The document discusses specialized Medium Access Control (MAC) methods in wireless communication, focusing on the motivations for their use and various access methods including SDMA, FDMA, TDMA, and CDMA. It explains how these methods allow multiple users to share a communication medium using techniques based on space, frequency, time, and code, while addressing challenges such as interference and synchronization. The document also highlights specific protocols like CSMA and variations for managing channel access.

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UNIT-II
(Wireless) Medium Access
Control
Dr R Jegadeesan Prof-CSE
Jyothishmathi Institute of Technology and
Science, karimnagar

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Contents
1. Motivation for Specialized MAC
• Hidden and Exposed Terminals
• Far and Near Terminals
2. Access Methods
• SDMA
• FDMA
• TDMA
• CDMA

3. 1. Motivation for Specialized MAC
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MULTIPLEXING
• A fundamental mechanism in
communication system and networks
• Enables multiple users to share a medium
• For wireless communication, multiplexing
can be carried out in four dimensions:
space, time, frequency and code

7. Access Methods
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2. Space Division Multiple Access (SDMA)
• Channels are assigned on the basis of
―space‖(but operate on same frequency)
• The assignment makes sure that the
transmission do not interfere with each
(with a guard band in between)

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3. FDMA: Frequency Division Multiple Access
1. Frequency domain is subdivided into several non-
overlapping frequency bands
2. Each channel is assigned its own frequency band (with
guard spaces in between)

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3. TDMA : Time Division Medium Access
• A channel is given the whole bandwidth for a
certain amount of time
– All senders use the same frequency, but at different
point of time
• Synchronization can be done by using
– Fixed Allocation Scheme or
– Dynamic Allocation Scheme

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15. Reservation Algorithms
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16. Packet Reservation Multiple Access (PRMA)
/ Implicit Reservation
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• In this each terminal on the network is able to
monitor the status of the channel before
transmitting information
• Variations:
– 1-persistent CSMA
– non-persistent CSMA -
– p-persistent CSMA
– CSMA/CA
– Elimination yield – non-preemptive multiple access
(EY-NMPA)
Carrier Sense Multiple Access Protocols (CSMA)

18. Multiple Access with Collision Avoidance (MACA)
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20. POLLING
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21. Inhibit Sense Multiple Access (ISMA)
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4. CDMA: Code Division Multiple Access
◼ separation of channels achieved by assigning each
channel its own code
◼ guard spaces are realized by having distance in
code space (e.g. orthogonal codes)
◼ transmitter can transmit in the same frequency
band at the same time, but have to use different
code
◼ Provides good protection against interference and
tapping
◼ but the receivers have relatively high complexity.

23. Spread Aloha Multiple Access(SAMA)
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24. Comparisons of S/T/F/CDMA
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25. Thank you