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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.
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CIS 725 Data Link 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 3-7 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
Alternating Bit Protocol - Premature timeouts 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
R: rbit = 0; do S ? (rbuff, b) if (b = rbit) then deliver(rbuff); rbit = ! rbit; S ! (ack, b) od
rbit sbit d,0 d,0 d,0 0 0 d,0 d,0 d,0 0 a,0 1 a,0 a,0 d,0 d,0 0 a,0 1 a,0 d,1
R: rbit = 0; do S ? (rbuff, b) if (b = rbit) then deliver(rbuff); rbit = ! rbit; S ! (ack, b) od
Alternating Bit Protocol - Premature timeouts 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 ; start timer ; start timer ; start timer ; cancel timer cancel timer
sbit d,0 d,0 FIFO channels 0 d,0 a,0 d,0 a,0 d,1 a,1 1 d,1 d,0 1 a,0 a,1 d,0
ABP with non-premature timeouts 0 4 -d0; ST timeout +d0 1 5 +a0 +a1 +d0 -a0 +d1 -a1 2 6 timeout -d1; ST +d1 3 7
Premature Timeouts +a1 0 4 -d0; ST timeout +d0 +a1 1 +d0 5 +a0 +a1 +d0 -a0 +d1 +a0 -a1 2 6 timeout -d1; ST +d1 +a0 3 7 +d1
Flow Control • Producer-consumer problem • Sliding window protocol - Go Back N - Selective retransmission
Go Back N S:: ns = 1; na = 1 do ns < na + W /\ avail(nl_buff) /\ sbuff[ns] = null sbuff[n] = nl_buff; R!(sbuff[ns],ns); ns++ [] sbuff[ns] != null R ! (sbuff[ns],ns); ns++ [] R ? ack(s) if s > na then na = s [] timeout ns = na od
R:: nr = 1 do S ? (D,x) if nr = x then deliver(D); nr++ S ! Ack(nr) od
Acks sent but not yet received nr na ns ns <= na + W outstanding data packets ns na 1 1 nr W = 3 d1 2 1 1 3 1 d2 2 d3 4 1 3 a2 a3 4 2 5 2 d4 d5