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
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
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
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
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
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
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.
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
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
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 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.
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
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
- Les Jones
Ascend - Bandwidth Control Protocols: A Brief Discussion of
BACP and MP+
Boardwatch January, 1998 - More Bandwidth Using Bonded Analog
Computer Retail Week - ISP speed bumps found in 100K race --
Obstacles can be overcome, say proponents of communication
Computer Retail Week - 100 K-bps Modems May Break ISP
Computer Shopper - Boca 112K Dynamic Duo: Two Modems are Better
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
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