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Chapter 4 Data Link Layer
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Announcements and Outline
Review Assignment Due Thurs., 9/8 Assessment #1 – Tues., 9/13 MC Short Answer Career Day, Tues. 9/20 Register Outline Data Link Layer Media Access Control Error Control Data Link Protocols 2
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Network Layers Computer 1 Computer 2
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Data Link Layer - Introduction
Responsible for moving messages from one device to another Controls the way messages are sent on media Organizes physical layer bit streams into coherent messages for the network layer Major functions of a data link layer protocol Media Access Control Error Control Message Delineation Network Layer Data Link Layer Physical Layer 4
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Recap – Media Access Control and Error Detection
Contention Error Control Source of errors Error Detection Parity Checks CRC-16 & CRC-32 5
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Error Correction Once detected, the error must be corrected
Error correction techniques Retransmission (or, backward error correction) Forward Error Correction 6
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Automatic Repeat reQuest (ARQ)
Process of requesting a data transmission be resent Main ARQ protocols Stop and Wait ARQ (A half duplex technique) Continuous ARQ (A full duplex technique) 7
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Stop and Wait ARQ Sender Receiver 8
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Stop and Wait ARQ – Timeouts
Sender Receiver 9
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Stop and Wait ARQ – Timeouts
Sender Receiver 10
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Continuous ARQ 11
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Flow Control with ARQ Ensuring that sender is not transmitting too quickly for the receiver Stop-and-wait ARQ Continuous ARQ 12
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Flow Control Example 13
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Data Link Protocols Classification Differ by Asynchronous transmission
Message delineation Frame length Frame field structure frame k-1 frame k frame k+1 14
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Asynchronous File Transfer
Used on: Point-to-point asynchronous circuits Typically over phone lines via modem Computer to computer for transfer of data files Characteristics of file transfer protocols Designed to transmit error-free data Popular File transfer Protocols Xmodem, Zmodem, and Kermit 15
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Asynchronous Transmission
Remember the Physical Layer…Bi-Polar Transmission 16
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Synchronous Transmission
Data sent in a large block Includes addressing information Includes a series of synchronization (SYN) characters Used to help the receiver recognize incoming data Synchronous transmission protocols categories Bit-oriented protocols: SDLC, HDLC Byte-count protocols: Ethernet Byte-oriented protocols: PPP 17
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Ethernet (IEEE 802.3ac & II – Ethernet 2)
Most widely used LAN protocol, developed jointly by Digital, Intel, and Xerox, now an IEEE standard Uses contention based media access control Byte-count data link layer protocol No transparency problem uses a field containing the number of bytes (not flags) to delineate frames Error correction: optional 18
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Ethernet II Frame 19
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Data Link Protocol Summary
Data Link Protocol Summary Protocol Size Error Detection Retransmission Media Access Asynchronous Xmission 1 Parity Continuous ARQ Full Duplex File Transfer Protocols XMODEM 132 8-bit Checksum Stop-and-wait ARQ Controlled Access XMODEM-CRC 8-bit CRC XMODEM-1K 1028 ZMODEM * 32-bit CRC KERMIT 24-bit CRC Synchronous Protocols SDLC 16-bit CRC HDLC Token Ring Stop-and wait ARQ Ethernet Contention SLIP None PPP * Varies depending on message length. 20
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Transmission Efficiency
An objective of the network: Move as many bits as possible with minimum errors higher efficiency and lower cost Factors affecting network efficiency: Characteristics of circuit (error rate, speed) Speed of equipment, Error control techniques Protocol used Information bits (carrying user information) Overhead bits ( used for error checking, frame delimiting, etc.) 21
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Transmission Efficiency
Total number of info bits to be transmitted Total number of bits transmitted = 22
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Transmission Efficiency of Protocols
Async Transmission: Ethernet II Transmission However, large packets likely to have more errors and are more likely to require retransmission wasted capacity 23
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Optimum Packet Size Trade-off between packet size and throughput
Acceptable range (more costly in terms of circuit capacity to retransmit if there is an error) (less likely to contain errors) 24
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Throughput A more accurate definition of efficiency
Total number of information bits received per second; takes into account: Overhead bits (as in transmission efficiency) Need to retransmit packets containing errors Complex to calculate; depends on: Transmission efficiency Error rate Number of retransmission Transmission Rate of Information Bits (TRIB) Used as a measurement of throughput 25
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TRIB = Number of info bits accepted
Total time required to get the bits Average number of non-info characters per block Ex: K=7 bits/character M = 400 char/block R= 4.8 Kb/s C = 10 char/block P = 1% T = 25 ms Probability that a block will require retransmission Info bits per character K (M – C) (1 – P) (M / R) + T TRIB = Time between blocks (in seconds) (propagation time + turnaround time) (a.k.a., reclocking time) 7(400-10)(1-0.01) (400/600)+0.025) = Kb/s Packet length in characters TRIB = Data xmission rate in char per second 26
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Implications for Management
Provide a few, widely used data link layer protocols for all networks Minimize costly customization Minimize costly translation among many protocols Less training, simpler network management Bigger pool of available experts Less expensive, off-the-shelf equipment 27
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Next Week: Tuesday – Assessment #1 Start Chapter 5 – Network and Transport Layers 28
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