CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 11/350,616 filed on Feb. 8, 2006, which is a continuation of application Ser. No. 09/847,005 filed on May 2, 2001 now U.S. Pat. No. 7,031,716, which is a continuation of application Ser. No. 09/594,662 filed on Jun. 15, 2000 now U.S. Pat. No. 6,282,406, which is a continuation of application Ser. No. 09/259,417 filed on Dec. 9, 1997 now U.S. Pat. No. 6,108,520, which is a continuation of application Ser. No. 08/608,629 filed on Feb. 29, 1996 now U.S. Pat. No. 5,729,827, which is a divisional of 08/264,973, filed Jun. 24, 1994, now U.S. Pat. No. 5,542,115, issued Jul. 30, 1996, entitled “PAGING METHOD AND APPARATUS”, naming Wong, et al. as inventors, all of these applications being incorporated by reference herein in their entirety. 


   BACKGROUND 
   1. Technical Field 
   This invention pertains to communications paging, and particularly to two-way paging method and apparatus. 
   2. Related Art 
   Over the last several decades, pagers have proven to be important communication devices for contacting remotely situated personnel. Whereas primitive pagers provided primarily only a tonal and/or vibratory output, more modern pagers have enhanced output capabilities such as message-bearing alphanumeric displays. 
   Paging systems have historically been one-way systems. That is, the user receives a paging message from a central terminal but has no way of responding to that message with the pager. Prior art attempts to provide two-way communication capabilities for a pager have included efforts to connect the pager to a telephone (e.g., to a mobile radio telephone). See, for example, U.S. Pat. No. RE 33,417 to Bhagat et al. (which combines an entire radio pager and radiotelephone linked through an automatic dialer) and U.S. Pat. No. 5,117,449 to Metroka, et. al. (which purports to combine paging and cellular radiotelephone functions in a single unit). 
   Some pagers have the capability of providing an acknowledgment or response to a paging signal. In some such “ack-back” systems, a user operates a reply input device (e.g., a toggle switch, pushbutton switch, or keyboard) when paged. Typically such ack-back systems involve a complex acknowledgement transmission scheme, involving numerous frequencies or frequency sub-bands. Hand-off of the pager, as the pager travels between differing geographic regions or “cells” served by differing central stations, becomes technically cumbersome when multitudinous frequencies are involved. 
   SUMMARY 
   A two-way paging system utilizes four local frequencies for transmissions between pager units and a central control station. A first local frequency carries a local clock; a second local frequency carries communications packets from the central control station to paging units; a third local frequency carries communication packets from the pager units to the central control station; and a fourth local frequency carries a status or request signal from the paging units to the central control station. Transmissions on the fourth local frequency are in accordance with a time divided slot allocation among pager units accessing the central control station. 
   For a two-way paging system having a plurality of central control stations servicing a corresponding plurality of cells, a total of eight frequencies are utilized within any one cell. Four of the utilized frequencies are the local frequencies, (which may differ from cell to cell), and four of the utilized frequencies are lower power common frequencies or switching frequencies which are used to switch or hand-off a pager unit traveling from one cell to another. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  is a schematic view of a central control station included in a paging system of an embodiment of the invention. 
       FIG. 2  is a schematic view of a pager unit included in a paging system for use with the central control station of  FIG. 1 . 
       FIG. 3  is a flowchart depicting steps executed by the central control station of  FIG. 1 . 
       FIG. 4  is a flowchart depicting steps executed by the pager unit of  FIG. 2  when in a transmit mode. 
       FIG. 5  is a flowchart depicting steps executed by the pager unit of  FIG. 2  when in a receive mode. 
       FIG. 6  is a timing diagram reflecting communications between the central control station of  FIG. 1  and the pager unit of  FIG. 2 . 
       FIG. 7  is a schematic view of a central control station included in a paging system of a second embodiment of the invention. 
       FIG. 8  is a schematic view of a pager unit included in a paging system for use with the central control station of  FIG. 7 . 
       FIG. 9  is a hybrid schematic view and timing diagram for representing switching operations for the paging system of the second embodiment of the invention. 
       FIG. 10  is a flowchart depicting steps executed by the pager unit of  FIG. 8  in connection with a channel switching operation. 
       FIG. 11  is a flowchart depicting steps executed by the central control station of  FIG. 7  in connection with a channel switching operation. 
       FIG. 12  is a schematic view of a format of a communications packet utilized with embodiments of the invention. 
       FIG. 13  is a schematic view illustrating a time divided slot allocation technique according to the invention. 
   


   DETAILED DESCRIPTION 
     FIG. 1  shows a central control station  20  according to a first embodiment of the invention;  FIG. 2  shows a paging unit  22  suitable for use with central control station  20 . 
   As shown in  FIG. 1 , 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  FIG. 1 , it should be understood that memory  54  includes a number of unillustrated memory devices, including (for example) a hard disk drive, RAM, and ROM.  FIG. 1  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  FIG. 12 , 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  FIG. 12  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  FIG. 2 , 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  FIG. 2 , 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 ). 
   Operation of First Embodiment 
   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  FIG. 13 , 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  FIG. 13 , a first time slot on frequency f 4  is assigned to a pager P 1 ; a second time slot is assigned to pager 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. 
     FIG. 3  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  FIG. 3  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  FIG. 12 , 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  FIG. 3  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  FIG. 3  are executed (prior to returning to loop  106 ). As will be seen hereinafter with respect to  FIG. 4 , 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  FIG. 12 , 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  FIG. 4 . Steps executed by a pager unit  22  in connection with its receive mode are depicted in  FIG. 5 . 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  FIG. 4 ), 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  FIG. 13 ) 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  FIG. 3 . 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  FIG. 12 . 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. 
     FIG. 5  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  FIG. 12 ) 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  FIG. 12 ) 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  FIG. 5 ) 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  FIG. 5 ) 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  FIG. 4 ). 
   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  FIG. 5 ) is followed by a loop back to step  304 . 
     FIG. 6  is a timing diagram showing the frequencies f 1 –f 4  and integration of the steps depicted in  FIGS. 3–5 , 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  FIG. 6 , “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  FIG. 4  and in the receiver mode as depicted in  FIG. 5 . In general,  FIG. 6  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 . 
   Structure of Second Embodiment 
     FIG. 7  shows a central control station  420  according to a second embodiment of the invention;  FIG. 8  shows a paging unit  422  suitable for use with central control station  420 . 
     FIG. 9  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  FIG. 9 , 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  FIG. 9 , CELL 1  and CELL 2  overlap in an overlap region shown in  FIG. 9 . 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  FIG. 9 , 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  FIG. 9 , 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 1  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  FIG. 12 . 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  FIG. 7 , central control station  420  resembles central control station  20  of the embodiment of  FIG. 1  (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  FIG. 7  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  FIG. 7 , computer  30  of  FIG. 7  (like that of  FIG. 1 ) 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  FIG. 8 , pager unit  422  resembles pager unit  22  of the embodiment of  FIG. 2  (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 ′. 
     FIG. 8  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. 
   Operation of Second Embodiment 
   As shown in  FIG. 9 , 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  FIG. 11 ) 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  FIG. 5 ). Step  518  of  FIG. 10  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  FIG. 11 . 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  FIG. 12 . 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  FIG. 10 , 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  FIG. 5 , 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 ,  FIG. 9  shows the relative timing of communications occurring on common frequencies C 1 –C 4 .  FIG. 9  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  FIG. 11 ) and by pager unit  422  (the channel switching routine of  FIG. 10 ). 
   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.
A network communication system is provided for communication between network nodes, including individual nodes and a central control station node. In the network, signals are transmitted from the central control node to identify one or more frequencies that the node may transmit. When a first individual node has data to transmit, the first individual node transmits a request signal to the central control node over one or more of its assigned frequencies. In response to the request signal, the central control node transmits a grant signal assigning one or more frequencies to allow the first node to transmit its data.