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Special Report:

Bonding: 112K, 168K, and beyond


What's the next stop beyond 56K? Most people are looking to digital technologies like ADSL, cable modems, and ISDN. Each of these systems offers higher speed and lower latency than 56K, but price and availability can prove to be major obstacles. In some geographic regions, analog phone lines will be the only available access technology for years to come.

For all the failings of analog phone lines, they're plentiful and cheap. Using an unsung technology called bonding, analog phone lines can deliver performance in excess of 100K or even 200K.

How it works

To understand how bonding modems work, we have to start by calling them by their real name: inverse multiplexing modems. A regular multiplexer ("mux") takes one signal and splits it into multiple signals. Electronics stores use multiplexers to show the same movie on a wall of televisions.

An inverse multiplexer takes multiple signals and bonds them into a single, usually stronger signal. Inverse multiplexing modems can bond multiple analog phone lines to double, triple, or quadruple the speed of a regular modem. The process is also referred to as multilink, channel aggregation, channel bonding, load balancing, and many other terms, but I'll refer to it here as bonding.

The idea behind bonding isn't new. RFC 1717, written in 1994, defines Multilink PPP, which is now used in virtually all modern ISDN equipment. It was later osboleted by RFC 1990. Multilink PPP allows ISDN devices to bond two 64K channels into a logical 128K channel. Another bonding protocol, appropriately called BONDING (an acronym for Bandwidth On Demand Interoperability Group), has likewise been around for years in ISDN equipment.

More sophisticated versions of Multilink PPP allow for intelligent phone line management. To conserve phone lines for voice calls, the equipment may only bring up a second or third line when extra bandwidth is needed, and drop the extra lines when the need for extra bandwidth passes. Examples of these improved include Ascend's MPP+ and the multi-vendor BACP (Bandwidth Allocation Control Protocol).

112K or 56K twice?

There's a fine point that should be brought up in the beginning. Bonding two 56K channels together provides up to 112K of bandwidth, but that 112K of bandwidth may or may not be the same as a 112K modem.

For instance, the Ramp Networks WebRamp M3 can bond three lines for a total bandwidth of 168K. However, each process (downloading a file via FTP, loading a Web page, sending an email message, etc.) is limited to 56K.

On the other hand, Boca Research's 112K Dynamic Duo bonds the two lines into a logical 112K connection. When shopping for bonding modems, make sure you know how the unit manages bandwidth.

Bonding: the technology of today

The first bonding analog modem to receive windespread attention was Transend's 67.2K modem. How is 67.2K possible, you ask? Isn't there just 64K of bandwidth in a voice circuit? Right you are! As noted in the press release and the InfoWorld story, the Transend modem is bonding two phone lines with two 33.6 connections.

If you're going to bond analog phone lines, why not use 56K modems instead of 33.6 modems? Answer: it's already been done. Angia Communications' TwinConnect modems were one of the first to use a pair of 56K modem connections for speeds of up to 112K for a single user.

The Ramp Networks WebRamp M3 uses three lines to reach speeds of up to 168K. MidCore's software uses four lines for speeds in excess of 200K. PCWEEK and TechWeb have stories. Note that these are network products, intended to provide access to multiple users, as opposed to providing high bandwidth to a single user. When used by a single user, each process (FTP, web surfing, etc.) is limited to 56K.

Diamond Multimedia's Shotgun technology bonds two phone lines intelligently to allow for voice calls and to mimimize use of the second line. When a second line is needed, it's brought up. When the additional bandwidth isn't needed, the second line is dropped. If you have call waiting, the Voice Priority feature will drop one line to allow the incoming call to ring through. Shotgun technology is built into the dual-modem SupraSonic II. Owners of existing SupraExpress 56K modems can download a free Shotgun upgrade and bond a second modem to their existing modem. The second modem doesn't have to be a Diamond Multimedia modem, or even a 56K modem, though the Voice Priority feature may not work when using any model other than the SupraSonic II.

Windows 98 will offer the option to bond multiple modems for use with Dial-Up Networking connections to the Internet. You can have this option today by downloading Microsoft's DUN 1.2 upgrade for Windows 95. For setup instructions, see Matt's Windows 95 Load Balancing page.

Who ya gonna call?

The big question about bonding modems is finding an ISP that supports this oddball configuration. For corporations, this isn't a problem. They can equip each office and each telecommuter with compatible modems. This is an especially attractive option in areas of the country where ISDN, frame relay, cable modems, and DSL are either unavailable or exorbitantly expensive.

That's all good and well for corporations, but consumers need support from their local ISPs. Today's ISPs don't support bonding except by special arrangement. That could change. Most ISPs today use Ascend MAX or U.S. Robotics Total Control terminal servers. Both are perfectly capable of inverse multiplexing. Almost no one is taking advantage of the built-in capability today, but it's in there, like a sleeping giant ready to awake when the ISP market changes.


You don't have to buy a new modem to take advantage of bonding. Several software solutions are available that work with any modems. However, you do have the option of buying a modem specifically designed for bonding.

Diamond Multimedia's Shotgun technology licenses Ascend's Multichannel Protocol Plus (MP+) to offer speeds up to 112K and intelligent use of the second phone line. The current Ascend MAX firmware supports Shotgun technology, though few ISPs have enabled the technology.

The Boca Research Dynamic Duo incorporates two modems on a single card. Like the Shotgun technology, the Dynamic Duo is designed to let incoming calls through by dropping one phone line without losing the Internet connection.


Recent versions of Windows 95 and all versions of Windows 98 support the option of using additional modems. (Users of early versions of Windows 95 can download the Dial-Up Networking upgrade. Look for version 1.2 or later.) Because it's free, this is the first thing to try. Many people have had trouble getting Dial-Up Networking to bond modems or ISDN devices, however.

Linux users can use the Equalizer driver.

Mac users may want to consider FCR Software's LinkUPP Turbo.

ISP issues

Bonding technology does offer some challenges to ISPs, not the least of which is billing, already a complicated problem for ISPs.

One major issue is that one phone line may connect to one modem chassis, while subsequent calls connect to a different chassis. This situation is referred to as spanning or stacking. This was already a problem with bonded ISDN calls, so vendors of ISP modem equipment have had time and incentive to address this issue. Some vendors (including Ascend, Cisco, and Livingston) have already fixed the problem in newer code releases.

For more information on spanning, see Diamond Multimedia's ISP page and the Computer Retail Week article below.


ADSL, cable modems, and ISDN put consumers at the mercy of price and availability. Because bonding modems work with regular analog phone lines, consumers are free to add an extra phone line when they need it. Bonding modems have the potential to be more popular than ADSL or cable modems for mass market Internet access in the short term, and are are likely to find a secure niche for high-speed telecommuting and remote office access, particularly in areas where the telecommunications infrastructure doesn't support digital technologies.

- Les Jones



Ascend - Bandwidth Control Protocols: A Brief Discussion of BACP and MP+

Boardwatch January, 1998 - More Bandwidth Using Bonded Analog Lines"

Computer Retail Week - ISP speed bumps found in 100K race -- Obstacles can be overcome, say proponents of communication devices

Computer Retail Week - 100 K-bps Modems May Break ISP Framework

Computer Shopper - Boca 112K Dynamic Duo: Two Modems are Better than One

Diamond Multimedia - Shotgun Information

HiWAAY.net Multichannel FAQ

Matt's Load Balancing with Windows 95

Microsoft - Dial-Up Networking 1.2 Upgrade

Multi-Tech Demonstrates 92-Kbps Modem Technology

News.com - Doubling dial-up speeds (Diamond Multimedia's Shotgun)

PC Week - "ISPs to provide 112K net access

PC Week - "Shotgun speeds analog communications"

PC World - "Bonded Modem Technology Taking Off Fast" (information about the Boca Research DynamicDuo)

PC World - Diamond Technology "Bonds" Modems

Reuters - "Diamond to double Internet phone speeds"

SLiRP Home Page

SmartLink Annouces Single-Chip 112Kbps Dual Line Analog Fax/Modem


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