-------------------------------------------------------------------------------- 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. DURING THE ABOVE DESCRIBED SEQUENCE THE DIALER SUB-CIRCUIT IS NOT USED, THE TONE SIGNALLING CIRCUIT IS NOT USED, AND SWITCH S1 REMAINS OPEN. INITIATING A CALL ----------------- BOTH THE DIAL NETWORK AND THE TONE SIGNAL GENERATORS ARE NEEDED TO PROVIDE THE PROPER BASIC SIGNALLING TONES BETWEEN THE CALLING TELEPHONE AND THE CENTRAL OFFICE. WHEN THE CALLER LIFTS THE SUBSET OFF THE CRADLE, DC CURRENT FLOWS IN THE T-R LOOP. THIS IS SENSED BY THE CENTRAL OFFICE AND INDICATES THAT A CALL HAS BEEN INITIATED. THE CENTRAL OFFICE LOCATES THE LINE INITIATING THE CALL AND PLACES A SELECTED TONE SIGNAL GENERATOR OUTPUT OF 480 HERTZ ON THE T-R LINE BY CLOSING SWITCH S1 AND PRODUCING A CURREENT INT HE PRIMARY OF TRANSFORMER TC. THIS PRODUCES A DIAL TONE IN THE CALLING HADNSET'S SPEAKER. SWITCHES S2 AND S3 ARE OPEN (S2 WILL REMAIN OPEN, SINCE NO RINGING CURRENT WILL BE DELIVERED TO THE CALLING TELEPHONE). WHEN THE CALLER HEARS THE DIAL TONE, THE NUMBER OF THE TELEPHONE TO BE CALLED WILL BE INDICATED AS AN ELECTRICAL SIGNAL BY A ROTARY DIAL CONTAINING SWITCH CONTACTS THAT OPENA ND CLOSE OR BY PRESSING TONE DIALING PUSHBUTTONS. THE DIAL TONE WILL BE INTERRUPTED WHEN THE FIRST DIGIT IS DIALED WITH A MECHANICAL DIALER. IN THE CASE OF THE TONE DIALING SYSTEM IT WILL BE REPLACED WITH THE TONE OF THE BUTTON PUSHED. MECHANICAL DIALER ----------------- IF A MECHANICAL DIALER (PULSE) IS USED, THE DIAL WILL BE ROTATED ACCORDING TO THE DIGIT REQUIREED AND ALLOWED TO ROTATE BACK TO ITS REST POSITION. AS IT DOES IT WILL INTERRUPT THE DC CURRENT IN THE T-R LINES TO CAUSE A NUMBER OF PULSES EQUAL TO THE NUMBER DIALED. THE DIAL IS MECHANICALLY DESIGNED TO PROVIDE 10 PULSES PER SECOND. THE SPEAKER IS USUALLY DISCONNECTED BY THE DIAL NETWORK TO KEEP FROM SOUNDING THE DIALING CLICKS. THE CENTRAL OFFICE CIRCUITS RECEIVE THE PULSES AND DETECT THE SEQUENCE OF NUMBERS DIALED AND STORES THEM. THEY USE THESE NUMBERS TO LOCATE THE CALLED TELEPHONE AND TO SET UP AN AVAILABLE TRANSMISSION PATH BETWEEN THE TWO STATIONS. WHEN THE CALLED TELEPHONE IS LOCATED, THE CENTRAL OFFICE CHECKS THE ON-HOOK STATUS OF THE CALLED TELEPHONE, AND RINGS IT IF THE TELEPHONE IS ON-HOOK (IDLE). IT ALSO PLACES A RINGING TONE THROUGHT H CLOSED S1 SWITCHES (S2 AND S3 ARE STILL OPEN) ON THE LINE SO IT IS HEARD AT THE CALLING TELEPHONE. THIS RINGING TONE IS DEVELOPED BY MODULATING A 440-HERTZ SINUSOID WITH A 480-HERTZ SINUSOID. THIS RESULTS IN A 920-HERTZ TONE INCREASED AND DECREASED IN INTENSITY AT A 40-HERTZ RATE. THE RINGING TONE CONTINUES UNTIL EITHE THE CALLED TELEPHONE ANSWERS OR THE CALLING TELEPONE HANGS UP. IF THE CALLED TELEPHONE ANSWERS, THE CENTRAL OFFICE CIRCUITS REMOVE THE RINGING CURRENT FORM THE CALLED SET, THE RINGING TONE FROM THE CALLING SET AND COMPLETES THE TRANSMISSION PATH BY CLOSING THE SWITCHES S3. BUSY TELEPHONE -------------- IF THE CALLED TELEPHONE IS BUSY (OFF-HOOK) THE TONE GENERATOR WILL SOUND ONE OF THREE POSSIBLE BUSY SIGNALS TO THE CALLING TELEPHONE. THE BUSY TONE IS A 140-HERTZ TONE GENERATED BY MODULATING A 480-HERTZ SINUSOID WITH A 620-HERTZ SINUSOID AND FILTERING OUT THE 1100-HERTZ SIGNAL (WHEN ONE SINEWAVE MODULATES ANOTHER, BOTH THE SUM AND DIFFERENCE FREQUENCY SINE WAVE SIGNALS RESULT). THIS TONE IS INTERRUPTED AT A RATE THAT INDICATES THE REASON FOR THE BUSY SIGNAL. A SIGNAL INTERRUPTED AT THE RATE OF 60 TIMES PER IMNUTE INDICATES THE CALLED LINE WAS BUSY. IF THE INTERRUPT RATE IS 1/2 OF THIS, THE TOLL LINE BETWEEN THE CENTRAL OFFICES WAS BUSY (FULL OF EXISTING CALLS.) IF THE INTERRUPT RATE IS DOUBLED TO 120 TIMES PER IMIUTE IT IMPLIES THAT ALL INTRA-OFFICE PATHS ARE BUSY. ONLY WITH THE 60 INTERRUPTS/MINUTE IS THE CALLER SURE THAT THE CALLED PARTY LINE IS BUSY. TONE DIALING ------------ WHEN THE HANDSET IS A TOUCH-TONE SERVICE TELEPHONE, THEN THE DIAL NETWORK IS MORE THAN JUST SWITCHES. IT CONTAINS TONE GENERATING CIRCUITS AS WELL. WHEN THE TONE DIALING TELEPHONE IS USED, THE TONE KEY OR PAD CAUSES A SIGNAL OF TWO FREQUENCIES TO BE PLACED ON THE LINE. THE FREQUENCIES AREE INDICATED BY THE INTERSECTION OF THE FREQUENCY LINES IN THE TONE MATRIX OF THE FOLLOWING DIAGRAM: 697--------1-------2-------3-------! ! ! ! ! ! ! ! ! ! ! ! ! 770--------4-------5-------6-------! ! ! ! ! ! ! ! ! ! ! ! ! 852--------7-------8-------9-------! ! ! ! ! ! ! ! ! ! ! ! ! 941--------*-------0-------#-------! ! ! ! ! ! ! ! ! ! ! ! ! 1209 1336 1477 1633 FOR EXAMPLE, PRESSING THE 5 KEY CAUSES A 770-HERTZ AND A 1336-HERTZ TONE TO BE SENT TO THE CENTRAL OFFICE (AND TO THE CALLED PARTY IF THE BUTTON IS PUSHED DURING A CONVERSATION). CENTRAL OFFICE CIRCUITS THAT DETECT AND DECODE THE TONES SET UP THE SWITCHING FOR THE COMMUNICATAONS PATH JUST AS FOR THE MECHANICAL DIALER. THE USE OF SUCH TONES SPEEDS UP THE DIALING OPERATION AND ALLOWS COMMAND AND CONTROL INFORMATION TO BE SENT TO THE CALLED LOCATION. BANDWIDTH --------- THE FREQUENCIES OF THE TONES GENERATED BY THE TONE KEYS AS WELL AS THE CONVERSATTQIONS OF THE TELEPHONE USERS MUST BE WITHIN THE BANDPASS CAPABILITIES OF THE TELEPHONE SYSTEM. THE BANDWIDTH OF A CHANNEL IN A TYPICAL TELEPHONE SYSTEM USED FOR CONVERSATIONS IS 4 KILOHERTZ. THE BANDPASS REGION ALLOWED FOR THE VOICE SIGNALS IS 200 HERTZ TO 3400 HERTZ. THE TONE DIALING SIGNALS FITS INTO THIS RANGE AS DO SOME SPECIAL CONTROL SIGNALS AT 2400 AND 2600 HERTZ. OTHER CONTROL SIGNALS FORM 3400 TO 3700 HERTZ FIT INTO THE OVERALL 4 KILOHERTZ BANDWIDTH OF THE TELEPHONE CHANNEL. THIS VOICE CHANNEL BANDWIDTH WILL BE AN IMPORTANT SYSTEM PARAMETER THROUGHOUT THIS CHAPTER. THE OTHER IMPORTANT PARAMETERS OF THE VOICE CHANNEL ARE THE SIGNALLING TONES AND THE DC CURRENTS THAT ARE USED BY THE SWITCHING AND CONTROL CIRCUITS OF THE CENTRAL OFFICES TO ESTABLISH THE COMMUNICATIONS PATHS BETWEEN MANY PAIRS OF TELEPHONES OVER A WORLDWIDE NETWORK. UNDERSTANDING THE BASIC CONCEPTS OF THE CENTRAL OFFICE CIRCUITS WILL HELP TO FURTHER UNDERSTAND THE TELEPHONE COMMUNICATIONS SYSTEMS. (CONTINUED IN PART II) -------------------------------------------------------------------------------- :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:- THE CONVENT TEXTFILES BBS 619-475-6187 10 MEGS 3/1200 BAUD NO PASS :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:- --------------------------------------------------------------------------------