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I have one location using at&t dsl that connects to the office vpn correctly but the speed is unacceptable. The internet speed is 2.5 mbps & when connected to the vpn the internet speed is unaffected. When I move the laptop to other wifi points - house, hotel cigar shop - everything works fine.

What do I need to change on the dsl modem to improve the vpn speed?

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Usually DSL modems are configured by the ISPs to have an unbalanced ratio so that downloads will be much faster than uploads. The reason for this is that people doing things like web browsing (which is what most users appear to do most of the time with their internet connections), and sending a request to a web server requires very little data compared to what often gets downloaded (when viewing videos, and listening to music or radio stations, this becomes quite pronounced because one simple request to a web server starts a steady and prolonged stream of data downloading).

So, where a ratio might be 10:1, a VPN would likely be effected because data transfer needs may be bi-directional (e.g., load a document, edit it, then save it) in nature unlike most web applications [that don't involve uploading].

You'll need to contact your ISP to find out what your specific ratio is because it may vary between ISPs. Chances are that the first-level support probably has no idea how to answer this question either, so you will probably have to get your question escalated to a higher level of technical support. (The "Network Operations Centre" might be who ends up answering your question.)

Some ISPs may provide an upgrade option (usually with a higher cost) to provide higher speed access. The cable providers usually do, but for DSL it's difficult to say because there are some known hard limits, and so you might have to look at other options that are not DSL based if you're already maxing out the technology.

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The modems have nothing to do with whether a circuit is synchronous or asynchronous, its the technology type. ADSL is asynchronous so the upload will always be much less, SDSL and IDSL are synchronous so both will be equal. The modem has nothing to do with it. ADSL2 provides the fastest possible upload speed on an asynchronous DSL connection, and that is 1.4 mbps. Your ISP cannot change this. If you need faster upload speed (and with a VPN, you probably do), you should go with cable, or get a good solid low latency connection like a T1 (latency will kill a VPN). – MaQleod Mar 22 '11 at 15:28
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@MaQleod ADSL is Asymmetric! You can't actually do anything until it synchronizes. Furthermore, maximum upload speed is in fact 3.3 Mb/s. – AndrejaKo Mar 22 '11 at 22:15
@andrejako: lack of sleep made me use ATM terminology instead... Asymmetrical is what I meant, thanks for the correction there. As for the 1.4, that is an older standard (ITU G.992.5) but its also the most common standard that most ISPs will offer. You are right that if the ISP as upgraded to ITU G.992.5 Annex M, then they might offer higher uploads, but anything over 1.4 will be very dependent on distance of the copper run (you'll need to have about 8000 or less feet between you and the CO) – MaQleod Mar 22 '11 at 22:35
@MaQleod Indeed. Another problem I've seen with ISPs is uneven distribution of equipment, so some ISPs do not offer higher upload speeds to customers who do have technical capabilities because most of the other costumers won't be able to achieve them. – AndrejaKo Mar 22 '11 at 22:39
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Not a sure thing, but you might look at your VPN encryption settings. 3DES is computationally intense compared to AES, for example, and it could be that you're bumping against a CPU limitation.

It really depends on the hardware/software combination on each end, so I can't give you a definitive answer.

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