8b/10b

Original paper by Franaszek and Widmer

In telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits is no more than 2, and that there are not more than five 1s or 0s in a row. This helps to reduce the demand for the lower bandwidth limit of the channel necessary to transfer the signal.

The code was described in 1983 by Al Widmer and Peter Franaszek in the IBM Journal of Research and Development. IBM was issued a patent for the scheme the following year. IBM's patent notwithstanding, the method, implementation and goals are very similar to Group Code Recording (GCR) used on floppy disks in some computers during late 1970s/early 80s.

Technologies that use 8b/10b

Now that the IBM patent has expired, the scheme has become even more popular and is the default DC-free line code for new standards.

Among the areas in which 8B/10B encoding finds application are

* PCI Express
* IEEE 1394b
* Serial ATA
* SAS
* Fibre Channel
* SSA
* Gigabit Ethernet (except for the twisted pair based 1000Base-T)
* InfiniBand
* XAUI
* Serial RapidIO
* DVI and HDMI (Transition Minimized Differential Signaling)
* DVB Asynchronous Serial Interface (ASI)
* HyperTransport