Step 2 is the hard part.

Matt Ginzton writes here.

Misleading DSL Sync Speeds, Revisited

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In response to my previous articles about frustrations with my DSL modem, sent me a replacement modem to see if it would fare better. It had one specific improvement but for the most part duplicated the previous results; however I promised to post an update, and during the extra testing I learned one more useful fact, so this is that update with that one useful fact.

That useful fact: if you have two DSL lines in bonded mode, but the DSL lines sync at different speeds, the Comtrend 5361 modem cares a lot about which order you connect the lines (which of the fast/slow lines is master and which is slave).

In more detail: the testing I was doing involved different ways of connecting the modem to the DSL lines (one line at a time or both lines bonded, if bonded then testing both ways to pair the incoming lines with the modem, with and without DSL filters, with and without additional indoor wiring, and with both Annex A and Annex M spectrum allocation modes).

If the modem is configured for Annex A operation and is connected to the MPOE with no filter and minimal indoor wiring, I see it provide the good results you’d expect: on either line alone, it transfers real bits at about 85% of that line’s claimed sync speed, and connecting both lines together, I get the sum of the individual speeds (or 85% of the combined sync speed, which is the same thing), and it doesn’t matter which line is master and which line is slave.

That’s all as it should be. Where it gets weird is if I deviate from that exact configuration. If I add a DSL filter, or enable Annex M, then the modem may deliver only 1/3 of the claimed sync speed, depending on the way I connect the 2 lines for bonded operation. Single-line operation still yields data transfers at 85% of sync speed, and if I connect it for bonded operation with the fast line as the master and slow line as slave I get the expected combined rate, but if I connect the slow line as the master and the fast line as the slave, real-world performance drops drastically. This is extra weird because the line order doesn’t affect the sync speed; in the bad ordering, the modem still claims a nice fast sync speed and just can’t transfer data nearly as fast as it should. I’m still at a loss to know where the performance is going or why.

Now, if you know this is a problem and know how to fix it, you can route around it. And most people probably don’t end up with 2 DSL lines that sync at different speeds, and I presume that if they sync at the same speed the modem doesn’t care how you connect them. But this weird behavior sure caused me a lot of grief and lost time.

On the bright side, the newer modem did perform correctly for single-line operation on either line, which does address my initial complaint.