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What is a static channel allocation in computer networks?
In wireless networks, a channel can be allocated between source and destination, and in between source and destination each router is connected to its neighbour router in peer to peer manner, the whole channel consists of a number of routers.
So some of the channels are used for static routing and some of them are for dynamic routing schemes.
Types of Channel Allocation
The different types of channel allocation schemes are as follows −
- Static channel allocation
- Dynamic channel allocation
- Hybrid channel allocation
Now let us discuss about static channel allocation −
Static channel allocation
The process of static channel allocation scheme is explained below in a stepwise manner −
Step 1 − If there are N users, the bandwidth is divided into N equal sized partitions, where each user is assigned with one portion. This is because, each user has a private frequency band.
Step 2 − When there is only a small and constant number of users, each user has a heavy load of traffic, this division is a simple and efficient allocation mechanism.
Step 3 − Let us take a wireless example of FM radio stations, each station gets a portion of FM band and uses it most of the time to broadcast its signal.
Step 4 − When the number of senders is large and varying or traffic is suddenly changing, FDM faces some problems.
Step 5 − If the spectrum is cut up into N regions and fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted. And if more than N users want to communicate, some of them will be denied permission for lack of bandwidth, even if some of the users who have been assigned a frequency band hardly ever transmit or receive anything.
Step 6 − A static allocation is a poor fit to most computer systems, in which data traffic is extremely burst, often with peak traffic to mean traffic ration of 1000:1, consequently most of the channels will be idle most of the time.
Step 7 − The poor performance of static FDM can easily be seen with simple queueing theory calculation.
Let us start with mean time delay T,
Send a frame onto a channel of capacity C bps.
Let us assume frames arrive randomly at an average arrival time λ frames/sec
Average length of the frame is 1/µ bits.
With the help of these parameters the service rate of channel is μC frame/sec
Standard queueing theory result is −
T= 1/( µC- λ)
Now divide the single channel into N independent subchannels, each with capacity C/N bps.
The mean input rate on each subchannel is λ/N.
Finally we get
TN=1/(µ(C/N) – (λ/N)) = N/( µC- λ) = NT
Example
If C is 100Mbps, the mean frame length 1/µ is 10,000 bits and the frame arrival time λ is 5000 frames/ sec then,
T=1/(µC- λ) = 200 μsec.
Techniques of Static Channel Allocation
Static channel allocation has two techniques and they are TDMA and FDMA.
Time Division Multiple Access − The time axis is divided into time slots of a fixed length. Each user is allocated a fixed set of time slots at which it can transmit. Then it requires that users be synchronized to a common clock. Remaining bits are required for synchronization.
Frequency Division Multiple Accesses − The available frequency bandwidth is divided into two frequency bands. A fixed band is allocated to each user. It requires a Security band between user frequency bands to avoid crosstalk.
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