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 Used widely in a variety of private and public networks which are not ISDN

2 X.25 Packet Flow SourceDestination Intermediate node

3 Frame Relay Packet Flow SourceDestination Intermediate node

4 Frame Relay u Control Signalling carried on separate logical connection from user data u Multiplexing and switching of logical connections take place at layer 2 not layer 3 u No hop-by-hop flow control or error control u Protocol functionality at user-network interface is reduced u 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-selectable TE functions LAPF core (Q.922) User PlaneControl Plane Physical Q.931/Q.933 LAPD (Q.921) I.430/I.431 User-selectable TE functions LAPF core (Q.922) User PlaneControl Plane

6 Control Plane Protocols u Q.933 protocol is used for control of connections u In ISDN, Control signalling uses LAPD protocol u It is also possible to use in-channel call control using Q.933 on top of Q.922

7 User Plane Protocols u LAPF (Q922) used for data transfer between users u LAPF Core functions: –Frame delimiting, alignment, transparency –Frame multiplexing / de-multiplexing –Frame integrity checking ( size, byte count, errors) –Congestion control u Functions are a sub-layer of data link layer u They provide a bare frame transfer service

8 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

9 Frame Relay Call Control u Subscriber must first be connected to a frame handler u This is called an access connection u When access connection is made, multiple logical channels can be multiplexed on the connection u These are called frame relay connections u They can be on-demand or semi-permanent

10 Frame Relay Call Control u Two types of access connection u 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 u 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

11 User Access TENTFHET Switched access connection Semi-permanent access connection Switched access TENTFHET Integrated access Local exchange

12 Frame Relay Connections u Analogous to virtual circuit in X.25 u Can be established when access connection established to frame handler u Multiple connections supported over single link –Called data link connections u Each connection has a unique Data link connection identifier (DLCI)

13 Frame Relay Connections u 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

14 Frame Relay Connections u Establishment and release of Logical connection is made by messages over dedicated call control logical connection with DLCI =0

15 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 NTISDN Frame Relay Network NT Setup 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 ack Connect ack Setup Connect ack Setup

16 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 NTISDN Frame Relay Network NT D-channel Q.931 exchange to release B-channel circuit - switched connection Release Disconnect Release complete Release complete Disconnect Release Disconnect Release complete Release complete

17 LAPF Frame Format Flag 1 octet Information variable length FCS 2 octets Flag 1 octet Frame Format Address octets Upper DLCIC/REA 0 EA 1DEBECNFECNLower DLCI 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

18 LAPF Frame Format u No control field exists in the frame u The connection can only carry user data u Therefore no in-band signalling exists u No error control or flow control exists since there are no sequence numbers

19 LAPF Frame Format u Address field carries DLCI u Address field length may be extended to 2, 3, or 4 octets u Length determined by EA bits - default is 2 octets u DLCI allows multiple logical connections to be multiplexed on single channel u DLCI can be 10, 17 or 24 bits depending on address field length

20 Congestion Control u No in-channel control signalling means no sliding window flow control u Congestion control is the joint responsibility of the network and the end- user u Network monitors congestion u User controls congestion by limiting flow of traffic at origin u Network discards packets as a last resort

21 Congestion Control Techniques FunctionKey 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

22 Discard Strategy u 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 u Network also negotiates excess burst size (Be) the maximum amount of data in excess of Bc it will attempt to transfer in normal conditions

23 Discard Strategy u Frame handler monitors traffic on a logical connection u If data rate exceeds Bc in time interval T it will set DE bit and forward packet u If data rate exceeds Bc+ Be in time interval T it will discard data

24 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

25 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

26 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

27 Congestion Avoidance u Network alerts end-systems to growing congestion u End-systems reduce offered load to network u Two methods exist in frame relay –Forward explicit congestion notification (FECN) –Backward explicit congestion notification (BECN)

28 Congestion Avoidance u Two bits, FECN and BECN exist in each frame address field u Any frame handler that detects may set either bit u Any frame handler receiving a frame with a bit set must forward the frame with the bit set u The bits therefore are signals to the end- user

29 Congestion Avoidance u The frame handler monitors outgoing queue lengths u Determines average queue length u If average exceed a threshold, then FECN bit or BECN bit or both is set u They may be set for certain logical connections or all depending on queue sizes

30 Congestion Avoidance u On receipt of BECN signal, user reduces rate of frame transmission u On receipt of FECN signal, user notifies peer user to reduce rate of frame transmission

31 Congestion Recovery u When higher-level end-end protocol detects frame loss it assumes congestion u This is called implicit signalling u Flow control may be used to recover u Gradual reduction of window size and gradual increase as frame loss disappears