Communication line backup system

Claim 2

The communication line backup system as claimed in claim 1, characterized in that said backup communication channel is formed by a B-channel within said integrated services digital network (8).

Claim 3

The communication line backup system as claimed in claim 1 or 2, characterized in that each of the first and second subscriber terminals (15) have a modem (32, 34), and the backup means (36) of each of said first and second backup units includes switching means (52, 53, 62) coupled to the communication line (6) and an ISDN line of said integrated services digital network (8), and control means (58, 80) for controlling switching of said switching means, said control means controlling said switching means to normally connect the modem to the communication line and to connect the modem to the ISDN line in response to a backup instruction which is received when a failure is generated on the communication line between the first and second subscriber terminals (2, 4).

Claim 4

The communication line backup system as claimed in claim 3, characterized in that each of said first and second backup units (15) include an ISDN interface circuit (60) which is coupled between said switching means (52, 53, 62) and the ISDN line, and said backup communication channel is formed by a B-channel within said integrated services digital network (8).

Claim 5

The communication line backup system as claimed in claim 4, characterized in that said backup communication channel is formed by a B-channel which is used for transmitting information and another B-channel which is used for receiving information.

Claim 6

A communication line backup system which provides a backup line for backing up a communication line (6) which connects first and second subscriber terminals (2, 4), characterized in that said communication line backup system comprises: a first backup unit (14) which is coupled to the first subscriber terminal (2); a second backup unit (16), coupled to the second subscriber terminal (4), for backing up a telephone line; an integrated services digital network (ISDN) (8) which is coupled to said first backup unit via ISDN channels; and a telephone line switching network (10) which is coupled between said second backup unit and said integrated services digital network, said second backup unit (16) being coupled to said telephone line switching network via two backup telephone lines, said first backup unit (14) including backup means (36) for forming a backup communication channel between the first and second subscriber terminals in response to a failure which is generated on the communication line between the first and second subscriber terminals, said backup communication channel being formed by a first channel which is within the ISDN channels established in said integrated services digital network and connects to one of the two backup telephone lines, and a second channel which is within the ISDN channels established in said integrated services digital network and connects to the other of the two backup telephone lines.

Claim 7

The communication line backup system as claimed in claim 6, characterized in that said backup communication channel is formed by B-channels within said integrated services digital network (8).

Claim 8

The communication line backup system as claimed in claim 6 or 7, characterized in that each of the first and second subscriber terminals (2, 4) have a modem (32, 34), and the backup means (36) of said first backup unit (36) includes switching means (52, 53, 62) coupled to the communication line (6) and an ISDN line of said integrated services digital network (8), and control means (58, 80) for controlling switching of said switching means, said control means controlling said switching means to normally connect the modem to the communication line and to connect the modem to the ISDN line in response to a backup instruction which is received when a failure is generated on the communication line between the first and second subscriber terminals.

Claim 9

The communication line backup system as claimed in claim 8, characterized in that said first backup unit (36) includes an ISDN interface circuit (60) which is coupled between said switching means (52, 53, 62) and the ISDN line, and said backup communication channel is formed by B-channels within said integrated services digital network (8).

Claim 10

The communication line backup system as claimed in any of claims 6 to 9, characterized in that said integrated services digital network (8) and said telephone line switching network (10) form a communication network (12) having an ISDN-PSTN inter-working function.