4B5B
Encyclopedia
In telecommunication
, 4B5B is a form of data communications Block Coding. 4B5B maps groups of four bit
s onto groups of 5 bits, with a minimum density of 1 bits in the output. When NRZI-encoded, the 1 bits provide necessary clock
transitions for the receiver. For example, a run of 4 bits such as 0000 contains no transitions and that causes clocking problems for the receiver. 4B/5B solves this problem by assigning each block of 4 consecutive bits an equivalent word of 5 bits. These 5 bit words are pre-determined in a dictionary and they are chosen to ensure that there will be at least two transitions per block of bits.
A collateral effect of the code is that more bits are needed to send the same information than with 4 bits. An alternate to using 4B5B coding is to use a scrambler.
Depending on the standard or specification of interest, there may be several 4b5b characters left unused. The presence of any of the "unused" characters in the data stream can be used as an indication that there is a fault somewhere in the link. Therefore, the unused characters can actually be used to detect errors in the data stream.
4B5B was popularized by Fiber distributed data interface
(FDDI) in the mid-1980s, and was later adopted by
The name "4B5B" is generally taken to mean the FDDI version. Other 4-to-5-bit codes have been used for magnetic recording are known as group code recording
(GCR).
On optical fiber, the 4B5B output is NRZI-encoded. FDDI over copper (CDDI) uses MLT-3 encoding
instead, as does 100BASE-TX.
Note that normal data symbols begin with at most one 0 bit and end with at most two, so there can be at most three 0 bits in a row. Control symbols are used in combinations that also preserve this rule. Thus, 4B5B encoding is a (0,3) RLL code
.
FDDI and 100BASE-TX begin frames with a JK pair. FDDI ends frames with a TT pair, while 100BASE-TX uses a TR pair.
(HDLC = High-Level Data Link Control
)
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, 4B5B is a form of data communications Block Coding. 4B5B maps groups of four bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s onto groups of 5 bits, with a minimum density of 1 bits in the output. When NRZI-encoded, the 1 bits provide necessary clock
Clock signal
In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...
transitions for the receiver. For example, a run of 4 bits such as 0000 contains no transitions and that causes clocking problems for the receiver. 4B/5B solves this problem by assigning each block of 4 consecutive bits an equivalent word of 5 bits. These 5 bit words are pre-determined in a dictionary and they are chosen to ensure that there will be at least two transitions per block of bits.
A collateral effect of the code is that more bits are needed to send the same information than with 4 bits. An alternate to using 4B5B coding is to use a scrambler.
Depending on the standard or specification of interest, there may be several 4b5b characters left unused. The presence of any of the "unused" characters in the data stream can be used as an indication that there is a fault somewhere in the link. Therefore, the unused characters can actually be used to detect errors in the data stream.
4B5B was popularized by Fiber distributed data interface
Fiber distributed data interface
Fiber Distributed Data Interface provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to . Although FDDI logical topology is a ring-based token network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its...
(FDDI) in the mid-1980s, and was later adopted by
- 100BASE-TX standardStandardizationStandardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
defined by IEEE 802.3u in 1995. - AES10-2003 MADI http://www.aes.org/publications/standards/ (Multichannel Audio Digital Interface)
The name "4B5B" is generally taken to mean the FDDI version. Other 4-to-5-bit codes have been used for magnetic recording are known as group code recording
Group Code Recording
In computer science, group code recording refers to several distinct but related encoding methods for magnetic media. The first, used in 6250 cpi magnetic tape, is an error-correcting code combined with a run length limited encoding scheme...
(GCR).
On optical fiber, the 4B5B output is NRZI-encoded. FDDI over copper (CDDI) uses MLT-3 encoding
MLT-3 encoding
MLT-3 encoding is a line code that uses three voltage levels...
instead, as does 100BASE-TX.
Encoding table
| Name | 4b | 5b | Description |
|---|---|---|---|
| 0 | 0000 | 11110 | hex data 0 |
| 1 | 0001 | 01001 | hex data 1 |
| 2 | 0010 | 10100 | hex data 2 |
| 3 | 0011 | 10101 | hex data 3 |
| 4 | 0100 | 01010 | hex data 4 |
| 5 | 0101 | 01011 | hex data 5 |
| 6 | 0110 | 01110 | hex data 6 |
| 7 | 0111 | 01111 | hex data 7 |
| 8 | 1000 | 10010 | hex data 8 |
| 9 | 1001 | 10011 | hex data 9 |
| A | 1010 | 10110 | hex data A |
| B | 1011 | 10111 | hex data B |
| C | 1100 | 11010 | hex data C |
| D | 1101 | 11011 | hex data D |
| E | 1110 | 11100 | hex data E |
| F | 1111 | 11101 | hex data F |
| Q | -NONE- | 00000 | Quiet (signal lost) |
| I | -NONE- | 11111 | Idle |
| J | -NONE- | 11000 | Start #1 |
| K | -NONE- | 10001 | Start #2 |
| T | -NONE- | 01101 | End |
| R | -NONE- | 00111 | Reset |
| S | -NONE- | 11001 | Set |
| H | -NONE- | 00100 | Halt |
Note that normal data symbols begin with at most one 0 bit and end with at most two, so there can be at most three 0 bits in a row. Control symbols are used in combinations that also preserve this rule. Thus, 4B5B encoding is a (0,3) RLL code
Run Length Limited
Run length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. This is used in both telecommunication and storage systems which move a medium past a fixed head. Specifically, RLL bounds the length of stretches ...
.
FDDI and 100BASE-TX begin frames with a JK pair. FDDI ends frames with a TT pair, while 100BASE-TX uses a TR pair.
The following character sets are sometimes referred to as command characters
| Control Character | 5b symbols | Purpose |
|---|---|---|
| JK | 11000 10001 | Sync, Start delimiter |
| II | 11111 11111 | Not Used |
| TT | 01101 01101 | FDDI end delimiter |
| TS | 01101 11001 | Not Used |
| IH | 11111 00100 | SAL |
| TR | 01101 00111 | 100BASE-TX end delimiter |
| SR | 11001 00111 | Not Used |
| SS | 11001 11001 | Not Used |
| HH | 00100 00100 | HDLC0 |
| HI | 00100 11111 | HDLC1 |
| HQ | 00100 00000 | HDLC2 |
| RR | 00111 00111 | HDLC3 |
| RS | 00111 11001 | HDLC4 |
| QH | 00000 00100 | HDLC5 |
| QI | 00000 11111 | HDLC6 |
| 00000 00000 | HDLC7 |
(HDLC = High-Level Data Link Control
High-Level Data Link Control
High-Level Data Link Control is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization...
)