>>Under no set of circumstances were you ever connecting standard modems over POTS (Plain Old Telephone System) lines at anything approaching 115Kbps -in the US, the FCC actually limits the maximum throughput of x2/Kflex/v.90 modems to a bit over 52K on the inbound channel.
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>>This is simply a case of looking at a meaningless number (in both cases) and reacting because the number is smaller, and therefoire, looks worse. The real measure is not the reported speed of the conenction, but objective measurement - connect using the two different modems and download an identical file; if the connection is direct, or if Internet conditions are similar using an Internet connection, you can get a more meaningful number - throughput - rather than reported connect speed.
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>>>TIA
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>What about if you're connecting through a pots port on an ISDN modem? Mine reports 115k also, and appears to be significantly faster than the 56k connection I get at work.

If you're connecting over POTS, you have an analog line; if you're connected to another ISDN connection, then you're using the digital line, which can carry 56K bidirectional (that's the effective data rate of a single B channel.) If you use both B channels of an ISDN link, you can get a data rate of ~112Kbps. But that's not POTS.

If you have a connection to an analog line over your ISDN modem, you may simply have a cleaner connection than you have at your office. But even pure digitial, a single B channel is limited to 64Kbps of raw transmission; with framing and other overhead, the effective data rate becomes about 56Kbps, assuming the connection is a pure digital (ISDN device to ISDN device over digital line.) Once you actually have an analog line in the mix, you become limited by the analog line itself; with the curent FCC rules, the raw bandwidth limit is about 52Kbps on the inbound channel and 33.6Kbps on the outbound channel. The numbers in both cases are raw bit rates, of course; with compression, your data rate might appear to be higher. That doesn't affect the raw data rate of the analog line, it's more a matter of how much repetitive data can be squeezed down via compression algorithms (I think that the compression on the 56K standards is still one of the v.42 protocols at best.)