This preview shows pages 1–11. Sign up to view the full content.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 2 Introduction Main Task of the data link layer: ± Provide error-free transmission over a link Network Layer Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer
Questions to Answer ± How to identify the start/end of a frame? ± How to detect transmission errors? ± How to perform flow control? ± How to perform error control? EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 3

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 4 Introduction ± The PDU at the Data Link Layer (DL-PDU) is typically called a Frame . A Frame has a header, a data field, and a trailer ± Example 01111110 8bits Address 8bits Control 8bits Data >=0 Checksum 16bits 01111110 8bits Header Trailer
EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 5 Framing ± Problem: Identify the beginning and the end of a frame in a bit stream ± Solution (bit-oriented Framing): A special bit pattern (flag) signals the beginning and the end of a frame (e.g., "01111110") ± Problem: ² The sequence '01111110' must not appear in the data of the frame 01111110 01111110 Data

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 6 Bit-Oriented Framing and Bit Stuffing ± 'Bit stuffing ' : If the sender detects five consecutive '1' it adds a '0' bit into the bit stream. The receiver removes the '0' from each occurrence of the sequence '111110' 0110111111111111111100 011011111 0 11111 0 11111 0 100 Original bit sequence: After stuffing bits at sender: After stuffing bits are removed by receiver: 0110111111111111111100 Stuffed bits
EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 7 Error Control Two basic approaches to handle bit errors: ± Error-correcting codes ² Used if retransmission of the data is not possible ² Data are encoded with sufficient redundancy to correct bit errors ² Examples : Hamming Codes, Reed Solomon Codes, etc. ± Error-detecting codes plus retransmission ² Used if retransmission of corrupted data is feasible ² Receiver detects error and requests retransmission of a frame.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 8 Error Detection Techniques ± Error Detection Techniques: ² Parity Checks ² Cyclic Redundancy Check
EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 9 Parity Checks General Method: ± Append a parity bit to the end of each character in a frame such that the total number of 1’s in a character is: ± even (even parity) or ± odd (odd parity) ± Example: With ASCII code, a parity bit can be attached to an 7-bit character ² ASCII "G" = 1 1 1 0 0 0 1 ² with even parity = 11100010 ² with odd parity = 11100011

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 10 Cyclic Redundancy Codes (CRC) General Method: ± The transmitter generates an n-bit check sequence number from a given k-bit frame such that the resulting (k+n)-bit frame is divisible by some number ± The receiver divides the incoming frame by the same number ± If the result of the division does not leave a remainder, the receiver assumes that there was no error
This is the end of the preview. Sign up to access the rest of the document.