A modem pool is a number of modems on the same card (such as a
multiport modem card) or many modems in an external chassis (something
like an external modem). The modems may be analog modems similar to
modems used for home/office PCs (can't send at 56k even if they are
"56k modems"). They also could be "digital modems" which can send at
56k. By 56k I actually mean all speeds above 33.6k as the 56k modem
can't quite do a true 56k speed. The "digital modems" require a
digital connection to the telephone line and don't use any serial
ports at all. All of these modem pools will require that you install
special drivers for them.
These are just several (or more) analog modems (the common
home/office modem) provided either on a plug-in card or in an external
chassis. Each modem comes with a built-in serial port. There is
usually a system of sharing interrupts or of handling interrupts by
their own electronics, thus removing much of the burden on the CPU.
Note that these modems are not "digital modems" and will thus not be
able to use 56k for people who dial-in.
Here is a list of some companies that make multiport modem cards. 8
modems/card is common. The cards listed claim to work with Linux and
the websites should point you to a driver for them.
"digital modems" are much different than the analog modems that
most people use in their PCs. They require a digital connection to
the telephone line and don't use serial ports for the interface to the
computer. Instead, they interface directly to the PC bus via a
special card (which may also contain the "digital modems"). They are
able to send at 56k, something no analog modem can do. They are often
a component of "remote access servers" or "digital modem pools"
The cables from the phone company that carry digital signals have been
designed for high bandwidth so that the same cable carries multiple
telephone calls. It's done by "time-division multiplexing". So the
first task to be done is to separate the phone calls and send each
phone call to its own "digital modem". There is also the task in the
reverse direction of combining all of the calls onto a single line.
These tasks are done by what is sometimes called a "... concentrator".
The digital modem takes the digital signal from the telephone company
and after processing it, puts it on the PC's bus (likely sending it to
a buffer in memory). Likewise, it handles sending digital signals in
the opposite direction to a digital telephone line. Thus it only
makes digital-to-digital conversions and doesn't deal in analog at
all. It thus is not really a modem at all since it doesn't modulate
any analog carrier. So the name "digital modem" is a misnomer but it
does do the job formerly done by modems. Thus Some "serial modems"
call themselves "digital signal processors", "remote access servers",
etc. and may not even mention the word "modem". This is technically
correct terminology.
Such a system may be a stand-alone proprietary server, a chassis
containing digital modems that connects to a PC via a special
interface card, or just a card itself. Digi calls one such card a
"remote access server concentrator adapter". One incomplete
description of what is needed to become an ISP is: See
What do I need to be an ISP?. Cyclades promotes their own products here so please do
comparison shopping before buying anything.
