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© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 1
What’s A Codeword Or Two Among Friends?
Ron Hranac
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 2
The World of Codewords
Questions often crop up about codewords, codeword errors, and acceptable percentages of DOCSIS upstream codeword errors. What the heck is this codeword stuff anyway?
A good place to start is to first define what a codeword is, and how it applies to the world of data transmission over cable networks. We’ll get to that shortly.
All of this falls under the umbrella of something known as forward error correction (FEC), a combination of techniques and algorithms used to identify and fix data transmission errors.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 3
Ensuring Data Transmission Integrity
In an ideal world, all bits would be received exactly as transmitted
Bit errors can and do occur, but there are ways to detect—and sometimes correct—those errors
The simplest form of error detection is parity checking
Appending a row of bits with a parity bit allows for simple error checking, but it doesn’t tell where in that row of bits the error occurred
Applying parity in two dimensions—that is, in rows and columns—not only tells us that an error occurred, but also where it occurred! This is known as block coding.
Examples of coding in DOCSIS include Reed Solomon (RS) and Trellis coding
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 4
Parity
One method of identifying errors is parity detection
For example, a parity bit can be appended to a group of seven bits
Assume that even parity is used. The appended bit will be a 1 if an odd number of 1s is in the original seven bits, and a 0 if there is an even number of 1s
Parity allows the detection of an odd number of errors, but doesn’t tell us which bit (or bits) is in error
0110100 1
7-bit word Parity bit
01101001
Transmitted word(including parity bit)
01001001
Error occurs
0100100 1
Receiver detects error because parity bit is
wrong for number of 1s now in 7-bit word
Total number of 1s is now even, hence the name “even parity”
Receiver cannot determine which bit is in error, only that an error
occurred.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 5
Block Coding
As mentioned previously, applying parity in two dimensions—that is, in rows and columns—not only tells us that an error occurred, but also where it occurred! This is known as block coding.
0110100111000100110011100110
0110100 1110001 0011001 1100110
0110100 11110001 00011001 11100110 0
0111010 0
0110100111100010001100111100110001110100
Parity bits
Original data stream
Groups of 7-bit words shown for clarity
Data arranged into a tabular form, and parity bits appended to rows
and columns
Transmitted data stream with parity
bits added
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 6
Block Coding
0110100111000100110011100110
0110100111100010001000111100110001110100
Received data streamwith parity bits removed
and errored bit corrected.
Data arranged into a tabular form in the receiver, and
parity bits in rows and columns are checked.
Transmitted data stream with parity bits and a bit error. The 0 shown in red was originally a 1.
0110100 11110001 00010001 11100110 0
0111010 0
Parity errors!
Adapted from example in Modern Cable Television Technology, 2nd Ed.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 7
Ensuring Data Transmission Integrity
ITU-T Recommendation J.83, Annex B, defines downstream data transmission in cable networks:
Source: ITU-T Recommendation J.83
We’re interested in the FEC part of this
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 8
Ensuring Data Transmission Integrity
In J.83, FEC has four processing layers comprising Reed Solomon coding, interleaving, randomization, and Trellis coding
Source: ITU-T Recommendation J.83
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 9
DOCSIS Downstream Codewords
As data enters a RS encoder, it is grouped into chunks called blocks or codewords, or more specifically, RS blocks or RS codewords (I’ll use the latter terminology throughout the remainder of this discussion).
Each downstream RS codeword is made up of 128 RS symbols, and each RS symbol is 7 bits.
Note that RS “symbols” are not the same kind of symbols that make up each symbol point in a QAM constellation.
7 bits = 1 RS symbol
0 1 1 0 0 1 0
≠
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 10
DOCSIS Downstream Codewords
122 of each RS codeword’s 128 symbols are data symbols, and the remaining six are parity symbols used for error correction.
ITU-T J.83, Annex B states that the data is “…encoded using a (128,122) code over GF(128)…” which shows each RS codeword consists of 128 RS symbols (first number in first parentheses) and the number of data symbols per RS codeword is 122 (second number in first parentheses), leaving six symbols per RS codeword for error correction.
DOCSIS downstream RS FEC is configured for what is known as “t = 3,” which means that the FEC can fix up to any three errored RS symbols in a RS codeword.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 11
DOCSIS Downstream Codewords
In DOCSIS downstream Reed Solomon FEC, 7 bits = 1 RS symbol, and 128 RS symbols = 1 RS codeword
7 bits = 1 RS symbol
In each RS codeword: 122 RS symbols = data symbols, 6 RS symbols = parity symbols
0 1 1 0 0 1 0
128 RS symbols = 1 RS codeword
RS symbol #1 RS symbol #2 RS symbol #3 RS symbol #4 RS symbol #127 RS symbol #128
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 12
Codeword Errors
What happens when there is, say, a burst of noise that causes a bit error or errors in one RS symbol?
It doesn’t matter to the RS decoder if one bit in that RS symbol is errored or all seven bits are errored—the entire symbol is considered broken.
= good RS symbol
= errored RS symbol
= errored RS symbol
= errored RS symbol
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 13
Codeword Errors
With a RS FEC configuration of “t = 3” the FEC decoder can fix up to any 3 errored symbols in a RS codeword
128 RS symbols = 1 RS codeword
When there are more than 3 errored symbols in a codeword the entire codeword is errored
This is a correctable codeword error
This is an uncorrectable codeword error
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 14
DOCSIS Downstream Codewords
There can be anywhere from three to 21 bit errors in a RS codeword that has three errored RS symbols.
For a t = 3 configuration, as long as there are no more than three errored RS symbols in a given codeword, the FEC decoder in the cable modem can fix all of them. That would result in what’s known as a correctable FEC error or correctable codeword error.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 15
DOCSIS Downstream Codewords
As soon as the number of errored RS symbols in a given RS codeword is four or more, the entire RS codeword is toast.
FEC can’t fix three RS symbols and leave the remaining ones broken; if more than three RS symbols are errored, the entire RS codeword is errored.
That errored codeword is now an uncorrectable codeword, and we have an uncorrectable FEC error or uncorrectable codeword error!
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 16
DOCSIS Upstream Codewords
DOCSIS downstream FEC is fairly straightforward, at least from the perspective of a fixed RS codeword size (128 RS symbols) and t = 3 configuration.
When DOCSIS 1.0 was introduced, upstream FEC supported codeword sizes ranging from 18 bytes (16 data or “k” bytes plus two parity bytes) up to 255 bytes (k bytes + parity bytes), and t = 0 to t = 10.
DOCSIS 2.0 brought improved FEC, supporting t = 0 up to t = 16.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 17
DOCSIS Upstream Codewords
The same basic principles apply in the upstream as is the case in the downstream: RS FEC can fix up to “t” errored symbols (bytes) per RS codeword, but if the number of errored symbols per codeword exceeds “t”, the entire codeword is considered uncorrectable.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 18
What’s Acceptable?
Which brings me to a common question: What’s an acceptable percentage or number of upstream codeword errors?
That’s definitely an it depends question, but let’s start with an ideal goal: No correctable or uncorrectable codeword errors at all in either the downstream or upstream!
One could argue that in a network performing so well that there are no codeword errors of any kind in the QAM signals, it would be possible to take advantage of eliminating FEC overhead altogether and picking up a little more throughput per channel.
Of course, this goal is unrealistic. There is no perfect cable network that is completely free of data transmission errors. Still, the fewer codeword errors, the better.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 19
Upstream Packet Loss
Let’s look at another metric for a moment.
I have for several years suggested that upstream packet loss—that is, packet loss in just the outside plant’s upstream transmission path—be limited to no more than about 1 percent when the traffic is plain old high-speed data.
If voice is thrown into the mix, upstream packet loss should not exceed about 0.1 to 0.5 percent, with the lower end of that range the better place to be.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 20
Codeword Errors vs. Packet Loss
How does upstream packet loss relate to codeword errors? That’s another it depends.
First, if uncorrectable codeword errors exist, that means that packet loss is happening. However, uncorrectable codeword errors do not necessarily track lost packets on a one-for-one basis.
For instance, one can configure upstream operation such that packet length equals the number of information bytes in a codeword.
One also could have a configuration in which the packet length is longer than the number of information bytes in one codeword, but less than in two codewords.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 21
Codeword Errors vs. Packet Loss
It might be worthwhile to work with your network operations center folks to come up with a packet loss estimate versus uncorrectable codeword errors in relation to the specific configuration used in your system.
From there, target maximum packet loss goals similar to my previously suggested numbers.
Once that’s done, start looking at trends over time.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 22
Packet Loss and Trends
Let’s say you find upstream packet loss to be less than 0.1 percent fairly consistently, but then you notice that it starts to trend upwards to 0.1 percent, 0.2 percent, 0.25 percent, and so on over some period of time.
That upward movement in packet loss suggests that something is degrading upstream performance and it’s getting worse.
Here the trend itself is arguably more important than the actual packet loss value, assuming the original value was reasonable to begin with.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 23
Other Troubleshooting Ideas
From a practical perspective, tracking uncorrectable codeword errors should be considered one of many tools used to troubleshoot problems. Others include modulation error ratio (MER), a CMTS’s Flap List metrics, and perhaps a third party spectrum monitoring tool.
Here are a couple examples of how evaluating more than one metric can better help the troubleshooting process.
Let’s say the CMTS’s reported upstream MER (also called “upstream SNR”) is good, but uncorrectable codeword errors are out of whack. That may indicate impulse noise or laser clipping.
Low MER and excessive uncorrectable codeword errors could indicate low carrier-to-noise ratio, or maybe the presence of some significant linear distortions (micro-reflections, amplitude ripple, group delay).
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 24
Tracking Trends
One can certainly track raw correctable and uncorrectable codeword error numbers—and many cable operators do—but another metric is to track codeword error ratio (CER)
The following example is excerpted from one of my Communications Technology magazine columns written back in 2003:
According to Motorola’s Marc Belland, using upstream uncorrectable codewords is one of the best ways to complement the CMTS’s [MER] estimate. Says Belland, “I know of several operators who use the following three management information base (MIB) values to calculate, using a script, what can be called codeword error ratio (CER) on their CMTS upstream ports. They are:
docsIfSigQUnerroredsdocsIfSigQCorrectedsdocsIfSigQUncorrectables
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 25
Tracking Trends (cont’d)
“A total of all three divided into the uncorrectables number will give you a CER at that point in time. Subsequent polls at user specified intervals—for example, every five minutes—can give you a trend as to whether the upstream is getting better or worse. Obviously if it’s getting worse you’ll need to pro-actively get someone to attend to the issue, depending on the rate of upstream degradation.”
Belland provided the following example, which could be ported to an Excel spreadsheet:
unerroreds: 1,562,456correctables: 803,867uncorrectables: 209,134total: 2,575,457loss %: 8.120%CER = 8.12E-02
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 26
Additional Resources
You’ll find some other useful background information and ideas in an older white paper authored by Cisco’s John Downey:
“Upstream FEC Errors and SNR as Ways to Ensure Data Quality and Throughput,” available at http://www.cisco.com/en/US/tech/tk86/tk319/technologies_white_paper09186a0080231a71.shtml
An updated version of the paper is available but not online. Contact John directly at [email protected] to get a copy of the newer version.
© 2011 Cisco Systems, Inc. All rights reserved. Cisco PublicCodeword Errors 27
Q and A