Flow Control and Data Compression Essays

Flow Control and Data Compression Essays Flow Control and Data Compression Essay Flow Control and Data Compression Essay Firstly flow control shall be looked at and the methods it undergoes when being implemented. Flow control is an efficient way of handling data when its to be sent to an external entity. For example this maybe the likes of a printer, further a modem may be the entity. All information that is to be sent to the entities must have a place to be stored. This is within the built in buffer, when the memory gets to full flow control is implemented and the jobs again can be stored. This ensures a faster speed of processing jobs whether they be print jobs or e-mails and helps maintain stability.Flow ControlAn example of flow control to be used will be the sending of information to a printer. A printer will contain what is known as a buffer. A buffer is there to aid the printer when to many jobs are sent. For example the user send 4 jobs to the printer these are then all queued in the buffer. This limit cannot be exceeded and if tried the printer will communicate with the computer and will say that it cannot receive any more information at the current time. It does this by saying x-off. When the list has been completed and printed all documents from the buffer the printer will say x-on and this resumes printing the 5th job that was unable to fit into the buffer.Another example of above would be the sending of an e-mail through the modem. If multiple e-mails are to be sent at once then they will be stored within the buffer. When the buffer is full and another job is sent the modem will say to the computer not to send any more. It does this by using CTS clear to send Once all jobs are finished from the modem it can carry out the jobs that were not able to fit into the buffer originally. It does this by saying RTS ready to sendData CompressionIntroductionThere are a few forms of data compression and they will be explained in a little more detail throughout. Data compression plays a big part in then world today. Compression is a key factor to system speeds and size and is pre dominantly used within communications; this is to ensure fewer bits are sent (therefore less bandwidth is used). When this has been accomplished faster communication speeds can be achieved. There are many forms of data compression however only a few have been standardised.Also a few negatives may be derived from data compression. When data has been compressed and sent over the network it is in a reduced form. This saves speed as previously said. The problem with this is the decompression methodology at the other end. If this algorithm/process is not very efficient then the overall speed may not decrease that significantly at all.Run Length EncodingIt is important to compress data to make file sizes smaller and increase speed times. When sending faxes users wish to send the information at the quickest speeds possible and this may only be done by the use of compression. For example if a page was made of black and white dots and was to be sent, instead of writing W W W W W W W W B B B W to represent white dots we may write 8W, this represents 8 white dots. There are also three black dots present and this would then be written as follows, 8W3B1W Overall this would compress a bigger file size immensely. It is called R.L.E run length encoding.White Space CompressionAnother example of data compression would be when uploading a web page using html coding. As html coding will contain masses of white on the page, for example spaces and so fourth, further there is a mass of unneeded tags. This can be compressed to make the file size smaller.Error Detection and CorrectionIntroductionError detection and correction is used to validate data when it is sent. There are many channels and places where errors in data can occur and there are forms of detection to help avoid this. The methods used shall be described below.When data is sent over the internet in the form of an instant message for example it will be sent (each character) in ASCII code. ASCII code is made of a 7 bit bi nary number and represents each character thats found on the keyboard. When a message is sent an eight bit is put at the beginning of the ASCII code, this is called a parity bit, and helps in error detection. There are many types of things that can cause errors and the parity bit helps fight this.Even parity: When the 7bit binary number is sent (1011000) we need to add parity. This would be in the form of a one; the number to be sent would then be 11011000. When this number is sent the recipient should be aware that the form being sent is an even parity. When received, the computer will be well aware if an error has occurred as the ASCII code will contain an odd number of 1s. This is the same with the Odd Parity Check. However with the odd parity check its essential to maintain an odd number of 1s.This process is simple and is not a sure way to detect errors. For example if a burst of errors were to occur, the ASCII code would turn into a even or odd respectively again, as was sent in the first place, so this is a floor in the check, further it only has a 50:50 chance of detecting burst errors.Error CorrectionError correction is a simple process. When sending information such as text, for example mother and donkey, there are a number of ways in which the computer can correct the errors that have occurred.WordCharacter DifferenceMother DonkeyCommentLikely MatchDonkey40Perfect Match to donkey but 4 to motherDonkeyHotter24Closer to mother than donkeyMotherMother04Perfect match to mother and maximum difference to donkeyMotherMither15Closer to mother than donkeyMotherMonkey31Closer to donkey than motherDonkeyCables55As close to mother as donkey. Cannot determine correct message.The process is simple. The programme will try to match the exact phrase to a list of comparisons and will then define what fits into the original word the best. This will work on the process of elimination and when complete can define what the exact word should be.