First steps towards an ITU-T V5.1 / V5.2 implementation

As some of you may know, I've been starting to collect "vintage"
telecommunications equipment starting from analog modems to ISDN
adapters, but also PBXs and even SDH equipment. The goal is to keep
this equipment (and related software) alive for demonstration and
practical exploration.

Some [incomplete] information can be found at
https://osmocom.org/projects/retro-bbs/wiki/

Working with PBXs to simulate the PSTN (ISDN/POTS) network is fine to
some extent, but it's of course not the real deal. You only get
S0-buses and no actual Uk0 like actual ISDN lines of the late 80ies and
90ies. You have problems with modems not liking the PBX dialtone, etc.

Hence, I've always wanted to get my hand on some more real-world
central-office telephone network equipment, and I finally have a source
for so-called V5.1/V5.2
access multiplexers. Those are like remote extension boxes for the
central office switch (like EWSD
or System 12). They
aggregate/multiplex a number of analog or ISDN BRI subscriber lines into
E1 lines, while not implementing any of the actual call control or ISDN
signalling logic. All of that is provided by the actual telephone
switch/exchange.

So in order to integrate such access multiplexers in my retronetworking
setup, I will have to implement the LE (local exchange) side of the V5.1
and/or V5.2 protocols, as specified in ITU-T G.964 and
G.965.

In the limited spare time I have next to my dayjob and various FOSS
projects, progress will likely be slow. Nonetheless I started with an
implementation now, and I already had a lot of fun learning about more
details of those interfaces and their related protocols.

One of the unresolved questions is to what kind of software I would want
to integrate once the V5.x part is resolved.

• lcr would probably be the most
  ISDN-native approach, but it is mostly unused and quite EOL.




• Asterisk or FreeSWITCH would of course be obvious candidates, but they
  are all relatively alien to ISDN, and hence not very transparent once
  you start to do anything but voice calls (e.g. dialup ISDN data calls
  in various forms).




• yate is another potential candidate. It
  already supports classic SS7 including ISUP, so it would be a good
  candidate to build an actual ISDN exchange with V5.2 access
  multiplexers on the customer-facing side (Q.921+Q.931 on it) and
  SS7/ISUP towards other exchanges.

For now I think yate would be the most promising approach. Time will
tell.

The final goal would then be to have a setup [e.g. at a future CCC
congress] where we would have SDH add/drop multiplexers in several
halls, and V5.x access multiplexers attached to that, connecting analog
and ISDN BRI lines from individual participants to a software-defined
central exchange. Ideally actually multiple exchanges, so we can show
the signaling on the V5.x side, the Q.921/Q.931 side and the SS7/ISUP
between the exchanges.

Given that the next CCC congress is not before December 2022, there is a
chance to actually implement this before then ;)