304 lines
17 KiB
Plaintext
304 lines
17 KiB
Plaintext
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UNDERSTANDING THE TELEPHONE SYSTEM
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FROM "UNDERSTANDING COMMUNICATIONS SYSTEMS" CHAPTER 6
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BY DON L. CANNON
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AND
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GERARD LUECKE
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TEXAS INSTRUMENTS PUBLISHING, 1984
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IN THE TELEPHONE SYSTEM, THE TWO-YWA COMMUNICATION IS CARRIED BY EITHER A
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TWO-WIRE OR A FOUR-WIRE SYSTEM. IN THE FOUR-WIRE SYSTEM ON PAIR OF WIRES IS
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CONNECTED FROM THE TRANSMITTER AT TELEPHONE A TO THE RECEIVER AT TELEPHONE B,
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AND THE OTHER PAIR OF WIRES IS CNNECTED FORM THE TRANSMITTER AT TELEPHONE B TO
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THE RECEIVER AT TELEPHONE A. IN THE TWO-WIRERE SYSTEM, WHICH USES ONE-HALF OF
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THE WIRING OF THE FOUR-WIRE SYSTEM AND THEREFORE IS LESS COSTLY, BOTH
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TRANSMITTER AND RECEIVERS AT TELEPHONES A AND B SHARE THE SAME PAIR OF WIRES FOR
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THE TWO-WAY COMMUNICATION. THE POWER FOR THE COMMUNICATION IS PROVIDED BY A
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BATTERY (OR AN APPROPRIATE POWER SOURCE WITH BATTERY BACKUP). THE BATTERY IS
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USUALLY PROVIDED AT THE CENTRAL OFFICE IN COMMERCIAL TELEPHONE NETWORKS, THOUGH
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IT CAN BE PROVIDED AT THE TRANSMITTER IN A PRIVATE OR SPECIAL PURPOSE FOUR-WIRE
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SYSTEM.
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THE TRANSMISSION LINKS IN MODERN TELEPHONE SYSTEMS MAY TAKE SEVERAL
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DIFFERENT FORMS, SUCH AS OVERHEAD WIRE, BURIED CABLE, MICROWAVE LINKS, SATELLITE
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LINKS, AND SO ON. WHICH OPTION IS USED FOR A GIVEN COMMUNICATION DEPENDS
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LAREGELY ON WHERE A AND B ARE LOCATED. IF A AND B ARE LOCATED CLOSE TO EACH
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OTHER, THE INFORMATION WILL BE CARRIED OVER WIRES ON TELEPHONE POLES OR IN
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OVERHEAD OR UNDERGROUND CABLES. IF A AND B ARE LOCATED ON THE SAME LAND MASS BUT
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A LARGE DISTANCE APART, THE INFORMATION COULD BE TRANSMITTED OVER TELEPHONE
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LINES, CABLES, MICROWAVE LINKS, OR EVEN SATELLITE LINKS). IF A AND B ARE
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SEPARATED BY AN OCEAN, THE TRANSMISSION MUST BE THROUGH TRANSOCEANIC CABLE,
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RADIO, OR SATELLITE LINKS, SINCE THESE ARE THE ONLY FEASIBLE WAYS TO GET
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INFORMATION FROM ONE CONTINENT TO ANOTHER. WHICHEVER APPROACH IS USED, A MEANS
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MUST BE PROVIDED FOR SELECTING THE PROPER PATHS OVER WHICH THE INFORMATION IS TO
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TRAVEL. THIS REQUIRES SOME WAY TO CONNECT CIRCUITS THROUGH SWITCHES CALLED
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NETWORK SWITCHING.
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NETWORK SWITCHING
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-----------------
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THE NETWORK SWITCHING PROCESS IDENTIFIES THE ORIGINATOR AND RECIPIENT OF A
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TELEPHONE CALL AND ROUTES THE CALL THROUGH SUITABLE TRANSMISSION LINKS TO ITS
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DESTINATION. REQUIREMENTS: EACH TELEPHONE IN THE SYSTEM MUST BE ASSIGNED A
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NUMBER TO INDICATE ITS LOCATION IN THE SYSTEM. THE SWITCHING NETWORK MUST
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RECOGNIZE WHICH TELEPHONE IS INITIATING THE CALL AND WHICH TELEPHONE IS TO
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RECEIVE THE CALL. FROM THIS INFORMATION, IT MUST SET UP THE CIRCUIT CONNECTIONS
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FOR A SIGNAL PATH THAT WILL SEND THE INFORMATION FROM THE SENDING (CALLING)
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TELEPHONE TO THE RECEIVING (CALLED) TELEPHONE. IN COMMERCIAL TELEPHONE SYSTEMS A
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SEVEN DIGIT NUMBER IS SUFFICIENT TO LOCATE INDIVIDUAL TELEPHONES WITHIN A
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METROPOLITAN AREA. DIGITS SPECIFY WHICH CENTRAL OFFICE SUPPLIES THE POWER TO THE
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TELEPHONES THAT ARE TO BE CONNECTED AND THE LAST FOUR DIGITS DETERMINE WHICH
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TELEPHONE OF A POSSIBLE 10,000 TELEPHONES IS CALLING OR IS BEING CALLED. THE
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CENTRAL OFFICE RECOGNIZES WHICH TELEPHONE IS CALLING (INITIATING THE CALL) WHEN
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THE TELEPHONE IS REMOVED FROM ITS HOOK OR CRADLE. IT RECOGNIZES WHICH TELEPHONE
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IS CALLED BY THE NUMBER THAT IS DIALED BY THE CALLING TELEPHONE. IF THE CALL IS
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TO A TELEPHONE OUTSIDE THE METROPOLITAN AREA, AN ADDITIONAL THREE DIGITS ARE
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USED TO DEFINE THE AREA CODE OR LOCATION OF THE CALLED METROPOLITAN AREA. THIS
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WOULD THEN BE A LONG DISTANCE CALL.
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IF CENTRAL OFFICE A HAS AN EXCHANGE CODE OF 123 AND CENTRAL OFFICE B HAS AN
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EXCHANGE CODE OF 456, THEN A CALL FROM TELEPONE NUMBER 123-0001 TO NUMBER
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123-0003 WOULD REQUIRE A CIRCUIT CONNECTED BY THE SWITCHED PATH. IF ON THE OTHER
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HAND THE TELEPHONE AT 123-0001 IS CALLING THE TELEPHONE NUMBER 456-0002, THE
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SWITCHING NETWORK WOULD HAVE TO SET UP A DIFFEREENT SWITCHED CIRCUIT PATH. THE
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NETWORK MUST PROVIDE THE SWITCHING CIRCUITRY ALLOWING FOR THE POSSIBILITY OF
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MANY CALLS OCCURRING AT THE SAME TIME WITHIN A CENTRAL OFFICE AND BETWEEN
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CENTRAL OFFICES. THUS, MANY PARALLEL SWITCH PATHS MUST BE AVAILABLE.
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FURTHERMORE, THE NETWORK ALSO PROVIDES DETECTION CIRCUITS FOR ALL OF THE
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SIGNALLING WHICH DETERMINES THE CALLING AND CALLED TELEPHONES AND ALL OF THE
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LINE CHECKING REQUIRED TO ESTABLISH THA THE LINES AND TELEPHONES ARE CLEAR TO
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COMPLETE THE COMMUNICATION. THESE SIGNALLING REQUIREMENTS DEPEND ON THE
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CHARACTERISTICS OF EACH TELEPHONE SET AND ON THE SIGNALLING SEQUENCE REQUIRED TO
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MAKE A TELEPHONE CALL.
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THE HANDSET
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-----------
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THE TELEPHONE HANDSET CONTAINS A MICROPHONE, A SPEAKER, SWITCH CONNECTIONS
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TO THE TELEPHONE SYSTEM, RINGING CIRCUITRY, AND A DIAL NETWORK. THE INSTRUMENT
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USED FOR TALKING INTO AND LISTENING WILL BE CALLED THE SUBSET. IT CONTAINS THE
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MICROPHONE AND THE SPEAKER. WHEN THE TELEPHONE IS NOT IN USE THE SUBSET RESTS ON
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THE CRADLE WHICH OPENS THE SWITCHES DENOTED SH (FOR SWITCH HOOK). THESE
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SWITCHES DISCONNECT THE TELEPHONE SUBSET FROM THE TELEPHONE SYSTEM. HOWEVER,
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THERE IS A CIRCUIT CONNECTION THAT IS MAINTAINED TO THE HANDSET. AN
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ELECTROMAGNET CALLED THE RINGER SOLENOID IS CONNECTED TO THE TELEPHONE LINE
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WIRES ON THE CENTRAL OFFICE SIDE OF THE SH SWITCH SO THAT THE CENTRAL OFFICE CAN
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RING THE TELEPHONE WITH AN AC SIGNAL WHEN IT IS CALLED. THE TELEPHONE WIRES ARE
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DENOTED AS THE T AND R LINES, FOR THE TERMS TIP AND RING, WHICH WERE RELATED TO
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PLUG CONNECTIONS USED IN THE ORIGINAL MANUAL (OPERATOR CONTROLLED) SWITCHING
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CENTRAL OFFICES OR EXCHANGES.
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WITH THE SUBSET IN THE CRADLE (OR HANGING TELEPHONES MAY BE ON A HOOK -
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THUS THE NAME SWITCH HOOK) NO DC CURRENT FLOWS FROM THE CENTRAL OFFICE 48-VOLT
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TALKING BATTERY THROUGH THE T-R LOOP OR THRU THE RING SOLENOID LOOP BECAUSE OF
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CAPACITOR C. THE CENTRAL OFFICE MONITORS THE DC CURRENT TO DETERMINE IF THE
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PHONE IS IDLE OR BUSY OR IS INITIATING A CALL. THE CENTRAL OFFICE CONTROLS THE
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SIGNALS THAT CAN BE DELIVERED TO A HANDSET THROUGH THE SWITCHES S1, S2, AND S3.
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WITH THE PHONE ON THE HOOK, SH IS OPEN AND ONLY THE RINGER CIRCUIT CAN FUNCTION.
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RINGING A CALLED TELEPHONE
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--------------------------
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THE RINGING SIGNAL IS A 20-47 HZ AC SIGNAL FROM THE CENTRAL OFFICE. PICKING
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UP THE HANDSET LETS DC CURRENT FLOW IN THE TALKING CIRCUIT, STOPS THE RING, AND
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CONNECTS THE REECEIVING TELEPHONE TO THE CALLING TELEPHONE.
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THE SIGNALS THAT CAN BE SENT ARE:
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SIGNAL TONE INTERRUPT RATE FREQUENCIES (HERTZ)
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----------- -------------- -------------------
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DIAL NONE 480
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RINGING NONE 440 MODULATED WITH 480
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BUSY 60 (CALLED LINE) 480 MODULATED WITH 620
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30 (TOLL LINE)
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120 (INTRA-OFFICE)
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ASSUME THAT THE TELEPHONE IS IDLE - THE SUBSET IS ON THE CRADLE OR ON THE
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HOOK. IT IS READY AND WAITING TO RECEIVE AN INCOMING CALL AS INDICATED BY THE
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LACK OF DC CURRENT FLOW IN ITS T AND R LINES. IF THIS TELEPHONE HAS BEEN
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SELECTED BY THE CENTRAL OFFICE TO RECEIVE A CALL, S2 WILL BE THROWN TO CONNECT
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THE RINGING SIGNAL TO THE T AND R LINES THROUGH THE TRANSFORMER TC. THE 110 VOLT
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AC SIGNAL FO 20 TO 47 HERTZ WILL CAUSE THE RINGER SOLENOID TO SOUND THE FAMILIAR
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TELEPHONE RING AT THE HANDSET. THIS RINGING WILL CONTINUE UNTIL THE CALLER HANGS
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UP OR UNTIL THE TELEPHONE IS ANSWERED BY LIFTIG THE SUBSET OFF THE SWITCH HOOK.
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SUCH ACTION WILL CAUSE DC CURRENT TO FLOW IN THE T-R LOOP THROUGH THE SUBSET.
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THE CENTRAL OFFICE CIRCUITRY WILL DETECT THE DC CURRENT FLOW AND REMOVE THE
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RINGING SIGNAL BY OPENING S2. IT ALSO WILL THROW SWITCH S3 TO CONNECT THE SET TO
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THE CALLING TRANSMISSION PATH.
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ANSWERING THE TELEPHONE
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-----------------------
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AS THE SPEAKER TALKS INTO THE SUBSET TO ANSWER THE PHONE, THE MICROPHONE OF
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THE HANDSET CAUSES THE CURRENT IN THE T-R LOOP TO VARY TO PRODUCE ELECTRICAL
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SIGNALS THAT CORRESPOND TO THE PATTERN OF THE SPEECH WAVEFORM. THE T-R LOOP
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CURRENT THROUGH THE MATCHING TRANSFORMER TS AND THE PRIMARY OF THE TRANSFORMER
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TC PRODUCES THE SPEECH SIGNALS IN THE SECONDARY CIRCUIT OF THE TRANSFORMER TC.
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THE SECONDARY OF THE TRANSFORMER TC IS CONNECTED TO THE CALLING TELEPHONE
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THROUGH THE CIRCUIT PATH FORMED BY THE CLOSING OF SWITCH S3 AND NETWORK
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SWITCHING CIRCUITS IN THE CENTRAL OFFICE.
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THE DIRECTION OF THE INFORMATON FLOW THROUGH THE TRANSFORMERS ALTERNATES AS
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ONE PERSON SPEAKS, THEN THE OTHER.
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NOW IF THE HANDSET IS CONSIDERED TO BE THE CALLING HANDSET, AFTER THE
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PERSON CALLING HAS DIALED AND THE CALLED TELEPHONE OWNER HAS LIFTED THE SUBSET
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AND SPOKEN INTO IT TO ANSWER THE TELEPHONE (AS DISCUSSED ABOVE), THE ELECTRICAL
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SIGNALS REPRESENTING THE SPOKEN WORD ARE NOW THE INPUT SIGNAL TO THE TRANSFORMER
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TC THROUGH TTHE CLOSED SWITCH S3. THE S3 SIDE OF THE TRANSFORMER IS NOW THE
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PRIMARY AND THE T-R LOOP SIDE IS THE SECONDARY. THE CURREENT IN THE PRIMARY
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PRODUCED BYTHE SPEECH SIGNALS INDUCE A CURRENT IN THE SECONDARY WHICH EXCITES
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THE SPEAKER OF THE HANDSET THROUGH THE MATCHING TRANSFORMER TS AND REPRODUCES
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THE SPOKEN WORD OF THE PERSON ANSWERING THE CALLED TELEPHONE IN THE EAR OF THE
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PERSON AT THE CALLING TELEPHONE. AS EACH PERSON SPEAKS AT EITHER THE CALLED OR
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CALLING TELEPHONE, THE SPOKEN WORD IS REPRODUCED FROM TRANSMITTER TO RECEIVER
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THROUGH THE COMPLETED COMMUNICATIONS CIRCUIT. THE SYSTEM REPRODUCES THE SPOKEN
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WORD IN BOTH DIRECTIONS THROUGH THE SAME CIRCUIT.
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WHENEVER A PERSON SPEAKS INTO THE MICROPHONE OF THE HANDSET THEY ALSO HEAR
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THEIR SPOKEN WORDS IN THE SPEAKER OF THE SAME HADNSET. THE MATCHING TRANSFORMER
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TS DETERMINES THE CORRECT SIGNAL LEVEL FOR THIS SIGNAL AS WELL AS THE CORREECT
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SIGNAL LEVEL FOR THE SIGNAL RECEIVED FROM THE SENDING TELEPHONE.
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DURING THE ABOVE DESCRIBED SEQUENCE THE DIALER SUB-CIRCUIT IS NOT USED, THE
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TONE SIGNALLING CIRCUIT IS NOT USED, AND SWITCH S1 REMAINS OPEN.
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INITIATING A CALL
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-----------------
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BOTH THE DIAL NETWORK AND THE TONE SIGNAL GENERATORS ARE NEEDED TO PROVIDE
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THE PROPER BASIC SIGNALLING TONES BETWEEN THE CALLING TELEPHONE AND THE CENTRAL
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OFFICE. WHEN THE CALLER LIFTS THE SUBSET OFF THE CRADLE, DC CURRENT FLOWS IN THE
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T-R LOOP. THIS IS SENSED BY THE CENTRAL OFFICE AND INDICATES THAT A CALL HAS
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BEEN INITIATED. THE CENTRAL OFFICE LOCATES THE LINE INITIATING THE CALL AND
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PLACES A SELECTED TONE SIGNAL GENERATOR OUTPUT OF 480 HERTZ ON THE T-R LINE BY
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CLOSING SWITCH S1 AND PRODUCING A CURREENT INT HE PRIMARY OF TRANSFORMER TC.
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THIS PRODUCES A DIAL TONE IN THE CALLING HADNSET'S SPEAKER. SWITCHES S2 AND S3
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ARE OPEN (S2 WILL REMAIN OPEN, SINCE NO RINGING CURRENT WILL BE DELIVERED TO THE
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CALLING TELEPHONE). WHEN THE CALLER HEARS THE DIAL TONE, THE NUMBER OF THE
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TELEPHONE TO BE CALLED WILL BE INDICATED AS AN ELECTRICAL SIGNAL BY A ROTARY
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DIAL CONTAINING SWITCH CONTACTS THAT OPENA ND CLOSE OR BY PRESSING TONE DIALING
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PUSHBUTTONS. THE DIAL TONE WILL BE INTERRUPTED WHEN THE FIRST DIGIT IS DIALED
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WITH A MECHANICAL DIALER. IN THE CASE OF THE TONE DIALING SYSTEM IT WILL BE
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REPLACED WITH THE TONE OF THE BUTTON PUSHED.
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MECHANICAL DIALER
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-----------------
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IF A MECHANICAL DIALER (PULSE) IS USED, THE DIAL WILL BE ROTATED ACCORDING
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TO THE DIGIT REQUIREED AND ALLOWED TO ROTATE BACK TO ITS REST POSITION. AS IT
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DOES IT WILL INTERRUPT THE DC CURRENT IN THE T-R LINES TO CAUSE A NUMBER OF
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PULSES EQUAL TO THE NUMBER DIALED. THE DIAL IS MECHANICALLY DESIGNED TO PROVIDE
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10 PULSES PER SECOND. THE SPEAKER IS USUALLY DISCONNECTED BY THE DIAL NETWORK TO
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KEEP FROM SOUNDING THE DIALING CLICKS. THE CENTRAL OFFICE CIRCUITS RECEIVE THE
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PULSES AND DETECT THE SEQUENCE OF NUMBERS DIALED AND STORES THEM. THEY USE THESE
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NUMBERS TO LOCATE THE CALLED TELEPHONE AND TO SET UP AN AVAILABLE TRANSMISSION
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PATH BETWEEN THE TWO STATIONS. WHEN THE CALLED TELEPHONE IS LOCATED, THE CENTRAL
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OFFICE CHECKS THE ON-HOOK STATUS OF THE CALLED TELEPHONE, AND RINGS IT IF THE
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TELEPHONE IS ON-HOOK (IDLE). IT ALSO PLACES A RINGING TONE THROUGHT H CLOSED S1
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SWITCHES (S2 AND S3 ARE STILL OPEN) ON THE LINE SO IT IS HEARD AT THE CALLING
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TELEPHONE. THIS RINGING TONE IS DEVELOPED BY MODULATING A 440-HERTZ SINUSOID
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WITH A 480-HERTZ SINUSOID. THIS RESULTS IN A 920-HERTZ TONE INCREASED AND
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DECREASED IN INTENSITY AT A 40-HERTZ RATE. THE RINGING TONE CONTINUES UNTIL
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EITHE THE CALLED TELEPHONE ANSWERS OR THE CALLING TELEPONE HANGS UP. IF THE
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CALLED TELEPHONE ANSWERS, THE CENTRAL OFFICE CIRCUITS REMOVE THE RINGING CURRENT
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FORM THE CALLED SET, THE RINGING TONE FROM THE CALLING SET AND COMPLETES THE
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TRANSMISSION PATH BY CLOSING THE SWITCHES S3.
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BUSY TELEPHONE
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--------------
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IF THE CALLED TELEPHONE IS BUSY (OFF-HOOK) THE TONE GENERATOR WILL SOUND
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ONE OF THREE POSSIBLE BUSY SIGNALS TO THE CALLING TELEPHONE. THE BUSY TONE IS A
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140-HERTZ TONE GENERATED BY MODULATING A 480-HERTZ SINUSOID WITH A 620-HERTZ
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SINUSOID AND FILTERING OUT THE 1100-HERTZ SIGNAL (WHEN ONE SINEWAVE MODULATES
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ANOTHER, BOTH THE SUM AND DIFFERENCE FREQUENCY SINE WAVE SIGNALS RESULT). THIS
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TONE IS INTERRUPTED AT A RATE THAT INDICATES THE REASON FOR THE BUSY SIGNAL. A
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SIGNAL INTERRUPTED AT THE RATE OF 60 TIMES PER IMNUTE INDICATES THE CALLED LINE
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WAS BUSY. IF THE INTERRUPT RATE IS 1/2 OF THIS, THE TOLL LINE BETWEEN THE
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CENTRAL OFFICES WAS BUSY (FULL OF EXISTING CALLS.) IF THE INTERRUPT RATE IS
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DOUBLED TO 120 TIMES PER IMIUTE IT IMPLIES THAT ALL INTRA-OFFICE PATHS ARE BUSY.
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ONLY WITH THE 60 INTERRUPTS/MINUTE IS THE CALLER SURE THAT THE CALLED PARTY LINE
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IS BUSY.
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TONE DIALING
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------------
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WHEN THE HANDSET IS A TOUCH-TONE SERVICE TELEPHONE, THEN THE DIAL NETWORK
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IS MORE THAN JUST SWITCHES. IT CONTAINS TONE GENERATING CIRCUITS AS WELL. WHEN
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THE TONE DIALING TELEPHONE IS USED, THE TONE KEY OR PAD CAUSES A SIGNAL OF TWO
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FREQUENCIES TO BE PLACED ON THE LINE. THE FREQUENCIES AREE INDICATED BY THE
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INTERSECTION OF THE FREQUENCY LINES IN THE TONE MATRIX OF THE FOLLOWING DIAGRAM:
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697--------1-------2-------3-------!
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! ! ! !
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! ! ! !
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! ! ! !
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770--------4-------5-------6-------!
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! ! ! !
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! ! ! !
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! ! ! !
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852--------7-------8-------9-------!
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! ! ! !
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! ! ! !
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! ! ! !
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941--------*-------0-------#-------!
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! ! ! !
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! ! ! !
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! ! ! !
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1209 1336 1477 1633
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FOR EXAMPLE, PRESSING THE 5 KEY CAUSES A 770-HERTZ AND A 1336-HERTZ TONE TO
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BE SENT TO THE CENTRAL OFFICE (AND TO THE CALLED PARTY IF THE BUTTON IS PUSHED
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DURING A CONVERSATION). CENTRAL OFFICE CIRCUITS THAT DETECT AND DECODE THE TONES
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SET UP THE SWITCHING FOR THE COMMUNICATAONS PATH JUST AS FOR THE MECHANICAL
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DIALER. THE USE OF SUCH TONES SPEEDS UP THE DIALING OPERATION AND ALLOWS COMMAND
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AND CONTROL INFORMATION TO BE SENT TO THE CALLED LOCATION.
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BANDWIDTH
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---------
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THE FREQUENCIES OF THE TONES GENERATED BY THE TONE KEYS AS WELL AS THE
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CONVERSATTQIONS OF THE TELEPHONE USERS MUST BE WITHIN THE BANDPASS CAPABILITIES
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OF THE TELEPHONE SYSTEM. THE BANDWIDTH OF A CHANNEL IN A TYPICAL TELEPHONE
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SYSTEM USED FOR CONVERSATIONS IS 4 KILOHERTZ. THE BANDPASS REGION ALLOWED FOR
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THE VOICE SIGNALS IS 200 HERTZ TO 3400 HERTZ. THE TONE DIALING SIGNALS FITS INTO
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THIS RANGE AS DO SOME SPECIAL CONTROL SIGNALS AT 2400 AND 2600 HERTZ. OTHER
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CONTROL SIGNALS FORM 3400 TO 3700 HERTZ FIT INTO THE OVERALL 4 KILOHERTZ
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BANDWIDTH OF THE TELEPHONE CHANNEL. THIS VOICE CHANNEL BANDWIDTH WILL BE AN
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IMPORTANT SYSTEM PARAMETER THROUGHOUT THIS CHAPTER. THE OTHER IMPORTANT
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PARAMETERS OF THE VOICE CHANNEL ARE THE SIGNALLING TONES AND THE DC CURRENTS
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THAT ARE USED BY THE SWITCHING AND CONTROL CIRCUITS OF THE CENTRAL OFFICES TO
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ESTABLISH THE COMMUNICATIONS PATHS BETWEEN MANY PAIRS OF TELEPHONES OVER A
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WORLDWIDE NETWORK. UNDERSTANDING THE BASIC CONCEPTS OF THE CENTRAL OFFICE
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CIRCUITS WILL HELP TO FURTHER UNDERSTAND THE TELEPHONE COMMUNICATIONS SYSTEMS.
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(CONTINUED IN PART II)
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:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-
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THE CONVENT TEXTFILES BBS 619-475-6187 10 MEGS 3/1200 BAUD NO PASS
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