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Data Communication and Networks

Data Communication and Networks. Lecture 4 Data Link Control (Part 2) September 29, 2005. Data Link Performance Issues. Performance is computed as a measure of the how efficiently a transmitter and receiver make use of the communications capacity of a give line (medium).

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Data Communication and Networks

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  1. Data Communication and Networks Lecture 4 Data Link Control (Part 2) September 29, 2005

  2. Data Link Performance Issues • Performance is computed as a measure of the how efficiently a transmitter and receiver make use of the communications capacity of a give line (medium). • We want to know how much of the potential capacity of the line a protocol can actually use. • This is called utilization, and it varies based on the flow control and error control mechanisms used. • First, let’s review these mechanisms.

  3. Stop and Wait • Source transmits frame • After reception, destination indicates willingness to accept another frame in acknowledgement • Source must wait for acknowledgement before sending another frame • 2 kinds of errors: • Damaged frame at destination • Damaged acknowledgement at source

  4. Figure 11.4

  5. Error-Free Stop and Wait T = Tframe + Tprop + Tproc + Tack + Tprop + Tproc Tframe = time to transmit frame Tprop = propagation time Tproc = processing time at station Tack = time to transmit ack Assume Tproc andTack relatively small

  6. Error-Free Stop and Wait (2) T ≈ Tframe + 2Tprop Throughput = 1/T = 1/(Tframe + 2Tprop) frames/sec Normalize by link data rate: 1/ Tframe frames/sec U = 1/(Tframe + 2Tprop) = Tframe = 1 1/ Tframe Tframe + 2Tprop 1 + 2a where a = Tprop / Tframe

  7. The Parameter a a = propagation time = d/V = Rd transmission time L/R VL where d = distance between stations V = velocity of signal propagation L = length of frame in bits R = data rate on link in bits per sec Rd/V ::= bit length of the link a ::= ratio of link bit length to the length of frame

  8. Stop-and-Wait Link Utilization • If Tprop large relative to Tframe then throughput reduced • If propagation delay is long relative to transmission time, line is mostly idle • Problem is only one frame in transit at a time • Stop-and-Wait rarely used because of inefficiency

  9. Error-Free Sliding Window ARQ • Case 1: W ≥ 2a + 1 Ack for frame 1 reaches A before A has exhausted its window • Case 2: W < 2a +1 A exhausts its window at t = W and cannot send additional frames until t = 2a + 1

  10. Figure 11.10

  11. Normalized Throughput 1 for W ≥ 2a + 1 U = W for W < 2a +1 2a + 1

  12. Stop-and-Wait ARQ with Errors P = probability a single frame is in error Nx = 1 1 - P = average number of times each frame must be transmitted due to errors U = 1 = 1 - P Nx (1 + 2a) (1 + 2a)

  13. Selective Reject ARQ 1 - P for W ≥ 2a + 1 U = W(1 - P) for W < 2a +1 2a + 1

  14. Go-Back-N ARQ 1 - P for W ≥ 2a + 1 U = 1 + 2aP W(1 - P) for W < 2a +1 (2a + 1)(1 – P + WP)

  15. High-Level Data Link Control • HDLC is the most important data link control protocol • Widely used which forms basis of other data link control protocols

  16. Frame Structure • Synchronous transmission • All transmissions in frames • Single frame format for all data and control exchanges

  17. Frame Structure Diagram

  18. Flag Fields • Delimit frame at both ends • 01111110 • 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

  19. Bit Stuffing • Example with possible errors

  20. Other DLC Protocols (LLC) • Logical Link Control (LLC) • IEEE 802 • Different frame format • Link control split between medium access layer (MAC) and LLC (on top of MAC) • No primary and secondary - all stations are peers • Two addresses needed • Sender and receiver • Error detection at MAC layer • 32 bit CRC • Destination and source access points (DSAP, SSAP)

  21. Other DLC Protocols (Frame Relay) (1) • Streamlined capability over high speed packet witched networks • Used in place of X.25 • Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF) • Two protocols • Control - similar to HDLC • Core - subset of control

  22. Other DLC Protocols (Frame Relay) (2) • ABM • 7-bit sequence numbers • 16 bit CRC • 2, 3 or 4 octet address field • Data link connection identifier (DLCI) • Identifies logical connection • More on frame relay later

  23. Other DLC Protocols (ATM) • Asynchronous Transfer Mode • Streamlined capability across high speed networks • Not HDLC based • Frame format called “cell” • Fixed 53 octet (424 bit) • Details later

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