William Stallings Data and Computer Communications 7 th Edition

1. William StallingsData and Computer Communications7th Edition Chapter 7 Data Link Control Protocols

2. Data Link Control Protocol (DLCP) • DLCP is a layer of logic above the physical layer used to control the sending of data over a data communication link. • Requirements and objectives of data link control: • Frame synchronization: to recognize beginning and end of the frame. • Flow control: can't send frames faster than the receiver can handle. • Error control: correct data should be delivered. • Addressing: on multipoint line, identify the destination. • Control and data on same link. • Link management: initiation, maintenance, and termination of data exchange.

3. Flow Control • The receiver must do a certain amount of processing before passing the data to the higher-level software. • Flow control technique ensures that sending entity does not overwhelm the receiving entity with data • Preventing buffer overflow • Transmission time • Time taken to emit all bits of a frame onto the medium. • Proportional to the length of the frame. • Propagation time • Time for a bit to traverse the link between source and destination.

4. Model of Frame Transmission

5. Stop and Wait Flow Control • The simplest form of flow control, is stop-and-wait: • Source transmits frame • Destination receives frame and replies with acknowledgement signal. • Source waits for ACK before sending next frame. • If an error occurs, destination can stop flow by not send ACK signal. • Works well for a few large frames • Few waiting times for ACK.

6. Fragmentation • Large block of data may be split into small frames, due to: • Limited buffer size on the receiver. • With smaller frames, errors detected sooner (when whole frame received) • On error, retransmission of smaller frames is needed (better) • Prevents one station from occupying a shared medium for long periods • Stop and wait becomes inadequate, with the use of multiple frames for a single message. • Only one frame at a time can be in transit.

7. Link utilization • Bit length of a link: • B = number of bits present on the link when a stream of bits fully occupies the link. • R = data rate of the link, in bps. • d = length, or distance, of the link in meters. • V = velocity of propagation, in m/s. • In situations where the bit length of the link is greater that the frame, serious inefficiencies result.

8. Stop and Wait Link Utilization Figure 7.2 Stop-and-wait link utilization (transmission time = 1; propagation time = a)

9. Sliding-Window Flow Control • The essence of the problem in the stop-and-wait, is that only one frame at a time can be in transit. • Sliding-window allows multiple frames to be in transit at the same time. • Receiver has buffer (window) space for W frames. • Transmitter can send up to W frames without ACK. • To keep track of which frames have been acknowledged, each frame has a sequence number. • ACK includes sequence number of next frame expected • Sequence number bounded by size of field (k bits) • Range of sequence numbers is 0 through 2k – 1. • Window size is needed to be less than 2k.

10. Sliding Window Diagram

11. Example Sliding Window

12. Sliding Window Enhancements • Receiver can acknowledge frames without permitting further transmission Receive Not Ready (RNR) • Must send a normal acknowledge to resume • If two stations exchange data, each needs to maintain two windows, one for transmit and one for receive. • Each side needs to send the data and ACK to the other. • To provide efficient support, use piggybacking • Each data frame includes a field that holds a sequence number of the frame plus a sequence number used for ACK. • If no data to send, use acknowledgement frame • If data but no acknowledgement to send, send last acknowledgement number again.

13. Error Control • Error control refers to both functions of detection and correction of errors. • Possibility of two types of errors: • Lost frames: a frame fails to arrive to the destination. • Damaged frames: bit error. • Components of error control techniques: • Error detection • Positive acknowledgment for error-free frames. • Retransmission after timeout (in case of lost frames) • Negative acknowledgement and retransmission for frames received with errors

14. Error Detection • Additional bits added by transmitter for error detection code • Parity • Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones. • Even number of bit errors goes undetected • Cyclic Redundancy Check (CRC) • For a block of k bits transmitter generates n bit sequence, known as frame check sequence (FCS). • Transmit k+n bits which is exactly divisible by some number • Receiver divides frame by that number, if no remainder, assume no error

15. Cyclic Redundancy Check • T = n-bit frame to be transmitted. • D = k-bit block of data, or message, the first k bits of T. • F = (n-k)-bit FCS, the last (n-k) bits of T. • P = pattern of n-k+1 bits; this is the predetermined divisor. • We would like T/P to have no remainder. It should be clear that: T = 2n-kD+F

16. CRC Example • Given: • Message D = 1010001101 (10-bits) • Pattern P = 110101 (6-bits) • FCS R = to be calculated (5 bits) • Thus, n = 15, k = 10, and (n-k) = 5. • The message is multiplied by 25, yielding 101000110100000. • This product is divided by P, remainder R = 01110. • R is added to 25D to give T = 101000110101110, which is transmitted. • On the receiver, the received frame is divided by P, if R=0 then it is assumed that there have been no errors.

17. Error Control Mechanisms: Automatic Repeat Request (ARQ) • The effect of ARQ is to turn an unreliable data link into reliable one. • Three versions of ARQ have been standardized: • Stop-and-wait ARQ • Go-back-N ARQ • Selective-reject ARQ

18. Stop and Wait • Based on stop-and-wait flow control technique. • Source transmits single frame and then waits for ACK. • Tow sorts of errors could occur: • Frame that arrives at the destination could be damaged • Transmitter has timeout • If no ACK within timeout, retransmit • If ACK damaged, transmitter will not recognize it • Transmitter will retransmit • Receiver gets two copies of frame • To avoid duplication, frames are alternately labeled with 0 or 1. Use ACK0 and ACK1 • Advantage: simplicity. • Disadvantage: inefficient line use.

19. Stop and Wait Diagram

20. Go-Back-N ARQ • Based on sliding-window flow control. • The number of unacknowledged frames outstanding is determined by window size. • If no error, the destination will ACK incoming frames as usual with (RR = receive ready) • If the destination detects an error, send a negative ACK (REJ = rejection); explained as follows: • Discard that frame and all future frames until the error frame received correctly • When the transmitter receives a REJ, it must go back and retransmit that frame and all subsequent frames

21. Go-Back-N Contingencies:1. Damaged Frame • Frame received with error • Receiver detects an error in frame i • Receiver sends REJ i • Transmitter gets REJ i • Transmitter retransmits frame i and all subsequent • Lost Frame (same as Damaged REJ) • Frame i lost • Transmitter sends i+1 • Receiver gets frame i+1 out of sequence • Receiver send REJ i • Transmitter retransmits frame i and all subsequent

22. Go-Back-N - Lost Frame (Last frame) • Frame i lost and no additional frame sent (frame i last frame). • Receiver gets nothing and returns neither ACK nor REJ. • Transmitter times out and sends ACK frame with that includes a bit known as P bit set to 1. • Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i ). • Transmitter then retransmits frame i.

23. Go-Back-N Contingencies :2. Damaged RR • Receiver gets frame i and send RR (i+1) which is lost. • Acknowledgements are cumulative, e.g. RR 6 means that all frames through 5 are acknowledged. • Transmitter may receive a subsequent RR > RR (i+1) before the timer associated with frame i expires. • If transmitter times out, it sends acknowledgement with P bit set as before. • This can be repeated a number of times before a reset procedure is initiated.

24. Go-Back-N Diagram • In this example 3-bit sequence number field is used. • Maximum window size = 23-1 = 7.

25. Selective Reject • Also called selective retransmission • Only rejected (negative acknowledged SREJ) frames are retransmitted, or those that time out. • Subsequent frames are accepted by the receiver and buffered • Advantage: • More efficient than go-back-N because it minimizes retransmission. • Disadvantages: • Receiver must maintain large enough buffer • More complex logic to be able to send s frame out of sequence.

26. Selective RejectDiagram

27. High Level Data Link Control (HDLC) • The most important data link control protocol. • ISO 33009, ISO 4335 • It is widely used. • It is the basis for many other important data link control protocols. • To satisfy a variety of applications, HDLC defines three types of stations, two link configurations, and three data transfer modes of operation.

28. Basic characteristics of HDLC:1. Station Types • Primary station • Controls operation of link • Frames issued by primary station are called commands • Maintains separate logical link to each secondary station • Secondary station • Under control of primary station • Frames issued called responses • Combined station • May issue commands and responses

29. Basic characteristics of HDLC:2. HDLC Link Configurations • Unbalanced Configurations • Consists of one primary and one or more secondary stations • Supports full duplex and half duplex transmission • Balanced Configurations • Consists of two combined stations • Supports full duplex and half duplex

30. HDLC Data Transfer Modes (I) • Normal Response Mode (NRM) • Unbalanced configuration • Primary initiates data transfer to secondary • Secondary may only transmit data in response to command from primary • Used on multi-drop lines, where host computer as primary and terminals as secondary

31. HDLC Data Transfer Modes (II) • Asynchronous Balanced Mode (ABM) • Balanced configuration (two combined stations) • Either station may initiate transmission without receiving permission from the other combined station. • Most widely used; • It makes more efficient use of a full-duplex point-to-point link because there is no polling overhead

32. HDLC Data Transfer Modes (III) • Asynchronous Response Mode (ARM) • Unbalanced configuration • Secondary may initiate transmission without explicit permission from primary • Primary responsible for line • rarely used

33. HDLC Frame Structure • HDLC uses Synchronous transmission • All transmissions in frames • Single frame format for all types data and control exchanges

34. Frame Structure

35. Flag Fields • Flag fields delimit frame at both ends with a unique pattern 01111110. • A single flag may close one frame and open another • Receiver hunts for flag sequence to synchronize • Bit stuffing used to avoid confusion with data containing 01111110 • 0 inserted after every sequence of five 1s • If receiver detects five 1s it checks next bit • If 0, it is deleted • If 1 and seventh bit is 0, accept as flag • If sixth and seventh bits 1, sender is indicating abort

36. Bit Stuffing • Example with possible error • (a) bit stuffing • (b) 1-bit error splits it in two. • (c) 1-bit error merges two frames into one.

37. Address Field • Identifies secondary station that sent or will receive frame • Usually 8 bits long, but may be extended to multiples of 7 bits • LSB of each octet indicates that it is the last octet (1) or not (0) • All ones (11111111) is broadcast

38. Control Field • First one or two bits of control filed identify frame type • Can identify three types: • Information frames: data to be transmitted to user (next layer up) • Flow and error control are piggybacked on information frames • Supervisory frames: provide the ARQ mechanism when piggybacking not used. • Unnumbered frames: supplementary link control • P/F: • In command frames (P bit): set to 1 to solicit a response frame from a peer HDLC entity. • In response frames (F bits): set to 1 to indicate the response frame transmitted as a result of a soliciting command.

39. Control Field Diagram

40. Information and FCS Fields • Information field: • Present only in information and some unnumbered frames • Must contain an integral number of octets • Variable length up to some system-define maximum. • FCS field: • Error detection detecting code calculated from the remaining bits of the frame (exclusive of flags) • 16 bit CRC • Optional 32 bit CRC, depends to line reliability.

41. HDLC Operation • HDLC operation consists of the exchange of information, supervisory and unnumbered frames • The operation involves three phases • Initialization: one side initializes the data link to exchange frames in an orderly fashion. • Data transfer: exchange user data and control information. • Disconnect: termination of the operation.

42. Some HDLC commands and responses • Information (I) C/R • Supervisory (S) • Receive ready (RR) C/R • Receive not ready (RNR) C/R • Reject (REJ) C/R • Selective reject (SREJ) C/R • Unnumbered (U) • Set asynchronous balanced/extend mode (SABM/SABME) C • Disconnect (DISC) C • Unnumbered acknowledgment (UA) R

43. Examples of Operation (1)

44. Examples of Operation (2)