System for connecting public network subscriber and private network subscriber

Claim 1

In a system for connecting a public network and a private network in Integrated Services Digital Network (ISDN) to make communications possible between a public network subscriber and a private network subscriber, comprising: a first exchange for connecting said public network subscriber and said public network, a second exchange for connecting said private network subscriber and said private network, and a plurality of third exchanges for connecting said public network and said private network, said first exchange comprising: first selecting means for selecting one of said third exchanges as an access point to said private network when receiving a private line number of said private network subscriber from said public network subscriber; and first set-up forming means for forming a first set-up message to be transmitted to said public network, said first set-up message including a destination number, a destination sub-address, and a calling party number, said destination number comprising the public line number of said access point selected by said first selecting means, said destination sub-address comprising said private line number received from said public network subscriber, and said calling party number comprising the public line number of said public network subscriber, said second exchange comprising: second selecting means for selecting one of said third exchanges as an access point to said public network when receiving the public line number of said public network subscriber from said private network subscriber; and second set-up forming means for forming a second set-up message to be transmitted to said private network, said second set-up message including a destination number, a destination sub-address, and a calling party number, said destination number comprising the private line number of said access point selected by said second selecting means, said destination sub-address comprising said public line number of said public network subscriber received from said private network subscriber, and said calling party number comprising the private line number of said private network subscriber, and each of said third exchanges comprising: storage means for storing the identification numbers registered previously, said identification numbers comprising registered public line numbers of said public network subscribers permitted access to said private network and registered private line numbers of said private network subscribers permitted access to said public network through said private network; decision means for deciding whether said calling party number read out from a received set-up message of said first and said second set-up messages is identical to one of said identification numbers; and connection permitting means for permitting connection between said public network and said private network only when said decision means decides that said calling party number is identical to one of said identification numbers.