Australian PC World - Speed up dial-up modems

Speed up dial-up modems

Tip
A little over a year ago, when the first crop of affordable V.34, 28.8Kbit/sec analog modems hit the market, we were told that this technology squeezed every last bit of bandwidth out of the existing telephone infrastructure. We were misinformed.
Two higher-performance features are showing up in affordable V.34 modems, thanks to new data-handling techniques: 33.6Kbit/sec data rates and 230Kbit/sec throughput. (Vendors are also talking about a third dial-up option said to offer rates up to 56Kbit/sec in one direction.)
The first of the options available today, 33.6Kbit/sec rates, is not overwhelming. If you need another modem, by all means purchase one capable of 33.6Kbit/sec; these cost little more than 28.8 models.
And if you already have a 28.8Kbit/sec modem, check with your modem's manufacturer about upgrades. You can upgrade U.S. Robotics Courier modems to 33.6Kbit/sec, for example, by downloading free software from the company Web site (http://www.usr.com/home/online/upgrades.htm). For most U.S. Robotics Sportster models, making the jump to 33.6 requires swapping an inexpensive ROM chip on the modem card.
But is it worth buying such a chip -- or a whole new modem -- just to gain 4.8Kbit/sec? Probably not. You get, at most, a 17 per cent increase in performance. And that assumes that you've got a similar modem on the other end and perfect line conditions for the transmission.
The second new feature -- 230Kbit/sec throughput through enhanced compression techniques -- is a much better bet.
While we may identify modems by their modem-to-modem throughput (such as 14.4, 28.8, and so on), they can achieve much higher effective throughput as a result of compression.
When you transfer a file, your computer sends the file through a serial port (or, with internal modems, firmware that emulates a serial port) and into the modem, where it's compressed and then sent out. The efficiency of compression depends on the file type. Text and graphics files usually yield the best compression ratios, but precompressed files such as ZIP files can't be compacted much more.
The handling of highly compressible data is also limited by how fast the serial port can pump data to the modem.
Until recently, the fastest PC serial ports supported a maximum of 115Kbit/sec, so 28.8 modems were limited to a maximum compression ratio of 4 to 1. But new enhanced serial ports that can handle rates of 230Kbit/sec, combined with beefed-up compression hardware on the modem itself, have pushed that theoretical peak up to 8 to 1.
So if you're sending or receiving highly compressible data, you could conceivably double your throughput. In fact, our early performance tests of such modems appear to confirm this. Is buying a new modem to obtain these higher rates worth it? If you send or receive lots of compressible files, yes.
One caution: the key to an enhanced serial port is the UART chip, which controls the flow of data across the serial port. To support 230Kbit/sec rates, the serial port should use at least a model 16650 UART; 16550 or earlier chips don't do the job.
External modems connect to a serial port that's controlled by a UART on the motherboard or on a serial card. If you're stuck with a slow UART -- and even most new motherboards currently use the 16550 -- consider buying a high-speed serial card.
Internal modems put a UART right on the modem card or emulate it in firmware. So if you have an upgradable internal modem, you don't need to worry about the UART's limitations: the software or hardware boost will upgrade the UART functions as needed.
Finally, whatever modem you buy, make sure the manufacturer has software drivers for Windows 95, since the standard serial port driver may not be able to handle the higher rates.
- Kirk Steers

Category: Hardware
Issue: Mar 1997
Pages: 177-181