Data Link Layer Chapter 4

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 Outline 4Data Link Layer 4.1 Media Access Control 4.2 Error Control 4.3 Data Link Protocols 2 Copyright 2010 John Wiley & Sons, Inc

Network Layers 3 Computer 1Computer 2

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 4

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 5

4.2.3 Error Correction Once detected, the error must be corrected Error correction techniques  Retransmission (or, backward error correction)  Forward Error Correction 6

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

Stop and Wait ARQ SenderReceiver 8

Stop and Wait ARQ – Timeouts SenderReceiver 9

Stop and Wait ARQ – Timeouts SenderReceiver 10

Continuous ARQ 11

Flow Control with ARQ Ensuring that sender is not transmitting too quickly for the receiver  Stop-and-wait ARQ  Continuous ARQ 12

Flow Control Example 13

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 14

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 15

4.3.1 Asynchronous Transmission 16 Remember the Physical Layer…Bi-Polar Transmission

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 17

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

Ethernet II Frame 19

Point-to-Point Protocol (PPP) 20

ProtocolSizeError DetectionRetransmissionMedia Access Asynchronous Xmission1ParityContinuous ARQFull Duplex File Transfer Protocols XMODEM1328-bit ChecksumStop-and-wait ARQControlled Access XMODEM-CRC1328-bit CRCStop-and-wait ARQControlled Access XMODEM-1K10288-bit CRCStop-and-wait ARQControlled Access ZMODEM*32-bit CRCContinuous ARQControlled Access KERMIT*24-bit CRCContinuous ARQControlled Access Synchronous Protocols SDLC*16-bit CRCContinuous ARQControlled Access HDLC*16-bit CRCContinuous ARQControlled Access Token Ring*32-bit CRCStop-and wait ARQControlled Access Ethernet*32-bit CRCStop-and wait ARQContention SLIP*None Full Duplex PPP*16-bit CRCContinuous ARQFull Duplex * Varies depending on message length. 4.3 Data Link Protocol Summary 21

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.) 22

4.3 Transmission Efficiency Total number of info bits to be transmitted Total number of bits transmitted = 23

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 24

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 25

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 26

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/character M = 400 char/block R= 4.8 Kb/s C = 10 char/block P = 1% T = 25 ms 7(400-10)(1-0.01) (400/600)+0.025) = Kb/s TRIB = 27

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 28

Next Week: Start Chapter 5 – Network and Transport Layers 29