2. Data Link Layer Design Issues
• Services Provided to the Network Layer
• Framing
• Error Control
• Flow Control
3. Functions of the Data Link Layer
• Provide service interface to the network layer
• Dealing with transmission errors
• Regulating data flow
• Slow receivers not swamped by fast senders
4. Functions of the Data Link Layer (2)
Relationship between packets and frames.
5. Services Provided to Network Layer
(a) Virtual communication.
(b) Actual communication.
8. Framing (2)
(a) A frame delimited by flag bytes.
(b) Four examples of byte sequences before and after stuffing.
9. 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.
10. Error Detection and Correction
• Error-Correcting Codes
• Error-Detecting Codes
• Key idea: redundant (additional) bits
(use only 2m out of 2n code words)
• Key metrics:
• how many errors can be
corrected/detected;
• what burst length
11. Hamming Distance
• Some codes words are valid; others are invalid
• Hamming distance between two code words is number of bits
that must be flipped to change from one to the other
• If Hamming distance is d then d single bit errors needed to change one
word to the other
• Hamming distance of a code is the minimum Hamming
distance between two valid code words
• Detecting one single-bit error requires a distance 2 code; how
does this generalize?
• Correcting one single-bit error requires a distance 3 code; how
does this generalize?
12. Parity Schemes
• Parity bits: choose a rule
• Even parity – each codeword has even number of 1’s
• Odd parity – each codeword has odd number of 1’s
• Always transmit according to the rule
• On receipt, if rule is violated, word is invalid
• Can also do “vertical parity” over whole block to achieve
single-bit error correction
13. CRC Schemes
• CRC – Cyclic Redundancy Check or polynomial code
• Consider bits of a message to be coefficients of a polynomial
M(x)
• 1011 – 1x3 + 0x2 + 1x1 + 1x0
• Of course real messages will be much longer and hence of higher degree
• Agree on a small-degree generator polynomial G(x) of degree r
• Note: G(x) has r digits to the right of the leading 1, hence r+1 total
• Divide xrM(x) by G(x) using modulo 2 division (no carries or
borrows) getting the remainder polynomial R(x)
• Transmit T(x) = xrM(x) - R(x); note that this has remainder 0
when divided by G(x)
• Receiver rejects frame if the remainder it computers is not 0
15. CRC Properties
• Easily computed with feedback shift register hardware
• Detects any single-bit error
• Proper choice of G(x) gives detection of any two bit errors
• Proper choice of G(x) gives detection of any odd number of bit
errors
• Detects any burst error of length <= r; Why?
• Received message is T(x)+E(x) for some error polynomial E(x)
• If E(x) represents a burst of length <= r then it can be written as xi(F(x))
where the degree of F(x) is < r
• If G(x) has an x0 term it can’t divide xi and no degree r polynomial
divides a polynomial of degree < r
16. Elementary Data Link Protocols
• Important to read pp. 200-204 – set up the
coding conventions used in what follows
• An Unrestricted Simplex Protocol
• No buffer limits, no errors
• A Simplex Stop-and-Wait Protocol
• Add buffer limits
• A Simplex Protocol for a Noisy Channel
• Add channel errors
21. A Simplex Protocol for a Noisy Channel
A positive
acknowledgement
with retransmission
protocol.
Continued
22. A Simplex Protocol for a Noisy Channel (ctd.)
A positive acknowledgement with retransmission protocol.
23. Sliding Window Protocols
• Why?
• Efficiency – bandwidth*delay product
• Efficiency – when errors occur
• A One-Bit Sliding Window Protocol
• A Protocol Using Go Back N
• A Protocol Using Selective Repeat
24. Sliding Window Protocols (2)
A sliding window of size 1, with a 3-bit sequence number.
(a) Initially.
(b) After the first frame has been sent.
(c) After the first frame has been received.
(d) After the first acknowledgement has been received.
27. A One-Bit Sliding Window Protocol (2)
Two scenarios for protocol 4. (a) Normal case. (b) Abnormal case.
The notation is (seq, ack, packet number). An asterisk indicates
where a network layer accepts a packet.
28. A Protocol Using Go Back N
Pipelining and error recovery. Effect on an error when
(a) Receiver’s window size is 1.
(b) Receiver’s window size is large.
38. A Sliding Window Protocol Using Selective Repeat (5)
(a) Initial situation with a window size seven.
(b) After seven frames sent and received, but not acknowledged.
(c) Initial situation with a window size of four.
(d) After four frames sent and received, but not acknowledged.
