ISDN stands for integrated services digital network. Voice and data are carried by bearer channels (B channels) occupying a bandwidth of 56 - 64 KBps (the latter being the standard). A data channel (D channel) handles signaling at 16 or 64 KBps, depending on the service type.  
  Two types of ISDN service are basic rate interface (BRI) and primary rate interface (PRI). BRI consists of two 64 KBps B channels and one 16 KBps data channel for a total bandwidth of 144 KBps. PRI is intended for users desiring greater bandwidth. It consists of 23 64 KBps B channels plus on 64 KBps D channel for a total bandwidth of 1536 KBps. In Europe, PRI consists of 30 64 KBps B channels and on 64 KBps D channel for a total bandwidth of 1984 KBps. It is also possible to support multiple PRI lines with one 64 KBps D channel using non-facility associated signaling (NFAS).  
  ISDN's greatest challenge to being accepted as a viable connection to the Internet is it's price. The first item is the ISDN modem. Some ISDN modems run as high as $400, but most are around $200 - $250. ISDN providers usually have to install some additional wiring, but at least they can utilize the standard phone lines that are already there. Service rate charges come from both the telephone company and the Internet service provider (ISP). The service rates can vary for the phone company where they charge a monthly fee plus 1-2 cents per minute. That does not sound like a lot, but, if you wanted 24/7 service, that runs roughly $650 / month.  

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