Next Previous Contents

3. Modem Pools, Digital Modems

3.1 Introduction

These are multiple modems that an ISP or the like might use. 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 nearly 56k (if you have a good line). 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.

3.2 Analog Modem Pools, Multiport Modem Cards

These are just many 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 this burden from 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.

Multiport Modem Cards:

3.3 Digital Modems

"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 near 56k, something no analog modem can do. They are often a component of "remote access servers" (RASs) 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 gets the digital signal from the telephone company. It converts the waveshape it represents back to the same data bytes that were sent from the sending PC. It puts these bytes on its 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 "digital modems" call themselves "digital signal processors", or "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.

Next Previous Contents