CatChat: Telecom Modem

CatChat: Telecom Modem

CatChat: Telecom Modem
Click here or on the image to view this entry in our collection.


Telecom NTU-LS X21 bis 9.6k Modem for leased lines
RS-232 25 Pin serial interface

Do you have any additional information or resources about this item? Did you own one, work with one or have you repaired one? Please join the discussion about this item in the ACMS collection here on CatChat. Alternatively, you can view other items in our collection here.
Created by CatalogBot

An old Telecom NTU (Network Terminating Equipment) technically this was not a modem as such, as it was part of the DDN (Digital Data Network) that Telecom/Telstra ran back in the 80es, 90es, and early 2000es.

It would connect back at the exchange to a ZDME - short for Zero order Digital Multiplex Equipment.
the ZDME would multiplex either 2400, 4800, or 9600 bps services together up to a 64Kbps, this in turn would feed a 1DME which then multiplexed up to 2,048 kbps (don’t think I am likely to be in an exchange for the next week or so, but I will get pictures when I can).
These higher order links would connect to various TDCCs (Time Division Cross Connects) around the country which would interconnect the lower speed services.

DDN was not a cheap service offering, (although almost nothing from Telstra was back then) but higher order links were monitored, as were TDCCes etc.
Also a test and monitoring system MACS/CAF (Sorry don’t remember what that stood for) could do remote testing, we could send a loopback request to the ZDME, or the NTU, and run a BERT (Bit Error Rate Test) to identify and locate faults remotely.

Again not my handwriting on the sticker, a regular DDN service was an NxxxxxxxN number, but from memory a YxxxxxxN number denoted a DDN link into the X25 Packet switching exchange Austpac.
Db25 and at that speed is RS232 but in this case is Synchronous RS232.

Memories slowly coming back, the single screw under the front panel allows you to remove the front panel, then the cards could be levered out to configure the NTU. IRC this version of the NTU needed line rate to be configured (As well as other parameters like controlled carrier etc) while the later cream coloured GEC made NTU would automatically work out line speed.

1 Like