ISDN Adapters » History » Revision 1
There are two architecturally different ways how ISDN can be attached to a computer:
- by so-called active ISDN adapter
- ISDN Terminal Adapters are a sub-class of the above
- by so-called passive ISND adapter
active ISDN adapters¶
Where active means that the adapter contains its own processor for handling the signaling protocol stack on the D-channel. Thus, the main CPU of the computer simply instructs that ISDN-processor to establish a call, and doesn't have to worry about all the low-level details.
Active ISDN adapters were the first ISDN adapters to be available, at a time where the main processor of a computer was too slow to be trusted with handling timing sensitive signaling on the public ISDN network. Many of those first generation adapters were ISA bus cards.
Examples are the ICN ISDN cards, as well as the AVM A1.
ISDN Terminal Adapters (TA)¶
An ISDN terminal adapter is an active ISDN adapter which provides a RS-232 serial port with hayes-style AT commands towards a dumb terminal or (more frequently) a computer. This approach meant that they were a drop-in replacement for analog modems, and all software developed in the earlier decades supporting modems over RS-232 with hayes AT commands could be used with ISDN. The terminal or computer (and its software) didn't really notice that it is now talking to ISDN instead of an analog modem.
For less popular / mainstream computers, TAs were often the only (or at leat the safe) choice to use ISDN.
passive ISDN adapters¶
Contrary to the active ISDN adapters, the passive ISDN adapters did not contain any on-board processor or software. Instead, they passed the raw ISDN signaling messages to the normal main CPU of the computer, where the driver and/or operating system were in charge of implementing the ISDN signaling protocols (typically Q.921 + Q.931).
This approach drastically reduced the bill of materials and hence the cost of ISDN interfaces, and made them more accessible to the mass market.
One benefit (from the open source point of view) is that there is no proprietary firmware implementing the protocol stack, but every last bit is under control of the open source software. People could finally fix bugs as they pleased, not having to rely on vendors to provide updates. On the other hand, this of course also meant that some people had regulatory concerns, as suddenly anyone could send malformed signaling to the public ISDN network, whether intentionally or accidentially.