ch04-mult_access

ch04-mult_access - Chapter 4: Multiple Access Control...

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    Chapter 4: Multiple Access Control Supcom Supcom Master F2006 Master F2006
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    Problem Statement Consider an audioconference where if one person speaks, all can hear if more than one person speaks at the same time, both voices are garbled How should participants coordinate actions so that the number of messages exchanged per second is maximized time spent waiting for a chance to speak is minimized This is the multiple access problem
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    Simple solutions Use a moderator a speaker must wait for moderator to call on him or her, even if no one else wants to speak. what if the moderator’s connection breaks? Distributed solution speak if no one else is speaking but if two speakers are waiting for a third to finish, guarantee collision Designing efficient schemes is surprisingly difficult !
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    Contexts for the multiple access problem Broadcast transmission medium message from any transmitter is received by all receivers Collision of messages are possible: need for retransmission and waiting mechanisms Objectives: maximize message throughput minimize mean waiting time Shows up in five main contexts: wired lan, wireless lan, packet radio, satellite communication, and cellular telephony
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    Contexts : computer networks
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    Contexts: telecommunications
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    Solving the problem First, choose a  base technology to isolate traffic from different stations Technology can be in time domain or frequency  domain Then, choose how to allocate a limited number  of transmission resources to a larger set of  contending users Choices: centralized vs distributed design,  circuit mode vs packet mode Techniques: collision detection, collision  avoidance, reservation, etc.
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    Choices Centralized vs. distributed design in a centralized solution: one of the stations is a  master  and the others are  slaves in a distributed solution, all stations are peers Circuit-mode vs. packet-mode do stations send steady streams or bursts of  packets? with streams, does not make sense to contend  for every packet allocate resources to streams   with packets, makes sense to contend for every  packet to avoid wasting bandwidth
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    Constraints Spectrum scarcity only a few frequencies available for long-distance  communication multiple access schemes must be careful not to waste  bandwidth Radio link properties radio links are error prone: fading, multipath interference hidden terminals :transmitter heard only by a subset of  receivers Capture: on collision, station with higher power  overpowers the other, lower powered station may never  get a chance to be heard
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    The parameter « a » The number of packets sent by a source before the  farthest station receives the first bit
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    Performance metrics
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ch04-mult_access - Chapter 4: Multiple Access Control...

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