Community TDM/ISDN/SS7 Network

This wiki page describes a work in progress creating a community TDM/ISDN/SS7 network, enabling retronetworking enthusiasts around the world to interconnect their TDM equipment, such as PBXs, telephony switches, RAS servers, access multiplers, etc.

For classic analog telephones, there is the C*NET ( This is basically the same idea but for one later generation of technology: Digital TDM/PDH/ISDN/SS7 networks. And of course one can connect analog telephone/modem equipment via ISDN/SS7, if that is desired.

Current Network

The network has a number of additional participants by now, but the general principle looks the same, updating the graph with every participant would make it rather complex without immediate gain:

How to contact us

Mailing List

We now have a public mailing list for discussion among interested parties. The list has a public archive

Don't be surprised by the lack of traffic on the ML, almost all communication happens on the IRC channel

IRC Channel

A lot of us are hanging out on the #retronetworking IRC channel on If you don't know much about IRC, feel free to use the provided webchat interface

Underlying Protocol / Hardware

The underlying transport protocol is being designed around the wiki page Proposed_efficient_TDMoIP. The initial hardware implementation is an icE1usb attached to some kind of [embedded] Linux sytem like a Raspberry Pi. The unique property about the icE1usb is that it contains a GPS-DO to ensure the same clock reference is used in all locations.


ISDN PRI (available)

ISDN (Q.921/Q.931) permit to connect equipment like PBXs. The hub offers the NT / central office function, while the client side implements the TE function.

A user connecting to the ISDN PRI service needs to be allocated telephone numbers in the dialling plan, see Phonebook..

On the hub side, the PRI lines and ISDN service are currently terminated in a yate instance.


ISDN BRI service is expected to initially only be obtained indirectly: By connecting a ISDN PRI capable PBX to the hub, and then locally connecting to ISDN BRI ports of the PBX.

At a later point, one could consider implementing a ISDN BRI version of the TDMoIP protocol and some software implementation for ISDN BRI cards that support NT mode. One notable topic is clocking: The hardware would either need a GPS-DO (like icE1usb has for E1) or at the very least a VCTCXO that can be disciplined by software to match the timing recovered from the IP packet side. See #5436 and #5417

TDM layer hub / cross-connect (future)

The TDM layer would allow to interconnect any E1 line (or even only some timeslots of a line) with any other E1 line. It doesn't interpret the payload but just passes it around.

Classic SS7 (available, not yet tested)

Classic SS7 (MTP2/MTP3 plus TUP/ISUP) would permit to connect STPs and SPCs, such as those of telephony exchanges / switches.

A user connecting to the SS7 service would need to be allocated point codes.

SCCP/MAP/TCAP (future)

SCCP/MAP/TCAP on top of SS7 can be used to provide interworking between private cellular networks.

Frame Relay (future)

A Frame Relay network could be implemented on top of the TDM hub. In this case, framed E1 would be used with groups of 64k timeslots (typically all 31) building the underlying FR transport medium.

POTS / analog telephony

There is no plan to directly support analog telephony. Rather, analog ports for phones, modems, etc. can be provided by means of ISDN PBXs connecting via ISDN PRI (and possibly later ISDN BRI)

Updated by laforge 4 months ago ยท 22 revisions

Add picture from clipboard (Maximum size: 48.8 MB)