This invention relates to communications systems, particularly cable systems which are capable of providing a broad range of services to users. Cable systems include community antennae television, [CATV], closed circuit television, [CCTV], and broad band cable systems in general.
U.S. Pat. No. 4,326,289 "Expandable Communications Systems" filed Feb. 28, 1980 discloses a broad band communications system employing a central station, a plurality of subordinate systems stations that can be selectively addressed by the central station, and a bilateral transmission medium, for example a coaxial cable. The cable links the subordinate stations of the system to the central station where a central controller is capable of addressing a specific subordinate station and an exchanging control and message information with any addressed station.
Each subordinate systems station has a main unit with a local input port and a local output port. Each systems station may be connected over a parallel bus to a number of expansion modules. These provide additional input and output ports to accomodate different communication services. Illustrative services include selective security surveilance, energy control, meter reading, text reading, data viewing, and the like.
Because the typical subordinate station is physically located on the premises of a customer, the station may be vulnerable to unauthorized tampering. Resistance against tampering is particularly desirable in the case of CATV where it is common practice to include security functions or payment per channel. The possibility of tampering may be deterred by locating the systems station beyond the customer's premises, for example on a utility pole or within a cable vault. It is advantageous, however, to provide a communications system that is able to accomodate a plurality of special communications services for various CATV customers, and is secure against unauthorized tampering. For that purpose U.S. Pat. No. 4,450,481 "Tamper-Resistant, Expandable Communications Systems" can be employed. Each subordinate systems station is formed by two main components. One component is a remote interrogator which is mounted in a secure location outside the customer's premises, for example on a utility pole, or within a cable vault. The remote interrogator contains authorization for various services and periodically polls the other component, which is the expansion transponder module. The transponder automatically transmits signals when activated by the interrogator. A transponder module can be located securely, but other modules can be located on the customer's premises to interface any of the specific service features desired by a customer. A securely located transponder module permits any associated subscriber drop to have duration-controlled access to other channels. For less stringent applications, where the transponder module is located on premises, periodic polling of the module by the remote interrogator assumes integrity of the system for such functions as meter reading.
Each remote interrogator, using a main communications channel, exchanges control and information signals with a central control computer located at the "headend" of the CATV system. This is preferably in a serial synchronous protocol in order to reduce signalling overhead. The control and information signals are carried in radio frequency channels which are passed by the CATV components. Any of the wide variety of modulation techniques may be employed to apply the signals to the radio frequency channels. Frequency shift or phase shift keying are preferred choices.
The remote interrogator unit, using a subcombination channel, also exchanges control and information signals with many expansion transponder modules, for example several thousand, over subscriber drop cables of over parts of the CATV trunk. The mode of signal control between the remote interrogator and the expansion modules is by serial, binary, or asynchronous transmission over a radio frequency channel and is passed by the intervening system components. Although there is a broad choice of modulation techniques, frequency shift keying is the preferred choice for subcommunications channels.
In particular, the communications system is expandable and provides multipurpose digital and analog communications by having a plurality of remote interrogator units connected in a branch of the system and isolated by band-stop filters to permit simultaneous polling without interference with other operations. In addition a frequency agile oscillator is used to provide premium service control.
When there are problems in the broad-band local area network, they become apparent by malfunctions or complete failure in individual data circuits. In many situations a number of individual data circuits fail simultaneously. The problem then becomes that of locating the specific problem or fault. In order to assure high data availability on data circuits, the faults must be located as quickly as possible