Chapter 4 Data Link Layer

Announcements and Outline Review Assignment Due Weds., 2/1 (Before Class!) Assessment #1 – Mon., 2/6 MC Short Answer Outline Data Link Layer Media Access Control Error Control Data Link Protocols http://gizmodo.com/hackers-locked-every-room-in-this-hotel-until-a-bitcoin-1791769502 2

Network Layers Computer 1 Computer 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 Network Layer Data Link Layer Physical Layer 4

Recap – Media Access Control and Error Detection Contention Error Control Source of errors Error Detection Parity Checks CRC-16 & CRC-32 5

Recap – Media Access Control and Error Detection 6

Recap – Media Access Control and Error Detection 7

Error Correction Once detected, the error must be corrected Error correction techniques Retransmission (or, backward error correction) Forward Error Correction 8

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) 9

Stop and Wait ARQ Sender Receiver 10

Stop and Wait ARQ – Timeouts Sender Receiver 11

Continuous ARQ 12

Flow Control with ARQ Ensuring that sender is not transmitting too quickly for the receiver Stop-and-wait ARQ Continuous ARQ 13

Flow Control Example 14

Data Link Protocols Classification Differ by Asynchronous transmission Message delineation Frame length Frame field structure frame k-1 frame k frame k+1 15

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 16

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 17

Ethernet II Frame 18

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.) 19

Transmission Efficiency Total number of info bits to be transmitted Total number of bits transmitted = 20

Transmission Efficiency of Protocols Ethernet II Transmission However, large packets likely to have more errors and are more likely to require retransmission  wasted capacity 21

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) 22

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 23

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) = 3.908 Kb/s Packet length in characters TRIB = Data xmission rate in char per second 24

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 25

Next Week: Monday – Assessment #1 Start Chapter 5 – Network and Transport Layers 26