Radio frequency communication network having adaptive communication parameters

0001TITLE: RADIO FREQUENCY COMMUNICATION NETWORK HAVING ADAPTIVE COMMUNICATION PARAMETERS

0002BACKGROUND OF THE INVENTION The present invention in a preferred implementation relates to improvements in radio data communication networks wherein a number of mobile transceiver units are to transmit data to a number of base stations under a wide range of operating conditions. To compensate for the wide range of operating conditions, adaptability has been provided using an exchange of parameters that define the nature of the network communication. The invention is preferably to be applicable as an upgrade of an existing data capture system wherein a number of hand-held transceiver units of an earlier design are already in the field representing a substantial economic investment in comparison to the cost of base stations, accessories and components. In installations spread over an extensive area, a large number of mobile portable transceiver units may be employed to gather data in various places and multiple base stations may be required. In a variety of such installations such as warehouse facilities, distribution centers, and retail establishments, it may be advantageous to utilize not only multiple bases capable of communication with a single host, but with multiple hosts as well. SUMMARY OF THE INVENTION

0003The present invention provides an improved data communication system which maintains RF communication links between one or more host computers and one or more base transceiver units, each of which may be communicative with many mobile portable transceiver units being moved about a warehouse complex for the collection of data. Specifically, the invention provides a data communication system for collecting and communicating data in the form of RF signals which has a plurality of RF transceivers that store and modify at least one variable operating parameter.

0004From the stored parameter(s) , each of transceivers control the operation of transmission and reception. The transceivers also evaluate the effect of the stored parameter based by analyzing each transmission received, and determine whether to make changes in the stored parameter. If changes are needed, the transceivers, modify and store the modified operating parameter and begin operation based thereon .

0005The operating parameters involve: 1) the size of data segments to be transmitted; 2) the length or frequency of the spreading code used for direct-sequence spread spectrum communication; 3) the hopping rate, coding, and interleaving for frequency-hopping spread spectrum communication; and 4) the type of RF source encoding used. In addition, the RF transceivers used in the data communication network of the present invention use system-default values to reset the operating parameters if a series of failed communication exchanges occurs, so that communication can be re-established.

0006It is therefore an object of the invention to provide an adaptive radio communication system which permits the interconnection of one or two host computer devices to a multiplicity of base transceiver units which may include both prior art existing installed units and new generation units capable of spread spectrum radio transmission. It is a further object of the invention to provide an adaptive RF data communication system which optimizes communication based on a set of operating parameters.

0007It is a further object of the invention to provide an adaptive RF data communication system which maintains communication based on a set of operating parameters for optimizing communication, wherein the operating parameters involve: 1) the size of data segments to be transmitted; 2) the length or frequency of the spreading code used for direct-sequence spread spectrum communication; 3) the hopping rate, coding, and interleaving for frequency-hopping spread spectrum communication; and 4) the type of RF source encoding to be used. It is a further object of the invention to provide a radio communication system network controller which via a communication exchange optimizes a set of operating parameters, yet returns the parameters to their previous or system-default values upon failed communication. These and other objects of the invention will be apparent from examination of the detailed description which follows.

0008DESCRIPTION OF THE DRAWING FIGURES Figure 1 is a block diagram of the prior art data communication system.

0009Figure 2 is a perspective view of the invention.

0010Figure 3 is a schematic representation of an exemplary radio communication system utilizing the invention. Figure 4 is a diagrammatic illustration of the control circuitry elements of the invention.

0011Figure 5 is a rear elevation view of the invention. Figure 6 is a diagrammatic illustration of the application specific integrated circuit of the invention.

0012Figure 7 is a block diagram showing an exemplary implementation of intelligent network and router transceiver units such as the network transceiver units of Figure 3.

0013Figure 8 is a diagram of an RF system utilizing a network controller according to Figures 2-6, with one of its network ports configured for communication with a second host, and another of its ports coupled with a multiplicity of RF transceivers via an adapter unit.

0014Figure 9 is a diagram illustrating the use of two network controllers according to Figures 2-6, configured for dual host computers each, and having their relatively high data rate extended distance network ports coupled with a multiplicity of intelligent network and router transceiver units implemented according to Figure 7.

0015Figure 10 is a diagram similar to Figure 9 but showing the pari of coupled network controllers interfaced to a common relatively high data rate system having multiple hosts (e.g.) a local area network of the Ethernet type or equivalent e.g. fiber optic type.

0016Figure 11 is a diagram similar to Figure 10 but indicating the network controllers being coupled to respective different high data rate multiple host systems (e. g. . token ring type local area networks or other individual networks e.g.. fiber optic loop networks of the collision-sense multiple-access type) . Figure 12 is a view similar to Figure 9 but intended to diagrammatically indicate a distribution of network and router transceivers and other elements of an on-line RF data collection system over an extensive area of a facility e.g. of one of the types previously mentioned.

0017Figure 13 shows an intelligent controller and radio base unit which unifies controller and radio components such as shown in Figure 7 into a single housing of the size represented in Figures 2 and

00185.

0019Figure 14 shows a diagrammatic illustration of the signal processing for two of four paiis of communication ports of the multiple base adapter of the RF data collection system illustrated in

0020Figure 8.

0021Figure 15 is a diagram of parts of an RF data collection system utilizing a network controller according to Figures 2-6 and a multiple base adapter according to Figure 14, with eight base transceiver units coupled to the multiple base adapter.

0022DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows an existing radio frequency data transmission system 10 wherein a base station transceiver means 11 has a number of mobile transceiver units such as 12A, 12B, ... , 12N in radio communication therewith. By way of example, the base station may be comprised of a radio base unit 14 such as the model RB3021 of Norand Corporation, Cedar Rapids, Iowa, which forms part of a product family known as the RT3210 system. In this case, the radio base 14 may receive data from the respective mobile RF terminals, e.g. of type RT3210, and transmit the received data via a network controller and a communications link 16 (e.g. utilizing an RS-232 format) to a host computer 17. The data capture terminals 12A, 12B, ... , 12N may each be provided with a keyboard such as 18, a display as at 19, and a bar code scanning capability.

0023Figure 2 provides a perspective view of the invention 40 in the preferred embodiment case 20. Front panel 22 is provided with display 24 and select key 26, up key 28 and down key 30. Power indicator 32 comprises a low power green light emitting diode which is energized when power is supplied to the invention 10. Error condition indicator 34 is a yellow LED which is software controlled to be energized if the invention 10 is in error condition.

0024Figure 3 discloses a diagrammatic illustration of a radio communication system in accordance with the present invention. Invention network controller 40 is coupled to host computer 42 such that data may be interchanged between the devices over host communications link 44, which may be either in an RS232C format or selectively in an RS422 format. The host communication link 44 couples to controller 40 at host port 46.

0025First communication port 48 of controller 40 provides means for coupling of network 50 to controller 40. Network 50 comprises a number of base RF transceiver units 52A, 52B and 53B, each of which may be selectively employed in the radio frequency communication of data from mobile transceiver units. It is to be understood that base transceiver units 52 are designed and equipped to be operable in the exchange of data with network controller 40 over network link 56 such that each base transceiver unit 52A, 52B, or 53C may independently exchange data with network controller 40 through first communication port 48.

0026Pi IΠ^ ¹"! IT r**. ^■* When first communication port 48 is intended for operation with a network such as network 50 of base transceiver units 52A, 52B and 53C, for example, network controller 40 is selectively operated to provide an RS485 interface at first communication port 48. First communication port 48 may be alternately selected to operate as an RS232C interface, as an RS422 interface, as a proprietary NORAND® Radio One Node Network interface or as a high speed V.35 interface. The selection of interface to be provided at first communication port 48 is front panel controlled, that is, the user may operate front panel keys 28, 30 and 26 (See Figure 2) to direct the proper interface to be provided at first communication port 48.

0027Base transceiver units 52A, 52B, and 52C are coupled to network link 56 by serial means, rather than parallel means, and each may be caused to transmit or to receive independently from the others while additionally being communicative with network controller 40 in a randomly chosen fashion.

0028It is further to be understood that interface translation is provided within controller 40 such that data communicated at first communication port 48 may be directed to host 42 at port 46 via properly chosen interface means as is required by the host 42 with which communication is intended. Like first communication port 48, second communication port 57 may be internally switched among interface choices of these types: RS232C, RS422, V.35, RS485 and proprietary NORAND® Radio One Node Network interface. In the illustrated arrangement of Figure 3, for example, second communication port 57 is coupled over third link 53 to previously installed base transceiver 54, which heretofore had been used in a prior art system as is illustrated in Figure 1. Because of limitations of base transceiver 54, it must communicate via RS232C interface format and therefore, second communication port 57 must be selected to operate in RS232C interface mode. However, when second communication port 57 is desired to communicate with a network via RS485 interface, front panel keys 26, 28 and 30 may be manipulated by the user to provide the RS485 interface availability at second communication port 57. Likewise, second communication port 57 may be selected to operate as an RS422 interface, as a V.25 interface, or as the proprietary NORAND® Radio One Node Network interface. Diagnostic port' 55 provides a fourth communication pathway for network controller 40, providing an asynchronous port operable at 300 to 19,200 baud as an RS232C interface. When desirable, diagnostic port 55 may be coupled by diagnostic link 58 to diagnostic device 60 for purposes of error diagnosis of controller 40 by diagnostic device 60, or for reprogramming of memory devices within controller 40 when desired. It is contemplated that diagnostic device 60 comprises a 16 or 32 bit microprocessor commonly known as a personal computer or "PC". The mode of coupling between diagnostic device 60 and network controller 40 may be direct or through remote means by use of a modem. Referring now to Figure 4, a central processing unit 70 is provided with at least four data communication ports, illustrated at numerals 71, 72, 73, and 74. First data communication port 71 may be selectively coupled to RS232 interface member 76 or V.35 interface member 78. The choice of whether RS232 interface member 76 or V.35 interface member 78 is chosen is dependent upon the operating characteristics presented by the

0029~ host computer, such as host computer 42 of Figure 3, with which network controller 40 will communicate. The choice of whether first communication port 71 is coupled to interface member 76 or to interface member 78 depends on the front panel selection made by the user by keys 26, 28, and 30 shown in Figure 2.

0030Second communication port 72 may be selectively coupled to RS232 member 80 or to RS485 interface member 82 or to RS422 interface member 84 or to NORAND® Radio One Node Network proprietary interface member 86. By use of front panel keys 26, 28, and 30 of Figure 2, the user may select second communication port 72 to be coupled to any one of interface members 80, 82, 84, and 86.

0031Third communication port 73 is identical to second communication port 72 in functionality, being selectively couplable to RS232 interface member 88, to RS485 interface member 90, to RS422 interface member 92 or to NORAND® Radio One Node Network proprietary interface member 94.

0032In the preferred embodiment of the invention 40, central processing unit 70 of Figure 4 comprises a Motorola™ 68302 integrated chip cooperative with an application specific integrated circuit. Central processing unit 70 employs novel features allowing the bidirectional use of a data communicative line of the Motorola™ 68302 chip and a single clock signal line to eliminate the need for coder-decoder members to be associated with the Motorola™ 68302 chip while allowing the use of only one pair of signal wires to be coupled to the RS485 interfaces 82 and 90 of Figure 4.

0033Fourth communication port 74 of central processing unit is coupled to asynchronous RS232 interface member 97 to be available for interconnection of a diagnostic device therewith.

0034Also coupled to central processing unit 70 are display member 24 and keyboard member 31 with which keys 26, 28, and 30 of front panel 22 (Figure 2) are interactive.

0035Memory elements including EPROM element 96, DRAM unit 98, FLASH memory unit 100 and EEPROM element 102 are intercoupled with each other and with central processing unit 70.

0036Power supply member 104 is selectively attachable to invention network controller 40. In order to avoid the necessity of different models of network controller 40 depending on the local electrical power ' utility's operating characteristics, power supply 104 is provided in optional models depending on the country in which it is to be used, power supply 104 being capable of providing satisfactory output power to network controller 40 regardless of the voltage or frequency of the input source provided to power supply 104.

0037The application specific integrated circuit

0038(ASIC) used in the invention network controller 40 is disclosed in Figure 6 and is identified by the numeral 120. ASIC 120 comprises a central processor unit interface 122 member which is coupled to the central processor unit bus by CPU bus link 124 which extends from ASIC 120. Also coupled to the CPU bus link 124 is dynamic random access memory (DRAM) timing element 126, which provides network controller 40 with timing signals for the DRAM member 98 illustrated in Figure 4 when memory refresh of the DRAM 98 is indicated. DRAM timing element 126 is also coupled exteriorly to the ASIC 120 to DRAM member 98 by DRAM link

0039127 . Central processing unit interface 122 is coupled to asynchronous signal processing element 128 by signal path 130. Asynchronous signal processing element 128 comprises a baud rate generator cooperative with a universal asynchronous receiver-transmitter.

0040Also coupled to central processing unit interface 122 is network clock and control member 132 which comprises a programmable network clock generator which can be selectively programmed to generate an optional clock speed for a network to be coupled through RS485 interfaces 82 and 90 seen in Figure 4. Network clock and control member 132 also provides detection means for detections of failure conditions dn a linked network and provides control signals to system components in response thereto, including interrupt signals to programmable interrupt coordinator circuitry included in central processing interface 122. Network clock and controller member 132 provides data encoding by the FMO standard, then the encoded data may be operated upon by RS485 interfaces 82 and 84 and transmitted and received by single twisted pair means to multiple serially networked base transceiver units exemplified by base transceiver unit 52A, 52B, and 52C illustrated in Figure 3.

0041Keyboard controller element 134 is coupled to central processing unit interface and provides a link exterior to ASIC 120 to keyboard 31 (See Figure 3) .

0042FLASH memory/EEPROM logic control member 136 is coupled to central processing unit interface 122 and comprises control functions for FLASH memory element 100 and EEPROM memory element 102 of Figure 3. Central processing unit interface 122 is also coupled by line 138 to latches exterior to ASIC 120.

0043It is to be understood that the base transceiver units 52A, 52B, and 52C illustrated in Figure 3 are communicative with mobile transceiver units by electromagnetic radio means. The mobile transceiver units may be associated with bar code scanning devices such as the NORAND® 20/20 High Performance Bar Code Reader whereby the scanning devices scan an object having a bar code associated therewith and collect information stored in the bar code, which information is then transmitted through the mobile transceiver units to base transceiver units such as base transceiver units 52A, 52B, and 52C or base transceiver unit 54 of Figure 3. The bar code data received by said base transceiver units is then transmitted in the example of Figure 3, over network 50 by base transceiver units 52A, 52B, or 52C, or over link 53 by base transceiver unit 54, to network controller 40 which performs the routing and delivery of the data to the stationary data processor, or processors, such as shown for example, by host 42 of Figure 3.

0044Description of Figures 7 through 11

0045Figure 7 shows a block diagram of a particularly preferred intelligent base transceiver unit known as the RB4000. It will be observed that the components correspond with components of the network controller of Figure 4, and similar reference numerals (preceded by 7-) have been applied in Figure 7. Thus, the significance of components 7-70 through 7-73, 7- 76, 7-82, 7-96, 7-98, 7-100 and 7-104 will be apparent from the preceding description with respect to Figure 4 and 6, for example. I/O bus 700 may be coupled with a spread spectrum transmission (SST) or ultra high frequency (UHF) transceiver 701 which may correspond with any of the transceivers of units 52A, 52B, 52C or 54 previously referred to. The network controller 70 could have a similar RF transceiver coupled with its data port 72 or 73 and controlled via input/output bus 400, e.g. for direct RF coupling with router transceivers such as 901, 901, Figure 9.

0046Referring to Figure 8, a network controller 40 is shown with port 71 configured for interface with a host port type SNA V. 35 56K/64K bits per second. Port 72 is shown as configured for communication with a personal computer of the PS/2 type operating asynchronously at 38.4K bits per second. Port 74 is coupled with a modem 8-60 providing for remote diagnostics and reprogramming of the network controller 40.

0047Port 73 of network controller 40 is shown as being connected with an adapter component 801 known as Norand Corporation MBA3000. In the operating mode indicated in Figure 8, the adapter 801 serves to couple controller 40 sequentially with four radio base transceiver units such as indicated at 811 through 814. Component 811 is a commercially available radio base known as the RB3021 of Norand Corporation. Base station 811 may communicate with a multiplicity of hand-held RF data terminals such as indicated at 821. Base 814 is indicated as being coupled with the adaptor 801 via RF broadband modems 831 and 832. Base units 813 and 814 may communicate with mobile transceiver units such as those indicated at 833 and 834. Figure 9 shows two network controllers 40A and 4OB each with its host ports configured as with the controller 40 of Figure 8. In this example, the second ports 72 of the controllers 40A and 4OB are configured for communication a relatively high data rate relatively along a distance network channel 56 which may have the characteristics of the serial channel 56 of Figure 3, for example, an RS485 channel operating at 384 kilobits per second (384K bps) . Network base transceivers 52A, 52B and 52C may correspond with the correspondingly numbered transceiver units of Figure 3, for example, and the network may have additional network transceivers such as 52D. Furthermore, the network transceivers may have RF coupling with router transceiver units such as indicated at 901, 902 and 903. Router transceiver unit 902 is illustrated as a RB4000 intelligent transceiver such as represented in Figure 7 and having its input/output bus 700 coupled with a peripheral.

0048Figure 10 is entirely similar to Figure 9, for example, except that ports 72 of the controllers 40A and 4OB are coupled with separate serial type high data rate network channels, and ports 73 of the respective network controllers are coupled to a very high speed network e.g. in the several megabits per second range such as an Ethernet local area network 1000. Suitable interfaces are indicated at 1001 and 1002.

0049Figure 11 is entirely similar to Figure 9 except that the ports 73 of the network controllers 40A and 40B are coupled with respective local area ring type networks which may be separate from each other and each have two or more hosts such as represented in Figure 9 associated with the respective ring networks such as token rings 1100A and 1100B. Suitable interface means are indicated at 1101 and 1102.

0050Description of Figure 12

0051Figure 12 shows, for example, two network controllers 40A and 40B, each with two host computer units such as 42-1A. Host 42-2 is shown with a printer or other peripheral PI which may generate bar codes, for example, for replacement of damaged bar codes or the like. Another printer P2 is shown associated with base 52C, again for example, for producing bar code labels where those are needed in the vicinity of a base station. In a large warehouse, relatively large distances may be involved for a worker to return to a printer such as PI to obtain a new bar code label. Thus, it may be very advantageous to provide a printer P2 at the base station 52C which may be relatively close to a processing location which requires printed labels, e.g. a processing location in the vicinity of hand-held terminal 12-2 in Figure 12. A base 52F may have a peripheral P3 associated therewith such as a large screen display, a printer or the like which may supplement the capabilities of a hand-held terminal, for example printing out new bar code labels at a convenient location, or providing a full screen display, rather than the more limited screen display area of the hand-held terminal 12-2.

0052If, for example, a base radio 52D which might be located at the ceiling level of a warehouse became inoperative at a time when qualified repair personnel were not immediately available, with the present system it would be feasible to provide a substitute base radio or base radios, for example, as indicated at 52D1 located at table level or the like.

0053With the present system, the base radio stations do not necessarily forward data communications received from a given terminal to a particular host. For example, hand-held terminal 12-2 may request a path to printer P2, and such a path may be created via base stations 52D1 and 52C. Station 52C upon receipt of the message form terminal 12-2 would not transmit the message to a host but would, for example, produce the desired bar code label by means of printer P2. Further, terminal 12-2 may have provision for digitizing a voice message which might, for example, be addressed to terminal 12-1. The system as illustrated would be operable to automatically establish a suitable path for example, via stations 52D1, 52C, 52B, 52E and 12-1 for the transmission of this voice message in digital form. Successive segments of such a voice message would be stored, for example, by the terminal 12-1, and when the complete message was assembled, the segments would be synthesized into a continuous voice message for the user of terminal 12-1 e.g. by means of a speaker 1201 also useful for sending tone signals indicating valid bar code read, etc.

0054In accordance with the present invention, a hardware system such as illustrated in Figure 12 may be physically laid out and then upon suitable command to one of the network controllers such as 42-2B, the entire system would be progressively automatically self-configured for efficient operation. For example, controller 40B could successively try its communications options with its output ports such as 71-73, determining for example, that host processors were coupled with ports 71 and 72, one operating on a 38.4 kilobit per second asynchronous basis and the other presenting a SNA port for the V.35 protocol at 64 kilobits per second. For example, on host, 42-1B might be a main frame computer, while the other host 42-2B might be a PS/2 type computer system. The controller 4OB having thus automatically configured itself so as to be compatible with the devices connected to ports 71 and 72, could proceed to transmit via port 73 a suitable inquiry message to the network channel 56. Although a polling protocol is preferred, each of the base stations could operate, for example, on a carrier- sense multiple-access (CSMA) basis or using a busy tone protocol to respond to the inquiry message from the controller 4OB, until each of the successive bases on the network had responded and identified itself. Each base, for example, would have a respective unique address identification which it could transmit in response to the inquiry message so as to establish its presence on the network.

0055The controller 4OB could then transmit auto configure commands to the successive bases in turn, instructing the bases to determine what peripherals and router bases such as 52D1, 52E and 52F were within the range of such base, and to report back to the controller. For example, bases such as 52C and 52F could determine the nature of peripherals P2 and P3 associated therewith so as to be able to respond to an inquiry form a terminal such as 12-2 to advise the terminal that a bar code printer, for example, was within direct RF range.

0056In the case of a breakdown of a component of the system such as 52D, it would merely be necessary to place a router device such as 52D1 at a convenient location and activate the unit, whereupon the unit could send out its own broadcast inquiry which, for example, could be answered by the base stations 52C and 52F, station 52C in turn, advising a relevant host or hosts of the activation of a substitute router station. Thus, the system is conveniently re-self- configured without the necessity for a technician familiar with the particular configuration procedure. As another example, where the base stations are operating utilizing spread spectrum transmission, the introduction of barriers (such as a new stack of inventory goods) to such transmission between a given base such as 52A and various terminals, could result in the base 52A contacting router 52E, for example, with a request to become active with respect to the blocked terminals.

0057<img file="WO9410774A1_D0001.tif" /> *- Description of Figure 13

0058Figure 13 shows an intelligent integrated controller and radio base unit 1300 which is integrated into a single housing or case 1301 corresponding to the case or housing 20 of Figure 2. the housing 1301 may be provided with an external antenna as diagrammatically indicated at 1302 with suitable RF coupling to the radio circuitry indicated at 1303. Components 13-70 through 13-74, 13-76, 13-78, 13-96, 13-97, 13-98, 13-100, and 13-102 may correspond with the correspondingly numbered components described with reference to Figure 4.

0059SUPPLEMENTARY DISCUSSION In accordance with the present disclosure, a network controller, or integrated network controller and radio unit is coupled to one or more host computers via a standard interface such as commonly encountered in practice (e.g. RS232, V. 35, Ethernet, token ring, FDDI, and so on). In this way, no specialized interface or adapter is required for the host.

0060Since the preferred network controller can connect to two hosts, if one host is detected to have failed, or in the event of a system crash, loss of communication link, or the like, the network controller can automatically switch to the second host. The second host may be a truly redundant system, or may be a simpler computer of the PC type (a so-called personal computer) that can simply store transactions until the main host is restored. As another example, a single host may have a second port coupled to a second port of the controller especially if a communication link failure may be a problem. For example, two ports of the network controller may be coupled by separate modems with separate phone lines, leading to separate ports of a single mainframe computer, for example an IBM3090. In a fully redundant system, two ports of a network controller may be connected respectively to two mainframe computers such as the IBM3090. The disclosed network controller can also connect one radio network to two hosts using RS232 or V.35 ports or to many hosts using a local area network such as Ethernet, token ring, or FDDI. A number of the disclosed network controllers (for example, up to thirty-two) can be connected together to interface many hosts to a single radio network. The hand-held portable terminals in such a network can then talk to any of the hosts they choose. For example where one port of the disclosed network controller is coupled via its RS232 interface to a mainframe computer such as the IBM3090, another of its ports may be coupled via an FDDI network with a super computer e.g. the

0061EET ftULE 26 Cray X-MP. Then mobile and/or portable terminals can access either the main frame or the super computer, or in general, any of the hosts that are connected to the network controller. As indicated in Figure 9, four hosts can be on one network. Referring to Figures 10 and 11, a multiplicity of hosts may be coupled with each local area network so as to be in communication with one or more of the disclosed network controllers. Furthermore, a single disclosed network controller can control two radio networks such as the one indicated at 50 in Figure 3. Where each network such as 50 is limited to thirty-two devices, the number of devices is doubled with the use of two radio networks. Two such radio networks may also be utilized for the sake of redundancy, with a provision for automatic switch-over from one radio network to the second if a problem develops on the first. Two radio networks may also facilitate the use of different radio technologies in one installation.

0062The various multi-drop local area networks referred to herein, for example at 7-82 in Figure 7 and as represented at 56, 56A, 56B, Figures 9 through 12, and at 13-82 in Figure 13 may comprise HDLC based local area networks operating at up to 2.5 megabits per second and using biphase space encoding (FMO) for clock recovery from data.

0063TE SHEET RULE _:5; The components 86 and 94, Figure 4, and component 13-11, Figure 13, provides a low-cost base radio interface using three pairs of twisted conductors. One pair provides a bidirectional RS485 data line. Another pair is used for the clock and has an RS422 electrical configuration, and is one directional from the radio to the controller. The third twisted pair is also RS422 and is used to communicate from the controller to the radio transceiver to effect mode selection.

0064An aspect of the invention resides in the provision of a network controller having port means selectively configurable for coupling in first mode with network RF transceiver units at a relatively high data rate such as 100 kilobits per second or higher, and for coupling in a second mode with network transceiver units at a relatively low data rate such as about twenty kilobits per second. Preferably a single port means such as 2, 3, or 5, 6, Figure 5, can be software configured to interface selectively in the first mode or in the second mode. It is presently less expensive to use multiple connectors per port rather than a single 37-pin connector for example.

0065Where a network controller such as 40 operates two high data rate networks, for example, one network of RF base transceivers could operate with the RTC protocol, and the second network could operate according to a different protocol. It will be apparent that many modifications and variations may be effected without departing from the scope of the teachings and concept of the present disclosure.

0066Description of Figures 14 and 15

0067Figure 14 is a block diagram of the circuitry for one pair of communication ports 1401 and 1403 of adapter 801 (fig.8) for use in coupling to base transceiver units. Three additional pairs of communication parts for coupling to six additional base transceiver units are provided in the embodiment of adapter 801 as exemplified by the Norand Corporation MBA3000 Multiple Base Adapter. It is to be understood that the circuit components coupled to each additional pair of communication ports of adapter 801 is identical to that shown for first port pair 1A/1A, that is ports 1401 and 1403 of Figure 14. The adapter 801 provides means for connecting the controller 40 (Fig. 8) at its port 73 to a multiplicity of radio base units illustrated in Fig. 8 as, for example, 811, 812, 813, 814, including in selected pairs. In the preferred embodiment of adapter 801, up to eight radio base units may be coupled through use of adapter 801 to a network controller 40, to be controlled by controller 40 in selected pairs thereof. The controller 40 may control the radio base units such as 811, 812, 813, 814, (Fig. 8) in simulcast mode, that is, with all base radios interrogating mobile transceiver units such as 821, 833, and 834 of Fig. 8 simultaneously, or with the base units being employed in pairs to interrogate the mobile transceiver units.

0068Referring again to Figure 14, the network controller 40 provides transmit data and baud rate select signals to adapter 801. Within adapter 801, the controller outputs are converted to TTL levels by TTL converter 1402 and they are then provided to buffer 1404 which provides the signals to paired RS232 transceivers 1406 and 1408, and to paired RS422 transceivers 1410 and 1412 which deliver the converted signals to ports 1401 and 1403 respectively. By this means, the controller's output signals are provided to a pair of output ports 1401 and 1403 in both RS232 and RS422 interface at the same time. An additional three output-port-pairs are provided which may be denominated 2A/2B, 3A/3B and 4A/4B, which ports are controlled and operated identically to ports 1A/1B identified in Fig. 14 as ports 1401 and 1402. The RS232 transceivers 1406 and 1408 and RS422 transceivers 1410 and 1412 and ports 1401 and 1403 are illustrative of all circuitry coupled to port pairs of adapter 801.

0069Similarly, signals provided to adapter 801 by base radios coupled to the output port pairs, e.g. ports 1401 and 1403 of Fig. 14, are first converted to TTL levels by the RS232 transceivers 1406 and 1408 or by the RS422 transceivers 1410 and 1412, depending upon which interface is presented by a pair of base radios at port 1401 and 1403. The signals provided to adapter 801 are then forwarded by the transceivers 1406 and 1408 or 1410 and 1412 at TTL levels to controller 40. A selection unit 1414 provides a push-to-talk selection signal to the RS232 transceivers 1406 and 1408 and to the RS422 transceivers 1410 and 1412 to provide PTT selection signals at ports 1401 and 1403 in both RS232 and RS422 format. It is to be understood that similar selection units are associated with remaining port pairs 2A/2B, 3A/3B. 4A/4B so that the ports may be independently operated.

0070The adapter 801 of Fig.8 is exemplified by the MBA3000 multiple base adapter unit manufactured by the NORAND Corporation of Cedar Rapids, Iowa. Because of the operation of the MBA3000 multiple base adapter by dual methods in either RS232 or RS422 signal environments, the MBA3000 may be incorporated into systems having existing installed base radios which present only an RS232 interface or it may be incorporated into systems having base radios some of which operate at RS422 and some at RS232. Fig. 15 illustrates a preferred arrangement of controller 40 and adapter 801 when used in an environment with multiple base radios in multiple warehouse environments. Controller 40 is coupled to adapter 801 which is coupled to paired bases 1511, 1512; 1513, 1514; 1515, 1516; and 1517, 1518; which are located in warehouses 1501, 1502, 1503 and 1504. By geographical separation in warehouse 1501, for example, base radios 1511 and 1513 provide substantial coverage of warehouse 1501 such that a mobile transceiver being used within warehouse 1501 would be communicated with by either base radio 1511 or 1513. By the use of adapter 801, controller 40 may cause interrogation simultaneously by base radios 1511, 1512, 1513, 1514, 1515, 1516, 1517, 1718, or it may cause sequential interrogation by radio pairs 1511/1512, 1513/1514, 1515/1516, or 1517/1518 in succession. When a mobile transceiver responds by RF communication means with a base radio, e.g. base radio 1511, the response is transmitted by base radio 1511 through coupling 1521 to adapter 801 which automatically converts the incoming response to RS232 interface as necessary, to make it suitable for reception by controller 40.

0071Through a system as exemplified in Fig.15, data collection from a number of roving mobile transceivers may be initiated by a network controller 40 through a four-warehouse environment. When base transceiver units 1511 and 1512 have been unsuccessful in establishing communication with the desired mobile transceiver unit, controller 40 will then cause bases 1513 and 1514 to attempt communication and if bases 1513 and 1514 are unsuccessful, controller 40 will proceed through the other base radio pairs, namely 1515/1516 and 1517/1518, as needed, to establish communication with the desired mobile transceiver unit.

0072The adapter 801 is provided to operate in either simulcast or sequential mode. In the normal or simulcast mode, adapter 801 allows the use of one to eight bases, where the bases are configured as four pairs of two bases. In this mode the adapter 801 simulcasts to a single base pair at a time and the four sets of base pairs are selected using a dynamic time-division multiplexing method. The user can configure the adapter 801 to use any of the eight base ports, using simulcasting or time-division multiplexing to best advantage.

0073There are two sets of base transceiver units, referred to as set A (identified as IA, 2A, 3A, and 4A) and set B (identified as IB, 2B, 3B, and 4B) . Within a set, the base transceiver units are selected by time-division multiplexing.

0074It can be seen in Fig. 15, that there are four pairs of base transceiver units defined as

0075cu τ TE SHEEⁱ (^ ^{E 2δ}) pairs 1A/1B, 2A/2B, 3A/3B, 4A/4B. Each base transceiver unit of a base pair is simulcasted to at the same time.

0076The hardware of the adapter 801 allows the selection of the base pairs (pair 1A/1B through 4A/4B) using control lines from the controller 40. Adapter 801 transmits to both base transceiver units of a base pair at the same time and receives independently from each base simultaneously. The use of adapter 801 allows an extension of the number of base transceiver units that can be used in a facility to allow for adequate coverage, it is important to understand how the base transceiver units operate when simulcasting is used, and when time-division multiplexing is used.

0077The adapter 801 distributes signals transmitted by controller 40 to base transceiver pairs at the same time, so if there is an overlap in the coverage for the two base transceiver units, there may be some interference. The amount of interference depends on the relative signal strengths; if the strength is similar in one spot the chance of interference is larger that if the signal strengths are different. This type of interference could be avoided in some configurations by splitting coverage areas of pairs of base transceiver units. Another method of covering the overlap area is to place another base (not one of the base pairs) to cover the overlap area. The radio signals from the mobile transceiver unit may be picked up fully or partially by either or both base transceiver units of a given pair. However the adapter 801 first tries to receive from one base transceiver unit, for example base 1511, and if unsuccessful, it then switches to try to receive from a second base transceiver unit, for example base transceiver unit 1513. If the information is successfully received from the first base transceiver unit, the information from the second base transceiver unit is ignored. Thus the controller assures data does not get sent to the host data processor in duplicate. The user may couple from one to eight base transceiver units to the adapter 801 and can then configure those base transceiver units as required to meet the installation's needs. Any combination of ports of the adapter 801 can be used. Thus the user can take advantage of the ability to simulcast or sequentially (via time-division multiplexing) access the base transceiver units 1511, 1512, 1513, 1514, 1515, 1516, 1517, and 1518.

0078Multipath Fading and Data Packet Size Parameters.

0079In a preferred embodiment, the data (or messages) to be sent through the RF communication link is segmented into a plurality of DATA packets and is then transmitted. Upon receipt, the DATA packets are reassembled for use or storage. Data segmentation on the RF link provides better communication channel efficiency by reducing the amount of data loss in the network. For example, because collisions between transmissions on an RF link cannot be completely avoided, sending the data in small segments results in an overall decrease in data loss in the network, i.e., only the small segments which collide have to be re¬ sent.

0080Similarly, choosing smaller data packets for transmission also reduces the amount of data loss by reducing the inherent effects of perturbations and fluctuations found in RF communication links. In particular, RF signals are inherently subject to what is termed "multi-path fading". A signal received by a receiver is a composite of all signals that have reached that receiver by taking all available paths from the transmitter. The received signal is therefore often referred to as a "composite signal" which has a power envelope equal to the vector sum of the individual components of the multi-path signals received. If the signals making up the composite signal are of amplitudes that add "out of phase", the desired data signal decreases in amplitude. If the signal amplitudes are approximately equal, an effective null (no detectable signal at the receiver) results. This condition is termed "fading".

0081Normally changes in the propagation environment occur relatively slowly, i.e., over periods of time ranging from several tenths (1/10*s) of seconds to several seconds. However, in a mobile RF environment, receivers (or the corresponding transmitters) often travel over some distance in the course of receiving a message. Because the signal energy at each receiver is determined by the paths that the signal components take to reach that receiver, the relative motion between the receiver and the transmitter causes the receiver to experience rapid fluctuations in signal energy. Such rapid fluctuations can result in the loss of data if the amplitude of the received signal falls below the sensitivity of the receiver.

0082Over small distances, the signal components that determine the composite signal are well correlated, i.e., there is a small probability that a significant change in the signal power envelope will occur over the distance. If a transmission of a data packet can be initiated and completed before the relative movement between the receiver and transmitter exceeds the "small distance", data loss to fading is unlikely to occur. The maximum "small distance" wherein a high degree of correlation exists is referred to hereafter as the "correlation distance".

0083As expressed in wavelengths of the carrier frequency, the correlation distance is one half (1/2) of the wavelength, while a more conservative value is one quarter (1/4) of the wavelength. Taking this correlation distance into consideration, the size of the data packet for segmentation purposes can be calculated. For example, at 915 MHz (a preferred RF transmission frequency), a quarter wavelength is about 8.2 centimeters. A mobile radio moving at ten (10) miles per hour, or 447 centimeters per second, travels the quarter wavelength in about 18.3 milliseconds. In such an environment, as long as the segment packet size remains well under 18.3 milliseconds, significant signal fluctuations during the duration of a packet transmission is unlikely. In such an preferred embodiment, five (5) millisecond data packet segments are chosen which provides a quasi-static multipath communication environment.

0084The faster the relative movement between a transmitter and a receiver the greater the effect of fading, and, therefore, the smaller the data segment should be. Similarly, if the relative movement is slower, the data segment can be larger. Slower fading effects which might be experienced between stationary transceivers in an office building due to the movement of people, mail carts, and the like. In a typical application of the present invention, the RF transceiver of a mobile unit may be secured with a bar-code scanner such as a deflected laser beam bar-code scanner or an instant CCD bar-code scanner. In such an example, the bar code data could be transmitted to the base station as the RF transceiver and a scanner device were being jointly transported by a vehicle (e.g. a forklift truck) to another site, or the RF transceiver and a scanner, e.g. as a unitary hand-held device, could be carried by the operator to another site as the bar code data was being transmitted to the base station. In such situations, fading is more pronounced.

0085If fading does not pose a problem on a given network, the overhead associated with segmentation, hand-shaking and reconstruction may not be justifiable. However, where fading exists, such overhead may be required.

0086In many communication environments, the degree of fading effects varies dramatically both from time to time and from installation to installation. In the preferred embodiment, transmitters and receivers communicate using an optimal data segment size parameter by adapting the size to conform to the communication environment of the network at any given time. For example, if a receiver detects repeated faulty transmissions, the data segment size parameter might be incrementally reduced (under the assumption that fading caused the faults) until the data throughput reaches an optimal level. Similarly, the size of the data segment can be reduced based on a measured indication of the degree of fading in the network.

0087One example of a receiver making such a measurement of fading can be found in the abandoned patent application of Ronald L. Mahany, U.S. Serial No. 07/485,313, filed February 26, 1990. Specifically, in that reference, a received signal strength indicator (RSSI) circuit is found in the receiver. The RSSI circuit samples the signal strength of a transmission. If the signal strength samples are evaluated in sequence and the trend analyzed, the degree of fading can be measured. If the signal strength samples decrease in value, it is likely that fading is present in the network. However, just because fading exists does not require segmentation. Only if fading causes the signal strength to drop below the level of the receiver's sensitivity is segmentation required.

0088A fixed threshold value that is located a safe margain above the receiver's sensitivity is

0089#-^**? used to determine whether to change the data segment size. If a trend in signal strength shows values falling below the threshold, the data segment size is decreased. If the threshold level is never reached, the segment size might be increased. In addition, the trend associated with a group of signal strength samples can be used to predict the optimal data packet size — the intersection of the signal strength samples with the threshold defines a segment length that, with a safe margain, can be used effectively used with the current degree of fading.

0090After receiving a data segment, the receiver sends to the transmitter indications regarding: 1) whether the data segment was received without fault; and 2) what the new optimal segment size should be. The transmitter responds by adjusting the data segment size and then sending the next segment. As can be appreciated, the data segments are adapted based on the previous transmission. Instead of adjusting on the basis of the reception of a single data segment (the previous transmission) , other techniques for adjustment are contemplated. For example, the transmitter may also utilize a threshold window (or weighted averaging) , inside of which the segment size will not be changed. Only if the requested change by the receiver falls outside of the threshold window will the segment size change. Similarly, the

0091-^• ip-c-rπ^*1 rrc SHEET (HULE 26) receiver might also utilize such a window — only requesting a change when the newly forecasted, optimal segment size falls outside of the window.

0092Direct-Sequence Spread Spectrum Parameters. As described above, the network controller provides an interface to both the older generation UHF radio transceivers and newer generation spread spectrum transceivers. A spread spectrum broadcasting system uses a sequential pseudo-noise signal to spread a signal that is in a relatively narrow band over a wider range of frequencies. It is the subject of standards issued by the Federal Communications Commission (FCC) that provide usable spectrum at low power levels for communication in limited areas such as warehouses, office buildings, and the like. The use of spread-spectrum techniques minimizes interference with others using the same channels in the spectrum. A transmitter using direct-sequence spread spectrum transmission uses a spreading-code of a higher frequency than that of the data rate to encode the data to be sent. This higher frequency is achieved by increasing the chip clock rate (wherein each chip constitutes an element of the spreading-code) . Using the same spreading code, the receiver decodes the received signal while ignoring minor faults which occurred in transmission, providing noise immunity and multipath signal rejection. The frequency and length of the spreading-code can be varied to offer more or less multipath signal rejection or noise immunity. Although it may result in improved communication, increasing the frequency or length of the spreading-code requires additional overhead which may not be justifiable unless necessary.

0093Frequency-Hopping Spread Spectrum Parameters.

0094Frequency-hopping is the switching of transmission frequencies according to a sequence that is fixed or pseudo-random and that is available to both the transmitter and receiver. Adaptation to the communication environment via an exchange in frequency-hopping operating parameters is possible, for example, via selective control of the hopping rate or through the use of coding or interleaving. The greater the degree of frequency selectivity of the fading envelope (i.e., when fading is significant only over a portion of the spectrum of hopping frequencies) , the greater the benefit of such adaptation.

0095Particularly, a parameter indicating the hopping rate can be varied to minimize the probability that the channel characteristics will detrimentally change during the course of a communication exchange. To vary the hopping rate

0096LE 25 is to vary the length of a hopping frame. Although multiple data (or message) exchanges per hopping frame is contemplated, the preferred hopping frame consists of a single exchange of data. For example, in a polling environment, the hopping frame might consist of: 1) a base station transmitting a polling packet to a roaming terminal; 2) the roaming terminal transmitting data in response; and 3) the base station responding in turn by transmitting an acknowledge packet. Each hopping frame exchange occurs at a different pseudo-randomly chosen frequency.

0097For optimization, the hop frame length is adjusted to be as long as possible, while remaining shorter than the coherence time of the channel by some safety margin. Although such adjustment does not eliminate the effects of fading, it increases the probability that the characteristics of the channel will remain consistent during each hopping frame. Thus, in the preferred embodiment, if the polling packet transmission is successfully received, the probability of successful receipt of the data (or message) and acknowledge is high. Another parameter for changing frequency- hopping performance is that of coding. Coding on the channel for error correction purposes can be selectively used whenever the probability of data or message loss due to fading is high. In particular, coding methods which provide burst error correction, e.g., Reed-Solomon coding, can be applied if the hop length is likely to exceed the coherence time of the channel. Such coding methods allow some portion of the data to be lost and reconstructed at the expense of a 30-50% reduction in throughput. The operating parameter for coding indicates whether coding should be used and, if so, the type of coding to be used. An operating parameter indicating whether interleaving should be used also helps to optimize the communication channel. Interleaving involves breaking down the data into segments which are redundantly transmitted in different hopping frames. For example, in a three segment exchange, the first and second segments are sequentially combined and sent during a first hopping frame. In a subsequent hopping frame, the second and third segments are combined and sent. Finally, the third and first segments are sequentially combined and transmitted in a third hopping frame. The receiving transceiver compares each segment received with the redundantly received segment to verify that the transmission was successful. If errors are detected, further transmissions must be made until verification is achieved. Once achieved, the transceiver reconstructs the data from the segments. Other methods of interleaving are also contemplated. For example, a simpler form of interleaving would be to sequentially send the data twice without segmentation on two different frequencies (i.e., on two successive hops).

0098As can be appreciated, interleaving provides for a redundancy check but at the expense of data or message throughput. The interleaving parameter determines whether interleaving is to be used and, if so, the specific method of interleaving.

0099In addition, any combination of the above frequency-hopping parameters might interact to define an overall operating configuration, different from what might be expected from the sum of the individual operating parameters. For example, selecting interleaving and coding, through their respective parameters, might result in a more complex communication scheme which combines segmentation and error correction in some alternate fashion.

0100Source Encoding Parameters (For Narrowband Applications) .

0101In the United States, data communication equipment operating in the ultra-high frequency (UHF) range under conditions of frequency modulation (FM) is subject to the following limitations. (1) The occupied band width is sixteen kilohertz maximum with five kilohertz maximum frequency deviation.

0102(2) The channel spacing is 25 kilohertz. This requires the use of highly selected filtering in the receiver to reduce the potential for interference from nearby radio equipment operating on adjacent channels.

0103(3) The maximum output power is generally in the range of ten to three hundred watts. For localized operation in a fixed location, however, transmitter power output may be limited to two watts maximum, and limitations may be placed on antenna height as well. These restrictions are intended to limit system range so as to allow efficient re-use of frequencies.

0104For non-return to zero (NRZ) data modulation, the highest modulating frequency is equal to one half the data rate in baud. Maximum deviation of five kilohertz may be utilized for a highest modulation frequency which is less than three kilohertz, but lower deviations are generally required for higher modulation frequencies. Thus, at a data rate of ten thousand baud, and an occupied bandwidth of sixteen kilohertz, the peak FM deviation which can be utilized for NRZ data may be three kilohertz or less.

0105Considerations of cost versus performance tradeoffs are the major reason for the selection

0106ULE 26 of the frequency modulation approach used in the system. The approach utilizes shaped non-return-to-zero (NRZ) data for bandwidth efficiency and non-coherent demodulation using a limiter-discri inator detector for reasonable performance at weak RF signal levels. However, the channel bandwidth constraints limit the maximum data "high" data rate that can be utilized for transmitting NRZ coded data. Significant improvements in system throughput potential can be realized within the allotted bandwidth by extending the concept of adaptively selecting data rate to include switching between source encoding methods. The preferred approach is to continue to use NRZ coding for the lower system data rate and substitute partial response (PR) encoding for the higher rate. The throughput improvements of a NRZ/PR scheme over an NRZ/NRZ implementation are obtained at the expense of additional complexity in the baseband processing circuitry.

0107Partial response encoding methods are line coding techniques which allow a potential doubling of the data rate over NRZ encoding using the same baseband bandwidth. Examples of PR encoding methods include duobinary and modified duobinary encoding. Bandwidth efficiency is improved by converting binary data into three level, or pseudo-ternary signals. Because the receiver decision circuitry must distinguish between three

0108E l RULE 26) instead of two levels, there is a signal to noise (range) penalty for using PR encoding. In an adaptive baud rate switching system, the effects of this degradation are eliminated by appropriate selection of the baud rate switching threshold.

0109Since PR encoding offers a doubling of the data rate of NRZ encoded data in the same bandwidth, one possible implementation of a NRZ/PR baud rate switching system would be a 4800/9600 bit/sec system in which the low-pass filter bandwidth is not switched. This might be desirable for example if complex low-pass filters constructed of discrete components had to be used. Use of a single filter could reduce circuit costs and printed circuit board area requirements. This approach might also be desirable if the channel bandwidth were reduced below what is currently available.

0110The preferred implementation with the bandwidth available is to use PR encoding to increase the high data rate well beyond the 9600 bit/sec implementation previously described. An approach using 4800 bit/sec NRZ encoded data for the low rate thereby providing high reliability and backward compatibility with existing products, and 16K bit/sec PR encoded transmission for the high rate may be utilized. The PR encoding technique is a hybrid form similar to duobinary and several of its variants which has been devised to aid decoding, minimize the increase in hardware complexity, and provide similar performance characteristics to that of the previously described 4800/9600 bit/sec implementation. While PR encoding could potentially provide a high data rate of up to 2OK bit/sec in the available channel bandwidth, 16K bit/sec is preferable because of the practical constraints imposed by oscillator temperature stability and the distortion characteristics of IF bandpass filters.

0111Exchanging Parameters.

0112All of the above referenced parameters must be maintained in local memory at both the transmitter and the receiver so that successful communication can occur. To change the communication environment by changing an operating parameter requires both synchronization between the transceivers and a method for recovering in case synchronization fails. In a preferred embodiment, if a transceiver receiving a transmission (hereinafter referred to as the "destination") determines that an operating parameter needs to be changed, it must transmit a request for change to the transceiver sending the transmission (hereinafter the "source") . If received, the source may send an first acknowledge to the destination based on the current operating parameter. Thereafter, the source modifies its currently stored operating parameter, stores the modification, and awaits a transmission from the destination based on the newly stored operating parameter. The source may also send a "no acknowledge" message, rejecting the requested modification.

0113If the first acknowledge message is received, the destination modifies its currently stored operating parameter, stores the modification, sends a verification message based on the newly stored operating parameter, and awaits a second acknowledge message from the source. If the destination does not receive the first acknowledge, the destination sends the request again. If after several attempts the first acknowledge is not received, the destination modifies the currently stored parameter, stores the modification as the new operating parameter, and, based on the new parameter, transmits a request for acknowledge. If the source has already made the operating parameter modification (i.e., the destination did not properly receive the first acknowledge message) , the destination receives the request based on the new parameters and responds with a second acknowledge. After the second acknowledge is received, communication between the source and destination based on the newly stored operating parameter begins.

0114ΛI • ^^.-^^j-T-i ιτr- O' <F-!"" /___>!__ r C\ If the destination does not receive either the first or the second acknowledge messages from the source after repeated requests, the destination replaces the current operating parameter with a factory preset system-default (which is also loaded upon power-up) . Thereafter, using the system-default, the destination transmits repeated requests for acknowledge until receiving a response from the source. The syste - default parameters preferably define the most robust configuration for communication.

0115If after a time-out period the second request for acknowledge based on the newly stored operating parameters is not received, the source restores the previously modified operating parameters and listens for a request for acknowledge. If after a further time-out period a request for acknowledge is not received, the source replaces the current operating parameter with the factory preset system-default (which is the same as that stored in the destination, and which is also loaded upon power-up) . Thereafter, using the common system-default, the source listens for an acknowledge request from the destination. Once received, communication is re¬ established.

0116Other synchronization and recovery methods are also contemplated. For example, instead of acknowledge requests originating solely from the destination, the source might also participate in such requests. Similarly, although polling is the preferred protocol for carrying out the communication exchanges described above, carrier- sense multiple-access (CSMA) or busy tone protocols might also be used.

0117As is evident from the description that is provided above, the implementation of the present invention can vary greatly depending upon the desired goal of the user. However, the scope of the present invention is intended to cover all variations and substitutions which are and which may become apparent from the illustrative embodiment of the present invention that is provided above, and the scope of the invention should be extended to the claimed invention and its equivalents. It is to be understood that many variations and modifications may be effected without departing from the scope of the present disclosure.