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
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
0
d,0
d,0
d,0
d,0
0
a,0
1
a,0
sbit
0
d,0
a,0
d,0
d,0
0
1
a,0
d,0
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 ; start timer
do
!sent /\ avail(nl_buff) 
sbuff = nl_buff; R ! (sbuff, sbit); sent = true ; start timer
[] timeout  R ! (sbuff, sbit) ; start timer
[] R ? (ack,b)  if b = sbit then
sent = false; sbit = !sbit ; cancel timer
cancel timer
od
d,0
sbit
0
d,0
a,0
d,1
a,1
d,0
d,0
a,0
1
d,0
d,1
1
a,0
a,1
d,0
FIFO channels
ABP with non-premature timeouts
0
4
-d0; ST
timeout
+d0
1
5
+a0
+a1
-a1
2
timeout
+d0
-a0
6
-d1; ST
3
+d1
7
+d1
0
+a1
Premature Timeouts
4
-d0; ST
timeout
1
+d0
+a1
+a0
+a1
+d0
+a0
2
timeout
-a1
-a0
+d1
6
-d1; ST
3
+d0
5
+d1
+a0
7
+d1