ECE155ALecture7

# ECE155ALecture7 - Computer Networks Lecture 7 Professor...

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1 Computer Networks Lecture 7 Professor Louise E. Moser Winter 2010

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2 Network Architecture The network architecture used in the textbook and this course MAC Sublayer
3 Medium Access Control (MAC) Sublayer Multi-access or random access communication channel, i.e., shared communication medium Key issue: Who gets to use the channel Channel allocation Static Dynamic

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4 Static Channel Allocation FDM or TDM Problem: Wasted bandwidth if user does not use the frequency band or time slot allocated Given C = channel capacity in bits/sec 1/ μ = mean frame length in bits/frame λ = mean arrival rate in frames/sec Then T = mean time delay with queuing = 1/( μ C – λ ) (from elementary queuing theory) T ' = mean time delay without queuing = 1/( μ C) Ex: C = 100 Mbps, 1/ μ = 10000 bits/frame, λ = 5000 frames/sec, then T = 200 μ sec/frame and T ' = 100 μ sec/frame
5 Static Channel Allocation Now let’s see what the delay is if we divide Single channel into N independent subchannels, as in FDM Each subchannel has capacity C/N bps Mean input rate is λ /N Transmit more slowly for a longer time Single time interval into N independent subintervals, as in TDM Each subinterval has full capacity but for only 1/N of the time, i.e., effective capacity is C/N bps Mean input rate is λ /N Transmit more quickly for a shorter time T = 1 / ( μ C/N – λ /N) = N / ( μ C – λ ) = NT Mean delay is N times worse than that for a single channel!

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6 Dynamic Channel Allocation Five key assumptions Station model N independent stations (also called terminals) Probability of a frame being generated in an interval Δ t is λΔ t (constant arrival rate λ ) Once a frame has been generated, the station is blocked until the frame is successfully transmitted Single channel that all stations share Collision – When two frames are transmitted simultaneously, the resulting signal is garbled Frame transmission time Continuous time – Begins at any instant Slotted time – Begins at the start of a time slot Carrier sense or not Carrier sense – Stations can tell if the channel is busy They do not send if the channel is busy No carrier sense – Stations just go ahead and send
7 Multiple Access Protocols Pure and Slotted ALOHA Carrier Sense Multiple Access Protocols (CSMA) Collision-Free Protocols Limited-Contention Protocols Wavelength Division Multiple Access (WDMA) Wireless LAN Protocols

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8 Pure ALOHA Basic idea Users transmit whenever they have data to send User does not listen before transmitting If frame is destroyed (which happens even if one bit overlaps), sender waits a random amount of time and sends again
9 Pure ALOHA Question: What fraction of transmitted frames escape collision? Let frame time = amount of time to transmit a frame

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## This note was uploaded on 05/20/2010 for the course ECE 155a taught by Professor Louisee.moser during the Winter '09 term at UCSB.

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ECE155ALecture7 - Computer Networks Lecture 7 Professor...

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