Frame Relay
Nirmala Shenoy
Information Technology Department
Rochester Institute of Technology
updated 12/2001
1
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
2
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
3
Frame Relay
• Purpose
Data rate
– Bursty traffic
Total data 15.44 Mbits
1.544mbps
Data rate
Data at a constant rate
6 Mbps
for 2 secs
Total data 15.44 Mbits
Bursty Data
updated 12/2001
10 secs
3.44 Mbps
for 1 secs
10 secs
4
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
5
Frame Relay
• Disadvantages
– Not high enough data rates
– Variable length frames
– Not suited to time sensitive applications
updated 12/2001
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Frame Relay
• Topology
LAN3
LAN2
Frame Relay
Network
router
router
SW
UNI
SW
SW
SW
Main
frame
router
LAN1
updated 12/2001
7
Frame Relay
• Topology – virtual circuits
Frame Relay
Network
A
C
DLCI =21
SW
SW
SW
SW
D
B
updated 12/2001
8
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
9
Frame Relay
• Route table at FR switch
Incoming
Interface DLCI
1
121
1
124
2
167
3
167
Outgoing
interface DLCI
2
041
3
112
3
367
1
192
2
DLCI 041
3
DLCI 112
SW
DLCI 124
DLCI 121
1
updated 12/2001
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Frame Relay
• Layers
Core data link
layer
Physical layer
ANSI
• Core data link function – LAPF
– Simplified HDLC
updated 12/2001
11
Frame Relay
• LAPF – PDU
– Flag
– Address field
•
•
•
•
•
•
DLCI
C/R (command response bit) not used
EA – extended address bit
FECN
BECN
DE - Drop eligibility bit
updated 12/2001
12
Frame Relay
• LAPF – PDU
– Information
– FCS – Frame check sequence
– Flag
flag
DLCI
6
Information
Address
FCS
C/R
EA
DLCI
FECN
1
1
4
1
updated 12/2001
flag
BECN DE EA
1
1
1
13
Frame Relay
• 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
updated 12/2001
14
Frame Relay
• 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
updated 12/2001
15
Frame Relay
• FECN & BECN
A
FECN BECN
0
0
B
A
FECN BECN
0
0
FECN BECN
0
1
B
FECN BECN
!
0
No congestion
Congestion A->B
A
FECN BECN
1
0
B
A
FECN BECN
0
1
Congestion B->A
FECN BECN
1
1
B
FECN BECN
1
1
Congestion –both directions
updated 12/2001
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Frame Relay
• Discard Eligibility bit
– This frame can be dropped during congestion
– A congestion control mechanism
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Frame Relay
• 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
updated 12/2001
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Frame Relay
• Traffic Control - Attributes
rate
– Excess burst size – Be
– Bits in excess of Bc that can be sent in T
– May not be transferred under congestion
Access rate
CIR
Be
Bc
T seconds
updated 12/2001
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Frame Relay
• Traffic Control
rate
Area < Bc DE=0
Bc+Be > Area >Bc DE =1
Area > Bc+Be discard
Access rate
Actual rate
CIR
Area = total bits sent in T seconds
T seconds
updated 12/2001
20
Frame Relay
• Traffic Control
• Forwarding of traffic
– Fast forward
– Leaking CIR
• Traffic control via leaky bucket
updated 12/2001
21
Frame Relay
• Traffic Control
– Control of output from leaky bucket – leak rate
– Timer (T) and counter for counting bits sent
Can not be sent
Counter
= 600
3000
bytes
2900
bytes
3000
bytes
2800
bytes
Counter
= 6,800
3400
bytes
Counter
=10,000
3200
bytes
First time slot – allocated bw = 10,000 bytes
New arrivals
3200
bytes
Counter
=3,400
3200
bytes
3200
bytes
3000
bytes
2900
bytes
3000
bytes
Next time slot – waiting packet sent, new arrivals
Output rate = 10,000 bytes
updated 12/2001
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Frame Relay
• Traffic Control – leaky bucket
Input rate
Be
Bc
Output rate
updated 12/2001
23
Frame Relay
• Traffic Control
• Use of the DE bit
– User setting
– Network setting, based on Bc, Be
– May not be used
• Policing & Traffic shaping
updated 12/2001
24
Frame Relay
• 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
updated 12/2001
25
Frame Relay
• UNI NNI inter-working
– ANSI T1.617
Multi network PVC
PVC segment
PVC segment
Frame relay
network
Router
SE
SE
S
S
UNI
Router
Frame relay
network
SE
S
SE
S
updated 12/2001
NNI
UNI
26
Frame Relay
• 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
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Frame Relay
• NNI operations
–
–
–
–
–
–
Adding a PVC notification
Detection of PVC deletion
UNI, NNI failures
PVC segment availability
Link verification
Node verification
updated 12/2001
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Frame Relay
• 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
updated 12/2001
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Frame Relay
• XA-FR topology
CPE
CPE
LEC
IC
LEC
CPE
CPE
FR-UNI
FR-ICI
FR-ICI
updated 12/2001
FR-UNI
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Frame Relay
• XA-FR
• Service parameters at FR-ICI –compliant
frames
• Performance objective
– Delay
– Accuracy
– Availability
updated 12/2001
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Frame Relay
• XA-FR – delay
–
–
–
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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
updated 12/2001
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Frame Relay
• 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
updated 12/2001
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Frame Relay
• XA-FR – availability
–
–
–
–
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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
updated 12/2001
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Frame Relay
• 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
updated 12/2001
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Frame Relay
• DLCI Values –
– Global addressing – unique destination address
– Semi-broadcast – copied to multiple routers
updated 12/2001
36
Frame Relay
• Frame Relay SVC operation
Set up message
Call proceeding
Call proceeding
connect
Connect ack
Router
Ingress
Node
Frame relay
network
updated 12/2001
Connect ack
Egress
Node
Router
37
Frame Relay
• 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)
updated 12/2001
38
Frame Relay
• Quality of service options
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–
–
–
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Residual Error rate
Frame related
Switched virtual call establishment delay
Clearing delay
Premature disconnect
updated 12/2001
39
Frame Relay
• 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
updated 12/2001
40
Frame Relay
• Summary
–
–
–
–
Extended use of HDLC technology
High speed WAN
Ideal to interconnect high speed LANs
Limitations
• Speed
• Frame size
– Good backbone technology
updated 12/2001
41
Frame Relay
• Summary
– Lower costs
• Operational
• End user
– Better sharing of resources
– New service offering
updated 12/2001
42