Monday, November 12, 2012

Notes - The Channel Allocation Problem

The following are notes from Computer Networks written by Tanenbaum 5th edition.

  • how to allocate a single broadcast channel among competing users
  • portion of wireless spectrum, or single wire
Static Channel Allocation
  • traditional way of allocating a single channel
    • FDM(Frequency Division Multiplexing)
      • N users Bandwidth divided into N equal sized portions
      • FM radio stations
      • small and constant number of users, steady stream or heavy load this is efficient
      • issues with bursty traffic, large and varying
      • if fewer than N users, spectrum wasted
      • more than N users denied permission
      • division inefficient, if users are quiet bandwidth is lost and no one else can use
    • Queuing theory result
    • where 
      • μ is number of bits in frame
      • λ is rate of frames/sec
    • Now if we divide this by N, so that the single channel is split into N independent subchannels, we notice that C/N, and λ/N causes the formula to change to N times the original formula
      • this means that dividing the channel into parts is N times worse than an orderly central queue
  • This is the same problem that occurs in FDM and TDM techniques and other static divisions of a channel
    • FDM(Frequency Division Multiplexing) TDM (Time Division Multiplexing)
    • Example, physical split of network 100Mbps into 10 10 Mbps we would get a mean delay jump from 2 microseconds to 2 milliseconds
Assumptions for Dynamic Channel Allocation
  • Assumptions made
    • Independent Traffic
      • model consists of N independent stations
      • generates frames for transmissions
      • generation in interval length is Δt is λΔt where λ is a constant
      • once generation is blocked station does nothing until frame is transmitted successfully
    • Single Chanel
      • all stations can commit and receive from single channel, but can be assigned different priorities
    • Observable Collisions
      • if two frames transmitted simultaneously, overlap in time can be detected
      • called collision
    • Continuous or slotted time
      • time assumed continuous or intervals
      • intervals called slots, transmissions begin at beginning of slot
    • Carrier sense or no carrier sense
      • can tell if channel is in use pre-sending or not
  • Assumption not particularly good, assumes frame arrivals are independent, generated unpredictably at a constant rate
  • packets come in bursts
  • Poisson models used
  • last three is engineering based on the system
  • slotted time used to improve performance, but requires a master clock which is not always available
  • carrier sense usually available on wired, but not on wireless
  • no guaranteed delivery can be expected

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