MEDIUM ACCESS CONTROL Wireless channel is a shared medium MAC coordinates transmission between users sharing the spectrum Goals: prevent collisions while maximizing throughput and minimizing delay Why Special MAC needed in Wireless Domain? Why not CSMA/CD?

HIDDEN & EXPOSED TERMINAL

NEAR AND FAR TERMINALS

MAC SCHEMES SDMA FDMA TDMA CDMA

Space Division Multiple Access Allocating Separate Space Eg.BS-MS User SDMA is always used in combinations Basis of SDMA – Cells,Sectorized Antennas which constitute the infrastructure implementing SDM.

Frequency Division Multiple Access Allocating Frequecies to Transmission Channel FIXED(Radion Station) /Dynamic (Demand Driven) FDMA+TDMA Hopping Pattern FDM-Simultaneous access to network Duplex Channel ;MS-BS;BS-MS

Contd.. Two Different Directions;Separated by two different frequencies(FDD) UPLINK & 915 MHZ DOWNLINK – to 960 MHZ 124 Channels

Contd…

Time Division Multiple Access Allocates certain time slot for communication Same frequency Synchronization must be done (s-r) Fixed and Dynamic -(WLC) Combined FDMA->Greater Flexibility & Transmission Capacity

Fixed TDMA Allocating Time Slots in a fixed pattern (Fixed Bandwidth ;BS-MS)

Contd.. Different slots for U/D Link in same frequency (TDD) Multiple Access & Duplex Channel(BS- MS) 1O ms the pattern is repeated. Each slot has the time duration of 417µs 64bit Duplex is inefficient for bursty data Too Static and too inflexible for data communication

Classical Aloha Scheme from University of HAWAII If TDM is applied without controlling access Each station can access medium at any time(Random Access Scheme-No Control)

Slotted Aloha Refinement by introducing Time Slots All Senders- Synchronized Transmission starts when each timeslot begins Still Access is not coordinated Suited for light loads(18-36%)

Carrier Sense Multiple Access Sensing the carrier and accessing the medium if it is idle Decreases the probability of Collision Hidden Terminals not detected Versions Non-Persistent CSMA Sense carrier & Starts Sending if it is idle otherwise repeat sensing after some time P-Persistent CSMA Sense the medium but only transmit with a probability of p.

Demand Assigned Multiple Access (Reservation Aloha) Reservation period followed by transmission period Eg.Satellite Systems ( Station on earth-Satellite ) Timeslot alloted successfully, no collision Satellite collects all requests & sends back reservation list indicating access rights EXPLICIT RESERVATION

PACKET RESERVATION MULTIPLE ACCESS(PRMA) DAMA with implicit reservation Implicit Reservation Certain slots form a frame Frame repeated in time BS broadcasts status of each slot All MS will know free & reserved slot

Each station has certain bandwidth and fixed delay RESERVATION TDMA Each station has certain bandwidth and fixed delay

Multiple Access with Collision Avoidance Solves Hidden Terminal Problem

MACA-Examples

BASESTATION FOR COORDINATION POLLING  ONE STATION TO BE HEARED BY ALL STATIONS One Master with several slaves Round Robin & Randomly Accessed Polling

Inhibit Sense Multiple Access Current state of the medium is signaled via a “busy tone” the base station signals on the downlink (base station to terminals) if the medium is free or not terminals must not send if the medium is busy terminals can access the medium as soon as the busy tone stops the base station signals collisions and successful transmissionsvia the busy tone and acknowledgements, respectively

Code Division Multiple Access- (CDMA) All terminals send on the same frequency probably at same time and can use the whole bandwidth of the transmission channel Uses codes to separate users and enable access to shared medium without interference

CDMA Orthogonal Codes 2 vectors are called orthogonal if the inner product is Zero (2,5,0) & (0,0,17) 2*0+5*0+0*17=0 (3,-2,4) &(-2,3,3) =0

BASIC FUNCTION OF CDMA Two senders A & B want to send data CDMA assigns 2 unique orthogonal code Key A k = B k = A d =>1; B d =>0 Assumptions 0 as -1 & 1 as 1

CDMA-Spreading Signal Sender Spreading the Signal (Multiplication of data bits) A s =A d *A k = 1* (-1,1,-1,-1,1,1) =(-1,1,-1,-1,1,1) Bs= Bd*Bk = -1 * (1, 1, -1, 1, -1, 1) = (-1,-1,1,-1,1,-1)

CDMA Both Signals are time,frequency C= A S +B S (-2,0,0,-2,2,0) Receiver wants A Code :C*AK=(-2,0,0,-2,2,0)*(-1,1,-1,-1,1,1) = =6 >0 ;Receiver detects as binary 1

CDMA Receiver wants B Code C*Bk=(-2,0,0,-2,2,0)*(1, 1, -1, 1, -1, 1) = = -6 <0 detects as ZERO

CDMA On Signal - I

CDMA On Signal - II

CDMA On Signal - III

CDMA On Signal - IV

CDMA On Signal - V

COMPARISON