There are three types of error-control options: toss the frame packet, return an error message to transmitter, and correct the error without the transmitter's help. This chapter is a discussion about the many ways to detect errors and what to do to remove or reduce the amount of damage these errors cause.
Noise and Errors:
White noise is a continuous type of noise. An example of this is the sound a radio transmitter makes when changing stations. We hear the static noise as we switch from one channel to another. This kind of noise is dependent on the degree of temperature. The more heat means more white noise. This is due to the increase in activity of the electrons. This type of noise is also called "thermal noise" or "Gaussian noise."
To reduce white noise from a digital signal, we need to pass the signal through a signal re-generator. Reducing white noise from an analog signal, we need to pass the signal through a set of filters.
Impulse noise is the opposite of white noise. The type of noise is a noncontinuous noise and is also considered to be one of the most difficult errors to detect because they occur randomly.
Crosstalk is caused by the coupling between two different signals. An example is when telephone users had that ability to hear other phone users conversations. It is said that high humidity and wet weather causes the increase in electrical crosstalk. This kind of interference can be reduced.
Echo is the reflective feedback of a signal as the signal moves through the specified medium. This is due to open ended coaxial cables or from junctions of connected wires. The signal will move the end of the cable and bounce back from the end of the cable. To reduce the amount of echo, many companies will provide an echo suppressor that restricts the signal to only pass in one direction.
Jitter is when the rises and falls of a digital signal begin to shift. This shifting will cause the signal to become blurry which will cause videos to flicker, audio transmissions to click and break-up, and transmitted computer data to contain various errors. Causes of this phenomena can be due to electromagnetic interference, crosstalk, passing the signal through too many repeaters, and the use of lower-quality equipment. To reduce this kind of interference, proper shielding can be installed and limiting the number of a signal being repeated can help.
Attenuation is the continuous loss of a signal's strength as it travels through a medium. Amplifiers can help reduce the amount of attenuation that occurs.
Error Prevention:
The side effect of noise during transmission is that the transmitting station will have to slowdown its transmission rate. This is because a modem will connect to another modem and conduct a "fallback negotiation." This means that if one of the modems is sending "garbled" data will be instructed to "fall back" to a slower rate to help improve the ability to distinguish one value from the next. However, there are many ways to prevent the occurrence of transmission errors. Many can install wiring with proper shielding, understand that wireless applications will share wireless frequencies, replace older equipment with newer up-to-date equipment, using proper numbering of digital repeaters and analog amplifiers, and to avoid pushing mediums beyond their recommended limits.
Error Detection:
Parity checks are the most basic error-detection techniques that are used with asynchronous connections. However, because these are the most basic types of error-detection these kinds of error checks are rarely used. Most parity checks let too many errors slip by undetected. There are two forms of parity checks: the first is a simple parity and the second is a longitudinal parity.
Simple parity, or also known as a vertical redundancy check, comes in two forms: even parity and odd parity. Parity checking is when a bit is added to a string of bits to create either even parities or odd parities. Even parity is caused when the binary 0's or 1's are added to the string will produce an even number. The odd parity will produce an odd number when the binary numbers are added to teh string.
Longitudinal Parity, or known as longitudinal redundancy check, will use additional parity bits to help solve the issues of the simple parity technique.
Arithmetic checksum is a technique where the characters that are to be transmitted will be "summed" together. These summed numbers are then added to the end of the message that is being transmitted. The receiving end will accept the message and perform the same summing operations to compare the receiving ends value with senders value. If both values match, there were no errors in the message. However, if the values do not match, this means that an error has occurred.
Cyclic redundancy checksum will add 8 to 32 check bits to a large packet of data. This kind of technique will treat the message as a polynomial. Operations to solve the polynomial will be performed on the receiving end of the message. If the division portion of solving the polynomial results in the remainder of 0, this means that there were no errors within the message. But, if the receiving end is unable to produce a remainder of 0, then there was an error within the message.
Error Control:
Error control will either use one of the following techniques to fix an error: toss the frame/packet, return a message to the transmitter to resend the data packet, or correct the error without re-transmission.
Toss the frame/packet is used to detect an error, but the error will be discarded without any thought to the where the error may have been cause. This kind of technique is normally used with mediums such as fiber optic cables. Because the fiber optic cable is used to transmit data from one end to the other, many will use the toss the frame/packet method because the error ratio is so low for this kind of technology.
Return a message is sending a message back to the transmitter to notify the error and to resend the same message without the error. There are two basic versions of returning a message: stop-and-wait error control and the sliding window. The stop-and-wait error control occurs when station A will send station B a message and then will "stop-and-wait" for a response from station B. The response will either confirm to station A that there was no error or it will notify station A of an error. Sliding window error control is produced as a flow control scheme that will allow a station to transmit a number of data packets at one time before receiving any kind of acknowledgement from the receiver.
Correcting the error is when the error is corrected. But, for this technique to occur there must be redundant data present within the message, which is called the forward error correction process.
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