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CIS 725 Data Link Layer. Physical Layer Figure 3-1 B. Forouzan, TCP/IP Protocol Suite.

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Presentation on theme: "CIS 725 Data Link Layer. Physical Layer Figure 3-1 B. Forouzan, TCP/IP Protocol Suite."— Presentation transcript:

1 CIS 725 Data Link Layer

2 Physical Layer

3 Figure 3-1 B. Forouzan, TCP/IP Protocol Suite

4 Figure 3-2 B. Forouzan, TCP/IP Protocol Suite

5 Figure 3-3 B. Forouzan, TCP/IP Protocol Suite

6 Figure 3-5

7 Figure 3-7 B. Forouzan, TCP/IP Protocol Suite

8 Data Link Layer Physical layer provides unreliable bit stream Services provided by DLL: - framing - message corruption - message reordering - message duplication - message insertion

9 Functions of the Data Link Layer Relationship between packets and frames.

10 Services Provided to Network Layer (a) Virtual communication. (b) Actual communication.

11 Services Provided to Network Layer Placement of the data link protocol.

12 Unacknowledged connectionless service Acknowledged connectionless service Acknowledged connection oriented service

13 Framing Converting a stream to frames (1) bit-oriented (2) character-oriented

14 Framing Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after destuffing.

15 Framing (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

16 Corruption Error detection Error correction - forward error control - feedback error control

17 Error codes Parity bit Block codes Hamming distance

18

19

20

21 Data transmission protocol S: do avail(nl_buff)  sbuff = nl_buff; deliver sbuff to PL; od R: do avail(pl_buff)  rbuff = pl_buff; deliver rbuff to NL od

22 S: do avail(nl_buff)  sbuff = nl_buff; R ! sbuff od R: do S ? rbuff  deliver rbuff to NL od

23 Error detection protocol S: sent = false do !sent /\ avail(nl_buff)  sbuff = nl_buff; R ! Sbuff; sent = true [] R ? ack  sent = false [] R ? nack  R ! sbuff od

24 Error detection protocol R: do S ? rbuff  if error(rbuff) then S ! nack else deliver(rbuff); S ! ack od

25 Reordering Detection of reordering Correction

26 Reordering: sender S: ns = 1; do avail(nl_buff[ns])  sbuff[ns] = nl_buff[ns]; R ! (sbuff[ns],ns); ns++ od

27 Reordering: receiver R: nr = 1; do S ? (D,x)  rbuff[x] = D; Recd[x] = true [] Recd[nr]  deliver(rbuff[nr]); nr++ od

28 Message Loss Detection of message loss - timeout non-premature timeout: if a timeout occurs then message is indeed lost premature timeout: if a timeout occurs then a message might have been lost

29 Message Loss detection S: sbit = 0; sent = false do !sent /\ avail(nl_buff)  sbuff = nl_buff; R ! (sbuff, sbit); sent = true [] timeout  R ! (sbuff, sbit) [] R ? (ack,b)  if b = sbit then sent = false; sbit = !sbit od Alternating Bit Protocol - Premature timeouts

30 R: rbit = 0; do S ? (rbuff, b)  if (b = rbit) then deliver(rbuff); rbit = ! rbit; S ! (ack, b) od

31 d,0 a,0 d,1 d,0 a,0 rbit sbit 0 1

32 R: rbit = 0; do S ? (rbuff, b)  if (b = rbit) then deliver(rbuff); rbit = ! rbit; S ! (ack, b) od

33 Message Loss detection S: sbit = 0; sent = false do !sent /\ avail(nl_buff)  sbuff = nl_buff; R ! (sbuff, sbit); sent = true [] timeout  R ! (sbuff, sbit) [] R ? (ack,b)  if b = sbit then sent = false; sbit = !sbit od Alternating Bit Protocol - Premature timeouts ; start timer ; cancel timer ; start timer cancel timer

34 d,0 d,1 d,0 d,1 d,0 a,1 a,0 0 sbit 1 1 a,1 d,0 FIFO channels

35

36 timeout d0; ST 5 +d0 3 -d1; ST 7 +d1 +d0 2 +a0 6 -a0 +a1 -a1 +d1 ABP with non-premature timeouts

37

38 d0; ST +a0 -d1; ST +a1 timeout d0 +d1 +d0-a0 -a1 +d1 +a0 +a1 +d0 +d1 Premature Timeouts


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