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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