Retronetworking » OCTOI - Osmocom Community TDM over IP

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h1. Community TDM/ISDN/SS7 Network

This wiki page describes some ideas about 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 (https://www.ckts.info/).  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.

h2. Underlying Protocol / Hardware

The underlying transport protocol is being designed around the wiki page [[e1-t1-adapter:Proposed_efficient_TDMoIP]].  The initial hardware implementation is going to be an [[e1-t1-adapter: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.

h2. TDM layer hub / cross-connect

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.

{{graphviz_link()

graph G {

  hub [shape=box, label="TDMoIP hub\ncross-connect\nin public internet"];

  subgraph cluster_1 {

    label="Hobbyist A";

    ad1 [label="Access Device\n"];

    pbx1 [label="PBX"];

    pbx1 -- ad1 [label="E1"];

  }

  subgraph cluster_2 {

    label="Hobbyist B";

    ad2 [label="Access Device\n"];

    frr [label="FrameRelay Device"];

    frr -- ad2 [label="E1"];

  }

  subgraph cluster_3 {

    label="Hobbyist C";

    ad3 [label="Access Device\n"];

    ras [label="RAS Server"];

    ras -- ad3 [label="E1"];

  }

  ad1 -- hub [label="TDMoIP\nInternet"];

  ad2 -- hub [label="TDMoIP\nInternet"];

  ad3 -- hub [label="TDMoIP\nInternet"];

}

}}

h2. Services

h3. Classic SS7

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.

h3. ISDN PRI

ISDN (Q.921/Q.931) would 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 would need to be allocated telephone nubmers in the dialling plan.

On the hub side, the PRI lines and ISDN service could be terminated in a yate or Freeswitch instance.

{{graphviz_link()

graph G {

  hub [shape=box, label="TDMoIP hub\ncross-connect\nin public internet"];

  subgraph cluster_1 {

    label="Hobbyist A";

    ad1 [label="Access Device\n"];

    pbx1 [label="PBX"];

    phone1a [label="Phone"];

    modem1b [label="Modem"];

    ta1c [label="ISDN TA"];

    pbx1 -- ad1 [label="E1"];

    phone1a -- pbx1 [label="POTS"];

    modem1b -- pbx1 [label="POTS"];

    ta1c -- pbx1 [label="ISDN-BRI"];

  }

  subgraph cluster_2 {

    label="Hobbyist B";

    ad2 [label="Access Device\n"];

    pbx2 [label="PBX"];

    phone2a [label="Phone"];

    modem2b [label="Modem"];

    ta2c [label="ISDN TA"];

    pbx2 -- ad2 [label="E1"];

    phone2a -- pbx2 [label="POTS"];

    modem2b -- pbx2 [label="POTS"];

    ta2c -- pbx2 [label="ISDN-BRI"];

  }

  subgraph cluster_3 {

    label="Hobbyist C";

    ad3 [label="Access Device\n"];

    pbx3 [label="PBX"];

    phone3a [label="Phone"];

    modem3b [label="Modem"];

    ta3c [label="ISDN TA"];

    pbx3 -- ad3 [label="E1"];

    phone3a -- pbx3 [label="POTS"];

    modem3b -- pbx3 [label="POTS"];

    ta3c -- pbx3 [label="ISDN-BRI"];

  }

  ad1 -- hub [label="TDMoIP\nInternet"];

  ad2 -- hub [label="TDMoIP\nInternet"];

  ad3 -- hub [label="TDMoIP\nInternet"];

}

}}

h3. SCCP/MAP/TCAP

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

h3. ISDN BRI

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 [[e1-t1-adapter: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 

h3. Frame Relay

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.

h3. 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)