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Data Link Control Protocols Overview: Stop-and-Wait Performance Analysis

This overview explores Data Link Control Protocols focusing on the Stop-and-Wait scheme. Learn about flow control, frame synchronization, error control, and link utilization in data transmission. Understand the performance metrics, including propagation time, transmission time, and link utilization. Discover how to manage data exchange efficiently through sliding window flow control. Dive into error-free sliding window analysis and error control techniques like Automatic Repeat Request (ARQ).

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Data Link Control Protocols Overview: Stop-and-Wait Performance Analysis

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  1. Data and Computer Communications Data Link Control Protocols

  2. Data Link Control Protocols • Need a logical layer above physical layer • To manage exchange of data over a link • Frame synchronization • Flow control • Error control • Addressing • Control and data on same link • Link management

  3. Flow Control • Ensure sending entity does not overwhelm receiving entity • By preventing buffer overflow • Influenced by: • Transmission time • Time taken to emit all bits into medium • Propagation time • Time for a bit to traverse the link • Assume here no errors but varying delays

  4. Model of Frame Transmission

  5. Stop and Wait • Source transmits frame • Destination receives frame and replies with acknowledgement (ACK) • Source waits for ACK before sending next • Destination can stop flow by not sending ACK • Works well for a few large frames • Becomes inadequate if large block of data is split into small frames

  6. Bit length of a link: B = R x d/V where: B = Length of the link in bits; the no. of bits present on the link at an instance in time when a stream of bits fully occupies the link R = Data Rate of the link in bps d = Length of the link in meters V = Velocity of propagation Define: a = B/L (L = number of bits in the frame) a<1 : Propagation time < Transmission time a>1 : Propagation time > Transmission time

  7. Stop and Wait: Performance • Aim: Determine maximum potential efficiency of a half-duplex point-to-point line using the stop-and-wait scheme • Example: Long message sent as a sequence of frames F1, F2, …., Fn • Station S1 sends F1. • Station S2 sends an acknowledgment. • Station S1 sends F2. • Station S2 sends an acknowledgment. • Station S1 sends Fn. • Station S2 sends an acknowledgment.

  8. Stop and Wait: Performance • Total time to send the data, T, can be expressed as T = nTF, where TF is the time to send one frame and receive an acknowledgment. TF = tprop + tframe + tproc + tprop + tack + tproc Where: • tprop = propagation time from S1 to S2 • tframe = time to transmit a frame (time for the transmitter to send out all of the bits of the frame) • tproc = processing time at each station to react to an incoming event • tack = time to transmit an acknowledgment

  9. Stop and Wait: Performance • Assumptions: • Processing time is relatively negligible • Acknowledgment frame is very small compared to a data frame • We obtain: T = n (2tprop + tframe) • Only (n x tframe) is actually spent transmitting data and the rest is overhead. • The utilization, or efficiency, of the line is:

  10. Stop and Wait: Performance • Parameter a: a = tprop / tframe • Then: • This is the maximum possible utilization of the link. • Again, • Propagation time = distance d of the link divided by the velocity V of propagation • Transmission time = length of frame in bits, L, divided by the data rate R • Therefore,

  11. Stop and Wait: Link Utilization

  12. Sliding Window Flow Control • Allows multiple numbered frames to be in transit • Receiver has buffer of length W • Transmitter sends up to W frames without ACK • ACK includes sequence no. of next frame expected • Sequence number is bounded by size of field (k) • frames are numbered modulo 2k • giving max window size of up to 2k - 1 • Receiver can ACK frames without permitting further transmission (Receiver Not Ready) • Must send a normal ACK to resume • If the link is full-duplex, ACKs can be piggybacked

  13. Sliding Window Diagram

  14. Sliding Window Example

  15. Error Free Sliding Window: Performance • Station A begins to emit a sequence of frames at time t = 0. • The leading edge of the first frame reaches station B at t = a. • The first frame is entirely absorbed by t = a+1 • Assume: Negligible processing time, B can immediately acknowledge the first frame (ACK) • Assume: The acknowledgment frame is so small that transmission time is negligible • ACK reaches A at t = 2a+1

  16. Error Free Sliding Window: Performance • Case 1: W ≥ 2a + 1 • Acknowledgment for frame 1 reaches A before A has exhausted its window. • A can transmit continuously with no pause and normalized throughput is 1.0. • Case 2: W < 2a + 1 • A exhausts its window at t = W and cannot send additional frames until t = 2a+1 • Normalized throughput is W time units out of a period of (2a+1) time units

  17. Error Free Sliding Window:Link Utilization • Utilization is expressed as:

  18. Error Free Sliding Window:Link Utilization

  19. Error Free Sliding Window:Link Utilization

  20. Error Free Sliding Window:Link Utilization

  21. Error Control • Detection and correction of errors such as: • Lost frames • Damaged frames • Common techniques use: • Error detection • Positive acknowledgment • Retransmission after timeout • Negative acknowledgement & retransmission

  22. Automatic Repeat Request (ARQ) • ARQ – Collective name for such error control mechanisms. • Versions include: • Stop-and-wait ARQ • Go-back-N ARQ • Selective-reject ARQ

  23. Stop-and-Wait ARQ • Source transmits single frame • Waits for ACK • Damaged frame • Receiver rejects frame • Transmitter has timeout • If no ACK within timeout, retransmit • Damaged ACK • Transmitter does not recognize, retransmits • Receiver gets two copies of frame • Uses alternate numbering for ACKs to distinguish: ACK0 and ACK1

  24. Stop and Wait • Example: Both types of errors • Pros and cons • Simple • Inefficient

  25. Stop and Wait: Performance • With no errors, max. utilization = 1/(1+2a) • Aim: Determine utilization with possibility that frames are repeated due to bit errors. • For error-free operation using stop-and-wait ARQ, where, Tp is the propagation time

  26. Stop and Wait: Performance • If error occurs, where Nr is the expected no. of transmissions of a frame. • Since Tt / Tf = (1+2a), we obtain: • Probability that it will take exactly k attempts to transmit a frame successfully: Pk-1(1-P) • P is the probability that a single frame is in error

  27. Stop and Wait: Performance • Therefore, for Stop-and-Wait,

  28. Go-Back-N ARQ • Based on sliding window • If no error, ACK as usual • Use window to control number of outstanding frames • If error, reply with rejection • Discard that frame and all future frames until error frame received correctly • Transmitter must go back and retransmit that frame and all subsequent frames

  29. Go-Back-N ARQ – Details • Damaged Frame • Error in frame i so receiver rejects frame i • Transmitter retransmits frames from i • Lost Frame • Frame i lost and either • Transmitter sends i+1 andreceiver gets frame i+1 out of seq and rejects frame i • Or transmitter times out and send ACK with P bit set, which receiver responds to with ACK i • Transmitter then retransmits frames from i

  30. Go-Back-N ARQ – Details • Damaged Acknowledgement • Receiver gets frame i, sends ACK (i+1) which is lost • ACKs are cumulative, so next ACK (i+n) may arrive before transmitter times out on frame i • If transmitter times out, it sends ACK with P bit set • Can be repeated a number of times before a reset procedure is initiated • Damaged Rejection • Reject for damaged frame is lost • Handled as for lost frame when transmitter times out

  31. Go-Back-N ARQ: Performance • Each error generated a requirement to retransmit K frames rather than 1 frame. where f(i) is the total no. of frames transmitted if the original frame must be transmitted itimes

  32. Go-Back-N ARQ: Performance • K = (2a + 1) for W ≥ (2a + 1) • K = W for W < (2a + 1)

  33. Go-Back-N ARQ:Link Utilization

  34. Selective Reject ARQ • Also called selective retransmission • Only rejected frames are retransmitted • Subsequent frames are accepted by the receiver and buffered • Minimizes retransmission • Receiver must maintain large enough buffer • More complex logic in transmitter • Hence less widely used • Useful for satellite links with long propagation delays

  35. Go-Back-N vsSelective Reject

  36. Selective Reject: Link Utilization • Same reasoning as for Stop-and-Wait ARQ • Nr = 1 / (1 – P)

  37. Comparative Link Utilization ARQ Utilization as a function of a (P = 10-3)

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