DataLink - Data Link Layer 1 Introduction Main Task of the...

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1 Data Link Layer
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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
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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
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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
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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
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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
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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.
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EL536: Principles of Communication Networks © Kang Xi, Polytechnic University 8 Error Detection Techniques ± Error Detection Techniques: ² Parity Checks ² Cyclic Redundancy Check
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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
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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
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This note was uploaded on 10/10/2011 for the course ECE 536 taught by Professor Sun during the Spring '11 term at NYU Poly.

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DataLink - Data Link Layer 1 Introduction Main Task of the...

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