Talk:8b/10b encoding

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Lowercase 'b' vs. Uppercase 'B'[edit]

Should this be "8b/10b" instead of "8B/10B" since little b means "bit"?--SFoskett 14:47, Sep 2, 2004 (UTC)

That's a good question. The 10GE FC draft spec refers to it as 8B/10B, but the PCI Express spec refers to it as 8b/10b. However this article gets named, we need a redirect from the other, IMO. MatthewWilcox 14:54, 24 Nov 2004 (UTC)
Finally renamed as most of the external links appear to use the lower-case variant. —Sladen 05:56, 27 August 2007 (UTC)

I studied also a 4b/5b encoding. Is it the same as this, referred to the byte (8 bits)? I can't find it in the Line code page. Maybe it is also known with some other name.--Luc4 Fri Sep 23 12:14:02 2005 (CEST)

I'm not especially familiar with 4b/5b encoding, but from what I gather here it appears to be somewhat similar but simpler. Aluvus 17:27, 16 June 2006 (UTC)
4b/5b is listed on that page... just as 4b5b. And yes, 4b/5b is similar but simplier. Mrand 18:26, 16 June 2006 (UTC)

8b/10b coding table[edit]

For eventual cut & paste into main article. (Go ahead and delete this section from talk if/when that's done.)

The 8 input bits of the 8B/10B code are conventionally identified by upper-case letters A through H, with A the low-order bit and H the high-order bit. Thus, in standard binary notation, the byte is HGFEDCBA. The output is 10 bits, abcdei fghj, where a is transmitted first.

Coding is done in alternate 5-bit and 3-bit sub-blocks. As coding proceeds, the encoder maintains a "running disparity", the difference between the number of 1 bits and 0 bits transmitted. At the end of sub-block, this disparity is ±1, and can be represented by a single state bit. The input and the running disparity bit are used to select the coded bits as follows:

5b/6b code
input RD = −1 RD = +1 input RD = −1 RD = +1
EDCBA abcdei EDCBA abcdei
D.00 00000 100111 011000 D.16 10000 011011 100100
D.01 00001 011101 100010 D.17 10001 100011
D.02 00010 101101 010010 D.18 10010 010011
D.03 00011 110001 D.19 10011 110010
D.04 00100 110101 001010 D.20 10100 001011
D.05 00101 101001 D.21 10101 101010
D.06 00110 011001 D.22 10110 011010
D.07 00111 111000 000111 D.23 10111 111010 000101
D.08 01000 111001 000110 D.24 11000 110011 001100
D.09 01001 100101 D.25 11001 100110
D.10 01010 010101 D.26 11010 010110
D.11 01011 110100 D.27 11011 110110 001001
D.12 01100 001101 D.28 11100 001110
D.13 01101 101100 D.29 11101 101110 010001
D.14 01110 011100 D.30 11110 011110 100001
D.15 01111 010111 101000 D.31 11111 101011 010100
K.28 001111 110000
3b/4b code
input RD = −1 RD = +1
HGF fghj
D.x.0 000 1011 0100
D.x.1 001 1001
D.x.2 010 0101
D.x.3 011 1100 0011
D.x.4 100 1101 0010
D.x.5 101 1010
D.x.6 110 0110
D.x.P7 111 1110 0001
D.x.A7 111 0111 1000

Note that, while in most cases, the codes that depend on RD change it by ±2, the encodings of D.07 and D.x.3 do not change it.

There are two encodings for D.x.7, "primary" and "alternate". The alternate code is used whenever using the primary code would result in bits eifgh being all the same, namely after D.11, D.13 and D.14 when RD=−1, and after D.17, D.18 and D.20 when RD=+1. This ensures that five consecutive identical bits never appear in a normal output symbol.

Using an additional 6B output value, called K.28, and/or the alternate D.x.A7 output in contexts where it would not be otherwise required, an additional 12 non-data "control symbols" can be formed. Some of these contain a "comma sequence" abcdeifg = 00111110 or 11000001, which never appears anywhere else in the bit stream, and can be used to establish the byte boundaries in the data stream.

Note that the comma sequence includes two leading identical bits, because five consecutive identical bits can appear as part of normal output when straddling two symbols. In particular, D.x.A7 followed by a symbol that begins with two identical bits such as D.03, D.11, D.12, D.19, D.20, D.28 or K.28. However, D.x.A7 is always preceded by a non-identical bit, so the patterns formed are ifghjabc = 10111110 or 01000001.

The control symbols are used by higher-level protocols to indicate boundaries or gaps (idle time) in the data stream. The names are assigned because their encoding is a slight variant of the corresponding D.x.y codes.

Control symbols
input RD = −1 RD = +1
abcdei fghj abcdei fghj
K.28.0  001111 0100 110000 1011
K.28.1* 001111 1001 110000 0110
K.28.2  001111 0101 110000 1010
K.28.3  001111 0011 110000 1100
K.28.4  001111 0010 110000 1101
K.28.5* 001111 1010 110000 0101
K.28.6  001111 0110 110000 1001
K.28.7* 001111 1000 110000 0111
K.23.7  111010 1000 000101 0111
K.27.7  110110 1000 001001 0111
K.29.7  101110 1000 010001 0111
K.30.7  011110 1000 100001 0111

(* K28.1, K28.5, and K28.7 are comma symbols, containing the comma sequence abcdeifg = 00111110 or 11000001. K28.7 must not appear after another K.28.7, or it would form a second false comma sequence.) —The preceding unsigned comment was added by (talkcontribs).

Wow, you put a lot of work into that. However, I think it may be a little excessive for an encyclopedic article. It is a great ref tho. Maybe link to it from the main article and put it in wikisource or something like that? — RevRagnarok Talk Contrib 12:04, 17 August 2006 (UTC)
I think they are absolutely appropriate for an encyclopedia! Cburnett 03:29, 19 December 2006 (UTC)

Running Disparity (RD)[edit]

At least one part of this more detailed explanation should be included in the article: the discussion of Running Disparity. This Acronym (RD) is given in the article, but it is never explained (note: RD is not yet defined in the above discussion either, though it is pretty obvious what it refers to). Also, I liked the following external links, should they be added?, DaraParsavand 18:39, 9 January 2007 (UTC)
I strongly agree about adding an explanation to Running Disparity (RD). Not only is the word used in the article but also the abbreviation in the table - and it is never explained. Actually I came here (to the discussion page) in hope of just finding this. IHMO the article is already quiet technical and adding this information seems appropriate and gives a good indication on "how" 8B/10B "works". MJost 13:22, 10 January 2007 (UTC)
I've added something as the Running Disparity (and DC-freeness) is the chief reason that brought me to 8b/10b encoding in the first place. The explanation needs some work to point out that only −ve/0/+ve disparity needs to be stored as even through the slew can be −2/−1/0/+1/+2 the direction of the next code will always take it towards zero. Sladen 14:12, 19 August 2007 (UTC)

Merge from Fibre Channel 8B/10B encoding[edit]

I propose a merge, since Fibre Channel 8B/10B encoding contains the same information (maybe in friendlier form). Nothing specific to Fibre Channel, really. --Kubanczyk 14:58, 22 August 2007 (UTC)

Agreed. They are the same. —Mrand T-C 02:03, 27 August 2007 (UTC)
However, the use of the control characters differs from implementation to implementation. Fibre Channel uses K28.5 at the beginning of every Ordered Set, while the other control chars are unused. Maybe that should be rolled into the main article?

Either Fiber channel uses a different scheme that I don't understand ("even/odd disparity") or it is plain wrong. If the latter, I think it could be deleted. I did change the link iom the "§fiber channel" page to point here. --Mschnell 14:05, 9 October 2007 (UTC)

I do not think it is good idea to merge the fiber channel article with this page. 8b/10b initially invented for fiber channel but I believe that its used more generally now for serial communication. It is used in many other applications (listed in article). Its good idea to keep the link between the two articles though. --Rahul —Preceding unsigned comment added by (talk) 18:49, 16 July 2008 (UTC)

IMHO, Disparity not zero after two Blocks[edit]

The article sais "This means that there are just as many "1"s as "0"s in a string of two symbols". I don't see how this is possible. when e.g encoding

0x00, 0x31, 0x31, 0x31, 0x31, 0x31, ...

0x00 introduces a disparity and 0x31 (=D17.1 => 1000111001) can't change it.

So the "relative disparity" is granted to get zero only with an infinite number of characters.

In fact the running disparity never is 0. It starts with either -1 or +1 and with any 6 or 4 bit block it gets incremented by either of -2, 0, or +2, the increment's sign being opposite to the value's.

A similar (but supposedly correct) version of the statement would be "This means that the difference between "1"s aand "0"s in a string of at least 10 bits is no more than 2". --Mschnell 07:35, 20 September 2007 (UTC)

Question : consider the following sequence D14.A7, D13.5, D20.2 (Initial RD = +1)[edit]

code Initial RD 6 bit code mid RD 4 bit code Final RD
abcdei fghj
D14.A7 +1 011100 +1 1000 -1
D13.5 -1 101100 -1 1010 -1
D20.2 -1 001011 -1 0101 -1
transmit sequence a first 011100 1000 101100 1010 001011 0101
bits 25:6 ....00 1000 101100 1010 0010.. ....

This arbitrary 20 bit substring has 7 ones and 13 zeroes.

IMHO this is not consistent with the statement in the first paragraph of the article : "This means that the difference between the counts of ones and zeros in a string of at least 20 bits is no more than two"

The statement appears true for 20 bits ending on a symbol boundary (as it would be for any sequence ending on a abcdei or fghj boundary with an integral number of symbols. With that interpretation, 20 bits would have no special meaning. As written the statement can reasonably be interpreted to include the counter example with unaligned bits. I have not seen this property stated for 8b10b elsewhere and would appreciate an expert restating this (or else explaining my misunderstanding).

(Note: newbie both to Wikipedia edits and the details of 8b10b encoding ) --kmrbrierley 07:56, 16 November 2015 (UTC)

IMHO, Table "Effect of Running Disparity" hard to understand and not necessary[edit]

1) In table "5B/6B code" the input RD is noted as "RD=-2" and "RD=+2". IMHO RD always is either +1 or -1 so here this should be the same as with table "3B/4B code": "RD=-1" and "RD=+1". I think best you just would say "RD=-" and "RD=+" in both tables.

Moreover it could be helpful to add the Disparity effect (-2, 0 or +2) with any result code entry in both tables:

3b/4b code
input RD = -1 RD = +1
HGF fghj
D.x.0 000 1011 (Dis=+2) 0100 (Dis=-2)
D.x.1 001 1001 (Dis= 0)
D.x.2 010 0101 (Dis= 0)
D.x.3 011 1100 (Dis= 0) 0011 (Dis= 0)
D.x.4 100 1101 (Dis=+2) 0010 (Dis=-2)
D.x.5 101 1010 (Dis= 0)
D.x.6 110 0110 (Dis= 0)
D.x.P7 111 1110 (Dis=+2) 0001 (Dis=-2)
D.x.A7 111 0111 (Dis=+2) 1000 (Dis=-2)

2) As the disparity is handeled equally when starting a 6 bit block and when starting a 4 bit block, you don't need to handle the disparity of complete 10 bit blocks at all.

So the table "Effect of Running Disparity" could be replaced by

Rules for Running Disparity
Previous RD RD of 6 or 4 Bit Code Next RD
- - Error
- 0 -
- + +
+ - -
+ 0 +
+ + Error

In the text you could state that for each byte to be coded the rule is applied to the 5B/6B part first and to the 3B/4B part afterwards with the RD resulting then to be the carry to the next byte operation. --Mschnell 15:26, 19 September 2007 (UTC)

IMHO, "five identical bits must not appear in normal code" is not true[edit]

When generating e.g. D.18.7, D.19.7 (i.e. D.18.A7, D.19.P7) after RD=-1, the bit sequence will be 010011 0111 110010 0001. Here we do have five identical bits. But we don't have one of the dedicate commas (with two zero bits before the five ones, which is not possible with D.x.A7) --Mschnell 16:51, 19 September 2007 (UTC)

I agree, as I understand it, and furthermore the assertion that the running disparity never exceeds the range -2 .. +2 is untrue since several of the zero-disparity codes start 00 or 11 (the former giving -3 disparity if -1 initially, the latter +3 if +1) MarkTillotson (talk) 15:08, 13 September 2013 (UTC)

For a lay person such as myself, this statement is a bit confusing: "This means that the difference between the count of ones and zeros in a string of at least 20 bits is no more than two, and that there are not more than five ones or zeros in a row."
Without knowing any of the technical details regarding the veracity of the statement, it's difficult to propose an alternative, but perhaps something like: "the difference between the count of ones as compared with the count of zeros" or "the difference between the number of ones and the number of zeros" would be better. Also, separating the last bit into its own sentence may be worthwhile. (talk) 15:58, 10 June 2014 (UTC)
Hm, the only thing that to me seems wrong with "the difference between the count of ones and zeros" is the singular form of "count". Will get it corrected, and it should be a bit better – if you agree. — Dsimic (talk | contribs) 15:04, 13 June 2014 (UTC)

IMHO, a separate table for K-symbols is not necessary[edit]

We better enhance the 3b/4b table appropriately. (4 result columns D-, D+, K-, K+ --Mschnell 15:31, 24 September 2007 (UTC)

After I got no response here, I did most of the changes I suggested. Sorry for forgetting to log in first :( :--Mschnell 09:04, 25 September 2007 (UTC)

Ideas for article improvement[edit]

Howdy all, the page is looking considerably better than it did not too long ago... good job everyone! Looking over it, here are a few thoughts I had:

  • Any useful content (or simpler phrasing) in the Fibre Channel 8B/10B encoding article should be merged in and that article set to redirect here
  • The "How it works" section should probably be moved down below the "Technologies that use 8b/10b" so that the encoding tables can be made to be a subsection of "how it works".
  • While the concept described in the sentence about the Ethernet transformer is correct, the overall statement looks technically wrong since 1000Base-T doesn't use 8b/10b.
  • Lastly, it seems like the intro could be re-written to be somewhat easier for laymen to understand.

Anyone interested, please feel free to be bold and do any (or all) of these! —Mrand T-C 14:12, 26 September 2007 (UTC)

while the first issue is beyond my nowledge and the last is hard to do for me as I'm not a native English speaker, I'll take a look at the two others tomorrow. --Mschnell 20:32, 27 September 2007 (UTC)

Differences with TMDS[edit]

According to the author the difference between IBM 8b10b and TMDS is subtle. But in my opinion the differences are significant:

  • TMDS selects between 3 different encoding schemes depending on audio/video/control data content in a particular time. There are also differences in encoding between TMDS data channels.
  • IBM 8b10b uses 5b6b and 3b4b subblocks while TMDS has a dedicated algorithm during the video data period.
  • TMDS has special coding for q_out[8] and q_out[9].
  • IBM 8b10b has much tighter control over the DC balance, while TMDS introduces the concept of a counter that tracks the disparity.

In my eyes the only thing in common is the translation from 8 bits to 10 bits.

What do you think about chancing this? —Preceding unsigned comment added by (talk) 21:30, 12 July 2008 (UTC)

  • It's a wiki! You are clearly better informed than the information presently available in the article. Please, dive in and update the article; or add web addresses for any references you have regarding the differences and somebody can help you or try to update the article themselves based on those new references. Once again, many thanks for taking the time to get involved! —Sladen (talk) 21:58, 12 July 2008 (UTC)

The paragraph on the subject at the moment is not very good. What about "Note that while most uses of the term 8b/10b refer to this particular code, this name is sufficiently generic that it is sometimes applied to other, incompatible codes that also expand 8 bits to 10 bits. One such example is Transition Minimized Differential Signaling, which is not related to the 8b/10b code described in this article."? Larry Doolittle (talk) 22:31, 22 June 2010 (UTC)

Running Disparity confusion[edit]

I'm by no means an expert on 8b/10b, but after doing some digging on the following statement it seems to be reversing cause and effect:

Obviously, if the six- or four-bit code has equal numbers of ones and zeros, there is no choice to make, as the disparity would be unchanged, considering the following exceptions. RD is positive at the end of the six-bit sub-block if the six-bit sub-block is 000111, and RD is positive at the end of the four-bit sub-block if the four-bit sub-block is 0011. RD is negative at the end of the six-bit sub-block if the six-bit sub-block is 111000, and RD is negative at the end of the four-bit sub-block if the four-bit sub-block is 1100.

To me, this reads as "Regardless of what value RD has, it will be positive after using 000111 or 0011 and negative after 111000 or 1100". If I'm not mistaken, the desired statement is "Even though the disparity of 000111 is 0, if RD is positive use 000111 or 0011, if it's negative use 111000 or 1100, and in both cases leave RD unchanged." I've gone back through the edits and it seems this general wording has been in place since 2012, so while I'm pretty sure my understanding is correct, I'm not quite 100%. I'm going to go ahead and change the wording of the article, but I'm leaving this comment here so somebody who knows better than I do is more likely to double check it.

Rezurok (talk) 22:11, 27 September 2016 (UTC)

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