Channel allocation is a process in which a single channel is divided and allotted to multiple users in order to carry user specific tasks. There are user’s quantity may vary every time the process takes place. If there are N number of users and channel is divided into N equal-sized sub channels, Each user is assigned one portion. If the number of users are small and don’t vary at times, then Frequency Division Multiplexing can be used as it is a simple and efficient channel bandwidth allocating technique.
Channel allocation problem can be solved by two schemes: Static Channel Allocation in LANs and MANs, and Dynamic Channel Allocation.
These are explained as following below.
1. Static Channel Allocation in LANs and MANs:
It is the classical or traditional approach of allocating a single channel among multiple competing users using Frequency Division Multiplexing (FDM). if there are N users, the frequency channel is divided into N equal sized portions (bandwidth), each user being assigned one portion. since each user has a private frequency band, there is no interference between users.
However, it is not suitable in case of a large number of users with variable bandwidth requirements.
It is not efficient to divide into fixed number of chunks.
T = 1/(U*C-L)
T(FDM) = N*T(1/U(C/N)-L/N)
T = mean time delay,
C = capacity of channel,
L = arrival rate of frames,
1/U = bits/frame,
N = number of sub channels,
T(FDM) = Frequency Division Multiplexing Time
2. Dynamic Channel Allocation:
In dynamic channel allocation scheme, frequency bands are not permanently assigned to the users. Instead channels are allotted to users dynamically as needed, from a central pool. The allocation is done considering a number of parameters so that transmission interference is minimized.
This allocation scheme optimises bandwidth usage and results is faster transmissions.
Dynamic channel allocation is further divided into:
- Centralised Allocation
- Distributed Allocation
Possible assumptions include:
Assumes that each of N stations independently produce frames. The probability of producing a packet in the interval IDt where I is the constant arrival rate of new frames.
Single Channel Assumption:
In this allocation all stations are equivalent and can send and receive on that channel.
If two frames overlap in time-wise, then that’s collision. Any collision is an error, and both frames must re transmitted. Collisions are only possible error.
Time can be divided into Slotted or Continuous.
Stations can sense a channel is busy before they try it.
- N independent stations.
- A station is blocked until its generated frame is transmitted.
- probability of a frame being generated in a period of length Dt is IDt where I is the arrival rate of frames.
- Only a single Channel available.
- Time can be either: Continuous or slotted.
- Carrier Sense: A station can sense if a channel is already busy before transmission.
- No Carrier Sense: Time out used to sense loss data.
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