The Data Link Layer - Asia University

The Data Link Layer

Chapter 3

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Data Link Layer Design Issues

? Network layer services ? Framing ? Error control ? Flow control

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Packets and Frames

Relationship between packets and frames.

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Network Layer Services

(a) Virtual communication. (b) Actual communication.

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Possible Services Offered

1. Unacknowledged connectionless service.

Ex: Ethernet Appropriate for low error rate and real-time traffic

2. Acknowledged connectionless service.

Ack/Timer/Resend Useful in unreliable channels, WiFi

3. Acknowledged connection-oriented service.

Guarantee frames are received exactly once and in the right order

Appropriate over long, unreliable links such as a satellite channel or a long-distance telephone circuit

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Framing Methods

1. Byte count: specify the number of bytes in the frame

2. Flag bytes with byte stuffing: Flag byte is used as both the starting and ending delimiter.

3. Flag bits with bit stuffing: Each frame begins and ends with a special bit pattern (ex:01111110)

4. Physical layer coding violations: use some reserved signals to indicate the start and end of frames.

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Framing (1)

Problem: the count can be garbled!! No way of telling where the next frame starts. Retransmission also does not help.

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

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Framing (2)

a) Problem: flag bytes may happens in the data, especially when binary data such as photographs or songs are transmitted.

? Solve by inserting a special escape byte before the flag byte in the data

b) Four examples of byte sequences before and after byte stuffing.

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Framing (3)

Stuff a 0 bit in the outgoing bit stream whenever it encounters five 1s in the data.

Bit stuffing. (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in the receiver's memory after

destuffing.

If the user data contain the flag pattern, 01111110, this flag is transmitted as 011111010. The side effect: the length of a frame depends on the contents of the data it carries.

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

Error Detection Codes (1)

1. Hamming codes. 2. Binary convolutional codes. 3. Reed-Solomon codes. 4. Low-Density Parity Check codes.

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, ? Pearson Education-Prentice Hall, 2011

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