Ș.l. dr. ing. Lucian-Florentin Bărbulescu

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1 Ș.l. dr. ing. Lucian-Florentin Bărbulescu 1

2 Forward Error Control sufficient additional bits are added to each message for error detection locate the position of the error correction is achieved simply by inverting the bit(s) that have been identified as erroneous 2

3 Forward Error Control the number of added bits is larger that the number needed just for error detection often less efficient than Feedback Error Control efficient mainly for: entertainment applications (live transmission) long distance transmission (some satellite links) 3

4 Forward Error Control Several methods, the basic: Multiple copies of data Hamming codes 4

5 Forward Error Control Multiple copies of data Each bit is replicated several times. If one bit is in error, majority rule can be applied. Eg.: data: transmitted: if one bit in error:

6 Forward Error Control Hamming code Several parity bits are added to the message If one bit is in error then the parity bits can be used to detect the error and positon of the bit 6

7 Forward Error Control Hamming code Eg.: data: bits b12, b11, b10, b9, b7, b6, b5, b3 4 parity bits: c8 even parity between b12, b11, b10, b9 c4 even parity between b12, b7, b6, b5 c2 even parity between b11, b10, b7, b6, b3 c1 even parity between b11, b9, b7, b5, b3 sent data:

8 Forward Error Control Hamming code Eg.: if b9 is in error -> received data: c8 is received as 0 and computed as 1 -> Error (1) c4 is received as 1 and computed as 1 -> Ok (0) c2 is received as 1 and computed as 1 -> Ok (0) c1 is received as 0 and computed as 1 -> Error (1) 1001 = 9 -> position of bit in error 8

9 Feedback Error Control the frame contains only error detection information a short confirmation message (ACK) is sent back to confirm error-free transmission In case of an error: no ACK is sent another short message might be sent (NAK) a retransmission of the frame is performed the process is known as Automatic Repeat Request (ARQ) 9

10 Automatic Repeat Request Two variations of the scheme: Idle RQ (send and wait) Continuous RQ (with two retransmission methods) Selective retransmission (selective-reject) Only the missing frames are retransmitted Go-back N All frames starting with the missing frames are retransmitted 10

11 Idle RQ 11

12 Continuous RQ 12

13 Continuous RQ selective retransmission 13

14 Continuous RQ selective retransmission 14

15 Continuous RQ go-back N 15

16 Continuous RQ go-back N 16

17 Flow Control Ensure that the receiver is not overwhelmed with data by the sender Two mechanisms Stop and wait implemented by Idle RQ Sliding window 17

18 Sliding window The sender has a limit of frames that can be emitted before an acknowledge is received window size If window size is 1 -> Idle RQ The window size defines the required buffers 18

19 Sequence numbers The frames are identified by adding an integer identifier to them The number of required identifiers and thus of bits needed for them is dependent on the retransmission method used 19

20 Performance issues stop and wait without errors where UU = nn tt ffffffffff nn(2tt pppppppp + tt ffffffffff ) = t frame -> time to transmit a frame t prop -> propagation time (delay) tt ffffffffff 2tt pppppppp + tt ffffffffff aa = tt pppppppp tt ffffffffff UU = aa 20

21 Performance issues stop and wait without errors where aa = dd VV LL RR = dddd LLLL d -> distance between emitter and receiver V -> signal speed (close to the speed of light) L -> number of bits in frame R -> bit rate UU = dddd LLLL 21

22 Performance issues Error-Free Sliding-Window where W -> window size 1 WW 2aa + 1 UU = WW 2aa + 1 WW < 2aa

23 Performance issues stop and wait with errors UU = TT ff NN rr TT tt where T f = time for transmitter to emit a single frame T t = total time that line is engaged in the transmission of a single frame N r = the expected number of transmissions of a frame UU = 1 NN rr 1 + 2aa 23

24 Performance issues stop and wait with errors NN rr = iipp ii 1 1 PP ii=1 where P = probability of a frame error = 1 1 PP UU = 1 PP 1 + 2aa = 1 PP dddd LLLL 24

25 Performance issues sliding window with errors Selective retransmission Go-back N 1 PP WW 2aa + 1 UU = WW 1 PP 2aa + 1 WW < 2aa + 1 UU = 1 PP 1 + 2aaaa WW 1 PP (2aa + 1)(1 PP + WWWW) WW 2aa + 1 WW < 2aa

26 Performance issues sliding window with errors 26

10.1 REVIEW QUESTIONS

CHAPTER 10 Data Link Control 10.1 REVIEW QUESTIONS 1. Transmission means to put a signal on a line. Communication is a meaningful and orderly relationship between devices that send and receive data. 3.