fig1 shows a central control station 20 according to a first embodiment of the invention ; fig2 shows a paging unit 22 suitable for use with central control station 20 . as shown in fig1 , central control station 20 includes central computer 30 ; transmitter 32 ; receiver 34 ; and computerized telephone answering system 36 . transmitter 32 transmits , via transmitting antenna 42 , two local frequencies , namely frequency f 1 and frequency f 2 . receiver 34 is connected to receiver antenna 44 for reception of two local frequencies , namely frequency f 3 and frequency f 4 . computerized telephone answering system 36 is connected to a bank of telephones 48 . central computer 30 of central control station 20 comprises a conventional computer equipped with typical components including a cpu 50 ; i / o interface 52 ; and memory 54 . although shown only generally in fig1 , it should be understood that memory 54 includes a number of unillustrated memory devices , including ( for example ) a hard disk drive , ram , and rom . fig1 shows that memory 54 has stored therein ( among other things ) a pager registration file 55 and a pager directory file 56 . pager files 55 and 56 are typically stored on a hard disk drive of central computer 30 , and upon start - up are loadable into a ram portion of memory 54 . central computer 30 of central control station 20 further includes a decoder 57 ( connected between receiver 34 and i / o interface 52 for decoding in - coming communications information from one or more pager units 22 ), as well as encoder 58 ( connected between i / o interface 52 and transmitter 32 for encoding out - going communications information ). central control station 20 also includes a clock unit 59 which generates a local clock signal f 1 clk ( which , in turn , is used to modulate frequency f 1 ). as illustrated further herein , cpu 50 of central control station 20 prepares communications packets for transmission on frequency f 2 . as generally illustrated in fig1 , the communications packets are of a predetermined format , having fields for identification of the central control station , for identification of the addressed pager unit ( s ) 22 , for an operation code , for ( optionally ) alphanumeric information , and for other conventional packet - type information such as checksum , error correction , and postamble . the preamble and postamble are specially chosen patterns which can be recognized and distinguished from data for the purpose of determining the beginning and ending of a packet . the alphanumeric information can be in a customary binary 8 - bit format . the format of fig1 is illustrative only , as such information as the order of the fields can be varied in other embodiments . central control station 20 communicates with a plurality of pager units 22 1 , 22 2 , . . . 22 n . only one such pager unit , generically referenced as pager unit 22 , is specifically illustrated and described herein , it being understood that the construction and operation of other pager units may be similar to the one illustration . as shown in fig2 , pager unit 22 includes a pager receiver antenna 60 which is connected to pager receiver 62 . pager receiver 62 is , in turn , connected through s / d converter 64 within pager computer 70 . receiver 62 receives the two local frequencies f 1 , and f 2 , which frequencies have been modulated to carry in - coming communications information ( described in more detail below ) to pager computer 70 . on a communications output side , pager computer 70 outputs out - going communications information to pager transmitter 72 via d / s converter 74 . transmitter 72 broadcasts , on pager antenna 76 , the out - going communications information on the two local frequencies f 3 and f 4 . as also shown in fig2 , pager computer 70 includes pager microprocessor 80 which is connected to each of an arithmetic processor ; a memory system 84 ( including both rom and ram ); and i / o interface 86 . i / o interface 86 is connected to a clock unit 87 . i / o interface 86 is also connected to receive in - coming decoded communications information from an 8 - bit decoder 88 and to output out - going uncoded communications information to an 8 - bit encoder 90 . decoder 88 is connected to receive in - coming coded communications information from s / d converter 64 ; encoder 90 is connected to output out - going coded communications information to d / s converter 74 . clock unit 87 is settable by suitable inputs thereto so that clock unit 87 generates a local clock signal f 1 clk having a frequency corresponding to its input . it should be understood that , in other embodiments , the function of clock unit 87 can be performed at least partially by microprocessor 80 using programmed execution . i / o interface 86 is also connected to supply an on / off signal on line 92 to pager transmitter 72 , as well as to facilitate input and output with numerous input / output devices . the input / output devices connected to i / o interface 86 include keyboard 93 ; beeper 94 ; vibrator 95 ; and lcd ( alphanumeric ) display 96 . upon manufacture , pager unit 22 is preprogrammed with an identification serial number ( e . g ., a 7 - digit alphanumeric pre - assigned id number ) which is stored in memory 84 ( rom ). pager unit 22 is activated ( e . g ., at the time of purchase ) by inserting a time slot assignment ( explained below ) both into a predetermined address in memory 84 of pager unit 22 and into pager directory file 56 ( stored in memory 54 of central control station 20 ). communication between central control station 20 and pager unit 22 occurs on the four local frequencies , in particular the frequencies f 1 , f 2 , f 3 , and f 4 mentioned above . the first frequency ( f 1 ) carries the local clock - aligning signal from central control station 20 to paging unit 22 . the second frequency ( f 2 ) carries a pager command and alphanumeric data from central control station 20 to paging unit 22 . the third frequency ( f 3 ) carries pager status data and alphanumeric data from paging unit 22 to central control station 20 . the fourth frequency ( f 4 ) carries a pager request signal from paging unit 22 to central control station 20 . in the illustrated embodiment , the frequencies f 1 - f 4 are preferably chosen so that f 1 ≠ f 2 ≠ f 3 ≠ f 4 . as explained in more detail below and illustrated in fig1 , in normal non - cell - switching operation , the pager request signal on frequency f 4 is transmitted in a predetermined time slot assigned to paging unit 22 . the predetermined time slot on frequency f 4 is related to the clock - aligning signal ( carried by frequency f 1 ) and assigned whereby the fourth frequency is utilizable by a plurality of other paging units . for example , as shown in fig1 , a first time slot on frequency f 4 is assigned to a pager p 1 ; a second time slot is assigned to page p 2 , and so on up to time slot n assigned to pager pn . in the illustrated embodiment , the number of time slots ( and accordingly the number of pagers ) may be as many as ten thousand or more . fig3 shows steps executed by cpu 50 of central control station 20 in processing communications to and from one or more paging units . the steps depicted in fig3 are indicative of instructions stored in a rom portion of memory 54 of central control station 20 . when central control station 20 is started up ( step 100 ), an initialization process ( step 102 ) is conducted . included in the initialization process is activation of transmitter 32 ( so that transmitter 32 can transmit at the two frequencies f 1 and f 2 ) and activation of receiver 34 ( so that receiver 34 can receive the two frequencies f 3 and f 4 ). moreover , frequency f , is modulated to carry the local clock - aligning signal generated by local clock 59 . then , at step 104 , the pager registration file 55 and the pager directory file 56 are loaded from hard disk into a ram section of memory 54 ( step 104 ). after initialization and loading of the files 55 and 56 , cpu 50 repetitively executes an instruction loop 106 . loop 106 involves checking to determine ( at step 108 ) whether a telephone message is being received ( via answering system 36 from one of the telephones in bank 48 ) and checking to determine ( at step 110 ) whether a pager message is being received ( via transmitter 32 from one of the pager units 22 ). as used herein , a message , whether originated from a telephone or from a pager , may require a plurality of packets for transmission from a central station 20 to a pager 22 or vise versa . in the ensuing discussion , transmission and reception of messages subsumes transmission and reception one or more packets . in general , the packetization of messages will be invisible to the user , meaning that a user enters a message without regard to the number of packets which might be required to transmit the message . the message typically ends with a user - entered message termination character or message delimiter character . the transmitting device ( either central station 20 or pager 22 ), allocates the message to one or more packets having a format similar to that of fig1 , with the last packet in the message bearing the message termination character . alternatively , the packets may be formatted in a manner to indicate the number of consecutively related packets emanating from a transmitter ( e . g ., there may be a separate packet field indicating the continuation number of related packets ). central computer 30 can distinguish between receipt of a telephone message ( at step 108 ) and a pager message ( at step 110 ) by virtue of the fact that i / o interface 52 generates different type of interrupts to cpu 50 depending on the type of message received . if it is determined at step 108 that a telephone message is being received , steps 112 , 114 , and 116 of fig3 are executed . in processing a received telephone message , at step 112 central computer 30 extracts out - going communications information from the predeterminately sequenced telephone - entered data . the telephone - entered data , entered via a touchpad of a calling one of the telephones in bank 48 , includes by convention an identification ( e . g ., telephone number ) of the calling telephone ; an identification of the called pager unit ( e . g ., the 7 - digit alphanumeric pre - assigned id number ); and any character data for transmission followed by a termination character . this out - going communications information is received at central computer 30 in standard dtmf format . at step 114 , using the id number of the called pager ( obtained at step 112 ) central computer 30 checks the pager registration file 55 and directory file 56 to determine whether the called pager unit is registered with central control station 20 . assuming that the called pager is so registered , at step 114 the central computer 30 also obtains from pager directory file 56 the slot assignment for the called pager unit . at step 116 , central control station 30 transmits communications information to the called pager unit . in this regard , central control station 20 prepares and transmits ( on frequency f 2 ) a communications message which includes , among other things , the id of the called pager unit and the character data received from the telephone for transmission of the pager unit 22 . after step 116 is executed , processing returns to loop 106 . if it is determined at step 110 that a pager message is being received , even numbered steps 132 - 140 of fig3 are executed ( prior to returning to loop 106 ). as will be seen hereinafter with respect to fig4 , a sending pager unit 22 transmits , in its assigned time slot , a request signal on frequency f 4 when the sending pager unit 22 desires to send a message . as central control station 20 is always monitoring frequency f 4 , a request signal carried by frequency f 4 from any pager unit 22 is noted . with reference to the local clock 59 , at step 132 cpu 50 determines in what time slot on frequency f 4 the request signal is detected . upon detection of the time slot at step 132 , at step 134 cpu 50 consults the pager directory file 56 to determine the identification number of the particular pager unit 22 which originated the request signal . with the identity of the requesting pager unit 22 now known , at step 136 central control station 20 authorizes the requesting pager unit 22 to transmit its message . in particular , cpu 50 directs preparation of a communications message for transmission on frequency f 2 . the particular communications packet prepared at step 136 includes an identification of the requesting pager unit ( the addressee of the packet ), as well as an operation code (“ op ” code ) which commands / authorizes the requesting pager unit 22 to send its message . at step 138 , central control station 20 receives a communications message on frequency f 3 sent from the sending ( e . g ., requesting ) pager unit 22 . the communications message prepared and sent by the sending pager unit 22 includes packets of similar format to that shown in fig1 , and includes an identification of a pager to which the message is ultimately addressed as well as its own identification . at step 138 , cpu 50 checks to ensure that the ultimate addressee pager unit is registered in pager files 55 and 56 . at step 140 , cpu 50 makes any necessary reformatting and / or information substitution in the message , and causes the message to be transmitted on frequency f 2 . the transmission on frequency f 2 required by step 140 includes the identification of the ultimate addressee ( e . g ., a pager unit 22 ) as well as an operation code indicating that the transmission includes a relayed message from another pager unit . steps executed by a pager unit 22 in connection with its transmission mode are depicted in fig4 . steps executed by a pager unit 22 in connection with its receive mode are depicted in fig5 . the term “ mode ” as used herein does not connote exclusivity at any particular moment , for it should be remembered that at all times pager unit 22 is receiving transmissions on frequencies f 1 and f 2 . in its transmission mode ( see fig4 ), after start - up ( step 200 ) microprocessor 80 of the transmitting pager unit 22 executes a loop 202 wherein user alphanumeric characters ( entered via keyboard 93 ) are repetitively fetched ( at step 204 ) until an end of message delimiter is detected ( at step 206 ). as entered , the characters fetched at step 204 are displayed on lcd display 96 . entry of the delimiter character at step 206 causes microprocessor 80 to exit loop 202 . by convention , the message must include an addressee id , which addressee id is likely the id of another one of the pager units to which the message entered in step 204 is directed . after entry of the message awaits entry from keyboard 93 of a transmit command at step 212 . assuming that the transmit command is entered at step 212 , microprocessor 80 prepares and sends a request signal on frequency f 4 . as indicated before , the request signal is transmitted on frequency f 4 in a time slot assigned to the requesting pager unit 22 . it should be kept in mind that pager unit 22 is all the while receiving the local clock - aligning signal on frequency f 1 , which enables microprocessor 80 to cause transmission of the request signal on frequency f 4 at a time corresponding to the specific time slot allotted to the particular sending pager unit 22 . in the above regard , in accordance with time division techniques , each pager unit 22 1 - 22 n ( e . g ., pagers p 1 - p n in fig1 ) is assigned a selected one of n number of time slots on frequency f 4 . after transmission of the request signal at step 214 , pager unit 22 awaits receipt of a transmit command from central control station 20 . preparation and transmission of the transmit command / authorization from central control station 20 is described with reference to fig3 . upon receipt of the transmit command / authorization from central control station 20 ( step 216 ), microprocessor 80 prepares ( at step 218 ) a communications message with one or more packets having a format much like that of fig1 . the addressee id and alphanumeric field of packets of the communications message is filled with the message entered in loop 202 . at step 220 , the sending pager unit 22 broadcasts the communications packet on frequency f 3 . if a transmit command is not entered at step 212 , or after transmission of the message at step 220 , microprocessor 80 awaits entry of at least one of several possible special function keys at step 222 . for example , the user may press a function key which requires storage of the message ( whether yet transmitted or not ) [ see step 228 ]. alternatively , the user may press function keys which facilitate editing or erasure of the message ( see steps 224 and 226 , respectively ). to complete the message and begin work on another message , a special function key for an exit operation ( step 230 ) must be pressed . fig5 depicts steps executed by microprocessor 80 of pager unit 22 when in a receive mode . after start - up ( step 302 ), and as indicated by step 304 , pager unit 22 receives transmissions from central control station 20 on frequency f 2 . once a complete packet is received ( determined at step 306 ), a check is made ( at step 308 ) whether the addressee id in the communications packet ( see packet format of fig1 ) is the id of the receiving pager unit 22 . if the determinations of either step 306 or 308 are negative , pager unit 22 awaits either completion of the communications packet ( in the case of step 306 ) or receipt of another communications packet ( in the case of step 308 ) by looping back to step 304 . assuming that the received communications packet is designated for this particular receiving pager unit 22 , at step 310 microprocessor 80 consults the operation code field of the communications packet ( see fig1 ) to determine if the operation code indicates that the message includes a command . if the operation code indicates a command , a command processing routine ( framed by broken lines 312 in fig5 ) is executed . assuming for the moment that the operation code does not indicate a command , at step 314 microprocessor 80 of pager unit 22 stores the alphanumeric field portion of the communications packet ( which at least partially forms the message ) in a ram portion of memory 84 . since a message communicated from central processing station 20 may require several communications packets for completion of the message ( with subsequent communication packets providing continuations of the message content ), microprocessor 80 checks at step 316 to ensure that the entire message has been received . if not , processing continues back at step 304 for reception of a further communications packet . upon reception of an entire communications message , at step 318 microprocessor 80 determines whether pager unit 22 is in a beep mode or a vibrate mode . in this regard , there are numerous ways of setting paging unit 22 to the desired mode , either by a specially dedicated switch on paging unit 22 or by data entry using keyboard 93 . if pager unit 22 is in a beep mode , microprocessor 80 outputs a signal which causes i / o interface 86 to issue a further signal to activate beeper 94 ( step 320 ). alternatively , if pager unit 22 is in a vibrate mode , microprocessor 80 outputs a signal which causes i / o interface 86 to issue a further signal to activate vibrator 95 ( step 322 ). at step 324 , microprocessor 80 directs i / o interface 86 to send the alphanumeric message data to lcd display 96 , so that the received message can be viewed by the user . after notification to the user ( either via beeper 94 and / or vibrator 95 ), and display ( on lcd 96 ) of the received alphanumeric data , microprocessor 80 returns to step 304 to check whether further communications packets are being received . the command processing routine ( framed by broken lines 312 in fig5 ) first determines ( step 330 ) which particular operation is being commanded . this determination is based on the content of the operation code , which is different for different command types . if the operation code indicates an error shut - down , execution jumps to an error shut - down sub - routine which begins at step 340 . if the operation code indicates a time slot change , execution jumps to a change time slot sub - routine which begins at step 350 . if the operation code requires transmitter shut - down , execution jumps to a transmitter shut - down sub - routine which begins at step 360 . if the operation code requires transmitter re - enablement , execution jumps to a transmitter reenable sub - routine which begins at step 370 . if the operation code requires clock re - set , execution jumps to a clock re - set sub routine which begins at step 380 . in connection with the error shut down sub - routine , at step 342 microprocessor 80 obtains an indication of error type from the communications packet . the error type is stored in memory 84 ( step 344 ) and then displayed on lcd display 96 ( step 346 ). then microprocessor 80 issues a command ( at step 348 ) to shut down pager unit 22 , which shut - down occurs at step 349 . in connection with the time slot changing sub - routine , at step 352 microprocessor 80 extracts , from the received communications packet , information indicative of the new time slot assigned to the receiving pager unit 22 . the new time slot is entered ( at step 354 ) into memory 84 and thereafter utilized ( until further change ) in connection with transmission of request signals on frequency f 4 ( see , for example , step 214 of fig4 ). the time slot changing sub - routine may also include other operations , if desired , including ( for example ) eliminating unused time slots ( thereby increasing scanning rate ); diagnosing and trouble shooting ; and avoiding interruption of service from malfunctioning or ill - functioning equipment . in connection with the transmitter shut down sub - routine , at step 362 microprocessor 80 directs i / o interface 86 to issue an off command to transmitter 72 . in connection with the transmitter re - enable sub - routine , at step 372 microprocessor 80 directs i / o interface 86 to issue an on command to transmitter 72 . in connection with the clock re - set sub - routine , at step 382 microprocessor 80 directs that clock 59 of pager unit 22 be set . after execution of steps 354 , 362 , 372 , or 382 , execution continues back to step 304 for processing of potential further communications packets . thus , unless an error shut - down is noted , each entry of the command processing routine ( framed by broken lines 312 in fig5 ) is followed by a loop back to step 304 . fig6 is a timing diagram showing the frequencies f 1 - f 4 and integration of the steps depicted in fig3 - 5 , particularly in the context of a request by a sending pager unit p 1 for sending a message to a sendee pager unit p 2 . as employed in fig6 , “ computer ” refers to central control station 20 . it should be understood that the sending pager unit p 1 and the sendee pager unit p 2 operate in both the transmission mode as depicted in fig4 and in the receiver mode as depicted in fig5 . in general , fig6 shows transmission of a message from pager unit p 1 ( via central control station 20 ) to pager unit p 2 ; transmission of a confirmation message from pager unit p 2 ( via central control station 20 ) to pager unit p 1 ; and transmission of a message from pager unit p 1 to central control station 20 indicating that pager unit p 1 received the confirmation message from pager unit p 2 . fig7 shows a central control station 420 according to a second embodiment of the invention ; fig8 shows a paging unit 422 suitable for use with central control station 420 . fig9 shows a wide area paging system including a plurality of central control stations s 1 - s 8 ( each identical to central control station 420 ), each preferably geographically centered within a respective cell . each central control station s 1 - s 8 broadcasts its own local frequencies , as well as a set of common or switching frequencies c 1 - c 4 . the common frequencies c 1 - c 4 are broadcast at a lower power , so that reception thereof occurs only in a relatively small neighborhood or common frequency reception region ( cfrr ) [ also referred to as a “ switching region ”] about the central control station . the local frequencies are broadcast at a significantly greater power for reception substantially throughout the cell . for example , in fig9 , central control station s 1 broadcasts its lower power common frequencies c 1 - c 4 to cfrr 1 and its higher power local frequencies f 1 - f 4 to cell ; central control station s 2 broadcasts its lower power common frequencies c 1 - c 4 to cfrr 2 and its higher power local frequencies f 5 - f 8 to cell 2 . as also shown in fig9 , cell 1 and cell 2 overlap in an overlap region shown in fig9 . station s 1 utilizes a set of local frequencies f 1 - f 4 ; station s 2 utilizes a different set of local frequencies f 5 - f 8 . both stations s 1 and s 2 utilize the same set of common or switching frequencies c 1 - c 4 . thus , each central control station utilizes two sets of frequencies , there being four frequencies in each set , resulting in a total of eight frequencies handled per station . thus , the second embodiment of the invention is suitable for a system having a plurality of central control stations 420 x where x = 1 , 2 , . . . m . each central control station 420 x transmits and receives a set of local frequencies f l1 , f l2 , f l3 , f l4 in an associated geographical area or cell , as well as the set of common or switch frequencies c 1 , c 2 , c 3 , c 4 . while the values of the local frequencies f l1 , f l2 , f l3 , f l4 , vary from cell to cell ( e . g ., differ for differing central control stations 420 x ), the values of the common or switch frequencies c 1 , c 2 , c 3 , c 4 are uniform through the system ( e . g ., for all central control stations 420 x ). although not shown in fig9 , it should be understood that the pattern of central control stations repeats in like manner in all compass directions in accordance with the prescribed geographical boundaries of the paging system . moreover , although not specifically illustrated in fig9 , it should also be understood that each central control station 420 has an associated cfrr . the common or switching frequencies c 1 - c 4 have an analogous function to the corresponding local frequencies f 1 - f 4 , respectively . in this regard , frequency c 1 carries a clock frequency transmitted by central control station ( s ), although the clock rate on common frequency c , preferably varies among central control stations . frequency c 2 is used to transmit information from central control station ( s ) to pager unit ( s ); frequency c 3 is used to transmit information from a pager unit to a central control station ; frequency c 4 is used by pager units to issue a request signal . frequency c 2 carries packets having a format similar to that of fig1 . in analogous manner to frequency f 2 , the packets carried by frequency c 2 may have command codes . among the c 2 command codes are a system command code ; a local frequency download command code ; a slot recognition command code ; and a slot assignment command code . as shown in fig7 , central control station 420 resembles central control station 20 of the embodiment of fig1 ( similar components being assigned the same reference numerals for simplicity ). however , central control station 420 is augmented by inclusion of a further transmitter , known as common frequency transmitter 432 , together with its common frequency transmission antenna 442 , for transmitting the common frequencies c 1 and c 2 . in contrast to the high power transmitter 32 , transmitter 432 is a low power transmitter . further , central control station 420 is augmented by inclusion of a further receiver , known as the common frequency receiver 434 , together with its common frequency receiver antenna 444 , for reception of the common frequencies c 3 and c 4 . central control station 420 of fig7 includes a clock unit 59 ′ which generates two clocking signals — a first or local clocking signal f l clk and a second or common clocking signal c 1 clk . the local clocking signal f l clk is used to modulate frequency f 1 ); the common clocking signal is used to modulate the common frequency c 1 . the central computers 30 of the central control stations 420 x are serially connected to one another by an output line 486 a and an input line 486 b . in particular , although not expressly shown as such in fig7 , computer 30 of fig7 ( like that of fig1 ) includes an i / o interface to which the serial lines 486 a and 486 b are connected . serial lines 486 a and 486 b are used , for example , to update contents of the pager registration file 55 and the pager directory file 56 . as shown in fig8 , pager unit 422 resembles pager unit 22 of the embodiment of fig2 ( similar components again being assigned the same reference numerals for simplicity ). however , pager unit 422 ( in like manner as central control station 420 ) is augmented by inclusion of a further transmitter , known as common frequency transmitter 572 , together with its common frequency transmission antenna 576 , for transmitting the common frequencies c 3 and c 4 . further , central control station 420 is augmented by inclusion of a further receiver , known as the common frequency receiver 434 , together with its common frequency receiver antenna 444 , for reception of the common frequencies c 1 and c 2 . the operational frequencies of transmitter 72 and receiver 62 are changeable in accordance with values transmitted on “ frequency control ” lines from computer 70 . in particular , the frequency control lines are connected to i / o interface 86 in computer 70 . as described in more detail below , when a pager unit 422 migrates into a new cfrr , signals are applied on the frequency control lines in order to switch pager unit 422 from the local frequencies of an old cell to the local frequencies of a new cell associated with the new cfrr into which pager unit 422 migrates . pager 422 includes a clock unit 83 ′ which is capable of separately generating local clocking signals f l clk and the common clocking signals f c1 clk for use by microprocessor 80 . these clocking signals are initiated and their frequencies set by appropriate respective inputs to clock unit 83 ′. fig8 also shows that pager unit 422 has data i / o unit 596 which includes both an alphanumeric graphic display and a pressure sensitive writing pad . the alphanumeric graphic display is a dot matrix device which can display characters and graphics . the writing pad has a 16 × 48 dot area . as shown in fig9 , a pager unit p 1 is assumed to have been operating in cell 1 and to have previously received the common frequencies c 1 - c 4 and local frequencies f 1 - f 2 from station s 1 . now pager unit p 1 travels on a route indicated by broken arrow - headed line route . in traveling along the route , pager unit p 1 continues to operate on local frequencies f 1 - f 2 , even as it travels through the cellular overlap region . however , when page unit p 1 enters a new common frequency reception region ( i . e ., cfrr 2 ), a switching or hand - off operation occurs . in the switching operation , as explained in more detail below , pager unit p 1 obtains common frequencies c 1 - c 4 from central control station s 2 and , as a result , can switch from the local frequencies f 1 - f 4 of cell 1 to the local frequencies f 5 - f 8 of cell 2 . in order to effect the switching or hand - off operation , pager unit p 1 executes a channel switching routine ; the central control station s 2 executes a switching enabling routine . in connection with the channel switching routine and the switching enabling routine , when pager unit p 1 moves into cfrr 2 , pager unit p 1 will receive the clocking signal on frequency c 1 from station s 2 . at such point , pager unit p 1 will automatically align its clock unit with the clocking signal from station s 2 . referring now to the channel switching routine executed by pager p 1 subsequent to start - up ( step 500 ), at step 506 pager unit p 1 obtains information characterizing the system centered about station s 2 . such characterizing information is referred to as system identification or system id information . at step 508 , microprocessor 80 of pager unit p 1 checks to determine if there is any new system id information acquired on frequency c 2 . that is , microprocessor 80 checks to determine if system id information is received on frequency c 2 ( which can occur only in a cfrr ) and , if so , compares the system id information to the immediately previously - stored system id information . if the previous and most recently - acquired system ids are the same , pager unit p 1 realizes that it is still in the jurisdiction of the same station ( e . g ., station s 1 ). if not , pager unit p 1 realizes that it has now wandered into a cfrr of a new station ( e . g ., station s 2 ) and , at step 510 , initiates a request on frequency c 4 for communication with the central control station ( e . g ., station s 2 ) for cell 2 . in the above regard , since pager unit p 1 has not yet been assigned a time slot for cell 2 , the request on frequency c 4 is randomly made . however , pager unit p 1 keeps track of the time slot in which it makes its request to the new central control station ( e . g ., station s 2 ). thereafter , pager unit p 1 continues to monitor ( step 512 ) communications packets from station s 2 on frequency c 2 , waiting for station s 2 to issue a message which references the time slot at which pager unit p 1 made its request of step 510 . in particular , page unit p 1 awaits a message from station s 2 on frequency c 2 that includes both a slot recognition command code and information stored in the same time slot which pager unit p 1 randomly generated . since the message including the slot recognition command code includes station s 2 as the sender and mirrors the slot randomly generated by pager unit p 1 , pager unit p 1 recognizes the message as being addressed to pager unit p 1 and considers issuance of such a message by station s 2 ( see step 612 of fig1 ) to constitute authority for pager unit p 1 to communicate further with station s 2 . in this regard , at step 514 microprocessor 80 of pager unit p 1 determines if there is a match between the time slot of a received message and the time slot at which the random request was made at step 510 . assuming a match is eventually found at step 514 , at step 516 pager unit p 1 sends a communications packet on frequency c 3 to station s 2 , with the communications packet including the identification or id of pager unit p 1 . using pager registration file 55 , station s 2 verifies that the id of pager unit p 1 is a valid id , and thereafter sends ( on frequency c 2 ) to pager unit p 1 a message with the command code local frequency download , which message informs pager unit p 1 of the values of the local frequencies handled by station s 2 ( e . g ., frequencies f 5 - f 8 ). thereafter , as also reflected by step 518 , station s 2 sends ( on frequency c 2 ) to pager unit p 1 a message with the command code slot assignment command code , which message informs pager unit p 1 of its slot assignment on frequency f 8 . microprocessor 80 then changes its slot allocation by steps which are similar to those discussed with the afore - mentioned change time slot routine ( see steps 350 , 352 , and 354 of fig5 ). step 518 of fig1 reflects reception of the local frequency values and reception of the slot assignment . after acquisition of all local frequencies and the slot assignment is completed ( step 520 ), microprocessor 80 implements ( at step 522 ) a switch to the new local frequencies ( e . g ., frequencies f 5 - f 8 ). in this regard , microprocessor 80 instructs i / o interface 86 to change transmitter 72 from frequencies f 3 , f 4 to frequencies f 7 , f 8 ; and to change receiver 62 from frequencies f 1 , f 2 to frequencies f 5 , f 6 . i / o interface 86 accomplishes the frequency changes by applying appropriate values on the frequency control lines connecting the i / o interface to transmitter 72 and receiver 62 , respectively . after the switch to new local frequencies at step 522 , microprocessor 80 loops back to step 506 , ultimately to determine when any further switching may be required . steps involved in the switching enabling routine executed by a central control station ( e . g ., station s 2 ) are depicted in fig1 . after start - up ( step 600 ), cpu 50 determines executes a loop 602 which enables cpu 50 to clean up its pager directory file 56 and to check if any new pager units have wandered into the cell which it administers . in particular , at step 604 cpu determines whether its central control station ( e . g ., s 2 ) has been advised by any other central control station ( e . g ., s 3 ) that a pager unit , formerly under the control of its central control station ( e . g ., s 2 ), has come under the control of the other central control station ( e . g , s 3 ). such advisement occurs on the serial links connecting the central control stations 420 x , and particularly input serial link 486 b . if such advisement occurs , the id for the wandered - away pager is deleted from the pager directory file 56 for station s 2 ( as reflected by steps 606 and 608 ). at step 610 , cpu 50 causes messages with a system command code to be transmitted on frequency c 2 . as indicated before , messages transmitted on frequency c 2 include a packet ( s ) having a format such as that shown in fig1 . the message with the system command code particularly includes the central station id number in its alphanumeric data field . at step 612 , central control station 420 checks to determine if a request signal has been transmitted by any pager unit 422 on frequency c 4 ( as occurred , for example , in context of the discussion of fig1 , particularly step 510 ). such a request signal would likely be issued from a pager unit 422 which has just wandered into the cfrr controlled by the central control station ( e . g ., into cfrr 2 controlled by station s 2 ). if no such request signal is detected , loop 602 is again repeated . in the event that a request signal is detected at step 612 , central control station 420 notes specifically the time slot on frequency c 4 at which the request occurred ( step 614 ). at this point , such time slot is the only way central control station 420 can identify the in - wandering pager unit 422 . central control station 420 desires for the in - wandering pager unit 422 to transmit its identification ( id ), but cannot specifically address the in - wandering pager other than with reference to the detected time slot . accordingly , at step 616 , central control station 420 prepares and transmits a message on frequency c 2 which has a slot recognition command code . the message including the slot recognition command code includes station s 2 as the sender and mirrors the slot randomly generated by pager unit p 1 ( e . g , the time slot at which the in - wandering pager unit 422 issued its request ). this transmission on frequency c 2 constitutes authority for pager unit p 1 to transmit its identification . step 618 denotes acquisition by central control station 420 of the identification ( id ) of the in - wandering pager unit 422 . at step 620 , central control station 420 checks its pager registration file 55 to determine if the pager id is a valid id . if not , an error message is generated and transmitted ( at step 622 ), followed by a command for pager unit p 1 to shut down ( see step 624 ). assuming that the identification of pager unit 422 was validated at step 620 , cpu 50 checks ( at step 630 ) its pager directory file 56 to locate an available time slot for the in wandering pager unit 422 , and then associates the available time slot with the id of the in - wandering pager unit 422 . then , at step 632 , using a message on frequency c 2 with a local frequency download command code , central control station 420 sends the values of its local frequencies ( e . g ., f 5 , f 6 , f 7 , f 8 ) to the in - wandering pager unit 422 . the central control station then ( at step 634 ) assigns to the in - wandering pager unit 422 a new time slot on its local frequencies using a message on frequency c 2 with a slot assignment command code . processing of the change time slot command by the in - wandering pager unit 422 is understood with analogous reference to fig5 , particularly steps 350 , 352 , and 354 . upon completion of step 634 , the in - wandering pager unit 422 is fully initiated into its new cell ( e . g ., cell 2 ), and has left the jurisdiction of its former control station ( e . g , cell 1 and station s 1 ). accordingly , at step 636 , cpu 50 requests its i / o interface to issue a command on serial line 486 a which advises ( using pager id ) that the in - wandering pager 422 is now under its jurisdiction , so that former jurisdictions ( e . g ., s 1 ) can delete this pager unit from their pager directory files 56 . such deletion is understood with reference to steps 604 - 608 as above - described . in addition to illustrating geographical location of pager p 1 , stations s 1 and s 2 , and cells cell 1 and cell 2 , fig9 shows the relative timing of communications occurring on common frequencies c 1 - c 4 . fig9 specifically relates the timing of communications transmissions to specific ones of the aforedescribed steps executed by central control station 420 ( the switching enabling routine of fig1 ) and by pager unit 422 ( the channel switching routine of fig1 ). although the central control stations 420 x use the same common frequencies c 1 - c 4 , there is no interference or confusion of these signals transmitted from the control stations 420 x . the common frequencies c 1 - c 4 are broadcast at a relatively lower power than the local frequencies f 1 - f 4 so that reception of the common frequencies c 1 - c 4 occurs only in a limited neighborhood ( cfrr ) about the central control station 420 x . accordingly , pager units 422 traveling through the system receive common frequencies c 1 - c 4 only in the limited and non - overlapping cfrrs . system operational characteristics , such as cell diameter , cfrr diameter , power level of the local frequencies ( e . g ., f 1 - f 4 ), and power level of the common frequencies ( c 1 - c 4 ) can be field adjusted to suit numerous factors , including particularly the terrain and topography of the geographical region covered by the system . by way of non - limiting example , in one embodiment , the radius of each cell is on the order of about 20 miles ; while the radius of each cfrr is on the order of about 10 miles or less . in the same example , the power for transmission of the local frequencies can be in a range of from about 3 watts to 1000 watts ; while the power for transmission of the common frequencies c 1 - c 4 is preferably less than 2 watts . thus , the invention provides a two - way paging system which operates independently from a telephone system for wireless data communication between users . the invention minimizes use of available frequencies allowed by the federal communications commission ( fcc ), using only four local frequencies f 1 - f 4 for any given cell and ( for expanded , multi - cellular coverage ) only four common or switching frequencies c 1 - c 4 . in order to minimize the number of frequencies ( e . g , channels ) utilized , techniques of time division sharing and synchronization are employed . a transmission power differential between the local frequencies and the common frequencies is also employed . these techniques allow data transmission to be kept separate from different pagers and thus eliminates merging of data . the switching technique of the present invention provides extended geographical coverage and minimizes paging time by increasing the number of frequencies utilized in a cell from four ( e . g , the four local frequencies ) to eight ( the four local frequencies plus the four common frequencies ). in connection with verification of pager id , it should be understood that a single pager registration file might be stored in a memory file only one of a plurality of central control stations , and that in such case verification would constitute issuing a search command ( on the serial links 486 ) to locate a pager id in the one ( remote ) memory file , with the results of the search being reported back to the inquiring central control station . the keyboards illustrated herein can , in some embodiments , be multi - language keyboards or writing pads which permit typing of english , chinese , or japanese languages , for example . the writing pad is especially useful in countries such as japan , thailand , the middle east or china where english - like alphabets are not used . the writing pad could also be used to sketch and transmit graphics . moreover , data compression / de - compression techniques can be utilized in connection with data transfer . while the invention has been particularly shown and described with reference to the preferred embodiments thereof , it will be understood by those skilled in the art that various alterations in form and detail may be made therein without departing from the spirit and scope of the invention . for example , it should be understood that repeaters may be employed within cells to facilitate transmission when a pager unit ventures far from a central control station .