CIS 725 Data Link Layer

Physical Layer

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

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

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

Figure 3-5

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

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

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

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

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

Unacknowledged connectionless service Acknowledged connectionless service Acknowledged connection oriented service

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

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.

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

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

Error codes Parity bit Block codes Hamming distance

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

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

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

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

Reordering Detection of reordering Correction

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

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

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

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

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

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

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

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

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

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

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