Prof.R.K.NADESH;SITE;VIT
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?
Prof.R.K.NADESH;SITE;VIT
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.
Prof.R.K.NADESH;SITE;VIT
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
Prof.R.K.NADESH;SITE;VIT
Contd..
• Two Different Directions;Separated by two different frequencies(FDD)
• UPLINK-890.2 & 915 MHZ
• DOWNLINK – 935.2 to 960 MHZ
• 124 Channels
Prof.R.K.NADESH;SITE;VIT
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
Prof.R.K.NADESH;SITE;VIT
Fixed TDMA
• Allocating Time Slots in a fixed pattern
(Fixed Bandwidth ;BS-MS)
Prof.R.K.NADESH;SITE;VIT
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
Prof.R.K.NADESH;SITE;VIT
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)
Prof.R.K.NADESH;SITE;VIT
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%)
Prof.R.K.NADESH;SITE;VIT
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.
Prof.R.K.NADESH;SITE;VIT
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
Prof.R.K.NADESH;SITE;VIT
PACKET RESERVATION MULTIPLE ACCESS(PRMA)PACKET RESERVATION MULTIPLE ACCESS(PRMA)DAMA with implicit reservationDAMA 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
Prof.R.K.NADESH;SITE;VIT
RESERVATION TDMAEach station has certain bandwidth and fixed delayEach station has certain bandwidth and fixed delay
Prof.R.K.NADESH;SITE;VIT
Multiple Access with Collision AvoidanceMultiple Access with Collision Avoidance
• Solves Hidden and Terminal Problem
Prof.R.K.NADESH;SITE;VIT
BASESTATION FOR COORDINATION
• POLLING
ONE STATION TO BE HEARED BY ALL STATIONS
• One Master with several slaves
• Round Robin & Randomly Accessed Polling
Prof.R.K.NADESH;SITE;VIT
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
Prof.R.K.NADESH;SITE;VIT
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
Prof.R.K.NADESH;SITE;VIT
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)
-6-6+12=0
Prof.R.K.NADESH;SITE;VIT
BASIC FUNCTION OF CDMA
• Two senders A & B want to send data
• CDMA assigns 2 unique orthogonal code
• Key Ak = 010011 Bk=110101
• Ad=>1; Bd=>0
• Assumptions
0 as -1 & 1 as 1
Prof.R.K.NADESH;SITE;VIT
CDMA-Spreading Signal
• Sender Spreading the Signal (Multiplication of data bits)
As =Ad*Ak
= 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)
Prof.R.K.NADESH;SITE;VIT
CDMA
• Both Signals are transmitted@same time,frequency
C= AS+BS
(-2,0,0,-2,2,0)Receiver wants A Code
:C*AK=(-2,0,0,-2,2,0)*(-1,1,-1,-1,1,1) = 2+0+0+2+2+0
=6 >0 ;Receiver detects as binary 1
Prof.R.K.NADESH;SITE;VIT
CDMA
• Receiver wants B Code
C*Bk=(-2,0,0,-2,2,0)*(1, 1, -1, 1, -1, 1)
= -2+0+0-2-2+0
= -6
<0 detects as ZERO