Abstract:
A data terminal apparatus comprising a wireless radio and a communications bridge is provided. The data terminal apparatus provides a link between data from a user equipment and a GSM network, connecting user equipment designed for circuit switched call link communications to a destination equipment via short message service or general packet radio service communications. According to an aspect of the invention, the data terminal apparatus simulates a circuit switched call link to the user equipment, thereby bridging legacy serial data communications systems with convenient wireless network technology. The result is a highly portable and easily integrated data terminal appartus for a wide variety of user equipment. In one embodiment, the invention is particularly advantageous in utility service monitoring applications.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. Nos. 09/400,623, entitled, “COMMUNICATIONS BRIDGE FOR CIRCUIT SWITCHED DATA TRANSFER SIMULATION”, and Ser. No. 09/400,624, entitled, “DATA COLLECTION SYSTEM”, both filed on the same day herewith and which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This application relates to the field of wireless communications equipment, and more particularly, data terminal apparatuses for bridging circuit switched and packet switched networks. 
     BACKGROUND 
     Monitoring the residential, commercial, and industrial complexes of buildings throughout the United States are a variety of user equipment. Examples of such user equipment include meter reading devices which measure consumption of various utility commodities such as natural gas, electricity and water via an electrical or electro-mechanical transducer. The meter reading devices are typically analog devices that record either a first reading and a second reading of the measured commodity over a period of time, or, alternatively, a cycling total (that is, a running total that recycles after a certain number is reached). 
     Generally, service personnel for the utility provider physically appear at or near the meter reading device to record consumption of the commodity each month. The recorded consumption from the meter reading device is then fed into a database used for billing purposes which in turn generates an invoice for the consumer based on user&#39;s consumption of the measured commodity. 
     In urban areas, the number of meter reading devices that need to be recorded is tremendous. Although the overhead associated with sending service personal to a desired location can be amortized by consolidation of meter reading devices at a particular location, for example, in a high-density residential development such as an apartment complex, the cost can still be significant. In rural areas, however, the cost is higher as meter reading cannot generally be amortized over a number of meter reading devices read at a single location. 
     Various techniques are employed by utility companies to reduce the cost of sending service personal to a physical site. 
     For example, a simple method is the use of stochastic techniques for extrapolating a measured quantity for a current reading from one or more past values or a moving or seasonal average. This technique is designed to reduce the frequency of meter reading. A disadvantage, however, is the fact that the extrapolated reading can be greatly under or over the actual consumption, such as the case where a consumer is simply not present and no services are used, or when an unusual weather pattern occurs and consumption is significantly increased. 
     Another technique is the use of radio-based meter reading devices. For example, each meter reading device includes a radio, the radio capable of broadcasting a meter reading to a nearby receiver. In the Middle East for example, such a system is often employed because service personnel are frequently denied access to a property when the property owner (a man) is not home. The radio based meter reading devices allow service personnel to drive near the radio meter reading device with a receiver device to read the meter. With such technology service personnel do not need to enter the property. An advantage of such a system is that, in rural areas, the time it takes service personnel to read the meters can be reduced. 
     For example, one system might require service personnel to physically drive by or near a collection of meter reading devices in order to communicate with the devices. The data collected in the “drive-by” would be later uploaded to a centralized data collection system. 
     Another solution might include periodic stations that collect wireless data from the devices. The periodic stations, in turn could include a land-line modem that communicates with the centralized data collection system by way of circuit switched calls. Such a solution offers an alternative to deploying service personnel, however, setting up phone lines to service the periodic stations can also be expensive. Moreover, circuit switched calls can also be expensive. 
     The problems mentioned above are exacerbated by deregulation of the utilities industry in the United States. It is now possible for several different suppliers of electricity to service a single metropolitan area. This, in turn, results in a non-contiguous patchwork of service areas that service personnel may have to monitor. No longer can it be assumed that all users in a particular geographic area receive electrical power from a single service provider. Indeed, on a single residential block every household may have a different service provider. Moreover, the alleged ease with which a consumer may switch service providers further complicates the circumstance. Accordingly, the ability to amortize the costs of collecting usage measurements is reduced. 
     SUMMARY OF THE INVENTION 
     A data terminal apparatus comprising a wireless radio and a communications bridge is provided. The data terminal apparatus provides a link between data from a user equipment and a GSM network, connecting user equipment designed for circuit switched call link communications to a destination equipment via short message service or general packet radio service communications. According to an aspect of the invention, the data terminal apparatus simulates a circuit switched call link to the user equipment, thereby bridging legacy serial data communications systems with convenient wireless network technology. In one embodiment, the invention is particularly advantageous in utility service monitoring applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a physical packaging of an embodiment of the present inventions; 
     FIG. 2 depicts physical packaging of an alternative embodiment of the present inventions; 
     FIG. 3 is a block diagram of a preprocessor unit and interface architecture; 
     FIG. 4 is a hardware schematic of the preprocessor unit; 
     FIG. 5 is a memory map of a presently preferred embodiment of the present inventions; 
     FIGS. 6A-C are block diagrams of embodiments of the present inventions including a user equipment; 
     FIGS. 7A-B depicts state diagrams for two of the interrupt service routines of the present inventions; 
     FIG. 8 is a flowchart depicting a main loop for a preprocessor driver; 
     FIGS. 9-12 are flowcharts depicting interrupt service routines for the preprocessor driver; 
     FIG. 13A is a block diagram of a system employing the present inventions; 
     FIG. 13B is a flow diagram showing a simulated circuit switched call set-up; 
     FIG. 13C is a flow diagram showing a simulated circuit switched call tear down; 
     FIG. 14 is a block diagram of an network monitoring system employing the present inventions; and 
     FIGS. 15A-C depict various protocol stacks used in embodiments of the present inventions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method and apparatus for circuit switched data transfer simulation is provided. According to an aspect of the inventions, a communications bridge (or interface) is provided that deceives a data collection device into believing that circuit switched communications are being performed. In a presently preferred embodiment, communications are actually performed by way of GSM short messaging services (hereinafter “SMS”). 
     According to one embodiment of the present inventions, the communications bridge is implemented by way of specially configured electrical hardware and software. However, according to another embodiment, the communications bridge is implemented by way of functionality added to an application layer of a GSM protocol stack on existing GSM modem hardware. Both embodiments are described herein. 
     Operational Overview 
     FIG. 13A is a block diagram of one embodiment of a system  1300  employing the present inventions. User equipment  1304  is a data collection device, such a meter reader collection station that receives data from a number utility meters. User equipment  1304  is communicatively coupled (e.g., by a serial data interface  1306 ) to a data terminal apparatus  1308 , which functions as a communications bridge. Data terminal apparatus  1308  is configured to communicate with a GSM network  1320  over a wireless interface (or “over-the-air” interface)  1324 , preferably by way of a standard GSM modem which is a component of the data terminal apparatus  1308 . For now, the left side of the GSM network  1320  will be called, for convenience, the originator equipment. The commands described below are preferably implemented from a modified Hayes AT command set. 
     To the right of GSM network  1320 , and also linked by a wireless interface  1328 , is a similar data terminal apparatus  1312  which is also communicatively coupled (e.g., by a serial data interface  1314 ) to user equipment  1316 . User equipment  1316  can include a second data collection station or other device for analyzing or relaying communications from the originator equipment. For convenience, the right side of the GSM network  1320  will be referred to as receiver equipment. Note that receiver equipment does not have to mirror the originator equipment, for example, the receiver equipment does not have to be coupled to the GSM network  1320  by way of a wireless interface  1328  and can instead be coupled by way of physical network connections. 
     FIGS. 13B-C depict a call setup and call tear down protocol for circuit switched call simulation. According to one embodiment, neither the originator user equipment  1304  nor the receiver user equipment  1316  will be aware that a non-circuit switched call was made. The data terminal apparatuses  1308  and  1312  simulate circuit switched call response to the user equipment and thus make the fact that a non-circuit switched exchange was performed transparent. 
     Turning first to FIG. 13B, it is aligned with FIG.  13 A and depicts a flow diagram for a simulated circuit switched call setup. Starting from user equipment  1304 , an ATD command  1332  is issued and serially passed to the data terminal apparatus  1308  over the serial data interface  1306 . The communications bridge handles the incoming ATD command  1332  and sends an SMS establish link message  1336  to the wireless radio. According to one embodiment, the data terminal apparatus and user equipment negotiate flow control so as to prevent input buffer overflows from data being transferred from the user equipment  1304  to the data terminal apparatus  1308 . 
     The wireless radio transmits the SMS establish link message  1336  over the wireless interface  1324  to the GSM network  1320 . The GSM network  1320  routes the SMS establish link message  1336  to the receiver wireless local loop  1328 . At the receiver wireless local loop  1328 , the SMS establish link message  1336  is then routed to data terminal apparatus  1312 , which receives the message at its wireless radio and then handles the message with its communications bridge. The communications bridge examines the message and notifies the user equipment  1316  of an incoming call with a ring indicator  1340 . 
     The data terminal apparatus  1308  communications bridge preferably keeps the phone number active for five minutes. This is to accord sufficient time to receive an acknowledgment of the SMS establish link message  1336  from the user equipment  1316 . 
     According to one embodiment, the AT command “ATA” (shown as ATA command  1344 ) is passed from the user equipment  1316  to the data terminal apparatus  1312 . The data terminal apparatus  1312  then sends an SMS link established message  1348  to the data terminal apparatus  1308 . Upon receipt of the SMS link establish message  1348 , the data terminal apparatus  1308  communications bridge sends a connect message  1352  to the user equipment  1304 . 
     Note that if more than one SMS establish link message  1336  is received from the GSM network  1320  by the data terminal apparatus  1312  before an ATA command  1344  is received from the user equipment  1316 , then the data terminal apparatus  1312  communications bridge responds to the most recent SMS establish link message  1336 . 
     Once the SMS link is established, data can be transferred between the user equipment  1304  and the user equipment  1316  over via short messaging services routed over the wireless local loop(s) by the data terminal apparatuses  1308  and  1312 . 
     With a virtual link between the originator equipment and the receiver equipment, data can be passed as if a regular circuit switched call is being performed. The operation is transparent to the user equipment, as the data terminal apparatus communications bridge handles all data packetization, handshaking, sequencing and error correction required by the particular application in which the equipment is employed. 
     Now turning to FIG. 13C, it is also aligned with FIG.  13 A. FIG. 13C depicts a call link tear down  1350  flow diagram for a simulated circuit switched call. According to one embodiment, the communications bridge in the data terminal apparatus  1308  waits approximately ten minutes for data or commands from the user equipment  1316 . If no data or commands are received in such time frame, then the call is considered “dropped”. However, receiving an escape sequence also causes the call to be dropped. 
     First, the escape sequence  1356  is received by the data terminal apparatus  1308 . An SMS disconnect link message  1360  is then transmitted over the wireless interface  1324  by the wireless radio in data terminal apparatus  1308 . 
     The GSM network  1320  receives the transmitted SMS disconnect link message  1360  and routes it over wireless interface  1328  to data terminal apparatus  1312 . Data terminal apparatus  1312  receives the SMS disconnect link message  1360  and it is processed by the communications bridge. The communications bridge, in turn, sends a disconnect indicator  1364  to the user equipment  1316  and then a link disconnected message  1368  back to data terminal apparatus  1308 . When the link disconnect message  1368  is received by the data terminal apparatus  1308 , the wireless radio drops the link. 
     Although described above with reference to an SMS embodiment, according to another embodiment, the communications bridge simulates circuit switched calls by way of general packet radio services (“GPRS”). The call setup and tear down are substantially similar to the methods described above (and below), however, rather than supplying a phone number after AT command “ATD”, an internet protocol address is supplied (e.g., “ATD114.32.0.108”). Once a connection is established, data packets passed over GPRS are formatted similar to the data packets over SMS. 
     Another difference between the SMS and GPRS approaches is that rather than having a 140 byte packet length and a baud rate of less than 300 baud (SMS), GPRS packets can have a 1500 Kbytes packet length, moreover, a much higher over-the-air rate, 170 Kbaud, is also possible. The addition of larger data packets and increased bandwidth allows easier integration of additional functionality into the communications bridge, such as relaxed flow control, handshaking for improved quality of service, multi-bit encryption, and other error recovery techniques (e.g., parity checks, CRC, etc.). While such features are possible in the SMS embodiment, the small packet size may require packet sequencing number and other header information to be sent with each packet, which would further slow communications. 
     One example of a system implementing the general architecture described above is a home automation application running on a personal computer (user equipment  1304 ), that interfaces data terminal apparatus  1308  via interface  1306 . At the opposite end of the home automation application resides a home network control center embodied in user equipment  1316 . The home automation application provides monitoring and control services to the home network control system, whereas the home network control system controls, for example, heating, ventilation, air conditioning, and security for a user&#39;s home. Commercially available home network control systems include Echelon Corporation&#39;s LONWORKS™ technology. 
     Another example of a system implementing the general architecture depicted in FIG. 13A is an automatic meter reading system. In such a system, user equipment  1304  is an automatic meter reader collection station that receives measurements of consumption of a metered commodity, such as electrical power or natural gas, for one or more automatic meter readers. Data measured by the automatic meter readers is sent to the collection station where it is in turn fed to the data terminal apparatus  1308 . User equipment  1316  can be a utility device that either actively polls the collection station via data terminal apparatus  1312 , or passively receives measured data from the data collection station via data terminal apparatus  1312 . 
     Physical Packaging 
     FIG. 1 depicts a perspective view of an embodiment of the physical packaging of a data terminal module (“DTM”)  100 . The data terminal module  100  includes an enclosure  104  that surrounds a data terminal sub-assembly (“DTSA”). The data terminal sub-assembly (not shown) is a circuit card that is configured to receive a preprocessor and wireless radio, which are described in detail below. 
     A coaxial cable connector  108 , a standard DB-9 connector  112 , a power connector  116  and a power indicator  120  are shown on the on the enclosure  104 . Each is connected, internally, to the data terminal sub-assembly. The coaxial cable connector  108  is configured to receive an antenna for the wireless radio. Two mounting sleeves  106  are notched into the enclosure  104 . The mounting sleeves  106  provide a path for connectors that are used to secure the data terminal module  100  to a desired location. 
     FIG. 2 depicts a perspective view of an embodiment of the physical packaging of a data terminal unit (“DTU”)  200 . The data terminal unit  200  includes a two-part enclosure. Case  204  carries electronics modules, such as data terminal sub-assembly  228  and power supply  232 . A heat sink  224  is placed at each inside corner of case  204  and is used to dissipate heat generated by the electronics modules. The outside surface of case  204  comprises a coaxial cable receptacle  240 , and an A/C power cord  244 . Also shown on the outside surface of case  204  are two hinged latches  216 . 
     The second part of the enclosure for data terminal unit  200  is a cover  208 . Cover  208  is connected to case  204  via hinges  248 , and is configured to sealably enclose the electronics modules carried in the case  204 . Latch connectors  212  receive hinged latches  216  to assist in this end. The cover  208  and the case  204  also include a number of connector receptacles  220  for additional protection. A patch antenna  236  is mounted to the cover  208 . The patch antenna  236  is coupled to the wireless radio contained in the data terminal sub-assembly  228 . 
     Details of a presently preferred patch antenna  236  and embodiments of an enclosure are described in U.S. patent application Ser. Nos. 09/316,457, entitled “CAPACITIVE SIGNAL COUPLING DEVICE”, and 09/316,459, entitled “RADIATING ENCLOSURE”, both filed May 21, 1999, which are incorporated herein by reference in their entirety. 
     According to an alternative embodiment, special electrical hardware is not employed in either the data terminal module  100  or data terminal unit  200 . In such an embodiment, application software is added to a standard GSM modem software stack. Accordingly, the data terminal apparatuses can be a specially configured GSM modem. 
     Preprocessor Embodiment 
     FIG. 3 is a block diagram of the preprocessor architecture. Preprocessor  300  is coupled to a first interface  328 , a second interface  336 , and a third interface  332 . Preferably, each of the interfaces includes an RS-232 port having a DB-9 or equivalent physical connector. COMA  304 , COMR  308  and DEBUG  312 , for example, can be implemented with such connectors. Communication lines  316 ,  320 , and  324  communicatively couple the physical connectors to the preprocessor  300 . 
     According to one embodiment, the first interface  328  connects to user equipment (e.g., telemetry equipment, automatic meter reading equipment, meter reader concentration point, utility meter control system, substation monitoring equipment, etc.). The user equipment is configured to collect measured data that monitors external activity. The second interface  336  is a physical connection to a wireless radio, more specifically a GSM modem having a baud rate of approximately 9600 bps or higher. The third interface  332  is preferably an open serial interface capable of receiving a terminal or test equipment for debugging and configuration purposes. According to one embodiment, the debug port services can be physically accessed through the first interface  328 . 
     Each RS-232 connector  304 ,  308  and  312  is shown coupled to the preprocessor  300  by unique communication lines  316 ,  320  and  324 . This is for simplicity and to represent a unique address for each communication port or serial interface. In fact, a single address and data bus can support the communication ports. 
     FIG. 4 is a hardware schematic a presently preferred embodiment of the preprocessor  300 . The preprocessor  300  comprises a microcontroller  404 , preferably Dallas Semiconductor part no. DS80C323 (16 MHz), a universal asynchronous receiver transmitter (“UART”)  416 , preferably an Exar Corporation part no. ST16C2450 (8 MHz), a non-volatile memory  424 , preferably Advanced Micro Devices part no. 29LV001B-70JC, and a volatile memory  428 , preferably IDT part no. 71V256SA-12PZ. Additional control logic  420  is desired, such as gate arrays and TTL logic, for maintaining timing (e.g., a clock divider for the UART  416 ), buffering, and logic levels. Power circuitry  412  provides power to the preprocessor  300  and any peripheral device (e.g., a wireless radio), and a crystal oscillator  408  (16 MHz) provides a clock signal. A main bus  432  communicatively couples the microcontroller  404 , with memories  424  and  428 , as well as control logic  420 . The main bus  432  includes both data, address and control lines, such as the same control lines  436  interconnecting the clock  408 , the UART  416  and the microcontroller  404 . 
     Additional lines  440 ,  444  and  448  are shown connected to UART  416 . These lines are for the first interface  328 , second interface  336 , and third interface  332 . Interrupts are received by the UART  416 , over lines  440 ,  444 , and  448 , which trigger exception/interrupt algorithms in the microcontroller  404 . A portion of the volatile memory  428  is used as a 1024 byte memory buffer for each input queue in UART  416  (thus, if two interfaces are used, 2048 bytes of memory are used). 
     FIG. 5 shows a memory map for 65 kilobytes of address space. The lower 49 kB address space  504  is for the non-volatile memory  424 , the next 12 kB of address space  508  is for volatile memory  428 , followed by 8 bytes of address space  512  for the first interface  328 , followed by 2 kB of address space  516  of reserved memory, 8 bytes of address space  520  for the second interface  336 , and another 2 kB of address space  524  for reserved memory. 
     FIGS. 6A-C depict various physical embodiments of the present inventions interfaced with user equipment (e.g., data collection unit  608 ). FIG. 6A shows a single user equipment  600  comprising a data collection unit  608  (e.g., an automatic meter reader), a preprocessor  604  and a wireless radio  612 . The preprocessor  604  is coupled to the data collection unit  608  via a first interface  616 , and to the wireless radio  612  via a second interface  620 . 
     FIG. 6B shows a user equipment  624  including the preprocessor  604 . Here, wireless radio  612  is in a separate physical packaging  628 . Here, the second interface comprises I/O interfaces  652  and  656  (e.g., RS-232 ports). The I/O interfaces  652  and  656  are connected via a serial cable  644 . I/O interface  656  is coupled to preprocessor  604  via connector  640 , and I/O interface  652  is coupled to wireless radio  612  via connector  648 . 
     FIG. 6C shows a user equipment  632 , which is similar to the system depicted in FIGS. 6A and 6B, however, the preprocessor  604  and wireless radio  612  are found in data terminal unit/module  636 . The same interface described above with reference to FIG.  6 B and the second interface  620  is shown in FIG.  6 C. However, in FIG. 6C the components found in the second interface  620  are found in the first interface  616  instead. FIG. 6C is most like the data terminal module  100  and data terminal unit  200  shown in FIGS. 1 and 2 respectively. 
     An operational overview of the techniques of the present invention are now presented. Generally speaking, the preprocessor  604  is a hardware component that includes a software driver. As described above, the preprocessor  604  can have a dedicated piece of hardware that executes the software driver, however, it is also possible for the software driver to be overlaid into an existing piece of hardware as an additional component of the software stack. For example, the preprocessor driver can be added to the user equipment software stack or to the wireless radio software stack. The preprocessor driver, as it is referred to herein, is generally an interrupt driven service routine that first identifies the source of an interrupt and second determines what process or interrupt service routine to execute based on any of the data accompanying the interrupt (or the interrupt itself). 
     It should be noted that the incoming data is preferably serial ASCII character data. Commands are preferably based on the well-known Hayes modem AT Command set, although additional special codes can be added to identify particular functionality described herein. Some of these codes are described below with references to Table 2. 
     According to one embodiment, the primary components of the preprocessor driver are saved in non-volatile memory  424  (e.g., section  504  of memory map  500 ) and are executed by microcontroller  404  as a sequence of instructions stored in a computer-readable format. For example, the sequences of instructions (e.g., op codes) are loaded into data and control registers within the microcontroller  404  from the non-volatile memory  424  (alternatively, the instructions can be copied from non-volatile memory  424  to a volatile execution memory before being executed). The sequences of instructions cause the microcontroller  404  in the preprocessor  300  to perform a series of acts based upon a combination of the sequences of instructions and the data received from the serial data interface (e.g., UART  416 ). Program variables needed by the preprocessor driver are either stored in available registers internal to the microcontroller  404 , or they are stored in volatile memory  428 . 
     Two state diagrams are described below with reference to FIGS. 7A-B. The descriptions are general and are further supported by the detailed flowcharts described below with reference to FIGS. 8-12. 
     A state diagram  700  for the first interface interrupt service routine is shown in FIG.  7 A. The default state is IDLE mode  704 . In the IDLE mode  704 , characters are received over the first interface  616  and tested for commands or events that trigger a state change, for example link commands  716  and  728 . If a command or event indicating a state change is not detected then the data characters are stored in a memory buffer until a state change does occur. 
     The circuit (“CKT”) mode  712  passes data from the first interface  616  directly to the second interface  620 , with little intervention from the preprocessor  604 , except to monitor for commands or events that may trigger another state change (e.g., an escape sequence). In the circuit mode  712 , an active virtual link is maintained between the wireless radio  612  and a public telephony switched network (“PSTN”) over a wireless local loop. The circuit mode  712  is maintained until an escape sequence  720  is detected. However, if a re-enter command  732  is detected following the escape sequence  720 , then the interrupt service routine will return to the circuit mode  712  and not to IDLE mode  704 . 
     The short message service (“SMS”) mode prepares and sends SMS messages comprising the data stored in the memory buffer from the first interface  616 , over the second interface  620 , and through the wireless radio  612 . From the wireless radio  612 , the SMS messages are carried over the GSM network and are then routed over other intervening networks to their ultimate destination. SMS messages generally have a 140 byte data structure. The first byte indicates an SMS message type, the second byte indicates the SMS message length, and the last  138  bytes comprise the SMS message body. The SMS message body comprises either character data, or commands, or both. The SMS messages types are described in Table 1. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 establish link 
                 requests that a virtual link 
               
               
                   
                   
                 be established for SMS data 
               
               
                   
                   
                 transfer 
               
               
                   
                 link established 
                 a reply to an establish link 
               
               
                   
                   
                 message indicating that the 
               
               
                   
                   
                 link is setup (the sending or 
               
               
                   
                   
                 receipt of this message 
               
               
                   
                   
                 causes the mode to change to 
               
               
                   
                   
                 SMS mode) 
               
               
                   
                 data link 
                 all data is transferred using 
               
               
                   
                   
                 this message type 
               
               
                   
                 disconnect link 
                 requests that a link be 
               
               
                   
                   
                 disconnected (the sending or 
               
               
                   
                   
                 receipt of this message 
               
               
                   
                   
                 causes the mode to change to 
               
               
                   
                   
                 IDLE mode) 
               
               
                   
                 link disconnected 
                 a reply to the disconnect 
               
               
                   
                   
                 link message 
               
               
                   
                   
               
             
          
         
       
     
     An escape sequence  724  causes the state to return from SMS mode  708  to IDLE mode  704 . 
     State diagram  750 , shown in FIG. 7B, depicts the states associated with the second interface interrupt service routine. The states described above with reference to the first interface interrupt service algorithm are substantially similar to those associated with the second interface interrupt service algorithm, the primary difference being that if an interrupt was received over the second interface, then it is not from the user equipment (e.g., data collection  608 ), but rather from remote equipment beyond the wireless radio  612 . 
     In IDLE mode  754 , data characters received at the second interface  620  (e.g., COMR  308 ) are passed straight through the first interface  616  (e.g., COMA  304 ). The data characters are, however, monitored for a link command  758  or  762 , which indicate a state change to SMS mode  766  or CKT mode  770 , respectively. Escape sequences  774  and  778  return the service routine to IDLE mode  754  from CKT mode  770  or SMS mode  766 . 
     FIG. 8 depicts the main loop  800  for the preprocessor driver. The main loop  800  begins by first initializing various operating parameters. For example, in act  804 , a watchdog timer, a second timer, the serial ports, and the UART  416  are initialized. 
     The watchdog timer is designed to reset the preprocessor  604  in the event that the software stalls or a processing error occurs. Preferably, the duration of the watchdog timer is set to 4.5 seconds. A second timer is used to extend the 4.5 second timeout for routines that take longer than the first watchdog timer. The second timer generates a 2 millisecond interrupt. 
     The serial ports (e.g., COMA  304  and COMR  308 ) are initialized to operated at 9600 baud, 8 data bits, 1 stop bit, and no parity, and the UART  416  is initially setup to run at 9600 baud. Memory buffer input queues have 1024 bytes each and store data characters received through COMR  308  and COMA  304 . 
     Next, in act  808 , the mode of the main loop is set to IDLE (e.g., for both the first interface  616  and the second interface  620 ). After act  808 , the interrupt service routine processing begins. 
     In act  812 , if an interrupt was received at the first interface  616 , then an exception occurs and processing continues to the first interface interrupt service routine in act  816 , which is described above with reference to FIG.  7 A and below with reference to FIG.  9 . 
     In act  820 , if an interrupt was received at the second interface  620 , then an exception occurs and processing continues to the second interface interrupt service routine in act  824 , which is described above with reference to FIG.  7 B and below with reference to FIG.  10 . 
     In act  828 , a test is performed to determine whether the elapsed time since the last data character was received over the first interface  616  (e.g., through COMA  304 ) when the first interface interrupt service routine is in SMS mode  708 . The test is referred to as the SMS timer expire event. If the SMS timer event has occurred, then in act  832  the memory buffer is prepared for SMS transmission, the SMS message is transmitted and the SMS timer is reset. Processing continues to act  836 . 
     In act  836 , if an interrupt was received at the third interface (e.g., debug interface  312  depicted in FIG. 3) then the debug interrupt service routine described below with reference to FIG. 11 is performed in act  840 . 
     In act  844 , a test is performed to determine whether values of a modem status register or a line status register (taken from wireless radio  612  by UART  416 ), have been updated. If either has been updated, then processing continues to act  848 , described below with reference to FIG.  12 . After, alternatively, acts  844  or  848 , the process continues to act  852 , at which point the loop is restarted at act  812 . 
     FIG. 9 is a flowchart depicting the acts performed by the preprocessor driver when servicing an interrupt over the first interface  616 . More specifically, the flowchart depicts the first interface interrupt service routine  900 , which services interrupts from the user equipment (e.g., data collection  608 ). 
     In act  904 , a test is performed to determine whether the present mode is CKT mode  712 . If the mode is CKT mode  712 , then the data received by the first interface  616  is passed through to the second interface  620  at act  908 . 
     In act  912 , a test is performed to determine whether an escape sequence was received. In particular, the test determines whether the character sequence “+++” was received through the first interface  616 . If the escape sequence was not received, then in act  916  the escape sequence counter is reset. However, if the escape sequence was received, then in act  920 , the mode is set to IDLE mode  704 . The preprocessor driver then returns to the main loop, namely act  820 . 
     In act  924 , a test is performed to determine whether the present mode is IDLE mode  704 . If the present mode is IDLE mode  704 , then the incoming data character from the first interface  616  is added to a memory buffer. Next, in act  932 , a test is performed to determine whether a process trigger (e.g., a carriage return, a CTRL-Z, or the memory buffer is full) has occurred. If a process trigger has occurred, then in act  936 , the memory buffer is parsed, interpreted and the appropriate acts performed. A list of exemplary interpreted strings and their results is shown in Table 2. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 AT˜EMU 
                 sets non-volatile memory to 
               
               
                   
                   
                 reflect the simulation mode -- 0 
               
               
                   
                   
                 indicates no simulation, 1 
               
               
                   
                   
                 indicates simulated circuit switch 
               
               
                   
                 ATDnnnnnnn 
                 sends an establish link SMS 
               
               
                   
                   
                 message to phone number nnnnnnn 
               
               
                   
                   
                 (when AT˜EMU set to 1) 
               
               
                   
                 ATH 
                 sends a disconnect link SMS 
               
               
                   
                   
                 message to the phone number 
               
               
                   
                   
                 established using the ATD command 
               
               
                   
                   
                 (when AT˜EMU set to 1) 
               
               
                   
                 ATS7? 
                 returns the value 30 to the first 
               
               
                   
                   
                 interface 616 
               
               
                   
                 AT0 
                 returns to CKT mode if the carrier 
               
               
                   
                   
                 detect signal on the wireless 
               
               
                   
                   
                 radio 612 is still asserted 
               
               
                   
                 AT + IPR = xxxx 
                 changes the baud rate on the 
               
               
                   
                   
                 wireless radio 612 and on both 
               
               
                   
                   
                 ports on the preprocessor 604 to 
               
               
                   
                   
                 xxxx (2400, 9600) 
               
               
                   
                 AT? 
                 displays this table to the debug 
               
               
                   
                   
                 port 
               
               
                   
                   
               
             
          
         
       
     
     It is noted that the AT-EMU command sets the simulation mode for the unit. This is important because the unit is capable of providing both true circuit switched calls or simulated circuit switched calls. Accordingly, if the mode is set to no emulation, then the ATD and ATH commands will operate as setup/control functions for a truly circuit switched call. However, when the mode is set to simulated circuit switched mode, the functions are unique—as described above. 
     After act  936 , the preprocessor driver returns to act  820 . 
     In act  940 , a test is performed to determine whether the present mode is SMS mode  708 . If the present mode is SMS mode  708 , then in act  944  the SMS timer is reset. In act  948 , another test is performed to determine whether there is sufficient room in the memory buffer to store additional data. If there is not sufficient room, then in act  952  an SMS message is sent thereby flushing a portion of the memory buffer. If, however, there is sufficient room in the memory buffer, then in act  956  any escape characters are handled, for example the occurrence of the string “+++” through COMA  304 , or a disconnect link SMS message received through COMR  308 . After handling the escape characters, the preprocessor driver returns to act  820 . 
     FIG. 10 is a flowchart depicting the acts performed by the preprocessor driver when servicing an interrupt over the second interface 620—i.e., the second interface interrupt service routine  1000 . 
     In act  1004 , a test is performed to determine whether the present mode is CKT mode  770 . If the present mode is CKT mode  770 , then any data characters received at the second interface  620  (e.g., COMR  308 ) are passed through the preprocessor  604  to the first interface  616  (e.g., to COMA  304 ) in act  1008 . Next, in act  1012  a test is performed to determine whether an escape sequence has been received. According to one embodiment, the escape sequence is the receipt of three consecutive plus signs, or the string “+++”. If the escape sequence is not found, then the escape counter is reset in act  1016 . However, if the escape sequence is found, then the mode is set to IDLE mode  754  in act  1020 . After acts  1016  or  1020 , the preprocessor driver continues to act  828 . 
     In act  1024 , a test is performed to determine whether the present mode is IDLE mode  754 . If the present mode is IDLE mode  754 , then data characters received at the second interface  620  are passed through to first interface  616  in act  1028 . In act  1032 , a test is performed to determine whether a link sequence is found in the data characters. According to one embodiment, the link sequence is the string “+CMTI:”, which indicates an incoming SMS message. If the link sequence is not detected, then the escape counter is reset at act  1036 . However, if a link sequence was detected, then the inbound SMS message is read in act  1040 . 
     Next, in act  1044 , a second test is performed to determine whether another link sequence is found in the SMS message (e.g., in the SMS message type field). For example, the next link sequence can be the “establish link” or “link established” messages described above with reference to Table 1. If no establish link or link established messages is found, then the remainder of the SMS message is output to the first interface  616  in act  1048 . However, if the establish link or link establish message is found, then the message is processed and the mode is set to SMS mode  766  in act  1052 . The preprocessor driver then continues to act  828 . 
     In act  1056 , a test is performed to determine whether the present mode is SMS mode  766 . If the present mode is SMS mode  766 , then a test is performed in act  1060  to determine whether a parse trigger has been received. According to one embodiment, parse triggers include a line feed or a carriage return, as well as a “buffer full” indicator. If no parse trigger has been received, then in act  1064  the data character is stored in the memory buffer and processing continues to act  828 . However, if a parse trigger has been received, then in act  1068 , the SMS message is parsed. 
     In act  1072 , a test is performed to determine whether the SMS message type is disconnect link. If the SMS message type is not disconnect link, then in act  1076 , the SMS message type is sent to the first interface  616 . However, if the SMS message is a disconnect link command, then the mode is set to IDLE mode  754  in act  1080 . After step  1080 , the preprocessor driver continues to act  828 . 
     FIG. 11 is a flowchart depicting the debug interrupt service routine  1100 . Under normal circumstances a third interface, shown in FIGS. 3 and 4, receives the debug interrupt. The debug interrupt is commonly associated with connecting a terminal device or laptop computer into the third interface  332 . The debug interrupt service routine  1100  is used for setup and diagnostic purposes. 
     In act  1104 , a data character received over the third interface  332  is added to the memory buffer. In act  1108 , the data character is tested to determine whether it is a carriage return. If the data character is not a carriage return, then the routine returns to act  844 . If the data character is a carriage return, then in act  1112  the memory buffer is parsed. In act  1116 , a test is performed to determine whether a command from the memory buffer is in a command list (e.g., Table 2). If the command is not in the command list, then in act  1120  an error message is reported over the third interface  332 . However, if the command is in the command list, then the command is executed in act  1124 . Thereafter, processing continues to act  844 . 
     FIG. 12 depicts a flowchart  1200  for handling the modem status register (“MSR”) and line status register (“LSR”) values gathered from the first interface  616  and second interface  620 . Essentially, a series of possible errors are analyzed and the appropriate action is taken, which includes, in some instances changing the preprocessor mode. 
     In act  1204 , a test is performed to determine whether the carrier detect (“CD”) is asserted on the second interface  620  (e.g., the wireless radio  612 ). If the carrier detect is asserted, then the carrier detect is asserted on the first interface  616  and the mode is set to the circuit mode in act  1208 . 
     In act  1212 , a test is performed to determine whether the carrier detect is dropped on the second interface  620 . If the carrier detect is dropped, then in act  1216  the carrier detect is then dropped on the first interface  616  and the mode is set to the idle mode. 
     In act  1220 , a test is performed to determine whether the ring indicator (“RI”) is asserted on the second interface  620 . If the ring indicator is asserted, then in act  1224  the ring indicator is asserted on the first interface  616 . 
     In act  1228 , a test is performed to determine whether the ring indicator is dropped on the second interface  620 . If the ring indicator is dropped, then in act  1232  the ring indicator is dropped on the first interface  616  as well. 
     In act  1236 , a test is performed to determine whether the clear-to-send (“CTS”) is asserted on the second interface  620 . If the clear-to-send is asserted, then in act  1240  the wireless radio  612  is initialized. After acts  1236  or  1240 , the preprocessor returns to act  852 . 
     The service routine embodied in flowchart  1200  is useful in that the service routine can change the mode of the preprocessor  604  at either the first interface  616  or the second interface  620 , in response to certain physical conditions, namely the assertion or dropping of one of the lines used to complete a virtual circuit. 
     Application Layer Embodiment 
     According to another embodiment, special purpose simulation hardware (e.g., preprocessor  300  shown in FIG. 3) is not integrated with an existing GSM modem. Rather, the functionality described above is implemented by way of software added to the application layer of a standard GSM protocol stack  1500  shown in FIG.  15 A. The same software can also be added to the general packet radio service (GPRS) protocol stacks  1580  (GPRS Class C) and  1590  (GPRS Class A), shown in FIGS. 15B and 15C, respectively. Like reference numerals in FIGS. 15A-C refer to like elements. 
     According to a presently preferred embodiment, the call setup and tear down functionality described above with reference to FIGS. 13A-C is performed by way of extensions to the application layer of the GSM protocol stack, for example, the GSM protocol stack already existing in a GSM modem. 
     In such an embodiment, computer-readable program code is compiled and loaded into a non-volatile storage medium. The code is later executed by one or more processors configured to handle the incoming AT commands from the user equipment or the wireless input in the form of SMS packets. The application layer extensions build a functional communications bridge for simulating circuit switched calls to the user equipment. An advantage of the application layer embodiment is that no special/single purpose hardware is required. Rather, functionality is added to an existing GSM modem by way of the software added to the GSM protocol stack. 
     FIG. 15A depicts a GSM protocol stack  1500 . The base components of GSM protocol stack  1500  are generally known in the art. For example, the base software components of GSM protocol stack  1500  are available from various venders such as debis Systemhaus in Berlin, Germany, CONDAT Datensystem Gmblt in Hannover, Germany, and other wireless communications vendors. According to one embodiment, the GSM protocol stack  1500  can be implemented in a variety of logic devices or in computer readable code executed by an embedded processor already part of the GSM modem. 
     The present inventions are preferably embodied in software code that comprises the AT command interface  1504 . The AT command interface is overlaid onto each of the various GSM protocol stacks  1500 ,  1580  and  1590 . Commands, as described above with reference to Table 2, are thus bridged between the user equipment  1304  and the GSM protocol stack—and, hence, the GSM network  1320  (FIG.  13 ). It is further noted that the AT command interface  1504  can also include event detection and notification software that detects alarms from the user equipment and handles them appropriately—for example by initializing a simulated circuit switched or circuit switched call. 
     Referring to FIG. 15A, the user equipment  1304  sends data over a GSM network  1320  using the Hayes standard AT command interface  1504 . The mobile network man machine interface (MN)  1512  receives the data and passes the data to the appropriate messaging service—e.g., a short message service (SMS)  1516 , a call control service (CC)  1520 , or a supplementary service (SS)  1524 . A registration element  1508  will provide the mobility management layer  1528  with necessary information about the data and the network. From each of layers  1508 ,  1516 ,  1520  and  1524  data flow is then directed to and from the mobility management layer (MM)  1528 . 
     The mobility management layer  1528  establishes, maintains, and releases connections between the user equipment  1304  and the GSM network  1320 . From the mobility management layer  1528 , data and control is passed to the radio resource management layer (RR)  1532 . The radio resource management layer  1532  establishes physical connections over the radio interface for call-related signaling and traffic channels between the user equipment  1304  and base station  1488  (FIG.  14 ). 
     Connected to the radio resource management layer  1532  is the physical layer (L 1 )  1540 . The physical layer  1540  processes call-related signaling and traffic channels directly from the radio resource layer  1532 , and also processes the data sent from the data link layer (L 2 )  1536 . 
     FIG. 15B is substantially similar to FIG. 15A, however, the “G” notation in the protocol stack layers indicates that the designated layers now refer to a general packet radio service (GPRS). GPRS uses a packet radio principle and can be used for carrying packet data protocol between the user equipment  1304  and the GSM network  1320 . GPRS provides additional services beyond what is offered with the standard GSM network, for example, GPRS can provide increased over-the-air data transfer rates and packet lengths. 
     An application program interface (API)  1544  is added to allow an application to control the subnetwork dependent convergence protocol (SNDCP)  1548 , which is responsible for segmentation and re-assembly of the data packets, encryption and decryption, and transmission control protocol (TCP) header and data compression. 
     Layers interfacing the AT command interface  1504  include the registration layer  1508  and man-machine interface layer  1512 , which in turn interface the SM layer  1552  and GSMS (GPRS short message service)  1556 . The SM layer  1552  and GSMS layer  1556  interface the GPRS mobility management (GMM) layer  1560 , and both the GMM and SNDCP layer  1548  interface the link layer control (LLC)  1564 , which handles the link layer information of the packet data. 
     Link layer control  1564  interfaces the GPRS resource management layer (GRR)  1568 . GPRS resource management layer  1568  in turn interfaces medium access control/radio link control (RLC/MAC) layer  1572 , which handles the physical link processing, as well as physical layer  1540 . 
     FIG. 15C shows the GPRS Class A protocol stack  1590 . The protocol stack  1590  is a merge of the GSM protocol stack  1500  (FIG. 15A) and the GPRS Class C protocol stack  1580  (FIG.  15 B), which is denoted by the dual reference numbers annotating the various layers of the protocol stack. The GPRS Class A protocol stack can operate standard GPRS and other GSM services simultaneously. 
     Data Collection System 
     FIG. 14 depicts an alternative system  1400  employing the present invention. User equipment  1404  comprises an application program, for example a telemetry, automatic meter reading, meter concentration point, utility meter control system, substation monitoring, home network control system, or other application. In particular, the present inventions can be used in conjunction with an event detection and notification application, such a fire alarm, gas alarm, burglar alarm, vending machine alarm, or another condition indicating a change of state of the user equipment  1404 , or some other device connected thereto. 
     An RS-232 interface  1448  with hardware flow control connects user equipment  1404  to data terminal module  100 , or alternatively data terminal unit  200 . Data terminal module  100  and data terminal unit  200  preferably comprise a debug port through which terminal equipment or a laptop computer  1412  can interface and perform installation or testing services with software tools  1484 . Optionally, the debug and configuration service can be accessed through the first interface  616 . 
     The data terminal module  100  and data terminal unit  200  communicate via a wireless radio to one or more antenna relays  1408 . Preferably the wireless radio is a GMS type modem. The wireless radio is configured to transmit and receive information between said data terminal module  100 , or data terminal unit  200  and GSM network  1488 . 
     At least one of the one or more antennas relays  1408  is connected to a base transceiver station (“BTS”)  1416 . The base transceiver station  1416  processes the inbound wireless data (e.g., forms data packets for the inbound wireless data) and routes it over a Ti line  1456  (or other leased line) to base station controller (“BSC”)  1420 . The base station controller  1420  authenticates service for the data terminal unit/module and directs the processed inbound wireless data over T 1  line  1456  to a mobile switching center (“MSC”)  1424 . The mobile switching center  1424  directs the processed inbound wireless data over a lease line  1456  to an appropriate networking station, for example an interworking function (“IWF”)  1432 , such as a PSTN bridge/router in the case of a circuit switched data path (identified by label “1”), or a short message service center (“SMSC”)  1428 , in the case of a short message service data path (identified by label “2”). 
     If the data path is a circuit switched data path, then from the interworking function  1432  the processed wireless data is passed over a public switched telephony network (“PSTN”)  1436  to circuit switched data interface  1436 . If, however, the data path is a short message service data path, then the short message service center  1428  can route the processed wireless data over a PSTN connection  1460  to interface  1436 , or over an packet switched network  1456  connected to an internet  1440 . In the case of routing over the internet  1440 , the short message service center  1428  handles all Internet Protocol packetization according to known Internet Protocol standards, such as publicly available Internet RFC  791 , which is incorporated herein by reference in its entirety. 
     A user application server  1444  retrieves the inbound wireless data from the PSTN interface  1436  via a modem connection  1468 . Alternatively, the user application server  1444  retrieves the inbound wireless data via an internet access/service provider  1476  connected to internet  1440 . 
     Optional terminal management software  1480  can be used by the user application server  1444  to provide outgoing data, command, or setup services from the user application server  1444  to the user equipment  1404  (though the data terminal module  100  or data terminal unit  200 ) in a reverse path as is described above. 
     The methods, techniques and apparatuses described herein are advantageous over prior data collection, monitoring and control equipment in that an application specific communication infrastructure is not needed. Rather, the inventions can be utilized with existing wireless communication networks, and especially with GSM networks supporting short message services. Moreover, the present inventions provide a system that minimizes service personnel physical intervention in the acquisition of measured data from user equipment. In the systems described herein, measured data can be collected remotely via user application server direct polling or by present or programmed intervals within the preprocessor unit. This represents a significant improvement over historical extrapolation and stochastic methods of measured data collection. 
     The inventions are described herein by way of example and not by way of limitation. The written description and drawings are illustrative of preferred embodiments but not the only embodiments of the present invention. 
     Accordingly, further embodiments of the invention will be apparent upon inspection of this specification by one of skill in the art. For example, the use of security or special identifiers to designate particular user equipment  1404 , data terminal modules  100  or units  200 , and terminal equipment  1412  can be employed to protect the system  1400  from unauthorized access. Additionally, other communications interfaces other than RS-232 can be employed in the present invention, for example, RS-485 and CEBus. Furthermore, the methods and techniques described above can be embodied in a distributed software environment wherein certain steps are performed by particular devices, or processing moved from the preprocessor to the user equipment or the wireless radio, or a combination of both.