2 ICS132-data-link-layer

2 ICS132-data-link-layer - Data Link Layer 1. 1 Data Link...

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ata Link Layer Data Link Layer
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ata Link Layer Functionality 1. Data Link Layer Functionality Recall: rame creation Frame creation Error detection and/or correction low control Flow control Creating the illusion of a reliable link
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odes of Operation 2. Modes of Operation Simplex alf Duplex Half Duplex Full Duplex
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1 implex Transmission 2.1 Simplex Transmission Data: one way only One-Way Street Data y { Link
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2 alf Duplex Transmission 2.2 Half Duplex Transmission Data: One direction at a time Sometimes Data { Link Sometimes
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3 ull Duplex Transmission 2.3 Full Duplex Transmission Data: Both ways simultaneously Forward Direction Data { Link Reverse Direction
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raming 3. Framing The data unit at the data link layer is the “frame” A frame is a group of bits, typically in equence sequence Issues: Frame creation Frame delineation
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1 rame Creation 3.1 Frame Creation Network Layer one-to-one mapping Data Link Layer control information network layer data unit frame boundaries
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rame Creation t d ) Frame Creation (cont’d) BAD Network Layer Data Link Layer
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2 rame Delineation 3.2 Frame Delineation How to tell when a new frame starts: haracter count Character count Frame tags with character stuffing rame tags with bit stuffing Frame tags with bit stuffing
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21 elineation b character co nt 3.2.1 Delineation by character count Character count lists the number of Control Field containing character count characters in the data field of the frame roblem: corrupted control fields Problem: corrupted control fields
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3.2.2 Frame tagging with character ffing stuffing rame tags se starting and ending characters (tags) to Frame tags Use starting and ending characters (tags) to mark boundaries of frame roblem: What if tag character occurs in the Problem: What if tag character occurs in the data or control portions of the frame?
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haracter stuffing Character stuffing Insert extra escape characters when a tag appears in data field STX DLE Start Tag ETX DLE g End Tag DLE DLE Character Stuffed DLE code
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haracter Stuffing Example Character Stuffing Example DLE I am a jerk trying to DLE crash your network! ETX DLE I am a jerk trying to DLE crash your network! ETX STX DLE DLE DLE ETX DLE Character Stuffing jy g y Character Unstuffing DLE I am a jerk trying to DLE crash your network! ETX
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23 rame tagging ith bit st ffing 3.2.3 Frame tagging with bit stuffing Bit strings may be used instead of character sequences to delineate frames ore efficient More efficient
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it stuffing Bit stuffing Each frame begins with a start and end bit sequence, e.g., 01111110 When sender’s data link layer sees five ’s in a row it stuffs a zero bit 1s in a row, it stuffs a zero bit The receiver “unstuffs” a zero after five onsecutive 1’s consecutive 1s.
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it Stuffing Example Bit Stuffing Example 00110010111111100010100 Bit Stuffing 0111111000110010111110110001010001111110 Bit Unstuffing
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rror Control 4. Error Control No physical link is perfect its will be corrupted Bits will be corrupted We can either: detect errors and request retransmission or correct errors without retransmission
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This note was uploaded on 03/24/2011 for the course CS 132 taught by Professor Suda,t during the Fall '08 term at UC Irvine.

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2 ICS132-data-link-layer - Data Link Layer 1. 1 Data Link...

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