Chapter 3 : Medium Access Control

What is meant by transmission of signal? Medium Access is part of which layer in OSI?

Why is special MAC required? Hidden and Exposed Terminal Problem Near and Far Terminal Problem

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

Algorithms in TDMA

Fixed TDMA Time slots Fixed Delay Efficient for constant Data Rate Inefficient for bursty Data Rate

(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

(Contd.)

Slotted Aloha Time Slots Transmission at the start of the load No control Max. throughput for an 36% load No transmission guarantee

(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

(Contd.)

Demand Assigned Multiple Access Reservation + Fixed TDMA Reservation period+ Transmission period Satellite Transmission Reservation List Explicit Reservation Scheme

(Contd.)

Packet Reservation Multiple Access (PRMA) Similar DAMA Implicit reservation Collision during Reservation Ensures guaranteed data rate

(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.

(Contd.)

Multiple Access with Collision Avoidance Request to send Hidden and exposed terminals No fixed access patter and co-ordinated

(Contd.)

Polling Master station and slaves Round Robin, Random or reservation schemes

used for polling List of stations during contention phase

(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

(Contd.)

Code Division Multiple Access Orthogonal codes Protection against tapping Examples

(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

Thank-you