DataLink

DataLink - 1 Data Link Layer Communication Networks © Kang...

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Unformatted text preview: 1 Data Link Layer Communication Networks © Kang Xi, Polytechnic Institute of NYU 2 Introduction Main Task of the data link layer: Provide error-free transmission over a link Ne twork Layer Da ta L ink Layer Phys ica l Layer Ne twork Layer Da ta L ink Layer Phys ica l 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? Communication Networks © Kang Xi, Polytechnic Institute of NYU 3 Communication Networks © Kang Xi, Polytechnic Institute of NYU 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 The ‘frame’ here is different from the concept of ‘frame’ in TDM. Example 01111110 8b its Address 8b its Con tro l 8b its Da ta >=0 Checksum 16b its 01111110 8b its Header T ra iler Communication Networks © Kang Xi, Polytechnic Institute of NYU 5 Framing Problem: Identify the beginning and the end of a frame in a bit stream One of the solutions (bit-oriented Framing): A special bit pattern (flag) signals the beginning and the end of a frame (e.g., "01111110") Problem: The sequence '01111110' must not appear in the data of the frame 01111110 01111110 Da ta Communication Networks © Kang Xi, Polytechnic Institute of NYU 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 11111 11111 100 O rig ina l b it sequence : A fter stu ffing b its a t sender: A fter stu ffing b its are removed by rece iver: 0110111111111111111100 S tu ffed b its Communication Networks © Kang Xi, Polytechnic Institute of NYU 7 Error Control Two basic approaches to handle bit errors: Error-correcting codes Used if retransmission of the data is not possible Data are encoded with sufficient redundancy to correct bit errors Examples : Hamming Codes, Reed Solomon Codes, etc. Error-detecting codes plus retransmission Used if retransmission of corrupted data is feasible Receiver detects error and requests retransmission of a frame. Communication Networks © Kang Xi, Polytechnic Institute of NYU 8 Error Detection Techniques Error Detection Techniques: Parity Checks Cyclic Redundancy Check Communication Networks © Kang Xi, Polytechnic Institute of NYU 9 Parity Check 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 ASCII "G" = 1 1 1 0 0 0 1 with even parity = 1110001 (total number of 1s is even) with odd parity = 1110001 1 (total number of 1s is odd) Communication Networks © Kang Xi, Polytechnic Institute of NYU...
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This note was uploaded on 04/06/2011 for the course EE 5363 taught by Professor Kang during the Spring '09 term at NYU Poly.

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DataLink - 1 Data Link Layer Communication Networks © Kang...

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