Data Link Control. Claude Rigault ENST Claude Rigault, ENST 11/3/2002. Data Link control 1

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1 Data Link Control Claude Rigault ENST Data Link control

2 Data Link Control Outline General principles of Data Link Control HDLC Data Link control 2

3 General principles of Data Link Control Functions of a data link protocol Framing Addressing Flow control Differentiation Error detection Error Correction Link initialization Data Link control 3

4 General principles of Data Link Control Frame Structure It is a series of bits that are taken into account as a group. It is a Data Link Layer PDU which contains header and trailer informations. Data Link control 4

5 General principles of Data Link Control Framing, transparency Beginning of frame : flag value 7E : 0 0 Transparency : Bit stuffing method after 5 consecutive, the transmitter sends an additional 0 The receiver suppresses systematically every 0 following 5 consecutive Data Link control 5

6 General principles of Data Link Control Stop-and-Wait Flow control 2 types of Supervisory Frames : RR(Receiver Ready) and RNR (Receiver not Ready) Principle : send a frame and wait for RR to send next frame INF RR INF Data Link control 6

7 General principles of Data Link Control Inefficiency of Stop-and-Wait If the propagation delay is long in respect to the transmission delay of a frame, a lot of time is spent where no transmission may take place, reducing the effective speed of transmission INF a a INF RR U = + 2a (assume RR negligible) Data Link control 7

8 General principles of Data Link Control Significant length of a medium Medium length : d transmission rate : r Signal velocity : v Transmission time : d v Bit length of the medium : rd v A frame = L bits Number of frames in the air a = vl rd Data Link control 8

9 General principles of Data Link Control Values of a a depends mostly on the rd product On LANs typical values of a are in a range of 0.0 to 0. On MANs and WANs typical values of a exceed and the efficiency is very reduced Data Link control 9

10 General principles of Data Link Control Pipelining Principle : send several frames without RR RR valid for a number of frames INF RR INF Data Link control 0

11 General principles of Data Link Control Sliding window flow control () Principle : send several frames without RR Maximum value is called the Window Frames are numbered modulo n (n=8, 28, ) If an RR is received with N(R) = i, then all frames up to (i-) have been received N(R) means : Next to Receive Frames already received Data Link control

12 Data Link control 2 Sliding window flow control (2) Frames already received Frames already received F5 F6 F7 RR General principles of Data Link Control

13 General principles of Data Link Control Sliding window flow control (3) To stop the transmission the receiver sends an RNR The RNR contains an N(R) = i then all frames up to (i-) have been received For example RNR 6 means : all frames up to 5 have been received, I cannot accept more frames This closes the window at the transmitter To reopen the transmitters window the receiver must send an RR here an RR 6 Data Link control 3

14 General principles of Data Link Control Piggybacking To gain efficiency information frames sent in the opposite direction also carry an N(R) sequence number RRs are used when the receiver has nothing to send in the opposite direction Data Link control 4

15 General principles of Data Link Control Error Detection The message field in the frame is followed by an n-bit FCS (Frame Check sequence). Consider the message field as a binary number M The value of the FCS is the remainder R of the division of 2 n 2 M by a number P of n+ bits n M = Q+ R P P Then the frame represents a number T= 2 n M+R The Division of T by P should give no Remainder n T M R = 2 + = Q+ R+ R= Q P P P P P Note that any binary number added to itself gives 0 Data Link control 5

16 General principles of Data Link Control Error Correction : ARQ ARQ : Automatic Repeat request ARQ mechanism is combined with the flow control mechanism : - Stop-and-Wait ARQ - Go-back-N ARQ - Selective reject ARQ Data Link control 6

17 General principles of Data Link Control Stop-and-Wait ARQ Remember : in stop-and-wait there are no frame numbers Principle : send a frame and wait for an ACK to send next frame Frame ACK Frame Data Link control 7

18 General principles of Data Link Control Stop-and-Wait ARQ : no ACK With every frame sent a timer is initiated Problem : if it is the ACK that has been lost, the receiver receives twice the same frame Frame Time out Same Frame Data Link control 8

19 General principles of Data Link Control Alternating Bit Protocol Principle : to avoid duplication of frames in case of lost ACK, frames are numbered modulo 2 ( bit). If 2 frames are received with the same bit, this means duplication Frame 0 ACK Frame ACK 0 Data Link control 9

20 General principles of Data Link Control Go-back-N ARQ Negative Acknowledgement : REJ. The receiver discards that frame and all future incoming frames until the frame in error is correctly received Frame 0 Frame 2 Frame 3 Frame 4 REJ 3 Frame 3 Frame 4 Data Link control 20

21 HDLC : Introduction Protocol in layer 2 of the OSI reference model developed by the (ISO). Goal: Error-free movement of data between network nodes, Issues:»flow control»ordering» error correction Data Link control 2

22 HDLC : a widespread protocol Main reasons for being widespread is that it supports: Half duplex and full duplex communication lines Point to point (peer to peer) and multi-point networks Switched or non-switched channels Data Link control 22

23 HDLC 3 types of stations : - Primary station : issues commands - Secondary station : issues responses - Combined station : issues both command and responses 2 link configurations Unbalanced : primary and or more secondary Balanced : 2 combined 3 data transfer modes Normal response Mode NRM : used with unbalanced mode, the primary polls the secondaries Asynchronous Balanced Mode ABM : any combined station may initiate transmission Asynchronous Response Mode Data Link control 23

24 3 types of stations () Three types of station are defined in HDLC: Primary Station» Control over the link, polls secondaries. Secondary Station» No control: command response stops command.etc. Combined Station» Able to send and receive commands and responses without any permission Data Link control 24

25 3 types of stations (2) Each secondary has one V(s) & one V(r) state variables. IMPORTANT, see later. Three V(s) & three V(r) state variables Data Link control 25

26 2 link configurations () Unbalanced Configuration A primary station and at least a secondary station Controlled/controlling configuration as described earlier. Can be used for full and half duplex(uni & bidirectional flow(simultaneous)). Allows both point to point networks and multipoint networks. Data Link control 26

27 2 link configurations (2) Asynchronous Response Mode(ARM) secondary station does not require permission from the primary to transmit. We do not need permission for frames to be transmitted we can reduce the overhead in the link. In h. duplex the secondary station must wait until it detects and idle channel before it can transfer any frames. Data Link control 27

28 Data transfer modes () Normal Response Mode(NRM) Unbalanced configuration(not possible with any other). Primary station initiates transfers to the secondary Secondary controlled by the primary and will only send responses when instructed to do so. Send if frame P= or UP frame; F= Data Link control 28

29 Data transfer modes (2) Asynchronous Response Mode(ARM) secondary station does not require permission from the primary to transmit. We do not need permission for frames to be transmitted we can reduce the overhead in the link. In h. duplex the secondary station must wait until it detects and idle channel before it can transfer any frames. Data Link control 29

30 Data transfer modes (3) Asynchronous Balanced Mode(ABM) Used with combined stations No permission grants are necessary Data Link control 30

31 Operational Modes Summary () HDLC Unbalanced Configuration Balanced Configuration.NRM 2.ARM. ABM Data Link control 3

32 Operational Modes Summary (2) The term asynchronous in ARM and ABM refers to the fact that any station can transfer frames without explicit permission. It does not refer to the format of the data in the link. Data Link control 32

33 Non Operational Modes () HDLC also defines three non-operational modes: Initialization Mode(IM) In the IM mode the secondary station's data link control program is in need of regeneration or it is in need of an exchange of parameters to be used in an operational mode. Data Link control 33

34 Non Operational Modes (2) The two disconnected modes(ndm and ADM) differ from the operational modes in that the secondary station is logically disconnected from the link (NB secondary station is not physically disconnected from the link!!!). IMP* No Info frames accepted Other frames accepted UP & UI, etc. Data Link control 34

35 Non Operational Modes (3) Asynchronous Disconnected Mode(ADM) Unbalanced and Balanced Config. ARM opportunity, if channel is idle it may transfer(h.d.) May ask for XID, DM, or Request IM Normal Disconnected Mode(NDM) Unbalanced Config. P= NRM Data Link control 35

36 HDLC general frame format Framing : a single flag field may be used as the closing flag for one frame and the opening flag for the next frame The flag is the already described 00 pattern with bit stuffing for transparency Trailer Header 6 bits 8 bits 8 bits 8 bits F FCS Payload (information) Control Address F Least significant bit first st bit sent Data Link control 36

37 Frame Fields Field Name Closing Flag ( F ) Frame Check Sequence( FCS ) Information ( I ) Control ( C ) Address ( A ), st bit tell us if +A. follows Flag ( F ) Size(in bits) 8 bits 6 or 32 bits Variable; Not used in some frames 8 or 6 bits 8 bits May be extended(n*7) 8 bits Data Link control 37

38 Transparency Transparency Property: it is able to carry any combination of bits as data without it being confused with control information. Accomplished with flags and bit stuffing Data Link control 38

39 Flags 00. Flags are continuously transmitted on the link between frames to keep the link active. (Synchronisation) Bit Stuffing Data Link control 39

40 Signals There are two sequences used in HDLC as signals for the stations on the link. Seven 's, but less than 5 signals an abort signal. The stations on the link know there is a problem on the link. 5 or more 's indicate that the channel is free. Data Link control 40

41 Transparency mechanism () It is logical to think that throughout transmission we are going to encounter bit sequences that are not flags with this same structure. HLDC uses a technique called bit-stuffing to differentiate this bit sequence from a flag field. Once the transmitter detects that it is sending 5 consecutive 's, it inserts a 0 bit to prevent a non-existent flag signal to be sent. Data Link control 4

42 Transparency mechanism (2) Data Link control 42

43 Address field Addressing : 8 bits. May be extended to multiples of 7 bits First bit (=) indicates if it is the last byte of the address field This field is not needed for point to point transmission (LAP B), but is always included for uniformity 0 0 Data Link control 43

44 Control field Frame differentiation : there are 3 types of frames distinguished by the format of the control field Information frames Supervisory frames Unnumbered frames Data Link control 44

45 Control Field Summary Information Transfer Commands - Information Information Transfer Responses - Information Supervisory Format Commands RR - Receive ready RNR - Receive not ready REJ - Reject (& N(R)) SREJ - Selective reject Supervisory Format Responses RR - Receive ready RNR - Receive not ready REJ - Reject S SREJ - Selective reject Data Link control 45

46 HDLC- Frame : Control Field Information Transfer Format 0 Supervisory Format 0 Receiver Ready, Receiver Not Ready Reject Selective Reject Data Link control 46

47 Information frames Bit has value 0 N(S) means Number Sent N(R) P N(S) 0 Data Link control 47

48 Supervisory frames Bit has value, bit 2 has value N(R) P/F RR : Receiver Ready N(R) P/F 0 0 RNR : Receiver Not Ready N(R) P/F 0 0 REJ : Reject Data Link control 48

49 Control Field, Supervisory frames RR and RNR are used in both NRM and ARM These frames are used both to indicate the willingness or otherwise of a secondary station to receive an information frame from the primary station, and for acknowledgment purposes. REJ and SREJ frames are used only in ABM permits simultaneous two-way communication across a point to point link. Frames are used to indicate to the other station that a sequence error has occurred, that is an information frame containing an out of sequence N(s) has been received. Data Link control 49

50 Unnumbered frames () Unnumbered Format: Perform link initialization, link disconnection and other link control functions. No Sequence Number 32 possible commands Data Link control 50

51 Unnumbered frames (2) Commands Unnumbered Format Commands SNRM & Extended SARM SABM DISC - Disconnect SIM - Set Initialization Mode UP - Unnumbered Poll (xchange of N(S)N (R) &add. UI - Unnumbered Information XID - Exchange identification RSET - Reset TEST -Test Responses Unnumbered Format Commands UA - Unnumbered Acknowledgment DM - Disconnected Mode RIM - Request Initialization Mode RD - Request Disconnect UI - Unnumbered Information XID - Exchange Identification FRMR - Frame Reject - Reports error that cannot be corrected by ret. TEST -Test Data Link control 5

52 Unnumbered frames (3) Bit has value, bit 2 has value P P F 0 0 SABM : Set Asynchronous Balanced Mode DISC : DISConnect UA : Unnumbered Acknowledgement Data Link control 52

53 Unnumbered frames (4) Bit has value, bit 2 has value F 0 FRMR: Frame Reject P DM : Disconnect Mode Data Link control 53

54 The Poll/Final Bit(P/F) () The Poll/Final Bit(P/F) Primary requires a status information provision from secondary. Secondary station responds to the P bit by transmitting a data or status frame to the primary station with the P/F bit set to F=. Data Link control 54

55 The Poll/Final Bit(P/F) (2) The Poll/Final Bit(P/F) 2 The secondary station wants to acknowledge the end of transmission under NRM i.e. final frame has been sent. Data Link control 55

56 HDLC- Frame: P/F bit Example Data Link control 56

57 HDLC- Frame : Address Field The Address Field Each station has a unique address. Unb.configuration, the Address field in both commands and responses refers to the secondary station. Bal.configuration, frame contains destination station address and sending station's address. Data Link control 57

58 HDLC- Frame : Information Field The Information Field This field is not always present in a HDLC frame. It is only present when the Information Transfer Format is being used in the control field. The information field contains the data the sender is transmitting to the receiver. Data Link control 58

59 HDLC- Frame : Frame Check Sequence The Frame Check Sequence Field This field contains a 6 bit, or 32 bit cyclic redundancy check. It is used for error detection. The CRC is calculated by performing a modulo 2 division of the data by a generator polynomial and recording the remainder after division. Polynomial used in HDLC is: CRC-6 = x6 + x5 + x2+ Data Link control 59

60 HDLC- Frame: FCS computation Data Link control 60

61 HDLC- State variables For the HDLC protocol it is necessary for each station to maintain an independent send state variable V(s) and receive state variable V(r). Data Link control 6

62 HDLC- Operations V(S)=Frame(I) frame being sent Each time a frame is sent this number shall be incremented by one. However, this number shall not exceed: V(s)-N(r)<Modulus : Window size N(R) of last received frame V(R)=Frame(I) next to receive The sequencing numbering scheme will give us the size of V(S)(8, 28). A sender will include a number N(S) in its sending frame which will be a copy of its V(S) state variable. V(R) shall be incremented when the expected frame(info) arrives. A sender will include a number N(R) in its sending frame which will be a copy of its V(R) state variable. Data Link control 62

63 HDLC- Example S tation A SABM P= Station B UA F= I N(S)=0 N(R)=0 I N(S)= N(R)=0 RR N(R)=2 I N(S)=2 N(R)=0 I N(S)=3 N(R)=0 I N(S)=4 N(R)=0 REJ N(R)=3 I N(S)=3 N(R)=0 I N(S)=4 N(R)=0 I N(S)=0 N(R)=4 I N(S)= N(R)=4 RR N(R)=2 DISC, P= UA, F= Data Link control 63

64 HDLC- Derived Protocols PPP LAPD LAPB LAPX SDLC (IBM) Data Link control 64

65 Data Link Control Glossary ACK : ACKnowledgement ARQ : Automatic Repeat request FCS : Frame Check Sequence N(R) : Next to receive RNR : Receiver Not Ready RR : Receiver Ready Data Link control 65

HDLC. King of the Link 2005/03/11. (C) Herbert Haas

HDLC. King of the Link 2005/03/11. (C) Herbert Haas HDLC King of the Link (C) Herbert Haas 2005/03/11 What is HDLC? High-Level Data Link Control Early link layer protocol Based on SDLC (Synchronous-DLC, IBM) Access control on half-duplex modem-lines Connectionoriented