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:
- Fixed assignment method
- Dynamic allocation method
- 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.
