outgoing)

– Requested Be (incoming & outgoing)

Frame Relay

updated 12/2001 39

• Quality of service options– Residual Error rate– Frame related– Switched virtual call establishment delay– Clearing delay– Premature disconnect

Frame Relay

updated 12/2001 40

• Features of emerging technologies?– Bursty data & high bit rate (not so high?!)– Quality of Service addressed– Frame based – real time suitability?– Flow control – implicit & minimal

• Drop traffic on congestion

– Error control – no• Drop traffic on error

– Payload integrity management – no– Band width on demand

Frame Relay

updated 12/2001 41

• Summary– Extended use of HDLC technology– High speed WAN – Ideal to interconnect high speed LANs– Limitations

• Speed

• Frame size

– Good backbone technology

Frame Relay

updated 12/2001 42

• Summary– Lower costs

• Operational

• End user

– Better sharing of resources– New service offering

Frame Relay