Although most PSTNs now use SS7 throughout their networks, some segments are still using conventional signaling. Conventional signaling in most of these cases consists of MF signaling.

SS7 must be able to function and internetwork with networks using conventional signaling. One of the primary issues is reservation of a voice circuit. Unlike conventional signaling, the voice circuit does not get connected until the distant party answers or at least until both exchanges have sent and received all the addressing information required to connect the call.

In MF signaling, the circuit is connected when the calling party completes dialing the digits and is used to signal the distant exchange. Therefore, some method must be established for enabling voice circuits to be reserved and tested in conjunction with the SS7 protocol.

To accomplish this, a circuit reservation procedure is used. Prior to sending an IAM, which is used to send addressing information necessary to establish the circuit connection, a circuit reservation message (CRM) is sent to the non-SS7 exchange. This is then converted to MF signaling by that exchange, and MF signaling may be used from that point on. The exchange acknowledges receipt of the CRM by sending in the backward direction (back to the originating exchange) a circuit reservation acknowledgment (CRA) message.

The originating exchange then can specify a continuity test to be invoked by the exchange using MF signaling by sending a continuity check request (CCR) message in the forward direction. This message...