Avaya S8XXX Servers that function as tandem nodes in an ETN can be interconnected using DS1 trunking facilities with ISDN. All signaling between the tandem switches is done with ISDN D-channel and normal ISDN protocol. The ISDN can also be used to connect ETN tandem and main servers or switches. In this case, the main server or switch collects all of the address digits from local users as well as users at other satellite and tributary switches, and originates a call over ISDN to the tandem server or switch.

Automatic Alternate Routing (AAR) and Automatic Route Selection (ARS) are used with ISDN and DS1 trunking facilities to access ETN facilities. AAR and ARS are used to collect the dialing information for the call that is originated from the main server or switch.