ERROR DETECTION AND CORRECTION
Even the best transmitter and receiver systems cannot guarantee 100% accuracy of data transmitted across an air link. There are just too many ways that Mother Nature can disrupt the signal; noise, multi path and fading are just a few causes of errors. To get an idea of what can happen to a data signal, try to copy RTTY signals that are weak or fluttery from ionospheric variations!
In recognition of the realities of radio propagation, data communications engineers have devised a number of strategies. The first challenge is to find out when an error has occurred! This is called error detection. Without some clue about what the data should have been, . however, there is no way to detect errors. To be able to discern transmission errors, information describing the data is sent along with the original data.
Error detection data can be as simple as the parity bit of ASCII data discussed earlier. The addition of the single parity bit is a great improvement over no error detection at all, at the cost of a small decrease in data rate. Parity cannot be used to detect any type of error, however. Parity can only detect errors that cause the number of even or odd bits to differ from that which the system has been configured to expect. You can probably imagine how changing one bit would cause the parity check to fail, but what if two bits are changed so that the net number of even or odd bits is unchanged? Parity checking can only detect single-bit errors.
Another popular technique of error detection, used by packet radio's AX.25 protocol, TCP/IP networking systems, and Ethernet (among many others) is checksums. Originally, this was just the sum of all the data values in a packet, sent as a single byte at the end of the packet. It was compared to the sum computed by the receiver. If the sum matched, the entire packet was judged to be good. The simple checksum has evolved into a more sophisticated technique called the cyclical redundancy check or (CRC). The two-byte CRC is a lot like a checksum and is evaluated by the receiving system in the same way as a checksum. Using a CRC detects most errors.
Once the system has detected an error, what it decides to do about it is another matter. This moves the process from error detection to error correction. The simplest form of error correction is ARQ (Automatic Repeat Request), introduced in the section on PACTOR earlier in this chapter. If the receiving system detects an error, it requests a retransmission of the corrupted packet or message by sending a NAK (Not Acknowledge) message to transmitting station. The information is retransmitted until the receiver responds with an ACK (Acknowledge) message. If the errors persist long enough, the system gives up and drops the connection.
Another popular error correction technique is to send some extra data about the information in the packet or message so that the receiving station can actually correct some types of errors. This technique is called Forward Error Correction or (FEC). The term "forward" stems from sending the error correction data "ahead" with the original information. The combination of the FEC data and the algorithm by which errors are detected and corrected is called an FEC code.
There are many types of FEC codes. Reed-Solomon, Hamming, BCH and Golay codes are all used in consumer electronics. FEC data is sometimes spread out over several data packets to account for fading. FEC is used with digitized voice to help preserve the quality of the received speech. This is why digital voice systems (such as mobile phones) tend to have good quality up to a certain error threshold and then become completely garbled - their FEC algorithm fails at that point.
One type of FEC builds the error correction into the structure of the transmitted data itself. Instead of allowing any possible sequence of symbols to be sent over the air link, the system called Viterbi encoding restricts the sequence to a smaller number. That way,
the receiving system has fewer possibilities to choose from when deciding what symbol was received. The receiver then reports the most likely sequence of symbols to have created the signal received the Viterbi path.
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What do the letters FEC mean as they relate to digital operation?
A. Forward Error Correction
B. First Error Correction
C. Fatal Error Correction
D. Final Error Correction
How is Forward Error Correction implemented?
A. By the receiving station repeating each block of three data characters
B. By transmitting a special algorithm to the receiving station along with the data characters
C. By transmitting extra data that may be used to detect and correct transmission errors
D. By varying the frequency shift of the transmitted signal according to a predefined algorithm
How does ARQ accomplish error correction?
A. Special binary codes provide automatic correction
B. Special polynomial codes provide automatic correction
C. If errors are detected, redundant data is substituted
D. If errors are detected, a retransmission is requested
What is the advantage of including a parity bit with an ASCII character stream?
A. Faster transmission rate
B. The signal can overpower interfering signals
C. Foreign language characters can be sent
D. Some types of errors can be detected
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