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## Wednesday, July 3, 2013

### What are five key assumptions in dynamic channel allocation?

Putting the available bandwidth in operation of the cellular telephone system to efficient use is an important problem to be considered for providing good service to the largest number of customers possible. The problem has gained a critical status owing to the rapid growth of the cellular telephones users.

- A communication channel is nothing but a band of frequencies which a number of users can use simultaneously if they are residing far apart from each other.
- There is a minimum distance at which no interference occurs between the users and it is known as the channel reuse constraint.
- A cellular telephone system divides the service area in to a number of regions commonly known as the cells.
- Each of the cells has its own base station for handling the calls concerned with that cell.
- The bandwidth of the communication channel is partitioned in to many channels permanently.
- The cells are then allocated these channels in such a way that the channel reuse constraint is not violated by the calls.
- There are a number of ways for allocating the channels.
- Few of them are better than the others when it comes to reliably making channels available to all the cells.

Few examples of channel allocation methods are:
1. Fixed assignment method
2. Dynamic allocation method
3. Reinforcement learning method
About Dynamic Method Allocation
- One type of dynamic method allocation is the BDCL or the borrowing with directional channel locking.
- Out of all the above mentioned channel allocation methods, the dynamic allocation is considered the best one according to some studies conducted.
- It is somewhat of the heuristic kind.
- In dynamic allocation, the channels are allocated in the same way as in the fixed assignment method but it permits borrowing channels from the other cells whenever required.
- It then arranges those channels in a specific order in each of the cells and this ordering is used in determining the channels for borrowing and reassigning the calls dynamically within the cells.
- There are static allocation techniques also but those don’t seem to work as well as the dynamic allocation techniques.

In dynamic channel allocation 5 assumptions are always made which we have discussed below:

Station model:
- There are N independent stations in the model and one frame is generated by each of the stations one at a time.
- It is blocked until the successful transmission of the previous frame.
- This means a station cannot queue multiple frames for transmission.
- For example, a transmission gap of 100 bits is required during the transmission of the consecutive frames.

Single channel assumption:
- The same medium is shared by all the stations.
- Through it all the stations can receive and transmit.

Collision assumption:
- A collision occurs whenever at the same time two frames are transmitted.
The two frames that collide have to be re-transmitted.

Transmission model:
- There are 2 types namely, the continuous time model and the slotted time model.
- In the former type transmission can be started at any given time.
- In the latter model, transmission starts with a time slot.

Carrier sense:
- It can also be classified in to 2 categories namely carrier sense and no carrier sense.
- Stations can know if a channel is occupied prior to using it. This is called carrier sense.
- In no carrier sense, the stations cannot know whether the channel is occupied or not before transmission.

- Also, it gets difficult for the dynamic allocation method for setting up the favorable usage patterns as the calls start saturating the system.