1 Frame Relay u Packet switching system with low overhead u Assumes very reliable high-quality physical network u Developed for use in ISDN networks u.

1

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

2

X.25 Packet Flow

Source Destination

216 1 152

143

13

4

12

5

6

11

9 8 107

Intermediate node

3

Frame Relay Packet Flow

Source Destination

21 8

2

7

3

6

5 4

Intermediate node

4

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

5

Frame Relay Protocol Architecture

Physical

Q.931/Q.933

LAPD (Q.921)

I.430/I.431

User-selectableTE functions

LAPF core (Q.922)

User PlaneControl Plane

Physical

Q.931/Q.933

LAPD (Q.921)

I.430/I.431

User-selectableTE 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 controlImplementedby end systemand network

Implementedby end systemnot network

Implementedby end systemand 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 FHET ET

Switched access connection

Semi-permanent access connection

Switched access

TE NT FHET

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)

13

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 RelayNetwork

NT

Setup

ConnectConnect ack

Setup

B-channel Q.933exchange to establishB-channel frame -mode connection

Frame relay Q.922exchange of user data on B-Channel

Connect

ConnectConnect ack

ConnectConnect 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 RelayNetwork

NT

D-channel Q.931exchange to releaseB-channel circuit -switched connection

Release

DisconnectDisconnect

Release

ReleasecompleteRelease

complete

Disconnect

Release Disconnect

Releasecomplete

ReleaseReleasecomplete

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LAPF Frame Format

Flag1 octet

Information variable length

FCS2 octets

Flag1 octet

Frame Format

Address2 - 4 octets

Upper DLCI C/R EA 0

EA 1DEBECNFECNLower DLCI

8 7 6 5 4 3 2 1

Address field 2 octets (default)

LegendEA Address field extension bitC/R Command/response bitDE Discard eligibility bit

FECN Forward explicit congestion notificationBECN Backward explicit congestion notificationDLCI 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 TechniquesFunction Key elements

Provides guidanceto network about

which framesto discard

DE bit

Provides guidanceto end-systems

about congestionin network

BECN bit

Provides guidanceto end-systems

about congestionin network

FECN bit

Technique

Discard Control

Backward explicitcongestionnotification

Forward explicitcongestionnotification

implicitcongestionnotification

Type

Discard Strategy

Congestionavoidance

Congestionavoidance

Congestionrecovery

End system inferscongestion 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 RateCIR

D = 0 Region

BitsTransmitted

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 RateCIR

D = 0 Region

BitsTransmitted

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 RateCIR

D = 0 Region

BitsTransmitted

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

31

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