lars@ACC.ARPA (12/17/85)
It looks like we will be hooking up terminals in new ways as digital PBXs sneak into our utility closets. DEC has already brought out a board to interface to the CPI port on the switches from Northern Telecom, thus bringing 24 terminals in one a double twisted pair, and others are not far behind, so I thought TELECOM should start discussing these things. If I understand this correctly (and who is to say I do), the CPI interface, and its competing sister, DMI (Digital Multiplex Interface) are really digital trunk lines, running at T1 rate, and the PBX thinks that the computer is just another PBX. The terminal must be RS-423, and it plugs in to the system just like a (digital) telephone instrument would, but where the telephone would deliver a steady flow of 8-bit A/D samples, the terminal delivers an asynchronous serial bit stream (which a UART can turn into 8-bit ASCII characters). A 64 Kbit bit stream will carry up to 32 Kbit of asynch traffic, so effective rate is 19.2Kbit (38.4 being too fast for the channel). Here are the things I don't understand: 1. It's been said that DMI is ISDN-compatible (23B+D) while CPI is not. Indeed, if CPI uses all 24 slots for Bearers, where is the signaling information ? 2. How does the terminal tell which CPU trunk it wants to get to ? Does this have to be preprogrammed into the switch ? (When I go off-hook, connect me to trunk group XX ?) 3. Do the interface cards (such as DECs CPI-32) allow the computer to dial out ? 4. Are there provisions for routing calls that come in on external modem lines into the CPU/PBX trunk ? I'd be thankful for any information you can provide. Given the turn-around time for the digest (is it weekly these days ?) we will probably get a lot of duplication unless you-all mail to me and then I summarise for the digest. / Lars Poulsen Advanced Computer Communications <Lars @ ACC.ARPA> ------