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 IN THE TELEPHONE SYSTEM, THE TWO-YWA COMMUNICATION IS CARRIED BY EITHER A
TWO-WIRE OR A FOUR-WIRE SYSTEM. IN THE FOUR-WIRE SYSTEM ON PAIR OF WIRES IS
CONNECTED FROM THE TRANSMITTER AT TELEPHONE A TO THE RECEIVER AT TELEPHONE B,
AND THE OTHER PAIR OF WIRES IS CNNECTED FORM THE TRANSMITTER AT TELEPHONE B TO
THE RECEIVER AT TELEPHONE A. IN THE TWO-WIRERE SYSTEM, WHICH USES ONE-HALF OF
THE WIRING OF THE FOUR-WIRE SYSTEM AND THEREFORE IS LESS COSTLY, BOTH
TRANSMITTER AND RECEIVERS AT TELEPHONES A AND B SHARE THE SAME PAIR OF WIRES FOR
THE TWO-WAY COMMUNICATION. THE POWER FOR THE COMMUNICATION IS PROVIDED BY A
BATTERY (OR AN APPROPRIATE POWER SOURCE WITH BATTERY BACKUP). THE BATTERY IS
USUALLY PROVIDED AT THE CENTRAL OFFICE IN COMMERCIAL TELEPHONE NETWORKS, THOUGH
IT CAN BE PROVIDED AT THE TRANSMITTER IN A PRIVATE OR SPECIAL PURPOSE FOUR-WIRE
SYSTEM.

     THE TRANSMISSION LINKS IN MODERN TELEPHONE SYSTEMS MAY TAKE SEVERAL
DIFFERENT FORMS, SUCH AS OVERHEAD WIRE, BURIED CABLE, MICROWAVE LINKS, SATELLITE
LINKS, AND SO ON. WHICH OPTION IS USED FOR A GIVEN COMMUNICATION DEPENDS
LAREGELY ON WHERE A AND B ARE LOCATED. IF A AND B ARE LOCATED CLOSE TO EACH
OTHER, THE INFORMATION WILL BE CARRIED OVER WIRES ON TELEPHONE POLES OR IN
OVERHEAD OR UNDERGROUND CABLES. IF A AND B ARE LOCATED ON THE SAME LAND MASS BUT
A LARGE DISTANCE APART, THE INFORMATION COULD BE TRANSMITTED OVER TELEPHONE
LINES, CABLES, MICROWAVE LINKS, OR EVEN SATELLITE LINKS). IF A AND B ARE
SEPARATED BY AN OCEAN, THE TRANSMISSION MUST BE THROUGH TRANSOCEANIC CABLE,
RADIO, OR SATELLITE LINKS, SINCE THESE ARE THE ONLY FEASIBLE WAYS TO GET
INFORMATION FROM ONE CONTINENT TO ANOTHER. WHICHEVER APPROACH IS USED, A MEANS
MUST BE PROVIDED FOR SELECTING THE PROPER PATHS OVER WHICH THE INFORMATION IS TO
TRAVEL. THIS REQUIRES SOME WAY TO CONNECT CIRCUITS THROUGH SWITCHES CALLED
NETWORK SWITCHING.

FURTHERMORE, THE NETWORK ALSO PROVIDES DETECTION CIRCUITS FOR ALL OF THE
SIGNALLING WHICH DETERMINES THE CALLING AND CALLED TELEPHONES AND ALL OF THE
LINE CHECKING REQUIRED TO ESTABLISH THA THE LINES AND TELEPHONES ARE CLEAR TO
COMPLETE THE COMMUNICATION. THESE SIGNALLING REQUIREMENTS DEPEND ON THE
CHARACTERISTICS OF EACH TELEPHONE SET AND ON THE SIGNALLING SEQUENCE REQUIRED TO
MAKE A TELEPHONE CALL.