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 Data Link Layer 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)

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 Link Layer: Introduction 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” 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 Adapters Communicating link layer implemented in “ adapter ” (aka NIC) E.g. Ethernet card, PCMCIA card, 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 sending node frame rcving node packet frame adapter link layer protocol

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 (t frame ) Time taken to emit all bits into medium Propagation time (t prop ) Time for a bit to traverse the link Link utilisation is the fraction of time a link is used for data

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

19 Utilization: Examples Satellite Link: Propagation Delay, t prop = 270 ms Frame Size = 4000 bits Data rate = 56 kbps  t frame = 4/56 = 71 ms  = t prop /t frame = 270/71 = 3.8 U = 1/(2 +1) = 0.12 Short Link: 1 km, t prop = 5 s, Rate=10 Mbps, Frame size= 500 bytes  t frame = 4k/10M= 400 s  =t prop /t frame =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 2 k Max window size 2 k - 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 McGraw-Hill © The McGraw-Hill Companies, Inc., 2004 A burst error means that 2 or more bits in the data unit have changed.

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

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

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 The following shows the actual bits sent 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 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 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: However, it is hit by a burst noise of length 8, and some bits are corrupted When the receiver checks the parity bits, some of the bits do not follow the even-parity rule and the whole block is discarded McGraw-Hill © The McGraw-Hill Companies, Inc., 2004

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

44 Cyclic Redundancy Check goal: choose r CRC bits, R, such that exactly divisible by G (divisor) (modulo 2 arithmetic) receiver knows G, divides 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 Assume: D = (d bits) G = 1001 (r+1 bits) What is the CRC?  Transmitted block is: (d+r bits)  011 (r bits) Stallings Section 6.3 CRC detect all burst errors  r bits

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

51 Go Back N - Diagram

52 Selective Reject ARQ Only rejected frames are retransmitted Subsequent frames are accepted by the receiver and buffered Minimizes retransmission Receiver must maintain large enough buffer

53 Selective Reject - Diagram

54 High Level Data Link Control (HDLC) The most important and widely used DL protocol Also the basis for many other DL protocols

55 HDLC Frame Structure 3 types of frame has same frame format: Information frame (I-frame) – carries network layer data Flow and error control piggybacked on information frames Supervisory frame (S-Frame) - ARQ when piggyback not used (i.e. RR, RNR, REJ) Unnumbered frame (U-frame) - link control frames

56 Flag Fields Delimit frame at both ends Receiver hunts for flag sequence to synchronise (i.e. frame synchronisation) Bit stuffing used to avoid confusion with data containing 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, accepts as flag If sixth and seventh bits 1, sender is indicating abort

57 Bit Stuffing

58 Control Field Different for different frame type First one or two bits of control field identify frame type

59 Control Field

60 Information Field Only in I- and some U-frames Must contain integral number of octets Variable length

61 Frame Check Sequence (FCS) Field Error detection code (i.e. redundant bits) 16-bit CRC Optional 32-bit CRC

62 HDLC Operation Three phases Initialization (Connection setup) Data transfer Disconnect (Connection Termination)

63 Examples of Operation (1) U-frames I-frames S-frames

64 Examples of Operation (2)

65 Summary Flow control and error control through the same scheme HDLC a basis for many other data link layer protocols Read Stallings sec. 6.2, 6.3, 6.5 and chap 7 Next: LAN Overview