IPDA with signaling survivability

With signaling survivability via the PSTN network, an alternative route for signaling via the ISDN network (PSTN) is set up as soon as a fault in the IP path is detected. The messages are then routed via a PSTN router to the central system (for example, router with IP > S2 or STMI2/4 board with WAML function) and an ISDN modem on the access point/OpenScape 4000 SoftGate. The signaling messages are sent via this alternative path once it is established (approx. 30 ... 60 seconds). This ensures that none of the messages that may be in a backlog are lost.

As a rule, an access point/OpenScape 4000 SoftGate with signaling survivability generally requires a telephone connection for this purpose. The connection should be in the public PSTN network. Alternatively, it may be in a private network, insofar as the connection is not routed via the (IP) network whose fault is to be bypassed via the alternative route.

The access point/OpenScape 4000 SoftGate is equipped with an RS 232/ V.24 interface on the HG 3575 module in order to connect a modem, via which the alternative signaling route into the telephone network is routed.

The counterpart to this modem is a special router at the OpenScape 4000 LAN segment, referred to here as the survivability router. This router also requires access to the telephone network.

If the LAN/WAN-based connection to the access point fails, a modem connection is established via CO. The access point is controlled via this connection until the LAN/WAN connection is reestablished.

If every access point features an individual modem (and the central system has reserved enough router connections via CO (PSTN)), signaling survivability can cover even the total failure of the WAN infrastructure. The only critical factor here are the connection between the active CC and the survivability router(s) in the OpenScape 4000 LAN segment and the independence of (failed) WAN from the PSTN used for survivability.

Details on the transmission methods between router and modem:

The transmission methods between the router and the modem must be compatible. Whenever possible, a digital link between the router and the modem should be used (ISDN modem), as analog modems require a long time to measure the route and agree on a transmission method. If the supervisory timer of the CC or access point/OpenScape 4000 SoftGate expires during the link establishment time, there is a danger of messages being lost. The access point/OpenScape 4000 SoftGate then resets itself. In addition, transmissions rates are substantially lower with analog modems than via ISDN. This can lead to delays with signaling messages and to the supervision timer expiring during bottleneck situations, in which case the access point/OpenScape 4000 SoftGate resets itself in the end.

Approximately 64 Kbps are generally considered to be sufficient for the maximum transmission capacity required in the signaling path. The transmission rate offered by ISDN modems comes sufficiently close to this value. In the case of analog modems, at most half of the transmission bandwidth can be achieved. This can lead to the limitations described. A transmission bandwidth of at least 28.8 Kbps must be guaranteed. A path is considered unsuitable for survivability if a lower bit rate is used for modem connects due to low-quality links.