UNIT 3 MULTIPLE ACCESS Adapted from lecture slides by Behrouz A. Forouzan © The McGraw-Hill Companies, Inc. All rights reserved

OUTLINE Multiple access mechanisms Random access Controlled access Channelization 2

SUBLAYERS OF DATA LINK LAYER 3

MULTIPLE ACCESS MECHANISMS 4

RANDOM ACCESS

Also called contention-based access No station is assigned to control another 6

ALOHA NETWORK 7

FRAMES IN PURE ALOHA 8

ALOHA PROTOCOL 9

EXAMPLE Calculate possible values of T B when stations on an ALOHA network are a maximum of 600 km apart T p = (600 × 10 3 ) / (3 × 10 8 ) = 2 ms When K=1, T B  {0ms,2ms} When K=2, T B  {0ms,2ms,4ms,6ms} : 10

ALOHA: VULNERABLE TIME 11

ALOHA: THROUGHPUT Assume number of stations trying to transmit follow Poisson Distribution The throughput for pure ALOHA is S = G × e −2G where G is the average number of frames requested per frame-time The maximum throughput  S max = when G= 1/2 12

EXAMPLE A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces  1000 frames per second  500 frames per second  250 frames per second 13

SLOTTED ALOHA 14

SLOTTED ALOHA: VULNERABLE TIME 15

SLOTTED ALOHA: THROUGHPUT The throughput for Slotted ALOHA is S = G × e −G where G is the average number of frames requested per frame-time The maximum throughput  S max = when G= 1 16

EXAMPLE A Slotted ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces  1000 frames per second  500 frames per second  250 frames per second 17

CSMA Carrier Sense Multiple Access  "Listen before talk" Reduce the possibility of collision  But cannot completely eliminate it 18

COLLISION IN CSMA 19 B C

CSMA: VULNERABLE TIME 20

PERSISTENCE METHODS What a station does when channel is idle or busy 21

PERSISTENCE METHODS 22

CSMA/CD Carrier Sense Multiple Access with Collision Detection Station monitors channel while sending a frame 23

ENERGY LEVELS 24

CSMA/CD: MINIMUM FRAME SIZE Each frame must be large enough for a sender to detect a collision Worst case scenario:  "A" is transmitting  "D" starts transmitting just before A's signal arrives 25 ABCD Long enough to hear colliding signal from D

EXAMPLE A CSMA/CD network has a bandwidth of 10 Mbps. If the maximum propagation time is 25.6 μs, what is the minimum size of the frame? 26

CSMA/CD: FLOW DIAGRAM 27

CSMA/CA Carrier Sense Multiple Access with Collision Avoidance Used in a network where collision cannot be detected  E.g., wireless LAN 28 IFS – Interframe Space

CSMA/CA: FLOW DIAGRAM 29 contention window size is 2 K -1 After each slot: - If idle, continue counting - If busy, stop counting

CONTROLLED ACCESS

CONTROL ACCESS A station must be authorized by someone (e.g., other stations) before transmitting Three common methods:  Reservation  Polling  Token passing 31

RESERVATION METHOD 32

POLLING METHOD 33

TOKEN PASSING 34

CHANNELIZATION

Similar to multiplexing Three schemes  Frequency-Division Multiple Access (FDMA)  Time-Division Multiple Access (TDMA)  Code-Division Multiple Access (CDMA) 36

FDMA 37

TDMA 38

CDMA One channel carries all transmissions at the same time Each channel is separated by code 39

CDMA: CHIP SEQUENCES Each station is assigned a unique chip sequence Chip sequences are orthogonal vectors  Inner product of any pair must be zero With N stations, sequences must have the following properties:  They are of length N  Their self inner product is always N 40

CDMA: BIT REPRESENTATION 41

TRANSMISSION IN CDMA 42

CDMA ENCODING 43

SIGNAL CREATED BY CDMA 44

CDMA DECODING 45

SEQUENCE GENERATION Common method: Walsh Table  Number of sequences is always a power of two 46