Frame Relay 33sp

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 Intermediate node 14

12 3

5

4

6 13

16

1

2

Source

15

11 9

8

7

Destination 2

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Frame Relay Packet Flow Intermediate node

2

3

7

1

82

Source

6

5

4

Destination 3

Frame Relay Control Signalling carried on separate logical connection from 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 -network interface is reduced  Large increase in throughput over X.25 

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Frame Relay Protocol Architecture

Control Plane

Plane

Plane

Q.931/Q.933

Control Plane Q.931/Q.933

-selectable TE functions

-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

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Plane Protocols  

LAPF (Q922) used for data transfer between s 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 

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Frame Relay Call Control  Two types of access connection 

Switched Access – on switched network where exchange does not have frame handling capability – Exchange provides switched access (demand or semipermanent) to remote frame handler



Integrated Access – connected to pure frame relay network or switched network with integrated frame handling in local exchange – has direct logical access to frame handler 10

Access Switched access connection

TE

NT

ET

ET

FH

Semi-permanent access connection

Switched access

TE

NT

ET

Integrated access

FH

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 ed 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

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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 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 data on B-Channel

15 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 complete Disconnect Release Release complete

Release Release complete

Disconnect Release Release complete

16 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

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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 17 DLCI Data link connection identifier

LAPF Frame Format No control field exists in the frame  The connection can only carry 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 19 

Congestion Control No in-channel control signalling means no sliding window flow control  Congestion control is the t responsibility of the network and the end  Network monitors congestion  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

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Discard Strategy 

Network agrees to 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

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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=1

Time Frame 3 DE=1

T

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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=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 end- 

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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, reduces rate of frame transmission  On receipt of FECN signal, notifies peer 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 

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