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Data Link Layer

Data Link Layer. Lesson 6 NETS2150/2850. Position of the data-link layer. McGraw-Hill. The McGraw-Hill Companies, Inc., 2004. LLC and MAC sublayers. McGraw-Hill. The McGraw-Hill Companies, Inc., 2004. IEEE Standards for LANs. McGraw-Hill. The McGraw-Hill Companies, Inc., 2004.

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Data Link Layer

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  1. Data Link Layer Lesson 6 NETS2150/2850

  2. Position of the data-link layer McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  3. LLC and MAC sublayers McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  4. IEEE Standards for LANs McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  5. The principles behind data link layer services: error detection, correction reliable data transfer, flow control sharing a broadcast channel: multiple access (see later) link layer addressing (see later) The Data Link Layer

  6. Lesson Outline • Introduction • Flow control • Stop and wait • Sliding Window • Error Detection • Parity Check • CRC • Error Correction • Stop and wait ARQ • Sliding Window ARQ

  7. Some terminology: End systems and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links wireless links L2-PDU is a frame,encapsulates a packet “link” Link Layer: Introduction data-link layer has responsibility of transferring frame from one node to adjacent node over a link Adapted from Kurose & Ross book

  8. Link Layer Services • Framing, link access: • encapsulate packet into frame, adding header, trailer • control channel access if shared medium • ‘physical addresses’ used in frame headers to identify source, dest • different from IP address! • Reliable delivery between adjacent nodes • seldom used on low bit error link (fibre, some twisted pair) • crucial for wireless links: high error rates

  9. Link Layer Services (2) • Flow Control: • pacing between adjacent sending and receiving nodes • Error Detection: • errors caused by signal attenuation, noise • receiver detects presence of errors • Error Correction: • receiver identifies and corrects bit error(s) • Half-duplex and full-duplex transmission • with half duplex, nodes at both ends of link can transmit, but not at same time

  10. link layer implemented in “adapter” (aka NIC) E.g. Ethernet card, PCMCIA card, 802.11 card sending side: encapsulates packet in a frame adds error checking bits, flow control, etc. receiving side looks for errors, flow control, etc extracts packet, passes to rcving node adapter is semi-autonomous Contains link & physical layers frame frame Adapters Communicating packet rcving node link layer protocol sending node adapter adapter

  11. Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  12. Flow Control • Ensuring the sending entity does not overwhelm the receiving entity • Preventing buffer overflow • Two schemes: • Stop and Wait • Sliding Window

  13. Stop and Wait • Source transmits frame • Destination receives frame and replies with acknowledgement • Source waits for ACK before sending next frame • Destination can stop flow by not send ACK • Works well for a few large frames

  14. Normal operation McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  15. Stop-and-Wait, lost ACK frame McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  16. Numbering frames prevents the retaining of duplicate frames McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  17. Stop and Wait Utilisation • Transmission time (tframe) • Time taken to emit all bits into medium • Propagation time (tprop) • Time for a bit to traverse the link • Link utilisation is the fraction of time a link is used for data

  18. RTT Light in vacuum = 300 m/s Light in fibre = 200 m/s Electricity = 200 m/s Speed of Signals Stop and Wait Utilisation (2)

  19. Utilization: Examples • Satellite Link: Propagation Delay,tprop = 270 ms Frame Size = 4000 bits Data rate = 56 kbps  tframe = 4/56 = 71 ms  = tprop/tframe = 270/71 = 3.8 U = 1/(2 +1) = 0.12 • Short Link: 1 km, tprop= 5 s, Rate=10 Mbps,Frame size=500 bytes  tframe= 4k/10M= 400 s  =tprop/tframe=5/400=0.012 U=1/(2 +1)=0.98

  20. Fragmentation • Large block of data may be split into small frames. Why? • Limited buffer size at rcvr • Errors can be detected sooner • On error, only retransmission of smaller frames is needed • Prevents one station occupying medium for long periods • However, stop and wait is inadequate to cater for long-delay links!!

  21. Sliding Window Flow Control • Allow multiple frames to be in transit • Receiver has buffer W-frame long • Transmitter can send up to W frames without ACK • Each frame is numbered • ACK includes number of next frame expected • Sequence number bounded by size of sequence # field (k-bit) • Frames are numbered modulo 2k • Max window size 2k - 1

  22. Sender and Receiver windows Last frame acknowledged

  23. Example Sliding Window

  24. Sliding Window Enhancements • Receiver can acknowledge frames without permitting further transmission (using Receive Not Ready or RNR frame) • Must send a normal acknowledge to resume (Receive Ready or RR frame) • Use piggybacking • When need to ACK and send data, use same data frame to carry ACK number

  25. Sliding Window Utilisation

  26. Sliding Window Utilisation (2)

  27. Model of Frame Transmission

  28. Data can be corrupted during transmission. For reliable communication, errors must be detected and corrected.

  29. Types of Error Single-Bit Error Burst Error

  30. In a single-bit error, only one bit in the data unit has changed. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  31. A burst error means that 2 or more bits in the data unit have changed. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  32. Error Detection Methods Parity Check Cyclic Redundancy Check (CRC) Checksum (see later)

  33. Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  34. Detection Methods McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  35. In even parity check, a parity bit is added to every data unit so that the total number of 1s is even (or odd for odd-parity).

  36. Example 1 Suppose the sender wants to send the word world. In ASCII the five characters are coded as 1110111 1101111 1110010 1101100 1100100 The following shows the actual bits sent 11101110 11011110 11100100 11011000 11001001 McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  37. Example 2 Now suppose the word world in Example 1 is received by the receiver without being corrupted in transmission. 11101110 11011110 11100100 11011000 11001001 The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). The data are accepted. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  38. Example 3 Now suppose the word world in Example 1 is corrupted during transmission. 11111110 11011110 11101100 11011000 11001001 The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are corrupted, discards them, and asks for retransmission.

  39. Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd. Even number of bit errors goes undetected!

  40. In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the whole block.

  41. Two-dimensional parity (even) McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  42. Example 4 Suppose the following block is sent: 10101001 00111001 11011101 11100111 10101010 However, it is hit by a burst noise of length 8, and some bits are corrupted. 1010001110001001 11011101 11100111 10101010 When the receiver checks the parity bits, some of the bits do not follow the even-parity rule and the whole block is discarded. 10100011 10001001 11011101 11100111 10101010 McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  43. Cyclic Redundancy Check r bits r+1 bits r bits McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

  44. Cyclic Redundancy Check • goal: choose r CRC bits, R, such that • <D,R> exactly divisible by G (divisor) (modulo 2 arithmetic) • receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! • can detect all burst errors less than r+1 bits • widely used in practice (HDLC, ATM)

  45. CRC Example CRC detect all burst errors  r bits Assume: D = 101110 (d bits) G = 1001 (r+1 bits) What is the CRC? 011 (r bits)  Transmitted block is: 101110011 (d+r bits) Stallings Section 6.3

  46. Error Control • Deliver frames without error, in the proper order to network layer • Handles Lost frames • And damaged frames • An automatic repeat request scheme: • Error detection • Positive acknowledgment • Retransmission after timeout • Negative acknowledgement and retransmission

  47. Automatic Repeat Request (ARQ) • Stop and wait ARQ • Sliding window schemes • Go back N ARQ • Selective reject (selective repeat) ARQ

  48. Stop and Wait ARQ • Source transmits single frame • And waits for ACK • If received frame damaged, discard it • Transmitter has timer • If no ACK within timeout, retransmit • If ACK damaged,transmitter will not recognize it • Transmitter will retransmit • Receive gets two copies of frame • Use ACK0 and ACK1 to distinguish

  49. Stop and Wait -Diagram

  50. Go Back N ARQ • Based on sliding window • If no error, ACK as usual with next expected frame • If error, reply with a rejection frame • Rcvr discards that frame and all future frames until errored frame received correctly • Transmitter must go back and retransmit that frame and all subsequent frames

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