noise, attenuation and error detection/correction standards
For data to be transferred via a digital network, the binary values must be converted to a voltage on the wire. Thus a value of 1 may be +5volts and a 0 may be -5 volts. When noise and attenuation is accounted for, a 1 may be interpreted as a 0 causing an error in transmission.
The degree of each effect is influenced by:
Noise
Noise is electrical interference on the wire that is carrying the data. (In radio systems, noise is Electro-Magnetic Radiation - EMF). In a digital system, the data is transferred by a voltage level on the wire for a given time frame. Noise is a change in the voltage over that period. If it gets very bad and especially when the period is very short, the data value cannot be determined or incorrect data is received.
Attenuation
This is a consequence of the voltage becoming less than that originally sent. Thus a +5 volts may be received as +3volts. This is a consequence of the resistance in the wire. When the wire is longer, the resistance is greater and so the received voltage is less. There is a limit to the length of the wire. At this distance, repeaters that amplify the voltage are used to boost the signal.
See Diagram P23 Data Communications, computer networks and OSI, Halsall, F.
Error Detection / correction
To overcome the problems of signal loss for whatever reason, error detection and correction systems have been developed. These include:
Parity Check
This is usually carried out on byte data. An extra bit of data is sent with along the data. The process is to add the bits and place a value in the extra bit that will make the sum of all bits either even or odd. With even parity all bits will add to an even number and odd for odd parity. When the data is received, parity checking takes place and if the check is not correct, the data is transmitted.
Note that combination of errors may go undetected.
Example
Suppose that the byte of date 10011101 is to be transmitted. The sum of all the bits is performed and one more bit value is added to the data based upon whether even or odd parity is required. For this data, if the parity is to be even, a one is added to make the total an even number; if the parity is to be odd, a zero is added to make the total an odd number.
Even parity (parity bit in yellow)
| 1 | 0 | 0 | 1 | 1 | 1 | 0 | 1 | 1 |
Odd parity (parity bit in red)
| 1 | 0 | 0 | 1 | 1 | 1 | 0 | 1 | 0 |
Block sum check
In frame oriented data transmission, a block of 8 bytes of data is transmitted. Each character is assigned a parity bit and, as well, the column data for the block of data is also assigned a parity value. When anomalies appear in the parity values, the entire block is retransmitted.
In the following example, even parity is across (yellow) and odd parity is down (red). There are no errors.
| 1 | 0 | 1 | 1 | 0 | 0 | 1 | 0 | 0 |
| 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 0 |
| 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
| 1 | 1 | 1 | 0 | 0 | 0 | 1 | 1 | 1 |
| 0 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 0 |
| 0 | 1 | 1 | 0 | 1 | 0 | 0 | 0 | 1 |
| 1 | 1 | 1 | 0 | 1 | 1 | 0 | 1 | 0 |
| 1 | 0 | 1 | 0 | 0 | 1 | 1 | 0 | 0 |
Cyclic Redundancy Check (CRC)
A single set of check digits is generated for a block of data by the transmitted. This is sent with the data. The receiver then does the same calculation for the same data and then compares this to the check value received. If an anomaly appears, the data is retransmitted. (This is also known as Frame Check Sequence (FCS).
Mike Leishman