Chapter 3
The Data Link Layer
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Design Issues
Controls communication between 2 machines directly connected
by “wire”-like link
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Services Provided to the Network Layer
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Framing: frame as an unit for physical layer to send in one go
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Error Control
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Flow Control, not sending faster than can be received
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Services Provided to Network Layer
Unacknowledged connectionless service.
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no recovering of lost or corrupted frame
• when the error rate is very low
• real-time traffic, like speech or video
Acknowledged connectionless service.
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returns information a frame has safely arrived.
• time-out, resend, frames received twice
• unreliable channels, such as wireless systems.
Acknowledged connection-oriented service.
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established connection before any data is sent.
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provides the network layer with a reliable bit stream.
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Framing, character count
Time gaps between frames are not suitable
•times are variable in networks
•received gaps can be smaller or larger
Need to be combined with other methods
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Framing, flag bytes
(a) A frame delimited by flag bytes.
(b) Four examples of byte sequences before and after stuffing.
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Framing, bit stuffing
Flag is 6 “1” bits
After 5 “1”’s a “0” is added
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Error-Detecting Codes, CRC
Cyclic redundancy Checks
related to polynomial theories
usually implemented in hardware
using 16 of 32 bit generators
-detect burst errors fewer than r+1 bits
-any odd number of bit errors
-up to an certain even number of errors
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Error-Correcting Codes
Use of a Hamming
code, adding extra bits,
to correct errors.
Error-correcting codes used in the early days, noisy lines
Later lines and electronics got better: error-detection and resend
Now used for wireless communication and high speed ethernet
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Protocols
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ACK (acknowledgements) for correct frames
possible NACK for lost or corrupted frames (piggybacked)
Pipelining
error correction by resend of lost or corrupted frames
Flow control
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Stop and Wait Protocols
A Simplex Protocol for a Noisy Channel
• sender waits for an ack for each send frame
• frames may be damaged or lost, same for acks
• no nacks for lost or error frames
• might be used to increase the data rate
• sender uses a timer and re-sends frame
• a 1-bit frame number (0 and 1) is needed for data and
ack frames, to separate a re-send frame from the
original one
• also called: alternating bit protocol
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Pipelining
sender
receiver
last bit transmitted, t = L / R
first packet bit arrives
RTT
last packet bit arrives, send ACK
ACK arrives, send next
packet, t = RTT + L / R
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Sender needs to buffer a send frame until its ack has
arrived
the buffer is a sliding window on all frames in the stream
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Go Back N
• receiver has no buffer
• it can only accept (and send to its higher layer) a correct frame
with the next expected sequence number
• sends an ack for that sequence number, this implies that all
previous frames have arrived
• otherwise it discards the frame and
• either sends nothing back or
• an ack for the last accepted frame
• or a nack for the next frame
• receiver timer to send ack (or nack) in separate frame
• do not wait too long for return frame to piggyback the ack
or nack
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Go Back N, sender
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buffer for a number of already send but not yet acked frames
a timer for the oldest frame in the buffer
resend in case of time-out, an ack for an already acked frame or a
nack
in case of a resend:
• all next already send frames must be resend
more than 1 bit for sequence numbers are needed:
• Nr_Buf <= Max_Seq+1
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Selective Repeat
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also receiver buffers frames:
• can store out-of-order correct frames
only 1 frame has to be resend in case of error or lost
nacks are usually used for error or lost frames
• just makes it faster
Nr_Buf <= (Max_Seq+1)/2 (0…Max_Seq)
sender needs timer of each send frame
receiver timer to send ack (or nack) in separate frame
• do not wait too long for return frame to piggyback
the ack or nack
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Selective Repeat buffers
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Go Back N vs Selective Repeat
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Protocol Verification
Modeling needed
• Finite State Machine Models
• Petri Net Models
Formal verification
• Uppaal
• PVS
2010
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