Technical Engineering Definitions
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

## Definitions

Checksum: A number that has been calculated as a function of some message, by adding up the bytes in the message. The [message] data is summed with out regard overflow. Perhaps this was what early checksums were. Today, however, although more sophisticated formulae are used, the term "checksum" is still used and interchangeable with the term CRC.

CRC: This stands for "Cyclic Redundancy Code". The term "CRC" seems to be reserved for algorithms that are based on the "polynomial" division. The essential mathematical operation in the calculation of a CRC is binary division, and the remainder from the division determines the CRC. CRC's cannot, however, be safely relied upon to verify data integrity. CRC types are often identified by the polynomial, which is the number used as the divisor. A number of pages on this site make reference to the CRC used for different electronic Interface Buses.

4B/5B Encoding The encoding method used for encoding 4-bit data bytes to 5-bit Transmission Characters. Data bytes are converted to Transmission Characters to improve the physical signal such that the following benefits are achieved: bit synchronization is more easily achieved, design of receivers and transmitters is simplified, error detection is improved, and control characters (i.e., the Special Character) can be distinguished from data characters. 4B/5B encoding prevent symbols with more than three 0's in succession from occurring in the stream. The encoding advantage is that it can use NRZ-I encoding without losing synchronization in case of long null sequences. The disadvantage is the 25% overhead due to conversion from 4 to 5 bits. Of the 32 different characters that the 4B/5B code can generate, only 16 characters are needed to transfer the payload, the remaining 16 are used as control characters.

5B/6B Encoding This encoding method used in 100Base-VG. Of the 32 six bit symbols, 20 are DC balanced. The remaining 12 six bit symbols are defined by two groups of 12 symbols with 2 ones, and 4 ones [to allow for unbalanced symbols to be transmitted without polarization]. This approach provides a balanced bit sequence comprising ones and zeroes, permitting a DC balanced transmission.

8B/10B Encoding The IBM patented encoding method used for encoding 8-bit data bytes to 10-bit Transmission Characters. Data bytes are converted to Transmission Characters to improve the physical signal such that the following benefits are achieved: bit synchronization is more easily achieved, design of receivers and transmitters is simplified, error detection is improved, and control characters (i.e., the Special Character) can be distinguished from data characters. In addition to 8B/10B encoding, data may be encoded using 5B/6B, or 3B/4B depending on the data width. This encoding is used by Fibre Channel, Gigabit Ethernet, 10 Gigabit Ethernet, and ATM transmission interfaces. Example format:

 - Data Byte 8B/10B 5B/6B 3B/4B 00 0000 0000 011000 1011 011000 0100 01 0000 0001 100010 1011 100010 1001 02 0000 0010 010010 1011 010010 0101 04 0000 0101 001010 1011 001010 0010 07 0000 0111 000111 0100 000111 0001 08 0000 1000 000110 1011 000110 -- 0F 0000 1111 101000 1011 101000 -- F0 1111 0000 100100 1110 -- -- FF 1111 1111 010100 1110 -- --

Bit Error Rate: [BER] is the ratio of received bits on an interface that contain errors. A bit error rate test is used to check the BER. The percentage of bits that have errors relative to the total number of bits received in a transmission, usually expressed as ten to a negative power. For example, a transmission might have a BER of 10 to the minus 6, meaning that, out of 1,000,000 bits transmitted, one bit was in error. Use the Equipment icon below to see BERT equipment manufacturers.