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Chapter 4 MAC Sublayer

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Chapter 4 MAC Sublayer

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  1. Chapter 4 MAC Sublayer By Dr.Ju Wang CMSC506, 2005

  2. 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)

  3. 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:

  4. Contention Based Protocols: ALOHA Illustration

  5. 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

  6. 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

  7. Performance of Pure ALOHA Protocol (cont)

  8. Performance of Pure ALOHA Protocol (cont) • Prob of zero transmission from t0 to t0+t • Vulnerable period is 2*t from last slides • Prob of successful send during t0 to t0+2t is • Throughput

  9. Throughput for ALOHA