The Data Link Layer. Functions of the Data Link Layer Provide service interface to the network layer Dealing with transmission errors Error detection.

The Data Link Layer

Functions of the Data Link Layer

• Provide service interface to the network layer

• Dealing with transmission errors• Error detection and correction codes• Positive and negative acknowledgements• Timers

• Regulating data flow• Slow receivers not swamped by fast senders

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.

Data Link Layer Services

• Unacknowledged connectionless service

• Acknowledged connectionless service

• Acknowledged connection-oriented service

Frame Delimiting

• Character count

• Flag bytes with byte stuffing

• Starting and ending flags, with bit stuffing

• Physical layer coding violations.

Framing

A character stream. (a) Without errors. (b) With one error.

Framing

(a) A frame delimited by flag bytes.(b) Four examples of byte sequences before and after stuffing.

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.

Error Detection and Correction

• Error-Correcting Codes

• Error-Detecting Codes

Error Probability

)()1(

)()0(

1

0

YYNpXP

YYNpXPP

th

the

Hamming Distance

• Hamming distance between codewords X and

Y, is the number of ones in

• The number of detected error is d if d+1 is the minimum Hamming distance between two codes.

• The number of detected error is d if 2d+1 is the minimum Hamming distance between two codes.

YX

Error Correction

• Codeword Y is calculated from generation matrix G, and block of data X:

Y=XG=[x1,x2,…,xm]·G

• At the receiver side the syndrom is found that detects and correct an error using check parity matrix H:

S=YHT

Error correction

• For generation and parity check matrices it should hold

GHT=0• If

G=[In-k|P]

Then

H=[-PT|Ik]

• Here I is unity matrix, n is the codeword length, and n-k is the data block length

Error correction: Hamming Code

• In Hamming code the codeword length is 2m-1, the number of added bits is 2m-1-m-1 • Parity check matrix comprises all possible column vectors. For example for m=3

1010101

1100110

1111000

Hamming Code

Use of a Hamming code to correct burst errors.

Hamming Code: Exercise

• Decode the bit-stream coded by Hamming code: 111100011001110010110

Hamming Code: Solution

• Correct the bit-stream coded by Hamming code: 111100011001110010110

• Correct stream is 111000011001100010110

000

111

001

111

011

101

001

110

010

100

0110100

1110011

0001111

Hamming Code: Exercise

• Find the generation matrix for Hamming code (7,4), and encode bit stream 001011100.

Hamming Code: Solution

• Find the generation matrix for Hamming code (7,4), and encode bit stream 001011100.

0011001

0000111

0101010

1001011

G

Error Detection Code: CRC

• Data block and codeword represented by polynomials.

• If data block is X(x)=b0+b1x+…+bm-1xm-1, codeword is Y(x)=X(x)xk-mod(X(x), G(x)), where G(x) is a generator polynomial.

• At the receiver side, codeword polynomial is divided by G(x). If the reminder is non-zero, an error is detected.

Error-Detecting Codes

Calculation of the polynomial code checksum.

Elementary Data Link Protocols

• An Unrestricted Simplex Protocol

• A Simplex Stop-and-Wait Protocol

• A Simplex Protocol for a Noisy Channel

Protocol Definitions

Continued

Some definitions needed in the protocols to follow. These are located in the file protocol.h.

Protocol Definitions(ctd.)

Some definitions needed in the

protocols to follow. These are located in the file protocol.h.

Unrestricted Simplex Protocol

Simplex Stop-and-

Wait Protocol

A Simplex Protocol for a Noisy Channel

A positive acknowledgement

with retransmission protocol.

Continued

A Simplex Protocol for a Noisy Channel (ctd.)

A positive acknowledgement with retransmission protocol.

Sliding Window Protocols

• A One-Bit Sliding Window Protocol

• A Protocol Using Go Back N

• A Protocol Using Selective Repeat

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.

A One-Bit Sliding Window Protocol

Continued

A One-Bit Sliding Window Protocol (ctd.)

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.

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.

Sliding Window Protocol Using Go Back N

Continued

Sliding Window Protocol Using Go Back N

Continued

Sliding Window Protocol Using Go Back N

Continued

Sliding Window Protocol Using Go Back N

Sliding Window Protocol Using Go Back N (2)

Simulation of multiple timers in software.

A Sliding Window Protocol Using Selective Repeat

Continued

Continued

A Sliding Window Protocol Using Selective Repeat (2)

A Sliding Window Protocol Using Selective Repeat (3)

Continued

A Sliding Window Protocol Using Selective Repeat (4)

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.

Protocol Verification

• Finite State Machined Models

• Petri Net Models

Finite State Machined Models

(a) State diagram for protocol 3. (b) Transmissions.

Petri Net Models

A Petri net with two places and two transitions.

Petri Net Models (2)

A Petri net model for protocol 3.

Example Data Link Protocols

• HDLC – High-Level Data Link Control

• The Data Link Layer in the Internet

High-Level Data Link Control

Frame format for bit-oriented protocols.

High-Level Data Link Control (2)

Control field of

(a) An information frame.

(b) A supervisory frame.

(c) An unnumbered frame.

The Data Link Layer in the Internet

A home personal computer acting as an internet host.

PPP – Point to Point Protocol

The PPP full frame format for unnumbered mode operation.

PPP – Point to Point Protocol (2)

A simplified phase diagram for bring a line up and down.

PPP – Point to Point Protocol (3)

The LCP frame types.