Chapter 4 MAC Sublayer

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Chapter 4 MAC Sublayer

• By Dr.Ju Wang
• CMSC506, 2005

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What is MAC

• Network assumption Broadcast channel
• One channel, many stations
• Competition, interference among stations.
• MAC Medium Access Control
• Also known as Multiple-Access Control
• The protocol used to determine who goes next on a
  shared physical media
• Classification of MAC protocols
• Channel allocation (centralized)
• Contention based protocols (distributed)
• Contention free protocols (distributed)

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Static Channel Alloc

• FDMA
• The whole spectrum is divided into sub-frequency.
  
• TDMA
• Each user has its own time slot.
• CDMA
• Simultaneous transmission, Orthogonal code
• Analogy

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Contention Based Protocols ALOHA Illustration
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ALOHA

• Users send whenever they want to send. If it
  fails, wait random time and resend it.
• Independent stations
• Single channel assumption
• Collision occurs
• Types of ALOHA
• Pure ALOHA stations transmit at any time
  (Continuous time)
• Slotted ALOHA Transmission can only occur at
  certain time instances
• carrier sense vs no carrier sense

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Performance of Pure ALOHA Protocol

• How many transmission attempt per frame time?
• Represented by random variable K
• K Includes new frame transmission and some
  retransmissions.
• K is very difficult to analysis
• Simplified model K is a Poisson Variable
• Mean is G frames/time-slot

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Performance of Pure ALOHA Protocol (cont)
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Performance of Pure ALOHA Protocol (cont)

• Prob of zero transmission from t0 to t0t
• Vulnerable period is 2t from last slides
• Prob of successful send during t0 to t02t is
• Throughput

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Throughput for ALOHA