Data Link Layer of OSI Model responsibilities

Networks: Data Link Layer 1
Data Link Layer

Data Link Layer
• Provides a well-defined service interface to the
network layer.
• Determines how the bits of the physical layer
are grouped into frames (framing).
• Deals with transmission errors (CRC and ARQ).
• Regulates the flow of frames.
• Performs general link layer management.

3 2 1
1 2 3 2 1
1 2
2
1
Medium
1
2
Physical layer entity
Data link layer entity
3 Network layer entity
Physical
Layer
Data link
Layer
Physical
Layer
Data link
Layer
A B
A B
Packets Packets
Frames
(a)
(b)
Figure 5.2
Leon-Garcia & Widjaja: Communication Networks
Copyright ©2000 The McGraw Hill Companies

1 2 3 4 5
Data Data Data
ACK/NAK
Data
1 2 3 4 5
Data Data Data Data
ACK/
NAK
ACK/
NAK
ACK/
NAK
ACK/
NAK
Figure 5.7
End to End
Hop by Hop
Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies

Tanenbaum’s Data Link
Treatment
• Concerned with communication between two
adjacent nodes in the subnet (node to node).
• Assumptions:
– Bits delivered in the order sent.
– Rigid interface between the HOST and the node
 the communications policy and the Host
protocol (with OS effects) can evolve separately.
– uses a simplified model.

Host
A
Host
B
Layer 4
Node
2
Node
1
Layer 4
Layer 2
frame
Data Link Layer Model
Assume Host has infinite supply of messages.
Node constructs frame from a single packet message.
Checksum is automatically appended in the hardware.
Protocols are developed in increasing complexity to help
students understand the data link layer issues.

Packet
sequence
Error-free
packet
sequence
Information
frames
Control
frames
Transmitter Receiver
CRC
Information
packet
Header
Station A Station B
Information Frame
Control frame
CRC Header
Figure 5.8
Basic Elements of ARQ

Tanenbaum’s Protocol Definitions
Continued 
Figure 3-9. Some definitions needed in the protocols to
follow. These are located in the file protocol.h.

Protocol
Definitions
(continued.)
Figure 3-9. Some
definitions needed in
the protocols to follow.
These are located in
the file protocol.h.

ack seq kind
info
buffer
physical layer
network layer
data link layer
frame
packet

Figure 3-10.
Unrestricted
Simplex
Protocol

Figure 3-11.
Simplex
Stop-and-
Wait
Protocol

(a) Frame 1 lost
A
B
frame
0
frame
1
ACK
frame
1
ACK
time
Time-out
frame
2
(b) ACK lost
A
B
frame
0
frame
1
ACK
frame
1
ACK
time
Time-out
frame
2
ACK
In parts (a) and (b) transmitting station A acts the same way, but part (b)
receiving station B accepts frame 1 twice.
Figure 5.9
Ambiguities with Stop-and-Wait
[unnumbered frames]

Transmitter Receiver
Slast
Rnext
0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
(0,0) (0,1)
(1,0) (1,1)
Timer
Global State:
(Slast, Rnext)
Error-free frame 0
arrives at receiver
ACK for
frame 0
arrives at
transmitter
ACK for
frame 1
arrives at
transmitter Error-free frame 1
arrives at receiver
Station A Station B
Rnext
Slast
Figure 5.11
State Machine for Stop-and-Wait

#define MAX_SEQ 1
typedef enum {frame_arrival, cksum_err, timeout} event_type;
include “protocol.h”
void sender_par (void)
{
seq_nr next_frame_to_send;
frame s;
packet buffer;
event_type event;
next_frame_to_send = 0;
from_network_layer (&buffer);
while (true)
{ s.info = buffer;
s.seq = next_frame_to_send;
to_physical_layer (&s);
start_timer (s.seq);
wait_for_event(&event);
if (event == frame_arrival) {
from_network_layer (&buffer);
inc (next_frame_to_send);
}
}
}
Protocol 3
(PAR) Positive ACK
with Retransmission
[Old Tanenbaum Version]

void receiver_par (void)
{
seq_nr next_frame_to_send;
frame r, s;
event_type event;
frame_expected = 0;
while (true)
{ wait_for_event (&event);
if (event == frame_arrival)
{ from_physical_layer (&r);
if (r.seq == frame_expected) {
to_network_layer(&r.info);
inc (frame_expected);
}
to_physical_layer (&s); /* Note – no sequence number on ACK */
}
}
}
Protocol 3
(PAR) Positive ACK
with Retransmission
[Old Tanenbaum Version]

A
B
frame
0 frame
0
ACK
frame
1
ACK
time
time-out
frame
2
Transmitting station A misinterprets duplicate ACKs
Figure 5.10
PAR [OLD] problem
Ambiguities when ACKs are not numbered

Simplex
Protocol
for a
Noisy
Channel
Figure 3-12.A positive acknowledgement with retransmission protocol.
Continued 

A Simplex Protocol for a Noisy Channel
Figure 3-12.A positive acknowledgement with retransmission protocol.

Sliding Window Protocols
[Tanenbaum]
• Must be able to transmit data in both directions.
• Choices for utilization of the reverse channel:
– mix DATA frames with ACK frames.
– Piggyback the ACK
• Receiver waits for DATA traffic in the opposite direction.
• Use the ACK field in the frame header to send sequence
number of frame being ACKed.
–  better use of the channel capacity.

• ACKs introduce a new issue – how long does
receiver wait before sending ONLY an ACK
frame.
We need an ACKTimer!!
 sender timeout period needs to set longer.
• The protocol must deal with the premature
timeout problem and be “robust” under
pathological conditions.

Each outbound frame must contain a sequence number. With n
bits for the sequence number field, maxseq = 2n - 1 and the
numbers range from 0 to maxseq.
Sliding window :: sender has a window of frames and maintains a
list of consecutive sequence numbers for frames that it is
permitted to send without waiting for ACKs.
receiver has a window that is a list of frame sequence numbers it
is permitted to accept.
Note – sending and receiving windows do NOT have to be the
same size.
Windows can be fixed size or dynamically growing and shrinking.

• Host is oblivious, message order at transport
level is maintained.
sender’s window :: frames sent but not yet
ACKed.
– new packets from the Host cause the
upper edge inside sender window to be
incremented.
– ACKed frames from the receiver cause
the lower edge inside window to be
incremented.

• All frames in the sender’s window must be
saved for possible retransmission and we
need one timer per frame in the window.
• If the maximum sender window size is B,
the sender needs B buffers.
• If the sender window gets full (i.e., reaches
its maximum window size, the protocol
must shut off the Host (the network layer)
until buffers become available.

receiver window
– Frames received with sequence numbers
outside the receiver window are not accepted.
– The receiver window size is normally static.
The set of acceptable sequence numbers is
rotated as “acceptable” frames arrive.
a receiver window size = 1  the protocol
only accepts frames in order.
There is referred to as Go Back N.

Standard Ways to ACK
1. ACK sequence number indicates the last
frame successfully received.
- OR -
2. ACK sequence number indicates the next
frame the receiver expects to receive.
Both of these can be strictly individual ACKs
or represent cumulative ACKing.
Cumulative ACKing is the most common
technique.

A
B
fr
0
time
fr
1
fr
2
fr
3
fr
4
fr
5
fr
6
fr
3
A
C
K
1 error
Out-of-sequence frames
Go-Back-4: 4 frames are outstanding; so go back 4
fr
5
fr
6
fr
4
fr
7
fr
8
fr
9
A
C
K
2
A
C
K
3
A
C
K
4
A
C
K
5
A
C
K
6
A
C
K
7
A
C
K
8
A
C
K
9
Figure 5.13
Go Back N
ACKing next frame expected

A
B
fr
0
time
fr
1
fr
2
fr
3
fr
4
fr
5
fr
1
fr
2
A
C
K
1
error
Out-of-sequence
frames
Go-Back-7:
fr
4
fr
5
fr
3
fr
6
fr
7
fr
0
N
A
K
1
A
C
K
3
A
C
K
4
A
C
K
5
A
C
K
6
A
C
K
7
A
C
K
2
Transmitter goes back to frame 1
Figure 5.17
Go Back N
with NAK error recovery

A
B
fr
0
time
fr
1
fr
2
fr
3
fr
4
fr
5
fr
6
fr
2
A
C
K
1 error
fr
8
fr
9
fr
7
fr
10
fr
11
fr
12
A
C
K
2
N
A
K
2
A
C
K
7
A
C
K
8
A
C
K
9
A
C
K
1
0
A
C
K
1
1
A
C
K
1
2
A
C
K
2
A
C
K
2
A
C
K
2
Figure 5.21
Selective Repeat
with NAK error recovery

Data Link Layer of OSI Model responsibilities

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