MOST SCANNER ENTUSIASTES ARE FAMILIAR WITH THE STANDARD MOBILE TELEPHONE SYSTEM; THIS SYSTEM HAS GONE THROUGH VERY LITTLE EVOLUTION IN THE PAST DECADE, AND IN THE US. IT HAS REMAINED A CONSIDERABLY LIMITED SERVICE. A LARGE METRO ARE MAY ONLY HAVE SEVERAL HUNDRED USERS, (NEW YORK HAS APPROX. 700 MOBILE TELEPHONE SUBSCRIBERS), DUE LARGELY TO LIMITAIONS IMPOSED BY SPECTRAL OVERCROWDING. SO-CALLED CELLULAR TECHNOLOGY PROMISES TO CHANGE ALL THAT, OFFERING SERVICE TO LITERALLY HUNDREDS OF THOUSANDS OF USERS. LAND MOBILE COMMUNICATIONS HAS SEEN A 10-12% ANNUAL GROWTH RATE FOR THE PAST TWO DECADES THE RESULT IS THAT THE 40,150,450 MHZ BANDS ARE OVER CROWDED. EVEN THE UTILIZATION OF THE NEW 900MHZ BAND WITH 30-40 TIMES MORE CHANNELS AVAIL. THAN ON OTHER BANDS IS A SHORT LIVED SOLUTION TO THE PROBLEM. CELLULAR TECHNOLOGY IS A RESONABLE ALTERNATIVE. CELLULAR TECHNOLOGY STANDARDS SUCH AS RECOMMENDED TRANSMISSION MODES (SSB, FM, DIGITAL, OR SPREAD SPECTRUM) HAVE YET TO BE AGREED UPON BUT WE CAN OUTLINE THE GENERAL PRINCIPLES BEHIND ITS IMPLIMENTATION AND USE.
THE CELL AND THE MOBILE CALL
THE ONLY FUNDIMENTAL POINT OF CELLULAR TECHNOLOGY IMPLIMENTATION ACTUALLY AGREED UPON TO DATE IS THAT A GIVEN SERVICE AREA WILL BE DIVIDED INTO IDENTICLE ADJACENT CELLS WITH NO OVERLAPS AND NO GAPS,HEXAGONS ARE POPULAR FOR DIAGRAMING INDIVIDUAL CELLS. AT THE CENTER OF AN INDIVIDUAL CELL IS A BASE STATION WHICH IS CONNECTED VIA LAND LINE TO A LOCAL MOBILE TELEPHONE SWITCHING OFFICE. CERTAIN FREQUENCY BANDS ARE ASSIGNED TO CERTAIN CELLS, BUT NOT SHARED WITH ADJACENT CELLS TO AVOID MUTUAL INTERFERENCE. A MOBILE UNIT WISHING TO MAKE A CALL WILL GO OFF-HOOK AND TRANS MITT DIGITAL SOURCE AND DESTINATION CODES. THESE TRANS MISSIONS TAKE PLACE ON SPECIAL SETUP CHANNELS AND ARE JSUT STRONG ENOUGH TO REACH THE BASE STATION IN THE LOCAL CELL.
THE BASE STATION THEN RECIEVES THE CODES AND FORWARDS A MESSAGE TO THE CENTRAL SWITCHING OFFICE WHICH IN TURN SENDS OUT A PAGING SIGNAL TO ALL CELLS IN SEARCH OF THE SECOND MOBILE UNIT WHOSE NUMBER HAS BEEN DIALED. WHEN THE DESTIANTION UNIT IS FINALLY FOUND, IT RESPONDES TO THE PAGING SIGNAL BY TRANSMITTING AN ACKNOWLEDGEMENT CODE TO ITS LOCAL BASE STATION ON A SETUP CHANNEL.
THE SWITCHING CENTER THEN ASSIGNES A PAIR OF FREQ'S (CHANNEL PAIR) TO EACH OF THE UNITS FOR ACTUAL VOICE COMMUNICATIONS TO TAKE PLACE. THESE CHANNEL PAIRS ARE NOT NECCESARILY THE SAME FOR THE RESPECTIVE CELLS THAT EACH MOBILE UNIT IS IN. THESE FREQ'S ARE ALSO RELAYED THROUGH THE BASE STATIONS AND THE CENTRAL SWITCHING OFFICE.
WHEN ONE UNIT MOVES INTO ANOTHER CELL, THINGS GET REALLY INTERESTING. UPON ENTRY INTO ANOTHER CELL THE MOBILE UNITS MUST TRANSMITT THROUGH A NEW BASE STATION. AN AUTOMATIC HANDOFF TO THE NEW BASE STATION IS CARRIED OUT BY ANOTHER EXCHANGE OF DATA THROUGH THE SETUP CHANNEL.
TERMINATION OF THE CALL IS A SIMPLE MATTER. WHEN THE CALL ENDS ON HOOK SIGNALS ARE EXCHANGED VIA THE SETUP CHANNELS BETWEEN THE MOBILE UNIT AND THE BASE STATION. THE MAIN VOICE CHANNELS ARE THEN CLEARED.
ADVANCED MOBILE TELEPHONE SYSTEM
IN 1979 AT&T BEGAN TEST MARKETING ITS VERSION OF A CELLULAR TELEPHONE SYSTEM IN CHICAGO,ILL. THIS SYSTEM IS CALLED ADVANCED MOBILE TELEPHONE SYSTEM (AMPS)
SOME 2100 SQ MILES OF THE METRO CHICAGO AREA ARE DIVIDED INTO 10 CELLS TO SERVE APPROX. 2000 CUSTOMERS. FULL DUPLEX IS POSSIBLE BY USING A PAIR OF ONE WAY CHANNELS SEPERATED BY 45MHZ TO CONNECT THE MOBILE UNITS WITH THE BASE STATIONS. THE RF RANGE IS 825-890 MHZ AND NORMAL NARROW BAND FM IS USED TO TRANSMITT VOICE. HAND OFF TO ADJACENT CELLS IS ACCOMPLISHED BY MONITORING SIGNAL STRENGTHS. WHEN THE CENTRAL SWITCHING OFFICE DETERMINES THAT A NEW BASE STATION RECIEVES THE MOBILE UNITS SIGNAL BETTER THAN THE PREVIOUS ONE THE SWITCHING OFFICE SIGNALS THROUGH THE VOICE CHANNEL FOR THE MOBILE PHONE TO SWITCH TO A NEW CHANNEL. COMMUNICATIONS DISRUPTION THROUGH THE SWITCHING PROCESS IS ONLY 50 MILLISECONDS.
AS WITH IMPS (DISCUSSED IN THE FEATURE ARTICLE ON THIS BBS (PIRATE-80)) THERE IS THE POSSIBILITY OF PHREAKING CALLS WITH IMPS OR AMPS SIMPLY BY MONITERING THE SETUP CHANNELS WITH A SCANNER AND RECORDING THE ID NUMBERS AND THE DIALED NUMBERS SINCE THEY ARE IN DIAL PULSE FORM AND AFTER U HAVE A NICE SET OF NUMBERS U WILL NEED A TRANSMITTER OF SUFFICIENT STRENGTH TO REACH THE BASE STATION (UNLICIENCED TRANSMITTER OF COURSE).
FINAL COMMENTS
MANY REGULATORY AND IMPLIMENTATION ISSUES REMAIN UNSOLVED. MODULATION ISSUES ARE THE BIGGEST PROBLEM TO BE SOLVED. SINGLE SIDEBAND AM, NARROW BAND FM, DIGITAL, AND SPREAD SPECTRUM TECHNIQUE'S ARE ALL BEING CONSIDERED.
BIBLIOGRAPHY
1. CELLULAR MOBILE TECHNOLOGY: THE GREAT MULTIPLIER BY GEORGE COOPER AND RAY NETTELTON. IEEE SPECTRUM, VOL 20 #6 JUNE 83 PP 30-37
2. MOBILE COMMUNICATIONS ENGINEERING BY WILLIAM C.Y. LEE
3.*** SPECIAL ISSUE ON AMPS. BELL SYSTEM TECHNICAL JOURNAL JAN. 79.
THIS FILE IS PRESENTED BY MT. <S><C><A><N><*><M><A><N> AND PIRATE-80 SYSTEMS....
