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
HIDDEN & EXPOSED TERMINAL
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NEAR AND FAR TERMINALS
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MAC SCHEMES
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SDMA
FDMA
TDMA
CDMA
Prof.R.K.NADESH;SITE;VIT
Space Division Multiple Access
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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
Contd…
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(BSMS)
• 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)
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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)
DAMA with implicit reservation
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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 TDMA
Each station has certain bandwidth and fixed delay
Prof.R.K.NADESH;SITE;VIT
Multiple Access with Collision Avoidance
• Solves Hidden and Terminal Problem
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MACA-Examples
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BASESTATION FOR COORDINATION
• POLLING
ONE STATION TO BE HEARED
BY ALL STATIONS
• One Master with several slaves
• Round Robin & Randomly Accessed
Polling
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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
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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
Prof.R.K.NADESH;SITE;VIT
CDMA On Signal - I
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CDMA On Signal - II
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CDMA On Signal - III
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CDMA On Signal - IV
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CDMA On Signal - V
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COMPARISON
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