Transcript The Data Link Layer

Chapter 3
The Data Link Layer
A. S. Tanenbaum Computer Networks
W. Stallings Data and Computer Communications
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Hybrid Model
a)
The hybrid reference model to be used in this book.
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Data Link Layer Design Issues
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Services Provided to the Network Layer
Framing
Physical addressing
Error Control
Flow Control
Access control.
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Services Provided to Network Layer
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Unacknowledged connectionless service.
No acknowledgement, no logical connection, used in very low
error rate channel and real-time traffic.
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Acknowledged connectionless service.
each frame acknowledged, used in unreliable channel
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Acknowledged connection-oriented service
a connection established, frames are numbed.reliable transmission
guaranteed.
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Functions of the Data Link Layer
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For the data link layer, break the bit stream up into
discrete frames and computer the checksum for each
frame (error control).
Four methods are used to break the bit stream.
1.Character count. (rarely used)
2.Flag bytes with byte stuffing
3.Starting and ending flags, with bit stuffing.
4.Physical layer coding violations.
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Framing (packetizing)
A character stream. (a) Without errors. (b) With one error.
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Framing (2)
(a) A frame delimited by flag bytes.
(b) Four examples of byte sequences before and after stuffing. 8
Framing (3)
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.
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Flow Control
Ensuring the sending entity does not overwhelm the
receiving entity
Feedback-based flow control
Rate-based flow control (network layer)
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Stop and Wait
Only one frame at a time can
be in transit.
1. Source transmits frame
2. Destination receives frame
and replies with
acknowledgement (if it is
correct, otherwise send
NAK.)
3.Source waits for ACK before
sending next frame
4.Destination can stop flow by
not send ACK
5.Source can retransmit if got
a NAK.
6.Works well for a few large
frames
7.Inefficient line utilization
for
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small frames
Sliding Window Protocols
Allow multiple frames to be
in transit
• Receiver has buffer W
long
• Transmitter can send up
to W frames without ACK
• Each frame is numbered
• ACK includes number of
next frame expected
• Sequence number
bounded by size of field
(k)
• Frames are numbered
modulo 2k
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Example Sliding Window
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Sliding Window Protocols (2)
Duplex communication: each station needs to
maintain two windows
Piggyback: num of frame and num of ack are
included in one frame.
Sliding-window flow control is more efficient than
stop-and-wait flow control.
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Error Control
Detection and correction of errors
• Lost frames: a frame failed to arrived to the
other side.
• Damaged frames: some bits are in error.
Automatic repeat request
– Error detection
– Positive acknowledgment
– Retransmission after timeout
– Negative acknowledgement
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Error Detection and Error Correction
a) Additional bits added by transmitter for error
detection and/or error correction code
b) Error detection is more often used in
protocol with ARQ.
High code rate, high reliability.
c) Error correction is used in specific situation.
(no reverse channel)
d) Hybrid ARQ: combination of error detection
and correction.
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Error control Scheme
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Stop-and –wait ARQ
1. Source transmits single
frame
2. Wait for ACK
3. If received frame
damaged, discard it
4. Transmitter has timeout
5. If no ACK within timeout,
retransmit
6. If ACK damaged,
transmitter will not
recognise it, Transmitter
will retransmit
7. Receive gets two copies
of frame
8. Use ACK0 and ACK1
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9. Simple but Inefficient
Go Back N
a)
b)
c)
d)
Based on sliding window
If no error, ACK as usual with next frame expected
Use window to control number of outstanding frames
If error, reply with rejection
– Discard that frame and all future frames until error
frame received correctly
– Transmitter must go back and retransmit that frame and
all subsequent frames
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Selective Reject (retransmission)
a) Only rejected frames are retransmitted
b) Subsequent frames are accepted by the receiver
and buffered
c) Minimizes retransmission
d) Receiver must maintain large enough buffer to
reordering.
e) More complex login in transmitter
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Example Data Link Protocols
• HDLC – High-Level Data Link Control
• The Data Link Layer in the Internet
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High-Level Data Link Control
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Frame format for bit-oriented protocols.
High-Level Data Link Control (2)
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The Data Link Layer in the Internet
PPP (Point-to-point protocol) handles error detection,
supports multiple protocols, allows IP addresses to be
negotiated at connection time.
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PPP – Point to Point Protocol
The PPP full frame format for unnumbered mode operation.
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Performance of ARQ
Stop-and-Wait Flow Control
The total time to send the data as
T = n(2tprop + tframe)
The utilization, or efficiency, of the line is
u
n  t frame
n(2t prop  t frame)
It is useful to define the parameter a = tprop/tframe .
Then
1
u
1  2a
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Error-free sliding-window flow control
1

u W
1  2a
W  1  2a
W  1  2a
where W is window size
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Frames are generated at node A and sent to node C through node B.
Determine the minimum transmission rate required between nodes B and
C so that the buffers of node B are not flooded, based on the following:
•The data rate between A and B is 100 kbps.
•The propagation delay is 5 μs/km for both lines.
•There are full-duplex lines between the nodes.
•All data frames are 1000 bits long; ACK frames are separate frames of
negligible length.
•Between A and B, a sliding-window protocol with a window size of 3 is
used. Between B and C, stop-and-wait is used.
•There are no errors.
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