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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 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