data link layer chapter 4. announcements and outline announcements review assignment due next...
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Data Link Layer
Chapter 4
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Announcements and Outline
Announcements• Review Assignment Due next Thurs., 9/17• Assessment #1 – Tues., 9/22
• MC• Short Answer
• Career Day, Weds. 9/16• Register
Outline4 Data Link Layer
4.1 Media Access Control4.2 Error Control4.3 Data Link Protocols
2Copyright 2010 John Wiley & Sons, Inc
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Network Layers
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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
Data Link Layer
Physical Layer
Network Layer
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Recap – Media Access Control and Error Detection
Media Access Control• Control• Contention
Error Control• Source of errors
Error Detection• Parity Checks • CRC-16 & CRC-32
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4.2.3 Error Correction
Once detected, the error must be corrected
Error correction techniques
Retransmission (or, backward error correction)
Forward Error Correction
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4.2.3.1 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)
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4.2.3.1 Stop and Wait ARQ
Sender Receiver
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4.2.3.1 Stop and Wait ARQ – Timeouts
Sender Receiver
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4.2.3.1 Stop and Wait ARQ – Timeouts
Sender Receiver
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4.2.3.1 Continuous ARQ
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4.2.3.1 Flow Control with ARQ
Ensuring that sender is not transmitting too quickly for the receiver
Stop-and-wait ARQ
Continuous ARQ
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Flow Control Example
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4.3 Data Link Protocols
Classification Asynchronous transmission Synchronous transmission
Differ by Message delineation Frame length Frame field structure
frame k frame k+1frame k-1
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4.3.1 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
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4.3.1 Asynchronous Transmission
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Remember the Physical Layer…Bi-Polar Transmission
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4.3.2 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
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4.3.2.1 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
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4.3.2.1 Ethernet II Frame
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4.3.2.2 Point-to-Point Protocol (PPP)
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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 132 8-bit CRC Stop-and-wait ARQ Controlled Access
XMODEM-1K 1028 8-bit CRC Stop-and-wait ARQ Controlled Access
ZMODEM * 32-bit CRC Continuous ARQ Controlled Access
KERMIT * 24-bit CRC Continuous ARQ Controlled Access
Synchronous Protocols
SDLC * 16-bit CRC Continuous ARQ Controlled Access
HDLC * 16-bit CRC Continuous ARQ Controlled Access
Token Ring * 32-bit CRC Stop-and wait ARQ Controlled Access
Ethernet * 32-bit CRC Stop-and wait ARQ Contention
SLIP * None None Full Duplex
PPP * 16-bit CRC Continuous ARQ Full Duplex
* Varies depending on message length.
4.3 Data Link Protocol Summary
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4.3 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.)
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4.3 Transmission Efficiency
Total number of info bits to be transmittedTotal number of bits transmitted
=
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4.3 Optimum Packet Size
Trade-off between packet size and throughput
(more costly in terms of circuit capacity to retransmit if there is an error)(less likely to contain errors)
Acceptable range
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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
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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
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TRIB =
K (M – C) (1 – P)
(M / R) + T
Info bits per character
Average number of non-info
characters per block
Probability that a block will
require retransmission
Time between blocks (in seconds) (propagation time +
turnaround time) (a.k.a., reclocking
time)
Packet length in
characters
Data xmission
rate in char per second
Number of info bits accepted
Total time required to get the bits
TRIB =
Ex:K=7 bits/characterM = 400 char/blockR= 4.8 Kb/sC = 10 char/blockP = 1%T = 25 ms
7(400-10)(1-0.01)
(400/600)+0.025)
= 3.908 Kb/s
TRIB =
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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
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Next Week:
Start Chapter 5 – Network and Transport Layers
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