3.3 Digital Modems, RAS
"Digital modems" are much different than the analog modems that most people use in their PCs. But they can communicate with analog modems at the other end of the phone line. ISP's use "digital modems" to send out data at almost 56k bps to 56k modems in homes and offices. The "digital modem" requires a digital connection to the telephone line (such as T1, EI, ISDN PRI, etc.). Except for some serial ISDN "modems", they don't use serial ports for the interface to the computer. Instead, they interface directly to a computer bus via a special card(s) (which may also contain the "digital modems"). They are often a component of "remote access servers" (RASs) or "digital modem pools"
You may already know that each time you make a telephone call from an analog device (a telephone or a modem) it gets converted by the telephone company to a digital signal. Then it's transmitted to near its destination as a digital signal and finally converted back into an analog signal just before it reaches it's destination. But it's also possible to receive this digital signal directly from the telephone company if you have what is called a "T1" line, etc.
The cables from the phone company that carry digital signals have been designed for high bandwidth so that the same cable carries many telephone calls. It's done by what's called "time-division multiplexing". A single phone call in a cable is carried on two different channels, one for each direction. So the RAS must connect each such channel-pair to the appropriate "digital modem" that services that phone call. Such tasks are done by what is sometimes called a "... concentrator".
Now the digital signal received by a "digital modem" may really represent an analog signal which has been sent to it by an analog modem. This is because when you send an analog signal (including ordinary voice) to the telephone company, it gets converted into digital by the phone company. One way for the digital modem to deal with this digital signal would be to convert it to an analog signal and then put that thru an analog modem to get the digital data sent by the analog modem. But why do all this work? Since the signal is already in digital form, why not process it digitally? That's how it's done. The digital signal is processed and converted to another digital stream of bytes which represents data bytes sent by the analog modem. A "digital signal processor" (DSP) is commonly used for this task. A CPU could also handle it but it would be heavily loaded.
Likewise, a "digital modem" must handle sending digital signals in the opposite direction from a RAS 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".
Such a RAS 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".
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