How Modem Bonding Works

It’s the hot new technology for 1998: modem bonding. Windows 95 supports it, many modem makers support it, and ISPs are beginning to support it.

Just What Is It?

Modem bonding is using two or more modems to simultaneously handle a single connection. The faster the modems, the higher your throughput. Likewise, the more modems you use, the higher your throughput.

The first bonded modems for the mass market offered two 33.6 kbps modems. The market advantages of 67.2 kbps vs. so-called 56k modems were obvious:

  • Works on any phone line, not just some.
  • 57.6-67.2 kbps throughput with a bonded modem beats 44 kbps or so with a 56k modem.
  • Allows you to disconnect one line to use the phone. (Downside: You need two phone lines for bonding!)

Now bonding has embraced both 56k standards, offering theoretical throughput beyond 100 kbps and real-world throughput in the 90 kbps range. Using a bank of four 56k modems would double that.

What’s the Catch?

The first catch is that you need two or more phone lines. That will probably involve an installation fee. On top of that, you’ll have a higher monthly phone bill with multiple lines.

The second catch is that you need two or more modems (sometimes two modems on a single card), and your older modem may not be compatible with bonding technology.

The third catch is that you need to have someone to connect to using bonded modems. So far, few ISPs have embraced this technology – but most have embraced 56k modems.

A fourth catch could be adding the additional modem(s) to your computer. You may have already filled all of your expansion slots or used all of your serial ports.

A fifth catch could be serial port speed for external modems. Older computers often had slow serial ports (by today’s standards) and could only be driven to 38.4 kbps or 57.6 kbps. These port speeds don’t provide the throughput needed to drive a 33.6k or 56k modem at full speed when you take into account data compression.

Your serial port should run at no less than twice the speed of the modem you are using. Running a 56k modem with the serial port set to 115.2 kbps provides roughly one-third better throughput than using the serial port at 57.6 kbps. Bumping that to 230.4 kbps on my Mac added another five percent or so to total throughput.

Of course, internal modems are unaffected by serial port speed. If you want to do bonding, you should look into multiple modems on a single card.

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