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. 

What is meant by Code Division Duplexing?

CDD or the Code Division Duplexing is a duplexing system that is perhaps the most spectrum – efficient one. In this article we discuss about this system. 

But first let us look at the scenario of the spectrum efficiency. 

- Elimination of interference is very complicated as well as an important issue when it comes to the implementation of the wireless communications. 

- With the increasing spectrum efficiency, interference becomes compulsory to be eliminated.

- Reducing the interference is very much important in order to increase the transmission data rate and the radio capacity of the wireless communications. 

- By duplexing, we mean that using two separate channels for the transmission of information in a two way communication in two different directions. 

- Currently we have only two working technologies in this regard namely:

1. Frequency division duplexing or FDD and
2. Time – division technology or TDD

- The former is the primary technology used in wireless because of its characteristic ability of interference isolation.

- The latter is kind of a spectral efficient technology.

- At present, frequency–division duplexing or FDD is mostly used by all the cellular systems for eliminating the interference that occurs in the adjacent cells. 

- A number of technologies are used today that have been successful in limiting the effects of the interference. 

- But they have not been successful in eliminating two particular types of interference namely:

1. Adjacent base – to – home mobile interference and
2. Adjacent mobile – to – home base interference

- Time Division Duplexing or TDD has not been put to use for the cellular systems since this one is much susceptible to different types of interference  

- TDD is not just subjected to the above mentioned two forms of interference rather it faces additional adjacent mobile to home and base to home interference.

- This limits the use of time division duplexing to the systems confined to a small area. 

- In Code–Division Duplexing, all these types of interference are eliminated making it the most spectrum efficient one. 

- However, proper CDD systems are not a perfect reality yet.

- CDD represents an innovative solution.

- Another best multiple access scheme is the CDMA (code – division multiple access) since it is more effective in eliminating the interference when compared to the other technologies. 

- Code – division duplexing is a combination of TDD and smart codes. 

- The CDD can be turned in to reality only with these smart codes. 

- CDMA based on these smart codes is termed as LAS – CDMA where LAS represents the smart codes set. 

- This technology holds the promise for increasing the spectral efficiency as well as capacity of the mobile networks. 

- For the efficient utilization of the spectrum, two technologies are required:

1. A multiple access scheme and
2. A duplexing system

- The industry has already established CDMA as the best out of three available (the other two are FDMA and TDMA). 

- Next, a duplexing system remains to be selected. 

- FDD has been the most common choice till now. 

- It makes use of different frequencies for handling the outgoing and incoming signals. 

- On the other hand, TDD operates on a single frequency but uses different time slots. 

- Using a TDD system provides a benefit of alternating sending and receiving in a single frequency. 

- Smart codes ensure that the code–division duplexing system is not like TDD but much better than that. 

- An interference limited environment was generated by the cellular systems using multiple frequencies in multiple cells. 

- Later, cells started using same frequency but still the environment was interference limited. 

- But the CDD system promises to change this environment in to one that is noise limited. 

What is meant by Holographic Data Storage?

Currently the conventional magnetic and optical data storage dominates the field of high – capacity data storage. But another technology called the holographic data storage holds the potential to lead this area. Now what is this technology? We shall discuss about it in this article. 

Difference between Conventional storage methods and Holographic Data Storage

- The conventional methods such as the optical storage and magnetic storage technologies depend up on the recording of the individual data bits on the distinct optical and magnetic on the medium’s surface. 

- In the holographic technology, the information is recorded throughout the medium’s volume.

- Multiple images can be recorded in the same medium area via utilization of the light at varying angles. 

- Further, in the conventional storage methods the recording takes place in a linear fashion whereas, 

- In holographic storage, millions of bits can be recorded and read in parallel thus increasing the rates of the data transfer more than what are offered by the conventional methods.

Features of Holographic Data Storage

Data Recording: 

- The information is stored in a thick photosensitive optical substance with the help of an optical interference pattern. 

- A laser beam is divided into two un-identical optical patterns (formed of light and dark pixels) and is projected towards the medium. 

- A multitude of holograms in one volume is recorded by making adjustments in the wavelength, reference beam angle, media position etc.

Data Reading: 

- For reading the stored data, the same reference beam is reproduced for the creation of the hologram. 

- This light beam is focused up on the photosensitive material that illuminates the interference pattern which leads to the diffraction of the light. 

- The pattern is then projected to the detector. 

- The data is then read in parallel at a rate more than that of millions. 

- This means high data transfer rate. 

- It takes less than 0.2 seconds for accessing the information from a holographic drive. 

- Holographic data storage has offered a solution to many companies for preserving and archiving the information. 

- The WORM (write – once, read many) approach provides assurance about the content security, risk of modification and overwriting of the information. 

- This technology provides hope for storage of data without degradation for 50 plus years which is quite more than the current options.

- However, if the same trend is followed, and it becomes possible to store data for 50 – 100 years in same format, that would still be irrelevant. 

- This is so because the format will be changing in less than 10 years.  

Types of Holographic Media

- Holographic media is of two types namely:

1.  The re-writable media and
2. The write once medium.

- In the former type, the changes can be reversed but in the latter can the changes are irreversible.

There is no point in the competition between the holographic storage and the hard drives since the former can find a market that is based up on the virtues like access speed. 

- The holographic data storage technology does seems to have a future in the video game market. 

In the year of 2009, the GE Global Research did came up with their own demonstration of the holographic storage medium that could stand the discs that had read mechanisms somewhat similar to that of the blu–ray disc players.