3.medium access control
DESCRIPTION
TRANSCRIPT
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Chapter 3 : Medium Access Control
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What is meant by transmission of signal? Medium Access is part of which layer in OSI?
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Why is special MAC required? Hidden and Exposed Terminal Problem Near and Far Terminal Problem
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Multiple Access Techniques
Space Division Multiple Access:- used for allocating space to users
Frequency Division Multiple Access:- comprises all algorithms allocating frequencies to transmission channel according to FDM
Time Division Multiple Access:- comprises technologies which allocate time slots for communication i.e., controlling TDM
FDMA + TDMA
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Algorithms in TDMA
Fixed TDMA Time slots Fixed Delay Efficient for constant Data Rate Inefficient for bursty Data Rate
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(Contd.)
Classical Aloha TDM without control Invented at Hawaii University Neither co-ordinates medium access nor resolve contention No central arbiter and no co-ordination Simultaneous transmission leads to collision Light load Max. throughput for an 18% load
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(Contd.)
Slotted Aloha Time Slots Transmission at the start of the load No control Max. throughput for an 36% load No transmission guarantee
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(Contd.)
Carrier Sense Multiple Access (CSMA) Sensing carrier to avoid collision Hidden Terminals Problem Back-off algorithms Types
Non-Persistent CSMA 1-Persistent CSMA p-Persistent CSMA CSMA/CA EY-NMPA
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(Contd.)
Demand Assigned Multiple Access Reservation + Fixed TDMA Reservation period+ Transmission period Satellite Transmission Reservation List Explicit Reservation Scheme
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(Contd.)
Packet Reservation Multiple Access (PRMA) Similar DAMA Implicit reservation Collision during Reservation Ensures guaranteed data rate
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(Contd.)
Reservation TDMA N mini-slots for reservation N*k data slots Guarantees min. bandwidth and delay Free slots allocates using round-robin or
uncoordinated.
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(Contd.)
Multiple Access with Collision Avoidance Request to send Hidden and exposed terminals No fixed access patter and co-ordinated
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(Contd.)
Polling Master station and slaves Round Robin, Random or reservation schemes
used for polling List of stations during contention phase
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(Contd.)
Inhibit sense multiple access (Digital sense multiple access) Base station signals busy medium Base station acknowledges successful transmission Collision is detected by absence of positive acknowledge
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(Contd.)
Code Division Multiple Access Orthogonal codes Protection against tapping Examples
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(Contd.)
Spread Aloha Multiple Access Aloha + CDMA Same orthogonal key High power, short period Phase difference Synchronization Max. throughput for 18% load Includes advantages of spread spectrum
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Thank-you