How ISDN can transmit 144 kbs while modem just 56 kbs over same line 1

[zenica]

How ISDN at basic rate can accross ordinary telephone line transmit datas at rate 2*64+16 kbs = 144 kbs while modem can only at max 56 kbs. I understand one B channel can support 64 kbs because digitalisation, D channel can support 16 kbs because it is transmited outband with different fruqency range than B channel. What is with rest 64 kbs - second B channel ? I assume that ISDN use second physical (from twisted pair) wire for one B channel which is used for electricity at ordinary phone or modem traffic but I am not sure.

Second question:
What additional equipment local telephone exchange (it's already digital) must have to support ISDN ? I assume some equipment for SS7 for D channel, and some store and forward capacity. Does anyone know what and names of that equipment ?

 

[Belushi]

ISDN actually only uses 1 pair of wire just like a regular phone line. Your actual throughput is never more than a maximum of 128kbps in each direction. The 16kbps channel is used for signaling andcan't be used by the consumer. The same wire is used for both POTs and ISDN, but they connect to different types of telephone switches on the other side. There are actually several different standards of ISDN which is part of the reason why it never really took off. For instance, you may see a completely different type of ISDN emulation from one area to another, especially oerseas.

Most of today's equipment can account for the different types of ISDN emulation. You need to have some type of terminal adapter which acts as a modem of sorts for ISDN. My personal favorite is the 3 COM 3C892 which incorporates the terminal adapter and a router all in one. Check it out.