| 摘要 |
A network in a four-wire communication system for verifying the source of a call seeking access to restricted telephone channels through a toll switch prior to granting such access comprises handshake circuits on the operator and subscriber sides, respectively, of the toll switch in both pairs of transmission lines, the operator input to the toll switch being on a dedicated trunk. The handshake circuits comprise two D.C. verification signal sources, the first being connectable to the transmission line pair (T lines) on the subscriber side of the toll switch, and the second being connectable to the receive line pair (TR lines) on the operator side of the toll switch. The first D.C. verification signal is applied to the T lines on completion of the dialing and is detected, in accordance with this invention, on the operator side of the toll switch by a first detector connected to the T lines in the dedicated trunk. The output of this detector causes the second D.C. verification signal to be applied to the TR lines in the dedicated trunk on the operator side of the toll switch. A second detector in the TR lines on the subscriber side of the toll switch is responsive to the second verification signal to close normally open contacts in the TR lines and complete the through connection. The absence of the first detector and/or the second verification signal source as in an unauthorized access attempt with a Blue Box or the like results in open TR lines and a defeat of that attempt.
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