3.  Modem Pools, Digital Modems

  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.


  3.1.  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:

  ·  MultiModemISI by Multi-Tech Systems.  56k or 33.6k, PCI or ISA, 4
     or 8 ports.  ISDN/56k hybrids.
     http://www.multitech.com/products/

  ·  RAStel by Moreton Bay Products. 56k PCI or ISA, 4 or 8 ports.  Also
     2 modems + 2 vacant serial ports.
     http://www.moretonbay.com.au/MBWEB/product/rastel/rastel.htm

  ·  RocketModem by Comtrol.  ISA 33.6k, 4 or 8 port.
     http://www.comtrol.com/SALES/SPECS/Rmodem.htm

  ·  AccelePort (RAS Family) by Digi.
     http:/www.dgii.com/digi.cfm?p=940564.pi.prd.00000046

  3.2.  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: 4. Serial Port and Modem Basics »