Frame Relay
 Packet
switching system with low overhead
 Assumes very reliable high-quality
physical network
 Developed for use in ISDN networks
 Used widely in a variety of private and
public networks which are not ISDN
1
X.25 Packet Flow
Intermediate node
12
5
6
11
16
1
2
Source
15
9
8
7
10
Destination
2
Frame Relay Packet Flow
Intermediate node
3
6
1
82
Source
5
4
Destination
3
Frame Relay
 Control
Signalling carried on separate
logical connection from user data
 Multiplexing and switching of logical
connections take place at layer 2 not layer 3
 No hop-by-hop flow control or error control
 Protocol functionality at user-network
interface is reduced
 Large increase in throughput over X.25
4
Frame Relay Protocol Architecture
Control Plane
User Plane
User Plane
Q.931/Q.933
Control Plane
Q.931/Q.933
User-selectable
TE functions
User-selectable
TE functions
LAPD (Q.921)
LAPD (Q.921)
LAPF core
(Q.922)
PhysicalI.430/I.431
LAPF core
(Q.922)
I.430/I.431Physical
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Control Plane Protocols
 Q.933
protocol is used for control of connections
 In ISDN, Control signalling uses LAPD protocol
 It is also possible to use in-channel call control
using Q.933 on top of Q.922
6
User Plane Protocols
 LAPF
(Q922) used for data transfer between users
 LAPF Core functions:
–
–
–
–
Frame delimiting, alignment, transparency
Frame multiplexing / de-multiplexing
Frame integrity checking ( size, byte count, errors)
Congestion control
Functions are a sub-layer of data link layer
 They provide a bare frame transfer service

7
Frame Relay and X.25
X.25
Implemented
by end system
and network
Implemented
by end system
not network
LAPF control
LAPB
LAPF core
I.430/I431
Implemented
by end system
and network
I.430/I431
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Frame Relay Call Control
 Subscriber
must first be connected to a
frame handler
 This is called an access connection
 When access connection is made, multiple
logical channels can be multiplexed on the
connection
 These are called frame relay connections
 They can be on-demand or semi-permanent
9
Frame Relay Call Control
 Two types of access connection

Switched Access
– User on switched network where exchange does not
have frame handling capability
– Exchange provides switched access (demand or semipermanent) to remote frame handler

Integrated Access
– User connected to pure frame relay network or
switched network with integrated frame handling in
local exchange
– User has direct logical access to frame handler
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User Access
Switched access connection
TE
NT
ET
ET
FH
Semi-permanent access connection
Switched access
TE
NT
ET
FH
Local exchange
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Integrated access
Frame Relay Connections
 Analogous
to virtual circuit in X.25
 Can be established when access connection
established to frame handler
 Multiple connections supported over single
link
– Called data link connections
 Each
connection has a unique Data link
connection identifier (DLCI)
12
Frame Relay Connections
 Data
transfer sequence
– Establish logical connection between two
endpoints and assign unique DLCI
– Exchange information in data frames - each
frame has a DLCI
– Release logical connection
13
Frame Relay Connections

Establishment and release of Logical
connection is made by messages over
dedicated call control logical connection
with DLCI =0
14
Frame Relay Control Signalling
NT
Setup
D-channel Q.931
exchange to establish
B-channel circuitswitched connection
B-channel Q.933
exchange to establish
B-channel frame mode connection
Frame Relay
Network
ISDN
Connect
Connect
ack
Setup
Setup
Connect
Connect
ack
Setup
Connect
Connect ack
NT
Connect
Connect
ack
Frame relay Q.922
exchange of user
data on B-Channel
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Message exchange for switched access to frame handler over ISDN
Frame Relay Control Signalling
NT
Frame Relay
NT
Network
Disconnect
ISDN
Disconnect
B-channel Q.933
exchange to release
B-channel framemode connection
D-channel Q.931
exchange to release
B-channel circuit switched connection
Release
Release
Release
complete
Release
complete
Disconnect
Release
Release
complete
Disconnect
Release
Release
complete
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Message exchange for terminating switched access to frame handler
LAPF Frame Format
Flag Address
1 octet 2 - 4 octets
Information
variable length
FCS
2 octets
Flag
1 octet
Frame Format
8
7
6
5
4
3
Upper DLCI
Lower DLCI
FECN
BECN
2
1
C/R
EA 0
DE
EA 1
Address field 2 octets (default)
Legend
EA Address field extension bit
C/R Command/response bit
DE Discard eligibility bit
FECN Forward explicit congestion notification
BECN Backward explicit congestion notification
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DLCI Data link connection identifier
LAPF Frame Format
 No
control field exists in the frame
 The connection can only carry user data
 Therefore no in-band signalling exists
 No error control or flow control exists since
there are no sequence numbers
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LAPF Frame Format
 Address
field carries DLCI
 Address field length may be extended to 2,
3, or 4 octets
 Length determined by EA bits - default is 2
octets
 DLCI allows multiple logical connections
to be multiplexed on single channel
 DLCI can be 10, 17 or 24 bits depending on
address field length
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Congestion Control
 No
in-channel control signalling means no
sliding window flow control
 Congestion control is the joint
responsibility of the network and the enduser
 Network monitors congestion
 User controls congestion by limiting flow
of traffic at origin
 Network discards packets as a last resort 20
Congestion Control Techniques
Type
Discard Strategy
Congestion
avoidance
Congestion
avoidance
Congestion
recovery
Technique
Function
Provides guidance
to network about
Discard Control
which frames
to discard
Provides guidance
Backward explicit
to end-systems
congestion
about congestion
notification
in network
Provides guidance
Forward explicit
to end-systems
congestion
about congestion
notification
in network
implicit
congestion
notification
Key elements
DE bit
BECN bit
FECN bit
End system infers Sequence numbers
congestion from
in higher-layer
frame loss
PDU
21
Discard Strategy
 Network
agrees to support a connection at a
certain data rate:
– Committed information rate (CIR) in bps
– Committed burst size (Bc) in bits over time T
 Network
also negotiates excess burst size
(Be) the maximum amount of data in excess
of Bc it will attempt to transfer in normal
conditions
22
Discard Strategy
 Frame
handler monitors traffic on a logical
connection
 If data rate exceeds Bc in time interval T it
will set DE bit and forward packet
 If data rate exceeds Bc+ Be in time interval
T it will discard data
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Discard Strategy
Bits
Transmitted
Discard Region
Bc+Be
DE = 1 Region
Bc
Access Rate
CIR
D = 0 Region
Frame 1
DE=0
Frame 2
DE=0
Time
Frame 3
DE=0 T
24
Discard Strategy
Bits
Transmitted
Discard Region
Bc+Be
DE = 1 Region
Bc
Access Rate
CIR
D = 0 Region
Frame 1
DE=0
Frame 2
DE=1
Time
Frame 3
DE=1
25
T
Discard Strategy
Bits
Transmitted
Discard Region
Bc+Be
DE = 1 Region
Bc
Access Rate
CIR
D = 0 Region
Frame 1
DE=0
Frame 2
DE=1
Time
Frame 3
Discard T
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Congestion Avoidance
 Network
alerts end-systems to growing
congestion
 End-systems reduce offered load to network
 Two methods exist in frame relay
– Forward explicit congestion notification
(FECN)
– Backward explicit congestion notification
(BECN)
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Congestion Avoidance
 Two
bits, FECN and BECN exist in each
frame address field
 Any frame handler that detects may set
either bit
 Any frame handler receiving a frame with a
bit set must forward the frame with the bit
set
 The bits therefore are signals to the enduser
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Congestion Avoidance
 The
frame handler monitors outgoing queue
lengths
 Determines average queue length
 If average exceed a threshold, then FECN
bit or BECN bit or both is set
 They may be set for certain logical
connections or all depending on queue sizes
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Congestion Avoidance
 On
receipt of BECN signal, user reduces
rate of frame transmission
 On receipt of FECN signal, user notifies
peer user to reduce rate of frame
transmission
30
Congestion Recovery
 When
higher-level end-end protocol
detects frame loss it assumes congestion
 This is called implicit signalling
 Flow control may be used to recover
 Gradual reduction of window size and
gradual increase as frame loss disappears
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