Slow modem transfer rate

Helpline Q&A Directory

I am puzzled by the transfer rate my computer achieves while downloading files from the web. I have a 113Kbps modem, yet the best connection rate I can achieve with it is a measly 3Kbps. I realise my computer is not cutting-edge, being a 400MHz Celeron with 128MB SDRAM and a V.90 modem, but it seems strange that I can only download at this speed. Is the phoneline the limiting factor? If not, what is?

There are several confusing points to consider. First, modem speeds are quoted in kilobits per second (Kbps), not kilobytes (KB). There are eight bits to a byte, plus start and stop bits where serial communications are concerned. Your web browser's download window shows the KBps speed. To convert this figure to kilobits, multiply by 10.

Internet communication protocols add an overhead of up to 10 percent of the data transmitted. You don't see this data, and it isn't counted in the download speed shown by your browser. It contains the information that enables the data you want to be routed to your computer across the internet.

Second, your V.90 modem's maximum speed is 56Kbps, not 113. If your software shows the modem connecting at a higher speed, such as 115,200Kbps, that's the speed of the serial port-to-modem connection, not the link between your modem and ISP. In fact, a V.90 modem will usually connect at 44Kbps to 48Kbps due to the limitations of your telephone line. A few lucky people may get 50Kbps.

Some people may only manage to connect at 28Kbps to 33Kbps because BT is using a DACS (digital-to-analogue converter) unit on the phoneline. This device enables a copper wire to be shared between two lines. If yours has a DACS unit fitted, you are unlikely to even achieve 3KBps. And it's often impossible to get BT to remove the unit as it is normally used when there's a lack of copper wires in your area.

The download rate reported by your software depends largely on whether the data you are downloading is very compressible. V.90 modems compress data before sending it over the phone network, and decompress it when sending it to your computer via the serial port. If the data is already compressed - for example, a ZIP file, JPG image file or MP3 audio - it can't be compressed any further. Therefore, the best speed you'll ever see is about 4Kbps. Any delays on the internet will reduce this figure. For instance, 3Kbps isn't too bad if you're downloading software from the USA.

Web pages, text files and other uncompressed files are often highly compressible, in which case you can see speeds of up to 11Kbps reported. The limiting factor is the modem-to-serial port speed; 115,200Kbps - or 11.5KBps - is the highest possible. USB modems don't have this restriction, so they may offer faster web browsing, although file downloads won't be any quicker.