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CS412 Introduction to Computer Networking & Telecommunication. Medium Access Control Sublayer. Topics. Introduction Channel Allocation Problem Multiple Access Protocols CDMA. Introduction. Broadcast networks Key issue: who gets to use the channel when there is competition Referred to as
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CS412 Introduction to Computer Networking & Telecommunication Medium Access Control Sublayer Chi-Cheng Lin, Winona State University
Topics • Introduction • Channel Allocation Problem • Multiple Access Protocols • CDMA
Introduction • Broadcast networks • Key issue: who gets to use the channel when there is competition • Referred to as • Multiaccess channels • Random access channels • MAC (Medium Access Control) sublayer • LANs • Satellite networks
Channel Allocation Problem • Channel Allocation • Static • Dynamic • Performance factors • Medium access delay • Time between a frame is ready and the frame can be transmitted • Throughput • #frames can be transmitted in unit time interval
Static Channel Allocation • FDM • Bandwidth divided into N equal sized portions for N users • Problems • #senders large • #senders continuously varies • bursty traffic • Discussion: #users > N ? < N ? = N ? • N times worse than all frames queued in one big queue
Static Channel Allocation • TDM • Each user is statically allocated every Nth time slot • Same problems as FDM • Under what circumstances are static channel allocation schemes efficient?
Dynamic Channel Allocation • Key assumptions 1. Station model • Independent • Work is generated constantly • One program per station • Station is blocked once a frame has been generated until the frame has been successfully transmitted 2. Single channel assumption
Dynamic Channel Allocation • Key assumptions 3. Collision Assumption • Collision: Two frames are transmitted simultaneously, overlapped in time and resulting signal garbled • Can be detected by all stations • No other errors
Dynamic Channel Allocation • Key assumptions 4. Time: either continuous or discrete (slotted) • Continuous • Frame transmission can begin at any instant • No "master clock" needed • Slotted • Time divided into discrete intervals (slots) • Frame transmissions begin at the start of a slot • #frames contained in a slot: 0 ? 1 ? >1 ?
Dynamic Channel Allocation • Carrier sense ("carrier" refers to electrical signal): either Y or N • Yes • A station can check channel before transmission • If busy, station idle • No • “Just do it" • Can tell if transmission successful later • LANs: carrier sense • Satellites -> not carrier sense (why?)
Multiple Access Protocols • ALOHA • Carrier sense multiple access protocols (CSMA) • CSMA w/ collision detection (CSMA/CD) • Collision-free protocols • Limited-contention protocols
ALOHA • Applicable to any contention system • System in which uncoordinated users are competing for the use of a single shared channel • Two versions • Pure ALOHA • Slotted ALOHA
Pure ALOHA • Let users transmit whenever they have data to be sent • Colliding frames are destroyed • Sender can always find out destroyed or not • Feedback (property of broadcasting) or ACK • LANs: immediately • Satellites: propagation delay (e.g., 270msec) • By listening to the channel • If frame is destroyed wait a random amount of time and retransmit (why "random"?)
Pure ALOHA Where are the collisions?
Pure ALOHA Frames are assumed to have the same size (same frame time) for analysis
Slotted ALOHA • Discrete time • Agreed slot boundaries • Synchronization needed • Performance • Which ALOHA has a shorter medium access delay? • Which ALOHA has a higher throughput?
Performance of ALOHA • Slotted ALOHA can double the throughput of pure ALOHA
Carrier Sense Multiple Access (CSMA) Protocols • Stations can listen to the channel (i.e., sense a carrier in the channel) • Types • 1-persistent CSMA • Nonpersistent CSMA • p-persistent CSMA
CSMA w/ Collision Detection (CSMA/CD) • Can listen to the channel and detect collision • Stop transmitting as soon as collision detected • Widely used on LANs (e.g., Ethernet) • Collision detection • Analog process • Special encoding is used
CSMA w/ Collision Detection (CSMA/CD) • Conceptual model • 3 states • Contention • Transmission • Idle • Minimum time to detect collision determines time slot • Depends on propagation delay of medium
Collision-Free Protocols • Model • N Stations: 0,1, ..., (N-1) • Question • Which station gets the channel after a successful transmission? • Protocols • Bit-map (i.e., reservation) protocol • Token passing protocol • Example: Token ring
Performance of Contention and Collision-Free Protocols • Contention • Low load => low medium access delay :) • High load => low channel efficiency :( • Collision-Free • Low load => high medium access delay :( • High load => high channel efficiency :)
Code Division Multiple Access (CDMA) • In FDMA, the bandwidth is divided into channels. • In TDMA, the bandwidth is just one channel that is timeshared. • In CDMA, one channel carries all transmissions simultaneously.
Figures 13.14 & 13.15Chip sequences and encoding rules The chip sequences must be orthogonal! AB = AC = AD = BC = BD = CD = 0 AA = BB = CC = DD = length of chip sequence
CDMA (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal