2 edits | Question on Single Line Failures with Bonded ADSL2+ Occasionally, a single line may have a problem that does not affect the other line to the same home. This may be evidenced by a complete loss of service on that line, or significant line problems that might cause the service to be so heavily degraded that it may as well be completely down. I would like to have information on what would happen in a Sonic.net bonded ADSL2+ installation in the event of such a failure for one of the two lines.
I would appreciate anyone providing me a link to a description of how this situation is handled by the various available modems for bonded service, if such a description is available online. If not, I would appreciate anyone providing answers to the following questions:
1. For the modem(s) that are available for bonded service, do each of them continue to operate on a single line if DSL is completely out on the other line? If so, does the continuation of service on the good line occur automatically? If not, what actions does the customer and/or Sonic.net need to take to continue service on the good line, pending repair of the lost line?
2. How do those modem(s) handle the case of a high error rate (i.e., other than a complete loss of service) on one of the lines? What actions does the customer and/or Sonic.net need to take to continue service on the good line in this instance?
3. Can a normal (i.e., non-bonded) ADSL2+ modem be used on either line of the bonded lines, in the event that the other line has failed or is experiencing very high packet loss? Can that be done without the intervention of Sonic.net?
Additionally, reports of any actual experience(s) with this scenario will be greatly appreciated, particularly citing the specific bonded modems involved. |
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 leiboldPremium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:2
 ·SONIC.NET
·Pacific Bell - SBC
| 1.) At the DSL protocol (lower level) the two lines are completely independent of each other. This means any operational difficulties on one line have absolutely no bearing on the other line. It also means that it is possible to use completely different settings for each DSL line. At the DSLAM side they are two normal DSL ports (the only constraint being that the two ports have to be on the same line card) and at the CPE side the DSL modem contains two single-line all-in-one ADSL chips. The bonding takes place at the next higher protocol level (ATM). On the sending side IP packets get put into ATM cells and transmitted via either of the two lines (whichever is ready to transmit) and on the receiving end the cells are reassembled into IP packets regardless through which line they came. No reconfiguration on either end is necessary if one line goes down.
2.) If one of the lines has a high error rate it will take much longer for a successful transmission of an ATM cell (due to retransmits) in the meantime the good line will transmit many more cells. This means that even so one of the lines is of poor quality it will still contribute to the overall bandwidth but since both lines are independent it will not hold up the good line. If you really wanted to, you could disconnect the bad line but I don't see why.
3.) I don't know that for sure. The DSLAM will try to negotiate a bonded connection and it depends on the modem firmware how it will handle a feature that it doesn't support. In theory both the modem and the DSLAM should fall back to a common supported mode of operation but this may work with some modems and not with others. While you may be able to use one non-bonded ADSL2+ modem with one line of a bonded pair, it is definitely impossible to use two non-bonded ADSL2+ modems on the two lines of a bonded pair. The bonding has to take place at the ATM level and with two separate modems you can't do that.
I have both the ZyXel and the Comtrend bonded modems but mostly use the Comtrend (Nexuslink 5631). I have seen it work fine with just one line at a time while experimenting.
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| The ZyXel isn't quite as forgiving to losing a line, at least from what I understand. It may have to be switched into Single Line mode in order to sync if one of the lines was to go down completely. Someone please correct me if I'm wrong, but this has been my experience. |
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 DaneJasperSonic.NetPremium,VIP
join:2001-08-20
Santa Rosa, CA
kudos:7 | reply to mxyztplk
Our bonded modems should:
Sync on a single line (either of them).
Sync on both lines (bonded).
Gracefully handle loss of either line.
Gracefully re-add that line if/when it comes back.
After loss of a single line, gracefully sync on one line after powerfail.
You can test to assure these conditions are met. If they are not, a firmware update might be needed, call support!
-Dane |
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2 edits | Thanks leibold, CCN and Dane. This is just the information that I was seeking. Incidentally, I think such information on the expected fault-tolerance of the bonded service should be included somewhere in the Sonic.net FAQs.
For my question 2, my concern was that continued transmissions on a line with a high error rate would result in significant delays and/or timeouts, and that the bonded firmware might not have an error threshhold that would force a faulty line to be automatically temporarily deconfigured. Does anyone know if such an error threshhold exists in the applicable firmware? Has anyone experienced any such degradation due to a high error rate on one of the lines of a bonded pair?
For my question 3, I am wishing to use my current normal (non-bonded) modem to act as a backup, in the event of a failure of the bonded modem, and also as a check on the performance of the bonded modem on each line.
Dane, is there anything special about each line of Sonic's bonded pairs that would inherently prevent a normal modem from being used on one of the lines?
If anyone has successfully used a normal (non-bonded) modem on a single line of a bonded pair, which normal modem did you use? |
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DaneJasperSonic.NetPremium,VIP
join:2001-08-20
Santa Rosa, CA
kudos:7 | I don't know how the bonded service behaves if one line is unstable, sorry. That's not something I've encountered.
Regarding using a non-bonded modem as backup, no, this will not work, our staff would need to take the circuit out of bonded mode for a standard modem to function.
-Dane |
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leiboldPremium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:2 Reviews:
·SONIC.NET
·Pacific Bell - SBC
| reply to mxyztplk
Regarding question 2 I'm happy to report that I never encountered that situation and sincerely hope that I never will.
My expectation however is that neither DSL modem nor DSLAM will ever completely disable a poorly performing line. As long as there are still some bits coming across that line intact they will reduce the data rate to match the capability of the line but continue to use it. On a really bad line there will be periodic downtime on the line while DSL modem and DSLAM are retraining to find any frequencies on which they can get an intact bit across but it will not be down permanently.
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