WK 4 copy of DataLink(2)

WK 4 copy of DataLink(2) - Click to edit Master subtitle...

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Unformatted text preview: Click to edit Master subtitle style 11 Data Link Layer EL536: Principles of Communication Networks Kang Xi, Polytechnic Institute of NYU Communication Networks Kang Xi, Polytechnic Institute of NYU 22 Introduction Main Task of the data link layer: n 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 EL536: Principles of Communication Networks Kang Xi, Polytechnic Institute of NYU Questions to Answer n How to identify the start/end of a frame? n How to detect transmission errors? n How to perform flow control? n How to perform error control? Communication Networks Kang Xi, Polytechnic Institute of NYU 33 EL536: Principles of Communication Networks Kang Xi, Polytechnic Institute of NYU Communication Networks Kang Xi, Polytechnic Institute of NYU 44 Introduction n 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 n The frame here is different from the concept of frame in TDM. n Example O bj1 0 1 EL536: Principles of Communication Networks Kang Xi, Polytechnic Institute of NYU Communication Networks Kang Xi, Polytechnic Institute of NYU 55 Framing n Problem: Identify the beginning and the end of a frame in a bit stream n One of the solutions (bit-oriented Framing): A special bit pattern (flag) signals the beginning and the end of a frame (e.g., "01111110") n Problem: The sequence '01111110' must not appear in the data of the frame O bj1 0 2 EL536: Principles of Communication Networks Kang Xi, Polytechnic Institute of NYU Communication Networks Kang Xi, Polytechnic Institute of NYU 66 Bit-Oriented Framing and Bit Stuffing n '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 EL536: Principles of Communication Networks Kang Xi, Polytechnic Institute of NYU Communication Networks Kang Xi, Polytechnic Institute of NYU 77 Error Control Two basic approaches to handle bit errors: Error-correcting codes n Used if retransmission of the data is not possible n Data are encoded with sufficient redundancy to correct bit errors n Examples : Hamming Codes, Reed Solomon Codes, etc. Error-detecting codes plus retransmission n Used if retransmission of corrupted data is feasible n Receiver detects error and requests retransmission of a frame. EL536: Principles of Communication Networks Kang Xi, Polytechnic Institute of NYU Communication Networks Kang Xi, Polytechnic Institute of NYU 88 Error Detection Techniques n Error Detection Techniques: Parity Checks Cyclic Redundancy Check EL536: Principles of Communication Networks Kang Xi,...
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WK 4 copy of DataLink(2) - Click to edit Master subtitle...

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