-------------------------------------------------------------------------------- UNDERSTANDING THE TELEPHONE SYSTEM FROM "UNDERSTANDING COMMUNICATIONS SYSTEMS" CHAPTER 6 BY DON L. CANNON AND GERARD LUECKE TEXAS INSTRUMENTS PUBLISHING, 1984 -------------------------------------------------------------------------------- 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. NETWORK SWITCHING ----------------- THE NETWORK SWITCHING PROCESS IDENTIFIES THE ORIGINATOR AND RECIPIENT OF A TELEPHONE CALL AND ROUTES THE CALL THROUGH SUITABLE TRANSMISSION LINKS TO ITS DESTINATION. REQUIREMENTS: EACH TELEPHONE IN THE SYSTEM MUST BE ASSIGNED A NUMBER TO INDICATE ITS LOCATION IN THE SYSTEM. THE SWITCHING NETWORK MUST RECOGNIZE WHICH TELEPHONE IS INITIATING THE CALL AND WHICH TELEPHONE IS TO RECEIVE THE CALL. FROM THIS INFORMATION, IT MUST SET UP THE CIRCUIT CONNECTIONS FOR A SIGNAL PATH THAT WILL SEND THE INFORMATION FROM THE SENDING (CALLING) TELEPHONE TO THE RECEIVING (CALLED) TELEPHONE. IN COMMERCIAL TELEPHONE SYSTEMS A SEVEN DIGIT NUMBER IS SUFFICIENT TO LOCATE INDIVIDUAL TELEPHONES WITHIN A METROPOLITAN AREA. DIGITS SPECIFY WHICH CENTRAL OFFICE SUPPLIES THE POWER TO THE TELEPHONES THAT ARE TO BE CONNECTED AND THE LAST FOUR DIGITS DETERMINE WHICH TELEPHONE OF A POSSIBLE 10,000 TELEPHONES IS CALLING OR IS BEING CALLED. THE CENTRAL OFFICE RECOGNIZES WHICH TELEPHONE IS CALLING (INITIATING THE CALL) WHEN THE TELEPHONE IS REMOVED FROM ITS HOOK OR CRADLE. IT RECOGNIZES WHICH TELEPHONE IS CALLED BY THE NUMBER THAT IS DIALED BY THE CALLING TELEPHONE. IF THE CALL IS TO A TELEPHONE OUTSIDE THE METROPOLITAN AREA, AN ADDITIONAL THREE DIGITS ARE USED TO DEFINE THE AREA CODE OR LOCATION OF THE CALLED METROPOLITAN AREA. THIS WOULD THEN BE A LONG DISTANCE CALL. IF CENTRAL OFFICE A HAS AN EXCHANGE CODE OF 123 AND CENTRAL OFFICE B HAS AN EXCHANGE CODE OF 456, THEN A CALL FROM TELEPONE NUMBER 123-0001 TO NUMBER 123-0003 WOULD REQUIRE A CIRCUIT CONNECTED BY THE SWITCHED PATH. IF ON THE OTHER HAND THE TELEPHONE AT 123-0001 IS CALLING THE TELEPHONE NUMBER 456-0002, THE SWITCHING NETWORK WOULD HAVE TO SET UP A DIFFEREENT SWITCHED CIRCUIT PATH. THE NETWORK MUST PROVIDE THE SWITCHING CIRCUITRY ALLOWING FOR THE POSSIBILITY OF MANY CALLS OCCURRING AT THE SAME TIME WITHIN A CENTRAL OFFICE AND BETWEEN CENTRAL OFFICES. THUS, MANY PARALLEL SWITCH PATHS MUST BE AVAILABLE. 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. THE HANDSET ----------- THE TELEPHONE HANDSET CONTAINS A MICROPHONE, A SPEAKER, SWITCH CONNECTIONS TO THE TELEPHONE SYSTEM, RINGING CIRCUITRY, AND A DIAL NETWORK. THE INSTRUMENT USED FOR TALKING INTO AND LISTENING WILL BE CALLED THE SUBSET. IT CONTAINS THE MICROPHONE AND THE SPEAKER. WHEN THE TELEPHONE IS NOT IN USE THE SUBSET RESTS ON THE CRADLE WHICH OPENS THE SWITCHES DENOTED SH (FOR SWITCH HOOK). THESE SWITCHES DISCONNECT THE TELEPHONE SUBSET FROM THE TELEPHONE SYSTEM. HOWEVER, THERE IS A CIRCUIT CONNECTION THAT IS MAINTAINED TO THE HANDSET. AN ELECTROMAGNET CALLED THE RINGER SOLENOID IS CONNECTED TO THE TELEPHONE LINE WIRES ON THE CENTRAL OFFICE SIDE OF THE SH SWITCH SO THAT THE CENTRAL OFFICE CAN RING THE TELEPHONE WITH AN AC SIGNAL WHEN IT IS CALLED. THE TELEPHONE WIRES ARE DENOTED AS THE T AND R LINES, FOR THE TERMS TIP AND RING, WHICH WERE RELATED TO PLUG CONNECTIONS USED IN THE ORIGINAL MANUAL (OPERATOR CONTROLLED) SWITCHING CENTRAL OFFICES OR EXCHANGES. WITH THE SUBSET IN THE CRADLE (OR HANGING TELEPHONES MAY BE ON A HOOK - THUS THE NAME SWITCH HOOK) NO DC CURRENT FLOWS FROM THE CENTRAL OFFICE 48-VOLT TALKING BATTERY THROUGH THE T-R LOOP OR THRU THE RING SOLENOID LOOP BECAUSE OF CAPACITOR C. THE CENTRAL OFFICE MONITORS THE DC CURRENT TO DETERMINE IF THE PHONE IS IDLE OR BUSY OR IS INITIATING A CALL. THE CENTRAL OFFICE CONTROLS THE SIGNALS THAT CAN BE DELIVERED TO A HANDSET THROUGH THE SWITCHES S1, S2, AND S3. WITH THE PHONE ON THE HOOK, SH IS OPEN AND ONLY THE RINGER CIRCUIT CAN FUNCTION. RINGING A CALLED TELEPHONE -------------------------- THE RINGING SIGNAL IS A 20-47 HZ AC SIGNAL FROM THE CENTRAL OFFICE. PICKING UP THE HANDSET LETS DC CURRENT FLOW IN THE TALKING CIRCUIT, STOPS THE RING, AND CONNECTS THE REECEIVING TELEPHONE TO THE CALLING TELEPHONE. THE SIGNALS THAT CAN BE SENT ARE: SIGNAL TONE INTERRUPT RATE FREQUENCIES (HERTZ) ----------- -------------- ------------------- DIAL NONE 480 RINGING NONE 440 MODULATED WITH 480 BUSY 60 (CALLED LINE) 480 MODULATED WITH 620 30 (TOLL LINE) 120 (INTRA-OFFICE) ASSUME THAT THE TELEPHONE IS IDLE - THE SUBSET IS ON THE CRADLE OR ON THE HOOK. IT IS READY AND WAITING TO RECEIVE AN INCOMING CALL AS INDICATED BY THE LACK OF DC CURRENT FLOW IN ITS T AND R LINES. IF THIS TELEPHONE HAS BEEN SELECTED BY THE CENTRAL OFFICE TO RECEIVE A CALL, S2 WILL BE THROWN TO CONNECT THE RINGING SIGNAL TO THE T AND R LINES THROUGH THE TRANSFORMER TC. THE 110 VOLT AC SIGNAL FO 20 TO 47 HERTZ WILL CAUSE THE RINGER SOLENOID TO SOUND THE FAMILIAR TELEPHONE RING AT THE HANDSET. THIS RINGING WILL CONTINUE UNTIL THE CALLER HANGS UP OR UNTIL THE TELEPHONE IS ANSWERED BY LIFTIG THE SUBSET OFF THE SWITCH HOOK. SUCH ACTION WILL CAUSE DC CURRENT TO FLOW IN THE T-R LOOP THROUGH THE SUBSET. THE CENTRAL OFFICE CIRCUITRY WILL DETECT THE DC CURRENT FLOW AND REMOVE THE RINGING SIGNAL BY OPENING S2. IT ALSO WILL THROW SWITCH S3 TO CONNECT THE SET TO THE CALLING TRANSMISSION PATH. ANSWERING THE TELEPHONE ----------------------- AS THE SPEAKER TALKS INTO THE SUBSET TO ANSWER THE PHONE, THE MICROPHONE OF THE HANDSET CAUSES THE CURRENT IN THE T-R LOOP TO VARY TO PRODUCE ELECTRICAL SIGNALS THAT CORRESPOND TO THE PATTERN OF THE SPEECH WAVEFORM. THE T-R LOOP CURRENT THROUGH THE MATCHING TRANSFORMER TS AND THE PRIMARY OF THE TRANSFORMER TC PRODUCES THE SPEECH SIGNALS IN THE SECONDARY CIRCUIT OF THE TRANSFORMER TC. THE SECONDARY OF THE TRANSFORMER TC IS CONNECTED TO THE CALLING TELEPHONE THROUGH THE CIRCUIT PATH FORMED BY THE CLOSING OF SWITCH S3 AND NETWORK SWITCHING CIRCUITS IN THE CENTRAL OFFICE. THE DIRECTION OF THE INFORMATON FLOW THROUGH THE TRANSFORMERS ALTERNATES AS ONE PERSON SPEAKS, THEN THE OTHER. NOW IF THE HANDSET IS CONSIDERED TO BE THE CALLING HANDSET, AFTER THE PERSON CALLING HAS DIALED AND THE CALLED TELEPHONE OWNER HAS LIFTED THE SUBSET AND SPOKEN INTO IT TO ANSWER THE TELEPHONE (AS DISCUSSED ABOVE), THE ELECTRICAL SIGNALS REPRESENTING THE SPOKEN WORD ARE NOW THE INPUT SIGNAL TO THE TRANSFORMER TC THROUGH TTHE CLOSED SWITCH S3. THE S3 SIDE OF THE TRANSFORMER IS NOW THE PRIMARY AND THE T-R LOOP SIDE IS THE SECONDARY. THE CURREENT IN THE PRIMARY PRODUCED BYTHE SPEECH SIGNALS INDUCE A CURRENT IN THE SECONDARY WHICH EXCITES THE SPEAKER OF THE HANDSET THROUGH THE MATCHING TRANSFORMER TS AND REPRODUCES THE SPOKEN WORD OF THE PERSON ANSWERING THE CALLED TELEPHONE IN THE EAR OF THE PERSON AT THE CALLING TELEPHONE. AS EACH PERSON SPEAKS AT EITHER THE CALLED OR CALLING TELEPHONE, THE SPOKEN WORD IS REPRODUCED FROM TRANSMITTER TO RECEIVER THROUGH THE COMPLETED COMMUNICATIONS CIRCUIT. THE SYSTEM REPRODUCES THE SPOKEN WORD IN BOTH DIRECTIONS THROUGH THE SAME CIRCUIT. WHENEVER A PERSON SPEAKS INTO THE MICROPHONE OF THE HANDSET THEY ALSO HEAR THEIR SPOKEN WORDS IN THE SPEAKER OF THE SAME HADNSET. THE MATCHING TRANSFORMER TS DETERMINES THE CORRECT SIGNAL LEVEL FOR THIS SIGNAL AS WELL AS THE CORREECT SIGNAL LEVEL FOR THE SIGNAL RECEIVED FROM THE SENDING TELEPHONE. ...
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