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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
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X.25 Packet Flow
Source Destination
2
16
1 15 2
12
5
6
11
9
8 10 7
Intermediate node
3
Frame Relay Packet Flow
Source Destination
2 1 8
3
6
5 4
Intermediate node
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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
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Frame Relay Protocol Architecture
Physical
Q.931/Q.933
LAPD (Q.921)
I.430/I.431
User-selectable
TE functions
LAPF core
(Q.922)
User Plane Control Plane
Physical
Q.931/Q.933
LAPD (Q.921)
I.430/I.431
User-selectable
TE functions
LAPF core
(Q.922)
User Plane Control Plane
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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
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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
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Frame Relay and X.25
LAPB
I.430/I431
X.25
LAPF core
I.430/I431
LAPF control
Implemented
by end system
and network
Implemented
by end system
not network
Implemented
by end system
and network
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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
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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 semi-
permanent) 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
TE NT FH ET ET
Switched access connection
Semi-permanent access connection
Switched access
TE NT FH ET
Integrated access
Local exchange
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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)
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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
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Frame Relay Connections
Establishment and release of Logical
connection is made by messages over
dedicated call control logical connection
with DLCI =0
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Frame Relay Control Signalling
Message exchange for switched access to frame handler over ISDN
D-channel Q.931
exchange to establish
B-channel circuit-
switched connection

NT ISDN Frame Relay
Network
NT
Setup
Connect
Connect
ack
Setup
B-channel Q.933
exchange to establish
B-channel frame -
mode connection
Frame relay Q.922
exchange of user
data on B-Channel
Connect
Connect
Connect
ack
Connect
Connect
ack
Setup

Connect ack
Setup
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Frame Relay Control Signalling
Message exchange for terminating switched access to frame handler
B-channel Q.933
exchange to release
B-channel frame-
mode connection

NT ISDN Frame Relay
Network
NT
D-channel Q.931
exchange to release
B-channel circuit -
switched connection
Release
Disconnect
Disconnect
Release
Release
complete
Release
complete
Disconnect
Release
Disconnect
Release
complete
Release
Release
complete
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LAPF Frame Format
Flag
1 octet
Information
variable length
FCS
2 octets
Flag
1 octet
Frame Format
Address
2 - 4 octets
Upper DLCI C/R EA 0
EA 1 DE BECN FECN Lower DLCI
8 7 6 5 4 3 2 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
DLCI Data link connection identifier
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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 end-
user
Network monitors congestion
User controls congestion by limiting flow
of traffic at origin
Network discards packets as a last resort
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Congestion Control Techniques
Function Key elements
Provides guidance
to network about
which frames
to discard
DE bit
Provides guidance
to end-systems
about congestion
in network
BECN bit
Provides guidance
to end-systems
about congestion
in network
FECN bit
Technique
Discard Control
Backward explicit
congestion
notification
Forward explicit
congestion
notification
implicit
congestion
notification
Type
Discard Strategy
Congestion
avoidance
Congestion
avoidance
Congestion
recovery
End system infers
congestion from
frame loss
Sequence numbers
in higher-layer
PDU
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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
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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
DE = 1 Region
Discard Region
Access Rate
CIR
D = 0 Region
Bits
Transmitted
Bc
Bc+Be
Time
Frame 1
DE=0
Frame 2
DE=0
Frame 3
DE=0
T
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Discard Strategy
DE = 1 Region
Discard Region
Access Rate
CIR
D = 0 Region
Bits
Transmitted
Bc
Bc+Be
Time
Frame 1
DE=0
Frame 2
DE=1
Frame 3
DE=1
T
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Discard Strategy
DE = 1 Region
Discard Region
Access Rate
CIR
D = 0 Region
Bits
Transmitted
Bc
Bc+Be
Time
Frame 1
DE=0
Frame 2
DE=1
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 end-
user
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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
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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