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1. l A a LINN reat ONISS30OHd dadana le YANNVOS H U S Patent 3dAL H3LLINSNVHL AGAL SAS T d 3dAL Eu 00 id sas Sais lt U S Patent June 27 1995 Sheet 2 of 2 5 428 666 102 INITIALIZE CURRENT NAM SCAN CONTROL CHANNEL AND EXTRACT SID FROM STRONGEST a 106 DOES SID MATCH CURRENT NAM SID 104 FIG 2 Yes No 108 410 ROAM ON ROAMING NAM 112 DOES SID MATCH NEXT NAM S SID Yes CURRENT NAM NAM WITH MATCHED SID a 118 PROCEED WITH CALL USING CURRENT NAM 5 428 666 1 AUTOMATIC NUMBER ASSIGNMENT MODULE SELECTION FOR MOBILE TELEPHONE This is a continuation application of U S Ser No 08 121 443 filed Sep 14 1993 now abandoned which is a continuation application of U S Ser No 08 039 513 filed Mar 29 1993 now abandoned which is a continua tion of U S Ser No 07 937 459 filed Aug 28 1992 now abandoned which is a continuation of U S Ser No 07 679 268 filed Apr 2 1991 now abandoned FIELD OF THE INVENTION This invention relates generally to telecommunica tion and more particularly to mobile telephones having a plurality of number assignment modules each desig nating a cellular system to which the mobile telephone subscribes BACKGROUND OF THE INVENTION Cellular telephone systems provide radio communi cation between a network of geographical2. each base station a plurality of radio channels to carry voice signals called voice channels and at least one control channel In turn the base station assigns the allocated voice channels to mobile units for use during voice telephone calls The assignments of the voice channels are made in response to requests from the mobile units to the base stations for communication service Typically the mo bile units request service from the home system base stations from which the mobile units receive the stron gest control signals over the control channels More specifically a mobile unit typically scans for service i e the mobile unit scans the strengths of con trol signals on pre determined control channels and for instance selects the channel having the strongest signal for further processing The mobile unit then examines the selected control signal to determine if it originated 5 20 40 45 50 65 2 from base station belonging to the mobile unit s home system Typically this is accomplished by the mobile unit extracting and examining the data contained in an over head message train contained within the se Jected control signal Each OMT contains system infor mation including the system identification code SID of the base station from which it was broad casted The mobile unit compares the extracted SID with the mobile units stored 51 If the two codes match the mobil
3. repeating steps D F until a correspondence is determined or until all number assignment modules have been selected 12 The method of claim 11 wherein i said scanning step includes identifying the control channels having both the strongest and the next strongest received control signals and ii said step of determining whether system identifica tion numbers correspond includes determining if either one of the system identification numbers derived from the strongest and the next strongest received control signals corresponds to the system identification number derived from the selected number assignment module and iii if so tuning to the control channel associated with the system identification number that corresponds to the number derived from the selected number assignment module and requesting communication service using the mobile identification number stored in the associated selected number assign ment module 13 The method of claim 11 further comprising the step of specifying an order in which the number assign ment modules are to be selected 14 The method of claim 11 further comprising the step of specifying an order in which the control chan nels are to be scanned 15 The method of claim 10 further including the steps of if in step D the numbers do not match selecting a next number assignment module and determining if there is a correspondence between the system iden tification number derived from the s
4. NAM if no match is found Moreover in a further variation roaming service can be obtained by instead of using only a single designated NAM as described above repeating the scanning as described for non roaming service starting with the CURRENT or the designated NAM and continuing with other NAM s as needed until a system providing roaming service can be obtained The foregoing description has been limited to a spe cific embodiment of this invention It will be apparent however that variations and modifications may be made to the invention with the attainment of some or all of its advantages Therefore it is the object of the appended claims to cover all such variations and modifi cations as come within the true spirit and scope of the invention What is claimed is 1 A mobile radio telephone capable of requesting communication services from any of a plurality of cellu lar telephone systems that each transmit and receive data over a plurality of voice channels and a plurality of control channels said radio telephone comprising A a tunable transceiver for sending and receiving a plurality of signals over a plurality of the channels the signals received over the control channels in cluding a plurality of control signals each of which is characterized by a signal strength B a memory which includes a plurality of number assignment modules each of said number assign ment modules storing data specifying 1 a mobile ide
5. communication service from one of said cellular telephone systems comprising the steps of A in each of a plurality of number assignment mod ules storing data specifying a mobile identification number a system identification number for an asso ciated cellular telephone system from which com munication services can be requested using said mobile identification number and a range of chan nels within the plurality of control channels B scanning the control channels specified in a first selected number assignment module C identifying from the control channels scanned the channel that has the strong received control sig nals D determining whether a system identification number derived from the strongest received con 55 60 65 10 trol signals matches the system identification num ber derived from the selected number assignment module and E if the numbers match tuning to the control chan nel identified in step C and requesting communica tion service from the associated cellular communi cation system using the mobile identification num ber stored in the selected number assignment mod ule 11 The method of claim 10 further including the steps of selecting next number assignment module and scanning the channels specified therein to identify the channel with the strongest received control signals if it is determined in step D that the system identification numbers do not correspond and G
6. radio telephone 10 also has a processing unit 20 for controlling operation of the transmitter 12 and re ceiver 14 in accordance with a program stored in a 45 50 55 60 65 4 program memory 24 preferably a programmable read only memory PROM A non volatile random access memory 30 stores information required for the operation of the radio telephone 10 including a plurality of number assignment modules NAM s 32 and information relating to NAM s 32 More specifically and for pur poses of illustration only the RAM 30 has four NAM s 32 designated NAM 00 NAM 01 NAM 10 AND NAM 11 Generally speaking however the RAM 30 can store NAM s 32 where n is a positive integer greater than 1 Each NAM 32 includes a first field 34 for storing a representation of a unique mobile telephone identifica tion number MIN a second field 36 for storing a representation of a system identification code 510 each SID being different from those stored in the other NAM s 32 and a third field 38 for storing a rep resentation of the type of the system As set forth in the above mentioned EIA RS 553 standards the systems can be either A type or type A type systems use channels 0 through 333 of which channels 313 to 333 are control channels which are typically scanned for service by scanner 39 of the mobile unit 10 in descending order On the other hand B type
7. systems use channels 334 through 666 of which channels 334 to 354 are control channels which are typically scanned for service in ascending order by mobile units More specifically NAM 00 stores MIN 00 SID 00 and Sys Type 00 NAM 01 stores MIN 01 SID 01 and Sys Type 01 NAM 10 stores MIN 10 SID 10 and Sys 10 and NAM 11 stores MIN 11 SID 11 and Sys 11 The RAM 30 also has a further set of fields 38 for storing a NAM priority directory or look up table The NAM priority directory cross references each NAM 32 with a priority ranking and in a preferred embodiment identifies the NAM 32 which is to be used by the radio telephone 10 during roaming More specifically the NAM priority directory has a first set of fields 42 for storing NAM identification codes or ID s i e in the above example the above mentioned 00 01 10 and 11 and a second set of fields 44 for storing an assigned priority in association with each NAM identification code e g 00 for the lowest priority and 11 for the highest priority The directory also has a third set of fields 46 for storing a designated roaming identification in associa tion with each NAM identification code The NAM that is to be used during roaming has a digital ONE in its associated designated roaming field 46 while the other NAM s have a digital ZERO in that field The radio telephone 10 further includes a user inter face 52 for interacting with
8. the processing unit 20 The user interface 52 can include a conventional keyboard 54 and display 56 In addition the user interface 52 provides other user activated controls such as an on off toggle 58 for disabling and enabling automatic NAM selection The on off toggle 58 can be implemented as a discrete switch a keyboard function key or as a soft ware routine executable by the processing unit 20 in response to a user selection that is entered e g on the keyboard 54 When automatic NAM selection is disabled the ra dio telephone 10 employs the NAM identified by the CURRENT NAM ID stored in a current buffer 64 as 5 428 666 5 described hereinabove and ignores the other NAM s stored in RAM 30 On the other hand when automatic NAM selection is enabled the radio telephone 10 can make use of any of the NAM s stored in the RAM 30 in accordance with an automatic NAM selection routine that is stored in the program memory 24 FIG 2 illustrates operation of the radio telephone 10 including automatic selection in accordance with the invention With automatic NAM selection enabled and in order to initiate a telephone call the mobile unit 10 initializes the CURRENT NAM in block 102 by defin ing the CURRENT NAM as the last NAM used by the radio telephone for communication That NAM is iden tified by accessing register 64 Alternatively the CUR RENT NAM can be defined as the NAM with the highest priority in the directory 38 Th
9. AU ATA US005428666A United States Patent 11 Patent Number 5 428 666 Fyfe et al 45 Date of Patent Jun 27 1995 54 AUTOMATIC NUMBER ASSIGNMENT 4 734 928 3 1988 Weiner et al 379 59 MODULE SELECTION FOR MOBILE 4 775 998 10 1988 Felix et al we 379 59 TELEPHONE 5 003 629 3 1991 Ness Cohn et al 455 54 5 020 001 5 1991 Kzrolopp et al 379 58 75 Inventors Kipling W Fyfe Calgary Susan 5 029 233 7 1991 Metroka 455 11 McIntyre Vancouver both of 5 044 010 8 1991 Frenkiel et al 379 61 Canada 5 101 500 3 1992 Mauri cct 455 33 73 Assignee NovAtel Communications Ltd OTHER PUBLICATIONS Calgary Canada The Bell System Technical Journal The Cellular Con 21 Appl No 292 300 cept Jan 1979 vol 58 No 1 7 User Manual for a telephone from Technophone Lid 22 Filed Aug 16 1994 dated Sep 1989 Motorola Your Portable Connection Jan 1989 Related U S Application Data Nokia Mobira Incorporated Nokia LXIIC May 1990 63 Continuation of Ser No 121 443 Sep 14 1993 aban doned which is a continuation of Ser No 39 513 Mar Washington Post Aug 10 1990 29 1993 abandoned which is a continuation of Ser Primary Examiner Curtis Kuntz No 937 459 Aug 28 1992 abandoned which is a or Willi continuation of Ser No 679 268 Apr 2 1991 abun illiam
10. Cumming doned Attorney Agent or Firm Cesari and McKenna 41 11 0 ABSTRACT 52 U S Cl 379 58 379 59 mobile radio telephones having a plurality of number 379 60 379 61 455 33 1 455 54 1 assignment modules NAM s selects automatically 58 Field of Search 379 58 59 60 61 one of the NAM s for use for communication In a pre 455 11 17 33 1 54 1 ferred embodiment the mobile radio telephone at 56 References Cited tempts to match system identification data in received U S PATENT DOCUMENTS 3 663 762 5 1972 Joel Jr 379 60 4 242 538 12 1980 Ito et al 4 658 435 4 1987 Childress 455 17 4 677 653 6 1987 Weiner et al MIN 5105 10 10 MIN 5105 SYS 11 44 TYPE 11 MEMORY PROM 24 USER e DISPLAY _ INTERFACE 52 Q ONOFF KEYBOARD control signals with system identification data stored in the NAM s and on encountering a match will use the NAM containing the matched system identification for telephone communication 16 Claims 2 Drawing Sheets TRANSMITTER i2 Y SCANNER PROCESSING UNIT 20 RECEIVER 14 44O NO Lol 39 zs 3OV H3NI 052 wisn 5 428 666 AHOWSIN WVHOOWd S v S u3AI03u m 2
11. M s for roaming Preferably the designated NAM is also stored in non volatile memory in the mo bile unit and the user makes the designation from among the available NAM s based on e g the preva lence of inter system roaming agreements roaming charges coverage area and other considerations With automatic NAM selection the mobile unit 20 25 30 switches into and out of operation without user 40 intervention In addition for the reasons described above the invention can achieve cost savings in con nection with use of mobile radio telephones BRIEF DESCRIPTION OF THE DRAWING The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which FIG 1 is a block diagram of a mobile radio telephone employing automatic number assignment module selec tion in accordance with the invention and FIG 2 is a flow chart depicting operation including automatic number assignment module selection of the mobile radio telephone of FIG 1 DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT FIG 1 shows a mobile radio telephone 10 employing automatic number assignment module selection in ac cordance with the invention The radio telephone 10 has a transmitter 12 and a receiver 14 for respectively transmitting and receiving communication signals via an antenna system 16 equipped for full duplex opera tion The
12. e unit sends a request over the corre sponding control channel to the base station for the assignment of a voice channel and two way communi cation can ensue over the assigned voice channel On the other hand if the two codes do not match then the mobile unit must wait until an acceptable con trol signal is received which may require that the mo bile unit move to another cell or that the mobile unit resort to roaming to establish telephone communica tion In roaming the mobile unit uses the services of a foreign cellular system a system that is not its home system and unfortunately the mobile unit incurs roaming fees for such services Moreover roaming is only available where the home system has a roaming contract with the foreign system Thus in certain areas outside the coverage area of the home system or where communication with home system base stations proves impossible or inadequate due to signal propagation con ditions or other reasons and where roaming is not avail able because there is no inter system roaming agree ment between the home and local cellular systems cellular services are not available The unavailability of service in those locales can present serious drawbacks to telephone users It has been proposed to provide mobile radio tele phones with multiple NAM s each storing a MIN and an associated SID This permits the mobile unit to subscribe for service on several cellular systems In es
13. en in block 104 the processing unit 20 scans the control channels indicated by and in the direction indi cated by the system type specified in the CURRENT third field 38 The identification of these chan nels and the direction of scanning for type A and type B systems can be stored in fields not shown of the RAM 30 Then the processing unit 20 extracts the SID from the OMT of the strongest of the control signals received over the scanned channels and in block 106 checks whether the extracted SID matches the SID stored in RAM 30 for the CURRENT NAM If the SID does not match the CURRENT SID then in block 108 the processing unit 20 deter mines whether any NAM s have not been checked If the outcome of this test is that all NAM s have been checked the radio telephone 10 commences roaming in block 110 using for example the NAM designated for roaming in the NAM priority directory 38 On the other hand if not all NAM s have been checked the radio telephone 10 repeats the scanning step in block 112 this time over control channels indi cated by and in the direction indicated by the system type specified in the NAM which is next in priority to those already examined as indicated in the priority directory 38 The processing unit 20 extracts the SID from the strongest of the scanned controls signals Then in block 114 the extracted SID is compared with the SID stored in RAM 30 for this next NAM If
14. g for each of the number assign ment modules order specifying data for specifying a pre determined order in which the control channels specified therein are to be scanned by said scanning tA 10 25 wherein said scanning means consecutively scans control channels specified in the selected number assignment module in the order specified by the associated order specifying data 6 The mobile radio telephone of claim 4 further 30 comprising means for storing data specifying the order in which the number assignment modules are to be selected by the processor 7 The mobile radio telephone of claim 6 wherein said processor if the system identification numbers do not correspond selects a next number assignment mod ule for said scanning means 8 The mobile radio telephone in accordance with claim 7 further comprising means for storing data speci fying for each of the number assignment modules an associated order in which the channels specified therein 35 are to be scanned 9 The mobile radio telephone in accordance with claim 4 further comprising memory means coupled to said processor for storing a designation of one of said number assignment modules for use during roaming 10 In a mobile radio telephone capable of requesting communication services from any of a plurality of cellu 45 lar telephone systems each system having a plurality of voice channels and a plurality of control channels a 50 method for requesting
15. he example a user subscribes to all four systems and has an automobile mounted mobile unit with four and otherwise incorporating the features shown in FIG 1 As noted above for radio telephone 10 the mobile unit 100 stores data representing various NAM parameters for each of the NAM s which for our pur poses here are set forth in the following NAM table NAM SYSTEM CHANNEL NO TYPE RANGE SIDp NI B 334 354 22222 N2 A 333 313 44444 N3 B 334 354 54321 N4 A 333 313 12345 Continuing the example the user lives within cell C1 of S1 and works within cell C4 of S2 and thus must pass from C1 to C2 to C3 and finally to C4 on the way to work On a particular day the user turns ON the mobile unit 100 while at his home and drives to his office 0n being turned ON the mobile unit 100 initially has for in stance 1 as its CURRENT NAM and scans the con trol channels associated with that NAM starting with channel 334 as indicated in the NAM table for 1 to identify the strongest control signal carried over those control channels Since the local system S1 is an A type system while N1 corresponds to a B type system the mobile unit 100 will not find that the scanned con trol channels contain control signals Using automatic NAM selection the mobile unit 100 will switch to the next NAM in its priority directory say N2 as its CURRENT NAM and repeat the fore going procedure except that this time the mobile unit
16. identifying from the channels scanned the con trol channel that has the strongest received control signal f D a processor coupled to said transceiver and said scanning means for i determining if a system identification number derived from the strongest received control sig nal corresponds to the system identification num ber derived from the selected number assignment module i causing if the numbers correspond said trans ceiver to tune to the control channel with the means 5 428 666 9 strongest control signal and request communica tion service from the associated cellular tele phone system using the mobile identification number derived from the selected number as signment module and iii determining if the numbers do not correspond whether the system identification number de rived from the strongest received control signal corresponds to the system identification number stored in a next number assignment module the processor repeatedly selecting a next number assignment module either until a correspondence of system identification numbers is determined said processor then causing said transceiver to tune to the associated control channel and re 15 quest service using the mobile identification number specified in the associated selected num ber assignment module or until the all of the number assignment modules have been selected 5 The mobile radio telephone of claim 4 further com 20 prising means for storin
17. in geographic areas for any of the i e the mobile unit 100 is outside the service areas of all the systems to which it subscribes Then in that case the mobile unit 100 uses the designated roaming NAM for roaming If the user chooses to disable automatic NAM selec tion for any reason the user can do so using the appro priate switch or toggle on the user interface When automatic selection is disabled the mobile unit 100 then resorts to using a single NAM e g the NAM with the highest priority in the priority directory In a variation of the procedure described above for scanning for service the mobile unit 100 can compare the SID extracted from the OMT of the control signal identified in each scan with the SID stored in all of the NAM s making the comparison in the order of NAM priority specified in the directory This avoids the ne cessity of scanning each time a new NAM is selected In another variation the mobile unit 100 as a result of scanning can identify plural control channels carrying adequately strong signals rather than the single stron gest control channel as in the above example and com pare the extracted SID for each channel with the SID stored in the CURRENT NAM In this case the mobile unit preferably compares the extracted SID for the strongest channel first and proceeds to compare the SID s of less strong channels in the descending order of their signal strengths prior to switching to a next
18. ly separated stationary base stations and mobile radio telephones e g mobile units installed in automobiles Each base station defines a particular cell of the cellular system and has at least one transceiver with the capability of calling to and receiving calls from mobile units in the corresponding cell Such a cellular telephone system is described in the Bell System Technical Journal January 1979 entitled The Cellular Concept page 15 et seq Cellular telephone systems are subject typically to governmental regulations and approvals and must com ply with standards such as for example those set forth for the United States of America in EIA RS 553 by Electronic Industries Association Washington D C U S A As set forth in those standards a central authority assigns a unique digital code for identifying each cellu lar telephone system called the system identification code SID When a mobile unit subscribes or regis ters for service on a particular cellular system that system is called its home system number assignment module on board the mobile telephone stores the SID that identifies the mobile unit s home system called the SID in a non volatile i e permanent memory The NAM also stores a mobile identification number MIN which represents the mobile unit s 10 digit telephone number in associa tion with the SID Typically a cellular telephone system allocates to
19. ntification number ii a system identification number that identifies an associated cellular tele phone system from which communication services 20 25 30 35 40 45 50 55 60 65 8 can be requested using the mobile identification number and iii a predetermined range of the plurality of control channels C scanning means coupled with said transceiver and said memory for scanning control channels said scanning means scanning the control channels within the range specified in a selected number assignment module and identifying from the con trol channels scanned the control channel that has the strongest received control signal said control channel with the strongest control signal being an identified control channel D a processor for comparing i a system identifica tion number that identifies the cellular telephone system associated with the identified control chan nel the system identification number being derived from control signals received over the identified control channel and ii the system identification number specified in the selected number assign ment module said processor responding to a corre spondence between the numbers by causing said transceiver to tune to the identified control channel and request service using the mobile identification number stored in the selected number assignment module 2 The mobile radio telephone of claim 1 wherein said processor includes a selecti
20. on means for selecting a next number assignment module for said scanning means said selection means selecting a next number assignment module if the system identification number from the number assignment module previously se lected does not correspond to the system identification number derived from the identified control channel 3 The mobile radio telephone of claim 2 wherein said memory includes means for storing order specify ing data that specify an order in which said selection means selects the number assignment modules 4 A mobile radio telephone capable of requesting communication services from any of a plurality of cellu lar telephone systems each system having a plurality of base stations each base station having a plurality of voice channels and a plurality of control channels said radio telephone comprising A a tunable transceiver for sending and receiving signals over a plurality of channels B a plurality of number assignment modules each number assignment module storing data specifying i a mobile identification number ii a system identification number of an associated cellular tele phone system from which communication services can be requested using the mobile identification number and iii a predetermined range of the plurality of control channels C scanning means coupled with said transceiver for scanning the control channels within the range specified in a selected number assignment module and
21. scans downwardly from 333 and identifies channel 315 as providing the strongest control signal Next the mobile unit 100 compares the SID received in the over head message train of the channel 315 signal ie 444444 with the SID stored for 2 i e 12345 Since the two SID s do not match the mobile unit 100 switches to the next NAM in the priority directory e g N3 NAM N3 corresponds to a B type system so the mobile unit scan will again not result in any control signal On the other hand the next NAM N4 corresponds to an type system and channel 315 will again be identified by the scan This time however the SID carried by the control signal s OMT will match the SID for N4 ie 12345 Accordingly phone service can commence using NAM N4 In accordance with standards the mobile unit 100 periodically verifies signal strength adequacy by repeat ing the foregoing procedure This is important because as the user drives to work the user passes out of the cell C1 of S1 and in the example into C2 of S2 Accord ingly as the user does so signal strength will decrease 5 428 666 7 and the mobile unit 100 will need to once again identify a suitable NAM this time for use in system S2 Without further explanation it should now be clear that NAM N1 will provide a suitable hit i e match on the corre sponding SID s and can be used for cellular service in C2 Of course it is possible that no match will occur in certa
22. sence the mobile unit has many different phone num bers and can have many different home systems each associated with one of the phone numbers In all known approaches the mobile unit can have however only a single NAM operative at a time For instance U S Pat No 4 734 928 suggests the use of multiple NAM s which the telephone user manually switches into operation e g whenever the user desires service from a different home system or desires to take advantage of inter system roaming agreements of a different home system Consequently a mobile unit equipped with multiple NAM s can provide the user with service over a broader geographic range of use In addition the user can avoid paying roaming charges by subscribing with a foreign system as a second home system This may permit many users to almost always be within one of its home systems Furthermore the user can take advan tage of billing rate differences offered by its various home systems so as to reduce telephone call charges Unfortunately manually switching from one to an other of the NAM s in a mobile unit requires that the user maintain a good working knowledge of the cellular systems including their geographic boundaries For at least that reason many users might find manual switch ing of NAM s too complicated or bothersome 5 428 666 3 SUMMARY OF THE INVENTION The invention resides in techniques for use by a multi NAM mobile radio telephone for au
23. they do not match then the radio telephone 10 returns to block 108 and checks whether any untested NAM s remain On the other hand if the extracted SID matches the SID for that NAM then in block 116 that NAM is made the CURRENT NAM After block 116 or if a match were found in block 106 the mobile unit 10 proceeds in block 118 with the telephone call using the CURRENT NAM and a voice channel assigned by the base station that originated the particular control signal whose SID was being tested for a match An example may further clarify the operation of auto matic NAM selection in accordance with the invention Consider a hypothetical metropolitan area served by four cellular telephone systems denominated as S1 S2 S3 and S4 The systems have contiguous overlapping cells respectively numbered C1 C2 C3 and C4 lo cated side by side Each cell is served by a base station of the corresponding system It should now be apparent that each of the systems is associated with a system type a control channel range an order of scanning control channels a specific control channel for the local base station and a system identifi 5 10 15 20 25 40 45 50 60 65 6 cation or SID This information is given in the follow ing systems table CELL SYSTEM SYSTEM CONTROL CHANNEL NO TYPE CHANNELS RANGE SID 1 A 315 313 333 12345 2 B 340 334 354 22222 S3 B 342 334 354 54321 54 331 313 333 44444 For t
24. tomatically select ing one of the NAM s for telephone communication In making the selection the mobile unit attempts to match system identification data in received control signals with system identification data stored in conjunction with any of the NAM s and on encountering a match will switch to the NAM containing the matched system identification for telephone communication More specifically in a preferred embodiment the mobile unit i scans certain control channels identified by system information stored on board in conjunction with a first of its NAM s to identify the strongest con trol signal ii extracts system identification data there from and iii attempts to match that data to system identification data stored in conjunction with the first NAM If a match exists that NAM is used for commu nication and if no match exists the mobile unit switches consecutively to a other NAM s and repeats the procedure for each until a match is found A NAM priority directory or look up table stored on board the mobile unit in non volatile memory deter mines the order in which the mobile unit examines the NAMv s for a match Preferably the priority order can be user set to reflect user preferences between cellular systems based on e g telephone charges and fees ser vices provided and other considerations If none of the NAM s provide a match the mobile unit can seek telephone service using a designated one of the NA
25. trongest re ceived control signal and the system identification number derived from this next number assignment module G repeating steps E and F until a correspondence is determined or until all number assignment modules have been selected 16 The method of claim 15 further including the steps of H if no correspondence is determined in step G selecting a next number assignment module and scanning the channels specified therein to deter mine the channel with the strongest control signals and 1 repeating steps D H until a correspondence is determined or until all number assignment modules have been selected
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