IES62964B2 - Dial-back modem - Google Patents

Abstract

A dial-back modem operates such that after successful identification of a caller and disconnection of that caller the modem waits for an incoming ring and dials the identified caller back after the end of such ring. This ensures that the telephoen line has been dropped by the caller and ensures that the call back is made to the correct number.

Description

Dial-Back Modem This invention relates to a dial back-modem.

The existing procedure used by dial-back modems for security dial back operates as follows. The calling party connects to the answering modem and is identified, usually by some sort of user ID and password. The answering modem contains a database with user IDs, passwords and associated dial back telephone numbers. Once the user is identified the answering modem hangs up and attempts to dial the telephone number from the database and connect to the original calling modem which should be attached and waiting for the incoming dial-back call.

A problem with this procedure is that the system can be fooled if a fake caller successfully enters a valid user ID/password to initiate a call-back but does not subsequently release the line (exchanges commonly hold open circuits if the caller does not hang up). The answering modem (now trying to dial back) cannot detect that the line has not been released, and providing that the calling end can generate the tones normally seen from the exchange, the dial-back modem will think it is dialling through the exchange to the correct dial-back number. Instead, however, it will be connected to the fake caller. Subsequently the fake caller can initiate a handshake and enter connected state, effectively bypassing the security dial-back. 2 9 6 < - 2 One solution to this problem is to use a second telephone line to make the call-back. However, this is expensive because it involves additional hardware in the modem to provide the second connection (relay and input port), and also the rental of a second line.

It is an object of the invention to increase the security of security call-back procedures on modems without requiring a second telephone line.

Accordingly, the present invention provides a dial-back modem in which after successful identification of a caller and disconnection of that caller the modem waits for an incoming ring and dials the identified caller back after the end of such ting.

The invention is based upon the recognition that the receipt by the modem of a ringing voltage, which for all practical purposes can only be generated by the local exchange and cannot be simulated by a remote caller, is a guarantee that the telephone line has been dropped.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a security call-back system including a modem according to the embodiment of the invention, and Figure 2 is a flow diagram of the operations occurring at the calling and answering modems respectively.

Referring to figure 1, a dial-back modem 10 and a remote calling modem 11 are connected to a public service telephone network (PSTN) 12. The dial-back modem 10 is connected to its local exchange 13 through a standard PSTN interface 14. The modem 10 has autocall and autoanswer functions 15 and 16 respectively, the former including tone detection and dialling routines and the latter including tone generation and ring detection functions. The modem 10 also has other standard modem functions 17 which do not need to be described.

The operation of the modem 10 will now be described with reference to figure 2. In figure 2, the right hand column shows the operations carried out at a remote calling modem, such as the modem 11, and the left hand column shows the operations carried out at the modem 10. The sequence of operations at the dial-back modem 10 are carried out fully automatically under microprocessor control, whereas the operations at the modem 11 may be carried out largely under user control in the conventional manner. Communications between the two modems are indicated by the generally horizontal arrows.

Initially, step 20, the modem 10 is idle, waiting for a call. The calling modem 11 dials the modem 10 and waits for an answer, steps 40 and 41. The incoming ring is detected by the modem 10 at step 21 and the call answered, step 22. Now the two modems establish protocol (handshaking, steps 23 and 42) to connect with one another, after which the caller is identified at steps 24 and 43. Now the modem 10 hangs up at step 25 and the remote modem 11 disconnects, step 44.

Thus far the procedure is similar to existing methods. However, whereas the conventional technique has the dial-back modem 10 immediately call back the calling modem, in the present case the modem 10 waits for an incoming ring, step 26.

The reason for this is to detect that the telephone line has been dropped so that the local exchange 13 will be in a state to allow for correct dialing of the call-back number. Since only the local exchange 13 is capable of generating ringing voltage, which consists of an AC voltage in the range 3 0 to over 100 volts and a frequency in the range of 14 to 70 Hz which is applied over the usual DC voltage source from the exchange, the receipt of a ringing voltage by the modem 10 is a guarantee that the line has been dropped. The caller cannot simulate the generation of the ringing signal.

Therefore, the caller dials again, step 45. The modem 10 detects the ringing signal, step 27, but does not answer it but simply waits for-the end of the ring signal, step 28. After the caller has disconnected again, step 46, the modem 10 calls the dial-back number corresponding to the remote modem 11, step 29, and waits for an answer, step 30. It is to be noted that the second call made by the remote modem 11 does not cost anything as it is not answered.

When the remote modem 11 sees the incoming ring from the modem 10, step 48, it answers the call, step 49. The two modems 10 and 11 now handshake again, steps 31 and 50, following which they can proceed with a secure session, steps 32 and 51.

The time between the modem 10 detecting the ringing ceasing at step 28 and making the call back at step 29 can be such that there is insufficient time for a bogus caller to make another call arriving simultaneously to the outgoing security call-back call. This guarantees the correct dial back connection is made.

Alternatively, a random time delay between steps 28 and 29 could be used, or the user validation call (the second call made by the modem 11) could be taken to book some fixed time of day in the future for the designated call back call.

Claims (4)

CLAIMS :

1. A dial-back modem in which after successful identification of a caller and disconnection of that 5 caller the modem waits for an incoming ring and dials the identified caller back after the end of such ring.

2. A modem as claimed in claim 1, wherein the modem dials back a random time after the end of the ring.

3. A modem as claimed in claim l, wherein the modem dials back at a fixed time of day after the end of the ring. 15

4. A dial-back modem substantially as described herein with reference to the accompanying drawings. F. R. KELLY & CO. Agents for the Applicant