Patent Publication Number: US-8995949-B2

Title: Proximity based selection of an implantable medical device for far field communication

Description:
TECHNICAL FIELD 
     Embodiments relate to far field communications between external devices and implantable medical devices. More particularly, embodiments relate to the selection of an implantable medical device for far field communication with an external device on the basis of proximity. 
     BACKGROUND 
     Conventionally, external devices such as clinician and patient programmers communicate with an implantable medical device (IMD) through a near field form of communication such as an inductive coupling. Due to the short range of the inductive coupling, a telemetry head is placed in close proximity to the IMD to establish the inductive link. Because the telemetry head has a very short range and consequently is in such close proximity to the IMD, there is essentially no risk of inadvertently communicating with a different nearby IMD. 
     Far field communication has become an alternative to the use of near field links between external devices and implantable medical devices. Far field communication uses frequencies that allow for electromagnetic signal propagation over significantly larger distances than the maximum distance of near field links. This increased range of signals allows an external device to communicate with the IMD without placing a telemetry head in close proximity to the IMD. However, the increased range of the far field communication creates issues that are not a concern for near field links. 
     In particular, far field communication by the external device creates the possibility that other IMDs besides the intended IMD are in communication range of the external device. Therefore, the external device must either have advance knowledge of an identifier of the desired IMD or the external device must receive a user selection from a list of IMDs that respond to a discovery signal by the external device. Requiring the user to select the proper IMD from a list adds extra time and burden to the process and also presents an opportunity for human error where the user may select the wrong device and/or application for an intended device. 
     SUMMARY 
     Embodiments address issues such as these and others by providing devices, systems, and methods that utilize physical proximity of an external component relative to an intended IMD to allow the external device to select the intended IMD for a far field communication session and ultimately eliminate pairing between external devices and unintended IMDs within range of far field communications. One or more various forms of proximity communication occur between the external device and the IMD during the establishment of the far field communication session. The various forms of proximity communication occur within a short distance from the IMD so that the proximity communication intentionally does not extend to any unintended IMDs that may be nearby. In some cases, the physical proximity may be removed once the far field communication has been appropriately confirmed through the proximity communication. The pairing of the intended IMD with the external device may in some cases then allow for additional benefits such as the ability of the external device to automatically run the correct application for the intended IMD. 
     A form of proximity communication may share unique information between the external device and the IMD that can then be verified using a far field communication. This allows the external device to confirm that the IMD is the intended one while unintended IMDs do not have access to the unique information and cannot verify the unique information through far field communication with the external device which allows the external device to filter out discovery responses from the unintended IMDs. A form of proximity communication may trigger the IMD to respond to a far field discovery request by the external device while unintended IMDs that do not receive a proximity communication at that time would not respond to the far field discovery request. A form of proximity communication may be used by one device to satisfy a challenge issued over a far field communication by the other device to confirm that the far field communication is between the devices that are in physical proximity to one another. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a typical operating environment for embodiments where an external device and an IMD utilize proximity communication to establish a far field communication session. 
         FIG. 2  shows components for one example of an external device embodiment. 
         FIG. 3  shows components for one example of an IMD embodiment. 
         FIG. 4  shows a first example of a procedure to establish a far field communication session where a unique value and/or key are shared via a proximity communication. 
         FIG. 5  shows a second example of a procedure to establish a far field communication session where a challenge is issued by the IMD during a discovery phase. 
         FIG. 6  shows a third example of a procedure to establish a far field communication session where the external device requests a challenge upon the far field communication session being established. 
         FIG. 7  shows a fourth example of a procedure to establish a far field communication session where the IMD issues a challenge upon the far field communication session being established. 
         FIG. 8  shows a fifth example of a procedure to establish a far field communication session where an encryption key is exchanged through a proximity communication. 
         FIG. 9  shows a sixth example of a procedure to establish a far field communication session where the external device considers far field signal strength when selecting the IMD for the far field communication session. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments provide for devices, systems, and methods that allow an external device to select an IMD for far field communication by using a proximity communication that is limited to the IMD of interest. In doing so, the external can eliminate those IMDs that are in range of far field communications but are not privy to the proximity communication. 
       FIG. 1  shows an environment that includes an external device  102 , such as a clinician programmer, patient programmer, or a remote/home monitoring device that is nearby a patient  108  who has an IMD  104 . The IMD  104  may be implanted within or mounted externally to the body  108  and may perform one or more medical tasks such as cardiac or neurological stimulation, physiological sensing, drug infusion, and the like. The IMD  104  may include components  106  such as stimulation or sensing leads or drug delivery catheters that extend from the IMD  104  and terminate at the target area of the body  108 . 
     The external device  102  ultimately communicates with the IMD  108  through a far field communication session utilizing far field signals  114  sent by the external device  102  and far field signals  116  sent by the IMD  108 . These far field signals  114 ,  116  may be radio frequency (RF) signals such as those of the Medical Implant Communications Service (MICS) band, the Industrial, Scientific, and Medical (ISM) band, or the short range device (SRD) band. The far field communication session may be used to program a medical therapy to the IMD, to obtain information from the IMD regarding therapy and patient information, and the like. 
     While the single IMD  104  is shown in  FIG. 1 , it will be appreciated that there may be other IMDs and/or other external devices nearby and in range of the far field signals  114  of the external device  102 . The external device  102  may not be aware of identification information of the intended IMD  104  in advance such that the external device  102  cannot immediately discern far field communications of the intended IMD  104  relative to far field communications of other IMDs. However, physical proximity can be established to allow proximity communication to occur between the external device  102  and the intended IMD  104 . Therefore, a procedure is provided that utilizes this physical proximity at the initiation of the far field communication session to avoid the external device  102  conducting a far field communication session with an unintended nearby IMD. To allow the external device  102  to select the intended IMD  104  for far field communication and avoid selecting an unintended nearby IMD, proximity communication signals  112  may be exchanged between a proximity communicator  110  and the IMD  104  during the establishment of the far field communication session. 
     The proximity communicator  110  may be of various forms and may be a separate component of the external device  102  or be integrated with the external device  102 , or a combination of both. For instance, the proximity communicator  110  may be a near field telemetry head that is tethered to the external device  102  by a communication path  118  such as a cable or wireless connection and that establishes an inductive link with the IMD  104 . As another example, the proximity communicator  110  may be an audible tone generator where the IMD  104  receives and recognizes different audible tones. As another example, the proximity communicator  110  may be a body thump device, such as a chest thump device, where the IMD  104  detects the thump through an on-board accelerometer or other vibration detector. As yet another example, the proximity communicator  110  may be a static field generating device such as an electromagnet or a permanent magnet being moved into and out of proximity with the IMD  104  by the clinician. 
     In some cases including the near field telemetry head, the audible signal generator, the body thump device, and the electromagnet, the proximity communicator  110  may be under control of the external device  102  through a tethered or wireless connection between the telemetry head  110  and the external device  102 . In some cases including the clinician providing the body thump or moving the permanent magnet, the proximity communicator  110  is under direct control of the clinician who may be following commands being issued by the external device  102  to provide or remove the proximity communication. 
     The proximity communication may range from being a simple present or absent signal to a more complex signal carrying data. Furthermore, the proximity communication may be a unidirectional communication mode in some embodiments, particularly where the communication is simple. This may reduce the cost and complexity of a device, particularly the IMD  104 . The proximity communication may be a bi-directional communication mode in other embodiments, such as where one device may send data through a proximity communication while the other device may send an acknowledgement through a subsequent proximity communication. This may improve the efficiency of the proximity communication procedure. 
       FIG. 2  shows components of one example of the external device  102 . The external device  102  includes a processor  202 , a memory  204 , and a storage device  206 . The external device  102  may also include local input/output (I/O) ports  208  such as to provide local screen displays and to receive user input via keyboard, mouse, and so forth. The external device  102  also includes far field communication circuitry  210  used to establish the far field communication session with the IMD  104 . The far field communication circuitry  210  may drive a signal propagation tool such as an RF antenna. The signal propagation tool may be included within the proximity communicator  110  so that the far field communication circuitry  210  instructs the signal propagation tool over the connection  118  or the signal propagation tool may be a separate external component or housed within the external device  102 . 
     In addition to the far field communication circuitry  210 , the external device  102  also includes proximity communication circuitry  212 . The proximity communication circuitry  212  may be of various forms to interact with the proximity communicator  110 . The link between the proximity communication circuitry  212  and the proximity communicator  110  may be a wired or wireless connection, for example using universal serial bus protocol, Bluetooth® protocol, or other such protocols, that provides data commands to circuitry within the proximity communicator  110  to produce the proximity communication signal. The proximity communicator  110  may then include a near field inductive driver circuit, a signal generator for producing audible tones, a motion signal generator for driving a body thump device, a field producing circuit for driving an electromagnet, and the like that are responsive to the data commands. Alternatively for a wired connection, these circuits may be included in the proximity communication circuitry  212  to drive the proximity communicator  110  directly. 
     The external device  102  may include additional communication capabilities that may be provided by far field communication circuitry  210  or by additional communication circuitry. For instance, the external device  102  may include Wi-Fi connectivity, public switched telephone network connectivity, and so forth to allow for remote communication, particularly where the external device  102  is a home/remote monitor. 
     The memory  204  may be used to store information in use by the processor  202 . For instance, the memory  204  may store therapy parameters that are input by a clinician or patient that are to be loaded into the IMD  104 . The memory  204  may also store programming that is used by the processor  202  to control the IMD selection procedure of the external device  102 . The memory  204  may be of various types, such as volatile, non-volatile, or a combination of the two. 
     The storage device  206  may be used to store information for a long term and may be of various types such as non-volatile so that the information is retained when the external device  102  is powered off. The storage device  206  may also store programming for the processor  202  that is implemented to control the IMD selection procedure. Examples of the storage device  206  include electronic, magnetic, and optical drives. The storage device  206  and the memory  204  are both examples of computer readable media that may store information in the form of computer programming, data structures, and the like. 
     The processor  202  performs logical operations to provide a sequence of far field and proximity communications and related decisions such as those of  FIGS. 4-9  to allow far field communication sessions with the IMD  104  to be established. The processor  202  may be of various forms. For instance, the processor  202  may be a general-purpose programmable processor that executes software that is stored on the storage device  206  or elsewhere. Other examples include a dedicated purpose hardware circuit or hard-wired digital logic. The processor  202  may be multiple separate components or processors, dedicated hardware/state machine, and the like. The processor  202  may communicate with the various other components through one or more data buses. 
       FIG. 3  shows components of one example of the IMD  104 . The IMD  104  includes a processor  302  and a memory  304 . The IMD  104  also includes medical circuitry  306  that performs a medical task such as stimulation, drug delivery, monitoring, and the like. The IMD  104  also includes far field communication circuitry  308  used to establish the far field communication session with the external device  102 . The far field communication circuitry  308  may drive a signal propagation tool such as an integral RF antenna. 
     In addition to the far field communication circuitry  308 , the IMD  104  also includes proximity communication circuitry  310 . The proximity communication circuitry  310  may be of various forms where for a given system, the type of proximity communication circuitry  310  matches the type of proximity communicator  110  that the external device  102  includes. Accordingly, the proximity communication circuitry  310  may be a near field inductive receiver, a microphone for receiving audible tones, an accelerometer or other vibration detection device, a field operable switch such as a magnetic reed switch, and the like. 
     The memory  304  may be used to store information in use by the processor  302  such as programming and data values. The memory  304  may store additional information including therapy parameters that are used to control the medical circuitry  306 . The memory  304  may be of various types such as volatile, non-volatile, or a combination of the two. The memory  304  is also an example of computer readable media that may store information in the form of computer programming, data structures, and the like. 
     The processor  302  performs logical operations to provide a sequence of far field and proximity communications and related decisions such as those of  FIGS. 4-9  to allow far field communication sessions with the external device  102  to be established. The processor  302  may be of various forms like those discussed above for the processor  202  of the external device  102  and as discussed above may be multiple separate components or processors, dedicated hardware/state machine, and the like. The processor  302  may communicate with the various other components through one or more data buses. 
       FIGS. 4-9  describe proximity based communications. While these examples show proximity communications being directed from an external device  102  to an IMD  104 , it will be appreciated that in some cases the roles may be reversed and the direction of the proximity communications may be reversed whereby the IMD  104  may send proximity communications rather than or in addition to the external device  102  doing so. 
       FIG. 4  shows a first example of a procedure to establish a far field communication session where a unique value and/or key are shared via a proximity communication. The proximity communication is of a type that can carry data. Furthermore, the proximity communication may be bi-directional so that an acknowledgement may be returned as a confirmation of receipt of the data so that a successful initial data transfer via the proximity communication can be completed as a prerequisite to attempting subsequent steps. 
     Initially, the external device  102  may send a proximity communication  104  that includes a value that is unique to the external device  102  to the IMD  104 . For example, the unique value may be a device serial number, hardware identification number, randomly generated number, a security key value, a combination, or other such values that may be unique to the external device  102 . Because this information is transferred through the proximity communication  402 , no other nearby IMD will receive this information. The external device  102  also sends a far field discovery communication  404  shortly before, during, or shortly after sending the proximity communication  402 . The IMD  104  as well as other nearby IMDs may receive and respond to this far field discovery communication  404 . 
     In one example, the IMD  102  may respond only to a discovery request that is within a certain time of receiving the proximity communication  402 , such as a simultaneous occurrence of the proximity communication  402  and the discovery communication  404  or within a predefined delay from one to the next. In this example, the IMD  104  and potentially other nearby IMDs as well are configured to respond by sending the unique value that each has received via a proximity communication and also by sending a value that is unique to the IMD. For example, this value may be a device serial number, hardware identification number, randomly generated number, a security key value, a combination, or other such values that may be unique to the IMD  104 . 
     Only the far field response communication  406  from the IMD  104  of interest will have the unique value that corresponds to the external device  102 . Other IMDs would either have no unique value of an external device to send or would send the unique value of a different external device. Furthermore, in some examples, only those IMDs that receive the discovery communication  404  within a specified time relative to a proximity communication, such as the proximity communication  402  received by the intended IMD  104 , bother to respond with a far field response communication such as the far field response communication  406  from the intended IMD  104 . 
     For each far field response communication, the external device  102  attempts to verify the shared unique value by determining whether the unique value being received matches the unique value that was previously sent over the proximity communication  402  at a query operation  408 . If a particular response does not include a matching value, then that particular response is ignored at operation  410 . For the response  406  which does have the matching unique value from the proximity communication  402 , the external device  102  then associates the value that is unique to the IMD  104  and that is included in the far field response communication  406  to the far field communication session being established at an association operation  412 . The external device  102  may also then execute the appropriate therapy program automatically based on the value that is unique to the IMD  104  where the external device  102  stores associations of such values to therapy applications. 
     The external device  102  then begins the session with the IMD  104 . The external device may communicate during the session by using the unique value of the external device  102  of which the IMD  104  is aware to identify the sender of transmissions and/or using the unique value of the IMD  104  to identify the intended recipient of transmissions. Likewise, the IMD may communicate during the session by using the unique value of the IMD  104  of which the external device  102  is aware to identify the sender of transmissions and/or using the unique value of the external device  102  to identify the intended recipient of transmissions. 
     The session may be made secure by encrypting the information with an encryption key. This encryption key may have been generated for the session by the external device  102  and included in the proximity communication  402  so that the IMD  104  already has the key. Alternatively, the key may be exchanged in another manner and/or at another time in the sequence such as by using a low power radio frequency communication to minimize the range. Furthermore, the IMD  104  may provide the key for the secure session rather than receiving the key from the external device  102 . 
     In some cases, such as for an external device  102  that is a patient programmer or home monitoring device, the external device  102  and the IMD  104  may be bonded whereby each device is aware of an identification value of the other that is used to address far field communications and already possesses the encryption key used to secure the far field communications. In that case, the proximity based initiation of the far field communication session by a process like that of  FIG. 4 , as well as  FIGS. 5-9  discussed below, may still be useful for various reasons. For instance, the initial proximity communication such as the proximity communication  402  may be used as a wake up signal for the far field communication circuits of the IMD  104 . Additionally, to ensure that the use of the external device  102  to initiate the communication session is legitimate, as opposed to being an accident or a malicious attempt, proximity must be established before the far field communication session can begin. However, in such a case, the far field communication session can begin pursuant to the processes of  FIGS. 4-9  but with omission of the discovery operations because the identification of the IMD  104  is known to the external device  102  so that it can immediately address an initial far field communication to the IMD  104 . 
     For embodiments using processes such as those of  FIGS. 4-9  where discovery via far field communications is attempted, the external device  102  and IMDs may be configured to apply collision avoidance and backoff algorithms. These algorithms allow devices to re-attempt to send and/or receive expected far field communications where two devices may attempt to send a far field communication at the same time such that neither transmission is received and acknowledged. A re-attempt to send the far field communication occurs by each of the sending devices but at different times on the second attempts because the backoff algorithm of each sending device randomly chooses the time for the re-attempt. This reduces the likelihood of collisions occurring multiple times. 
       FIG. 5  shows a second example of a procedure to establish a far field communication session. In this particular example and as further discussed below, a challenge may be issued by the IMD during a discovery phase to provide confirmation that the correct IMD has been selected. Other examples of providing confirmation or otherwise selecting the appropriate IMD are discussed with reference to  FIGS. 6-9 . 
     Initially, the external device  102  provides a proximity communication  502  in the form of a signal. As discussed above, the proximity communication  502  may be provided by a third party such as the clinician acting at the request of the external device  102  such as to pass a magnet nearby the IMD  104  to provide a form of the proximity communication  502 . In either case, this signal may be simple in terms of carrying no data but merely being on or off. Alternatively, this signal of the proximity communication  502  from the external device  102  may be more sophisticated including the ability to carry data such as communication identifiers, encryption key data, challenge data, and so forth. In either case, the IMD  104  may or may not have the ability to send a return proximity communication. 
     The proximity communication of these embodiments of  FIGS. 4-9 , such as the proximity communication  502  from the external device  102  may serve one or more purposes. For instance, the proximity communication  502  may serve as a trigger for the IMD  104  to respond to far field communication. Likewise, the proximity communication  502  may serve as a wake-up signal to the far field communication abilities of the IMD  104 . This may be useful where the IMD  104  deactivates the far field communication abilities during periods of non-use and reactivates those abilities upon receiving a proximity communication  502 . The opposite may also be true, where the IMD  104  uses the far field communication abilities to monitor for a far field wake up signal that then wakes up the proximity communication abilities of the IMD  104 . In that case, the external device  102  may send a far field communication in advance of providing the proximity communication. 
     The external device  102  also sends a far field discovery communication  504 . This far field discovery communication  504  may occur at some point shortly after the proximity communication  502 , particularly in examples where the proximity communication  502  serves as a wake-up signal to the far field communication abilities of the IMD  104 . The far field discovery communication  504  may occur shortly before or during the proximity communication  502 , particularly in examples where the far field communication abilities of the IMD  104  are already functioning prior to the proximity communication  502 . 
     The far field discovery communication  504  serves as an inquiry to all IMDs within range and triggers the receiving IMDs to provide a response that identifies the IMD such as by including a value unique to the IMD  104  in the response to the discovery. In one example, the far field discovery communication  504  specifies a condition for responding. The condition may be that the IMD responds only if the IMD is receiving the far field discovery communication  504  within a predefined time relative to receiving the proximity communication. In one example, the predefined time may be zero, such that the IMD must receive the proximity communication  502  at the time the far field discovery communication  504  is received in order to provide a response. 
     This condition may be specified by setting a bit value within the discovery request, where the predefined time is preconfigured within the logic of the IMD  104 . This condition may alternatively be specified in a more complex manner such as by indicating the predefined amount of time within the request. 
     In another example, the criteria for responding are preconfigured within the IMD  104 . So, in this case, the far field discovery communication  504  may omit any conditions, and the external device  102  may rely on the IMDs that receive the far field discovery communication  504  to properly determine whether to respond based upon the preconfiguration. 
     In this example, the IMD  104  detects whether the far field discovery communication has arrived within the predefined amount of time relative to receiving the proximity communication  502  at a query operation  506 . For those IMDs where no proximity communication  502  has been received or has been received such that the far field discovery communication  504  is outside of the allowed window of time, the far field discovery communication  504  is ignored at an operation  508 . For the intended IMD  104 , the proximity communication  502  is received and the far field discovery communication arrives within the predefined time relative to the proximity communication  502  so that a far field response communication  510  is returned. 
     The far field response communication  510  may specify the value that is unique to the IMD  104 . The external device  102  may then utilize this unique value to establish a communication session with the IMD  104 . However, there is the possibility that multiple IMDs provide a response, including the intended IMD  104  as well as the other nearby IMDs who may have also had a proximity signal from other external devices present at the appropriate time relative to the far field discovery communication  504  from the external device  102 . In that case, the external device  102  may not determine which IMD  104  is the correct one from the far field responses alone. The external device  102  may instead rely on a challenge procedure in order to ultimately confirm that the intended IMD  104  is the one that the external device  102  is communicating with via far field communications. 
     In this example, the challenge procedure may be provided by each IMD that is responding including a challenge within the far field response communication  510 . The challenge may specify that the external device  102  provide a proximity communication that includes a challenge response. The challenge response may be of various types and may depend upon the type of proximity communication that is in use. For instance, where the proximity communication is a simple on or off state of a signal, the challenge may be to provide an on-off sequence, or to be on at only a certain time or for only a certain duration. As another example, where the proximity communication is capable of providing data, the challenge may be to repeat a particular data value or sequence. 
     In this example, the external device  102  may respond to receiving several responses by choosing one of the responses and attempting to satisfy the challenge. The choice may be based on time the response was received, strength of the response signal, a random selection, and the like. When choosing whether to respond to any one of the received responses, the external device  102  may detect whether each response is timely at a query operation  512  and ignore the response at an operation  514  if not. 
     The external device  102  may proceed to setup the communication session along with responding to the challenge for the chosen response. Initially, for the chosen response, the external device  102  may proceed to associate the unique value of the IMD received in the response to the session at an association operation  516 . The external device  102  may also then execute the appropriate therapy program automatically based on the value that is unique to the IMD where the external device  102  stores associations of such values to therapy applications. 
     The external device  102  then determines that the challenge response is necessary at a query operation  518 . A proximity communication  520  that provides the response to the challenge is sent if requested, and then the external device  102  attempts to begin a communication session with the selected IMD via an exchange of far field communications  528 . In embodiments where no challenge was requested, such as where the external device  102  is using some other technique for selecting the correct IMD that has responded, then the external device may proceed with the far field communications  528  without sending the challenge response  520 . 
     The IMD  104 , upon sending the far field response communication  510  may then detect that a challenge has been sent at a query operation  522 . For embodiments where no challenge is performed such as where the external device  102  uses some other technique for selecting the correct IMD  104  that has responded, then the IMD  104  may proceed with the far field communications  528  to establish the communication session whereby the IMD  104  responds to far field communications that include the unique value of the IMD  104  as the recipient. 
     For embodiments where the IMD  104  has issued the challenge to the external device  102 , the IMD  104  may then detect whether the shared challenge is verified by detecting whether the challenge response has been received and whether the challenge response matches the challenge that was issued at a query operation  524 . Because the IMD  104  is receiving proximity communications from the external device  102 , the IMD  104  will receive the proximity communication  520  that includes the challenge response regardless of whether the external device  102  is responding to the challenge by the IMD  104  or a challenge by another nearby IMD. However, if the external device  102  is responding to a challenge by another nearby IMD, then because the challenge from each IMD is different the challenge response being provided to the IMD  104  will not match the challenge that was issued by the IMD  104 . Accordingly, the IMD  104  will ignore all subsequent communication at an operation  526  because the external device  102  has selected another nearby and unintended IMD rather than the intended IMD  104 . The IMD  104  may continue to ignore subsequent communication until the discovery process of  FIG. 5  re-starts with another proximity communication  502  and far field discovery communication  504 . 
     The other nearby IMD that has been incorrectly selected by the external device  102  will not receive the proximity communication  520  from the external device  102  that includes the challenge response because the other nearby IMD is not within range of the proximity communication  520 . If this other IMD does not receive a challenge response, then this other nearby IMD also ignores all subsequent communication until the discovery process of  FIG. 5  re-starts with another proximity communication  502  and far field discovery communication  504 . However, the external device that is providing the proximity communication to this other IMD may have properly selected this other IMD and may provide a proximity communication that does satisfy the challenge of this other IMD so that a proper communication session may be established between them. 
     In the event the external device  102  has not adequately responded to the challenge, the external device  102  will need to re-attempt to discover, select, and establish a session with the intended IMD  104 . However, where the external device  102  has selected the IMD  104  from the set of far field responses and responds by providing the proximity communication  520  that does satisfy the challenge by the intended IMD  104 , then the external device  102  and the IMD  104  will both begin the far field communication session via subsequent far field communications  528 . 
     The external device may communicate during the session by using the unique value of the external device  102  of which the IMD  104  has been made aware via a far field communication to identify the sender of transmissions and/or using the unique value of the IMD  104  to identify the intended recipient of transmissions. Likewise, the IMD  104  may communicate during the session by using the unique value of the IMD  104  of which the external device  102  is aware to identify the sender of transmissions and/or using the unique value of the external device  102  to identify the intended recipient of transmissions. 
     As with the example of  FIG. 4 , the session may be made secure by encrypting the information with an encryption key. This encryption key may have been generated for the session by the external device  102  and included in a far field communication so that the IMD  104  obtains the key. As one example, the key may be exchanged by using a low power radio frequency communication to minimize the range. Furthermore, the IMD  104  may provide the key for the secure session rather than receiving the key from the external device  102 . To the extent the devices have a capable manner of using more complex proximity communications, the key may be exchanged through proximity communication rather than through far field communication. 
       FIG. 6  shows a third example of a procedure to establish a far field communication session. In this particular example and as further discussed below, a challenge may be issued by the IMD upon the start of a far field communication session to provide confirmation that the correct IMD has been selected. 
     Initially, the external device  102  provides a proximity communication  602  in the form of a signal. As discussed above, the proximity communication  602  may be provided by a third party such as the clinician acting at the request of the external device  102  such as to pass a magnet nearby the IMD  104  to provide a form of the proximity communication  602 . In either case, this signal may be simple in terms of carrying no data but merely being on or off. Alternatively, this signal of the proximity communication  602  from the external device  102  may be more sophisticated including the ability to carry data. In either case, the IMD  104  may or may not have the ability to send a return proximity communication. 
     As with  FIG. 5 , the proximity communication  602  from the external device  102  may serve one or more purposes. For instance, the proximity communication  602  may serve as a trigger for the IMD  104  to respond to far field communication. Likewise, the proximity communication  602  may serve as a wake-up signal to the far field communication abilities of the IMD  104 . 
     The external device  102  also sends a far field discovery communication  604 . This far field discovery communication  604  may occur at some point shortly after the proximity communication  602 , particularly in examples where the proximity communication  602  serves as a wake-up signal to the far field communication abilities of the IMD  104 . The far field discovery communication  604  may occur shortly before or during the proximity communication  602 , particularly in examples where the far field communication abilities of the IMD  104  are already functioning prior to the proximity communication  602 . 
     The far field discovery communication  604  serves as an inquiry to all IMDs within range and triggers the receiving IMDs to provide a response that identifies the IMD such as by including a value unique to the IMD  104  in the response to the discovery. In one example, the far field discovery communication  604  specifies a condition for responding. The condition may be that the IMD responds only if the IMD is receiving the far field discovery communication  604  within a predefined time relative to receiving the proximity communication. In one example, the predefined time may be zero, such that the IMD must receive the proximity communication  602  at the time the far field discovery communication  604  is received in order to provide a response. 
     This condition may be specified by setting a bit value within the discovery request, where the predefined time is preconfigured within the logic of the IMD  104 . This condition may alternatively be specified in a more complex manner such as by indicating the predefined amount of time within the request. 
     In another example, the criteria for responding are preconfigured within the IMD  104 . So, in this case, the far field discovery communication  604  may omit any conditions, and the external device  102  may rely on the IMDs that receive the far field discovery communication  604  to properly determine whether to respond based upon the preconfiguration. 
     In this example, the IMD  104  detects whether the far field discovery communication has arrived within the predefined amount of time relative to receiving the proximity communication  602  at a query operation  606 . For those IMDs where no proximity communication  602  has been received or has been received such that the far field discovery communication  604  is outside of the allowed window of time, the far field discovery communication  604  is ignored at an operation  608 . For the intended IMD  104 , the proximity communication  602  is received and the far field discovery communication arrives within the predefined time relative to the proximity communication  602  so that a far field response communication  610  is returned. 
     The far field response communication  610  may specify the value that is unique to the IMD  104 . The external device  102  may then utilize this unique value to establish a communication session with the IMD  104 . However, there is the possibility that multiple IMDs provide a response in this example as well. These responding devices may include the intended IMD  104  as well as the other nearby IMDs who may have also had a proximity signal from other external devices present at the appropriate time relative to the far field discovery communication  604  from the external device  102 . In that case, the external device  102  may not determine which IMD  104  is the correct one from the far field responses alone. The external device  102  may instead rely on another challenge procedure in order to ultimately confirm that the intended IMD  104  is the one that the external device  102  is communicating with via far field communications. 
     In this example, the challenge procedure may be provided by the external device  102  entering into a communication session with a selected one of the responding IMDs. The choice of IMD may be based on time the response was received, strength of the response signal, a random selection, and the like. The external device  102  then attempts to satisfy the challenge and then proceeds with the session if the challenge is satisfied or terminates the current session due to a failed challenge. If a challenge is failed, the external device  102  then starts a session with the next selected responding IMD to attempt to satisfy that challenge and this process continues until a challenge for a responding IMD is satisfied. As an alternative, the challenge procedure may be provided by the external device  102  entering into separate and simultaneous communication sessions with all of the responding IMDs. The external device  102  then attempts to satisfy the challenge of each one in sequence where sessions with failed challenges terminate and the session with the satisfied challenge proceeds. 
     When choosing whether to respond to any one of the received responses by establishing a communication session, the external device  102  may detect whether each response is timely at a query operation  612  and ignore the response at an operation  614  if not. The external device  102  may then proceed to setup the far field communication session(s) for the chosen response or alternatively for each of the responses. 
     The external device  102  may proceed to associate the unique value of the IMD received in the response to a given far field communication session at an association operation  616  and being far field communications  618 . The external device  102  may also then execute the appropriate therapy program automatically based on the value that is unique to the IMD where the external device  102  stores associations of such values to therapy applications. 
     The external device  102  may communicate during the session by using the unique value of the external device  102  of which the IMD of the session has been made aware via a far field communication to identify the sender of transmissions and/or using the unique value of the IMD of the session to identify the intended recipient of transmissions. Likewise, the IMD of the session may communicate during the session by using the unique value of the IMD of the session of which the external device  102  is aware to identify the sender of transmissions and/or using the unique value of the external device  102  to identify the intended recipient of transmissions. 
     As with the example of  FIG. 4 , each session may be made secure by encrypting the information with an encryption key, where each session may utilize a different key. This encryption key may have been generated for the session by the external device  102  and included in a far field communication so that the IMD of the session obtains the key. Furthermore, the IMD  104  may provide the key for the secure session rather than receiving the key from the external device  102 . 
     Upon starting the communication session, the external device  102  of this particular example then sends a far field communication  620  that includes a challenge request that is addressed with the unique value of the IMD of the session. The challenge request triggers the IMD of the session to return a challenge and to monitor for a challenge response via a proximity communication. In some embodiments, the IMD may detect whether the proximity challenge is timely and if not, then ignore subsequent far field communications from the external device  102 . 
     When appropriate, the IMD of each of the sessions sends the far field communication  622  that may include the unique value of the external device  102  and includes the challenge. The challenge may specify that the external device  102  provide a proximity communication that includes a challenge response. As discussed above for the example in  FIG. 5 , the challenge response may be of various types and may depend upon the type of proximity communication that is in use. For instance, where the proximity communication is a simple on or off state of a signal, the challenge may be to provide an on-off sequence, or to be on at only a certain time or for only a certain duration. As another example, where the proximity communication is capable of providing data, the challenge may be to repeat a particular data value or sequence. 
     The external device  102  then sends a proximity communication  624  for the session or sessions that provides the response to the challenge. The proximity communication  624  may also be addressed with the unique value of the IMD of the session. Then the external device  102  attempts to continue the communication session with the IMD of the session via an exchange of subsequent far field communications  630 . 
     The IMD of the session detects whether the shared challenge is verified by detecting whether the challenge response has been received and matches the challenge that was issued at a query operation  626 . Because the IMD  104  is receiving proximity communications from the external device  102 , the IMD  104  will receive the proximity communication  624  that includes the challenge response regardless of whether the external device  102  is responding to the challenge by the IMD  104  or a challenge by another nearby IMD. In the situation where the IMD  104  has yet to issue the challenge either because the external device  102  has yet to establish the communication session with the IMD  104  or because it is not yet the turn of the IMD  104  to receive a challenge request, then the IMD  104  may simply ignore the proximity communication  624 . 
     In that case, the proximity communication that includes the challenge response is a result of the external device  102  responding to a challenge by another nearby IMD. This other nearby IMD that has been incorrectly selected by the external device  102  will not receive the proximity communication  624  that includes the challenge response because this other nearby IMD is not within range of the proximity communication  624 . Thus, this other nearby IMD will detect that a matching response has not been received at a query operation  626 . As a result, the other nearby IMD also ignores all subsequent communication at an operation  628  until the discovery process of  FIG. 6  re-starts with another proximity communication  602  and far field discovery communication  604 . 
     In the event the external device  102  has not adequately responded to the challenge, the external device  102  will need to re-attempt to discover, select, and establish a session with the intended IMD  104 . However, where the external device  102  has selected the IMD  104  from the set of far field responses and responds by providing the proximity communication  624  that does satisfy the challenge by the intended IMD  104  at the query operation  626 , then the external device  102  and the IMD  104  will both continue the far field communication session via subsequent far field communications  630 . The external device  102  may then cease attempting to satisfy any remaining challenges of other responding IMDs, and those IMDs ignore subsequent communications until the discovery process of  FIG. 6  re-starts. 
       FIG. 7  shows a fourth example of a procedure to establish a far field communication session. In this particular example and as further discussed below, a challenge may be issued by the IMD  104  at the onset of the far field communication session to provide confirmation that the correct IMD has been selected. Additionally or alternatively, the IMD  104  may issue the challenge at one or more subsequent times during the communication session which allows the IMD  104  to periodically confirm that far field communications are with the intended external device  102 . 
     Initially, the external device  102  provides a proximity communication  702  in the form of a signal. As discussed above, the proximity communication  702  may be provided by a third party such as the clinician acting at the request of the external device  102  such as to pass a magnet nearby the IMD  104  to provide a form of the proximity communication  702 . In either case, this signal may be simple in terms of carrying no data but merely being on or off. Alternatively, this signal of the proximity communication  702  from the external device  102  may be more sophisticated including the ability to carry data. In either case, the IMD  104  may or may not have the ability to send a return proximity communication. 
     As with  FIG. 5 , the proximity communication  702  from the external device  102  may serve one or more purposes. For instance, the proximity communication  702  may serve as a trigger for the IMD  104  to respond to far field communication. Likewise, the proximity communication  702  may serve as a wake-up signal to the far field communication abilities of the IMD  104 . 
     The external device  102  also sends a far field discovery communication  704 . This far field discovery communication  704  may occur at some point shortly after the proximity communication  702 , particularly in examples where the proximity communication  702  serves as a wake-up signal to the far field communication abilities of the IMD  104 . The far field discovery communication  704  may occur shortly before or during the proximity communication  702 , particularly in examples where the far field communication abilities of the IMD  104  are already functioning prior to the proximity communication  702 . 
     The far field discovery communication  704  serves as an inquiry to all IMDs within range and triggers the receiving IMDs to provide a response that identifies the IMD such as by including a value unique to the IMD  104  in the response to the discovery. In one example, the far field discovery communication  704  specifies a condition for responding. The condition may be that the IMD responds only if the IMD is receiving the far field discovery communication  704  within a predefined time relative to receiving the proximity communication. In one example, the predefined time may be zero, such that the IMD must receive the proximity communication  702  at the time the far field discovery communication  704  is received in order to provide a response. 
     This condition may be specified by setting a bit value within the discovery request, where the predefined time is preconfigured within the logic of the IMD  104 . This condition may alternatively be specified in a more complex manner such as by indicating the predefined amount of time within the request. 
     In another example, the criteria for responding are preconfigured within the IMD  104 . So, in this case, the far field discovery communication  704  may omit any conditions, and the external device  102  may rely on the IMDs that receive the far field discovery communication  704  to properly determine whether to respond based upon the preconfiguration. 
     In this example, the IMD  104  detects whether the far field discovery communication has arrived within the predefined amount of time relative to receiving the proximity communication  702  at a query operation  706 . For those IMDs where no proximity communication  702  has been received or has been received such that the far field discovery communication  704  is outside of the allowed window of time, the far field discovery communication  704  is ignored at an operation  708 . For the intended IMD  104 , the proximity communication  702  is received and the far field discovery communication arrives within the predefined time relative to the proximity communication  702  so that a far field response communication  710  is returned. 
     The far field response communication  710  may specify the value that is unique to the IMD  104 . The external device  102  may then utilize this unique value to establish a communication session with the IMD  104 . However, there is the possibility that multiple IMDs provide a response in this example as well. These responding devices may include the intended IMD  104  as well as the other nearby IMDs who may have also had a proximity signal from other external devices present at the appropriate time relative to the far field discovery communication  704  from the external device  102 . In that case, the external device  102  may not determine which IMD  104  is the correct one from the far field responses alone. The external device  102  may instead rely on another challenge procedure in order to ultimately confirm that the intended IMD  104  is the one that the external device  102  is communicating with via far field communications. 
     In this example, the challenge procedure may be provided by the external device  102  entering into a communication session with a selected one of the responding IMDs. The choice of IMD may be based on time the response was received, strength of the response signal, a random selection, and the like. The external device  102  then attempts to satisfy the challenge and then proceeds with the session if the challenge is satisfied or terminates the current session due to a failed challenge. If a challenge is failed, the external device  102  then starts a session with the next selected responding IMD to attempt to satisfy that challenge and this process continues until a challenge for a responding IMD is satisfied. As an alternative, the challenge procedure may be provided by the external device  102  entering into separate and simultaneous communication sessions with all of the responding IMDs. The external device  102  then attempts to satisfy the challenge of each one in sequence where sessions with failed challenges terminate and the session with the satisfied challenge proceeds. 
     When choosing whether to respond to any one of the received responses by establishing a communication session, the external device  102  may detect whether each response is timely at a query operation  712  and ignore the response at an operation  714  if not. The external device  102  may then proceed to setup the far field communication session(s) for the chosen response or alternatively for each of the responses. 
     The external device  102  may proceed to associate the unique value of the IMD received in the response to a given far field communication session at an association operation  716  and begin sending far field communications  718 . The external device  102  may also then execute the appropriate therapy program automatically based on the value that is unique to the IMD where the external device  102  stores associations of such values to therapy applications. 
     The external device  102  may communicate during the session by using the unique value of the external device  102  of which the IMD of the session has been made aware via a far field communication to identify the sender of transmissions and/or using the unique value of the IMD of the session to identify the intended recipient of transmissions. Likewise, the IMD of the session may communicate during the session by using the unique value of the IMD of the session of which the external device  102  is aware to identify the sender of transmissions and/or using the unique value of the external device  102  to identify the intended recipient of transmissions. 
     As with the example of  FIG. 4 , each session may be made secure by encrypting the information with an encryption key, where each session may utilize a different key. This encryption key may have been generated for the session by the external device  102  and included in a far field communication so that the IMD of the session obtains the key. Furthermore, the IMD of the session may provide the key for the secure session rather than receiving the key from the external device  102 . 
     Upon starting the communication session(s), the IMD of a session in this particular example begins detecting whether it is time to challenge the external device  102  at a query operation  720 . For instance, the IMD of a session may be configured to challenge the IMD immediately upon the communication session being established. In this case, the IMD of a session may challenge the external device  102  to provide confirmation at the onset of the communication session without the external device  102  first having to request the challenge. Furthermore, the IMD of a session additionally or alternatively determines that such a challenge is necessary at some later time during the session so that confirmation throughout the session may occur. It will be appreciated that in some examples, the external device  102  may request one or more challenges, as in  FIG. 6 , while the IMD  104  may respond to those and may also generate one or more challenges to the external device  102  without receiving a request as in  FIG. 7 . 
     In the example of  FIG. 7 , the IMD of the session sends the far field communication  722  that includes the challenge. The challenge may specify that the external device  102  provide a proximity communication that includes a challenge response. As discussed above for the example in  FIG. 5 , the challenge response may be of various types and may depend upon the type of proximity communication that is in use. For instance, where the proximity communication is a simple on or off state of a signal, the challenge may be to provide an on-off sequence, or to be on at only a certain time or for only a certain duration. As another example, where the proximity communication is capable of providing data, the challenge may be to repeat a particular data value or sequence. 
     The external device  102  then sends a proximity communication  724  for the session that provides the response to the challenge. Then the external device  102  attempts to continue the communication session with the IMD of the session via an exchange of subsequent far field communications  730 . 
     The IMD of the session detects whether the shared challenge is verified by detecting whether the challenge response has been received and matches the challenge that was issued at a query operation  726 . Because the intended IMD  104  is receiving proximity communications from the external device  102 , the intended IMD  104  will receive the proximity communication  724  that includes the challenge response regardless of whether the external device  102  is responding to the challenge by the IMD  104  or a challenge by another nearby IMD. In the situation where the IMD  104  has yet to issue the challenge because the external device  102  has yet to establish the communication session with the IMD  104 , then the IMD  104  may simply ignore the proximity communication  724 . 
     In that case, the proximity communication that includes the challenge response is a result of the external device  102  responding to a challenge by another nearby IMD. This other nearby IMD that has been incorrectly selected by the external device  102  will not receive the proximity communication  724  that includes the challenge response because this other nearby IMD is not within range of the proximity communication  724 . Thus, this other nearby IMD will detect that a matching response has not been received at a query operation  726 . As a result, the other nearby IMD also ignores all subsequent communication and terminates the session at an operation  728  until the discovery process of  FIG. 7  re-starts with another proximity communication  702  and far field discovery communication  704 . 
     In the event the external device  102  has not adequately responded to the challenge, the external device  102  will need to re-attempt to discover, select, and establish a session with the intended IMD  104 . However, where the external device  102  has selected the IMD  104  from the set of far field responses and responds by providing the proximity communication  724  that does satisfy the challenge by the intended IMD  104  at the query operation  726 , then the external device  102  and the IMD  104  will both continue the far field communication session via subsequent far field communications  730  until the time for the next challenge by the IMD  104  arrives. Upon satisfying the first challenge by the IMD  104 , the external device  102  may then cease attempting to satisfy any remaining challenges of other responding IMDs, and those IMDs ignore subsequent communications until the discovery process of  FIG. 7  re-starts. 
       FIG. 8  shows a fifth example of a procedure to establish a far field communication session. In this particular example, the external device  102  relies upon the uniqueness of an encryption key for the secure far field session with the IMD  104  together with the security of the proximity communication as a manner of confirming that the intended IMD  104  has been selected for the far field communication. 
     Initially, the external device  102  provides a proximity communication  802  in the form of a signal. As discussed above, the proximity communication  802  may be provided by a third party such as the clinician acting at the request of the external device  102  such as to pass a magnet nearby the IMD  104  to provide a form of the proximity communication  802 . In either case, this signal may be simple in terms of carrying no data but merely being on or off. Alternatively, this signal of the proximity communication  802  from the external device  102  may be more sophisticated including the ability to carry data. In either case, the IMD  104  may or may not have the ability to send a return proximity communication. 
     As with  FIG. 5 , the proximity communication  802  from the external device  102  may serve one or more purposes. For instance, the proximity communication  802  may serve as a trigger for the IMD  104  to respond to far field communication. Likewise, the proximity communication  802  may serve as a wake-up signal to the far field communication abilities of the IMD  104 . Furthermore, in some cases, the proximity communication  802  may provide the encryption key to the IMD  104 . 
     The external device  102  also sends a far field discovery communication  804 . This far field discovery communication  804  may occur at some point shortly after the proximity communication  802 , particularly in examples where the proximity communication  802  serves as a wake-up signal to the far field communication abilities of the IMD  104 . The far field discovery communication  804  may occur shortly before or during the proximity communication  802 , particularly in examples where the far field communication abilities of the IMD  104  are already functioning prior to the proximity communication  802 . 
     The far field discovery communication  804  serves as an inquiry to all IMDs within range and triggers the receiving IMDs to provide a response that identifies the IMD such as by including a value unique to the IMD  104  in the response to the discovery. In one example, the far field discovery communication  804  specifies a condition for responding. The condition may be that the IMD responds only if the IMD is receiving the far field discovery communication  804  within a predefined time relative to receiving the proximity communication. In one example, the predefined time may be zero, such that the IMD must receive the proximity communication  802  at the time the far field discovery communication  804  is received in order to provide a response. 
     This condition may be specified by setting a bit value within the discovery request, where the predefined time is preconfigured within the logic of the IMD  104 . This condition may alternatively be specified in a more complex manner such as by indicating the predefined amount of time within the request. 
     In another example, the criteria for responding are preconfigured within the IMD  104 . So, in this case, the far field discovery communication  804  may omit any conditions, and the external device  102  may rely on the IMDs that receive the far field discovery communication  804  to properly determine whether to respond based upon the preconfiguration. 
     In this example, the IMD  104  detects whether the far field discovery communication has arrived within the predefined amount of time relative to receiving the proximity communication  802  at a query operation  806 . For those IMDs where no proximity communication  802  has been received or has been received such that the far field discovery communication  804  is outside of the allowed window of time, the far field discovery communication  804  is ignored at an operation  808 . For the intended IMD  104 , the proximity communication  802  is received and the far field discovery communication arrives within the predefined time relative to the proximity communication  802  so that a far field response communication  810  is returned. 
     The far field response communication  810  may specify the value that is unique to the IMD  104 . The external device  102  may then utilize this unique value to establish a communication session with the IMD  104 . However, there is the possibility that multiple IMDs provide a response in this example as well. These responding devices may include the intended IMD  104  as well as the other nearby IMDs who may have also had a proximity signal from other external devices present at the appropriate time relative to the far field discovery communication  804  from the external device  102 . In that case, the external device  102  may not determine which IMD  104  is the correct one from the far field responses alone. The external device  102  may instead rely on an exchange of a unique encryption key via a proximity communication. 
     In this example, the key encryption procedure may be provided by the external device  102  entering into a communication session with a selected one of the responding IMDs. The choice of IMD may be based on time the response was received, strength of the response signal, a random selection, and the like. The external device  102  then attempts to exchange the encryption key via a proximity communication and then begin far field communications using the key. If far field communications fail, the external device  102  then starts a session with the next selected responding IMD to attempt to exchange the encryption key and then begin far field communications. As an alternative, the exchange of the key may be provided followed by the external device  102  entering into separate and simultaneous far field communication sessions with all of the responding IMDs. The external device  102  then attempts to communicate using data encrypted by the encryption key via the far field communication session with each one where sessions with failed far field communication attempts terminate and the session with the successful far field communication proceeds. 
     When choosing whether to respond to any one of the received responses by establishing a far field communication session, the external device  102  may detect whether each response is timely at a query operation  812  and ignore the response at an operation  814  if not. The external device  102  may then proceed to setup the secure far field communication session(s) for the chosen response or alternatively for each of the responses. 
     The external device  102  may proceed to associate the unique value of the IMD received in the response to a given far field communication session at an association operation  816  and then send a proximity communication  818  that includes the encryption key. The external device  102  may also then execute the appropriate therapy program automatically based on the value that is unique to the IMD  104  where the external device  102  stores associations of such values to therapy applications. 
     The external device  102  may then begin sending secure far field communications  818 . The external device  102  may communicate during the session by using the unique value of the external device  102  of which the IMD of the session has been made aware via a far field communication to identify the sender of transmissions and/or using the unique value of the IMD of the session to identify the intended recipient of transmissions. Likewise, the IMD of the session may communicate during the session by using the unique value of the IMD of the session of which the external device  102  is aware to identify the sender of transmissions and/or using the unique value of the external device  102  to identify the intended recipient of transmissions. 
     Upon starting the communication session(s), the IMD of a session in this particular example begins detecting whether any incoming communications can be decrypted. Because the intended IMD  104  is receiving proximity communications from the external device  102 , the intended IMD  104  will receive the proximity communication  818  that includes the encryption key regardless of whether the external device  102  is attempting to communicate with the IMD  104  or with another nearby IMD. 
     In one example where the external device  102  is proceeding with one secure far field session at a time, the external device  102  may provide the proximity communication  818  before each attempt at a secure far field communication session. In that case, if the IMD  104  has yet to receive a secure far field communication from the external device  102  because it is not yet the turn of the IMD  104 , then the IMD  104  may simply ignore the proximity communication  818  after a timeout period. In another example where the external device  102  is proceeding with one secure far field session at a time, the external device  102  may provide the proximity communication  818  a single time and then rely on the intended IMD  104  to retain the encryption key until it is the turn of the intended IMD  104  to begin secure far field communications. Furthermore, in that case the key exchange may be provided at the initial proximity communication  802  as opposed to providing the proximity communication  818 . 
     A secure far field communication attempt being sent by the external device  102  may be addressed to another nearby IMD. This other nearby IMD that has been incorrectly selected by the external device  102  has not received the proximity communication  818  that includes the encryption key because this other nearby IMD is not within range of the proximity communication  818 . Thus, this other nearby IMD will not be able to decrypt the secure far field communication attempt. As a result, the other nearby IMD will not respond which terminates the session with that other IMD. This other IMD may then wait for the discovery process of  FIG. 8  re-starts with another proximity communication  802  and far field discovery communication  804 . 
     In the event the external device  102  has not successfully established secure far field communications with the IMD  104 , the external device  102  will need to re-attempt to discover, select, and establish a session with the intended IMD  104 . However, where the external device  102  has selected the IMD  104  from the set of far field responses and responds by providing the proximity communication  818 , then the external device  102  and the IMD  104  will both begin exchanging secure far field communications  820 . 
       FIG. 9  shows a sixth example of a procedure to establish a far field communication session. In this particular example, the external device  102  relies upon a strength of far field signal between the external device  102  and the responding IMDs as a manner of confirming that the intended IMD  104  has been selected for the far field communication. This may be a valid manner of confirmation considering that the odds of a nearby IMD receiving a proximity communication at the proper time in order to trigger the response to the discovery message coupled with a strength of signal for that nearby IMD being stronger than the strength of the intended IMD  104  are relatively low. 
     The signal strength of interest may vary from one example to the next. For instance, the signal strength may be from the perspective of the IMD for a far field communication sent by the external device  102  such as a discovery message. That signal strength may be reported by the IMD in a response to the discovery message. The signal strength may be from the perspective of the external device  102  for a far field communication sent by the IMD such as the response to the discovery message. The signal strength may be based on a combination of these values and may be based on other far field communications between the external device  102  and the IMD as well. 
     Initially, the external device  102  provides a proximity communication  902  in the form of a signal. As discussed above, the proximity communication  902  may be provided by a third party such as the clinician acting at the request of the external device  102  such as to pass a magnet nearby the IMD  104  to provide a form of the proximity communication  902 . In either case, this signal may be simple in terms of carrying no data but merely being on or off. Alternatively, this signal of the proximity communication  902  from the external device  102  may be more sophisticated including the ability to carry data. In either case, the IMD  104  may or may not have the ability to send a return proximity communication. 
     As with  FIG. 5 , the proximity communication  902  from the external device  102  may serve one or more purposes. For instance, the proximity communication  902  may serve as a trigger for the IMD  104  to respond to far field communication. Likewise, the proximity communication  902  may serve as a wake-up signal to the far field communication abilities of the IMD  104 . 
     The external device  102  also sends a far field discovery communication  904 . This far field discovery communication  904  may occur at some point shortly after the proximity communication  902 , particularly in examples where the proximity communication  902  serves as a wake-up signal to the far field communication abilities of the IMD  104 . The far field discovery communication  904  may occur shortly before or during the proximity communication  902 , particularly in examples where the far field communication abilities of the IMD  104  are already functioning prior to the proximity communication  902 . 
     The far field discovery communication  904  serves as an inquiry to all IMDs within range and triggers the receiving IMDs to provide a response that identifies the IMD such as by including a value unique to the IMD  104  in the response to the discovery. In one example, the far field discovery communication  904  specifies a condition for responding. The condition may be that the IMD responds only if the IMD is receiving the far field discovery communication  904  within a predefined time relative to receiving the proximity communication. In one example, the predefined time may be zero, such that the IMD must receive the proximity communication  902  at the time the far field discovery communication  904  is received in order to provide a response. 
     This condition may be specified by setting a bit value within the discovery request, where the predefined time is preconfigured within the logic of the IMD  104 . This condition may alternatively be specified in a more complex manner such as by indicating the predefined amount of time within the request. 
     In another example, the criteria for responding are preconfigured within the IMD  104 . So, in this case, the far field discovery communication  904  may omit any conditions, and the external device  102  may rely on the IMDs that receive the far field discovery communication  904  to properly determine whether to respond based upon the preconfiguration. 
     In this example, the IMD  104  detects whether the far field discovery communication has arrived within the predefined amount of time relative to receiving the proximity communication  902  at a query operation  906 . For those IMDs where no proximity communication  902  has been received or has been received such that the far field discovery communication  904  is outside of the allowed window of time, the far field discovery communication  904  is ignored at an operation  908 . For the intended IMD  104 , the proximity communication  902  is received and the far field discovery communication arrives within the predefined time relative to the proximity communication  902  so that a far field response communication  910  is returned. 
     The far field response communication  910  may specify the value that is unique to the IMD  104 . The external device  102  may then utilize this unique value to establish a communication session with the IMD  104 . The far field response communication  910  may also specify a signal strength such as that as detected by the responding IMD for the far field discovery communication  904 . Additionally or alternatively, the external device  102  may collect the signal strength of the far field response communication  910 . There is the possibility that multiple IMDs provide a response in this example as well, and the external device  102  collects signal strength information for each response. These responding devices may include the intended IMD  104  as well as the other nearby IMDs who may have also had a proximity signal from other external devices present at the appropriate time relative to the far field discovery communication  904  from the external device  102 . In that case, the external device  102  may not determine which IMD  104  is the correct one from the far field responses alone. The external device  102  may instead rely on the collected signal strength related to the responding IMDs. 
     When choosing whether to respond to any one of the received responses by establishing a far field communication session, the external device  102  may detect whether each response is timely at a query operation  912  and ignore the response at an operation  914  if not. As stated above, the external device  102  collects the signal strength information for each of the responding IMDs. The external device  102  may continue to receive and collect the signal strength information until determining at a query operation  916  that a response period has ended. The external device  102  may then determine for each responding IMD whether that IMD is associated with the strongest signal at a query operation  918 . These operations are iterated for to account for each IMD  104 . 
     If a responding IMD is not associated with the strongest signal, then the external device ignores the response at the operation  914 . However, for the responding IMD that is associated with the strongest signal, the external device  102  proceeds to associate the unique value of that responding IMD that was received in the far field response communication  910  to the far field communication session at an association operation  920 . The external device  102  may also then execute the appropriate therapy program automatically based on the value that is unique to the IMD  104  where the external device  102  stores associations of such values to therapy applications. 
     The external device  102  may then communicate during the session by using the unique value of the external device  102  of which the IMD  104  has been made aware via a far field communication to identify the sender of transmissions and/or using the unique value of the IMD  104  to identify the intended recipient of transmissions. Likewise, the IMD  104  may communicate during the session by using the unique value of the IMD  104  of which the external device  102  is aware to identify the sender of transmissions and/or using the unique value of the external device  102  to identify the intended recipient of transmissions. 
     To further ensure that the IMD that has been selected based on signal strength is the intended IMD, other techniques may then be performed. For instance, a challenge procedure initiated by the external device  102  as in  FIG. 6  or initiated by the IMD  104  as in  FIG. 7  may be conducted. As another example, the encryption key may be exchanged via proximity communication as in  FIG. 8 . 
     Regardless of whether a procedure similar to that of  FIG. 8  is used for further confirmation, the session may be made secure by encrypting the information with an encryption key. This encryption key may have been generated for the session by the external device  102  and included in a far field communication so that the IMD  104  obtains the key. As one example, the key may be exchanged by using a low power radio frequency communication to minimize the range. Furthermore, the IMD  104  may provide the key for the secure session rather than receiving the key from the external device  102 . To the extent the devices have a capable manner of using more complex proximity communications, the key may be exchanged through proximity communication rather than through far field communication as discussed above for  FIG. 8 . 
     With regard to  FIGS. 4-9 , it will be appreciated that the roles of the external device  102  and IMD  104  may be reversed. As one example, in relation to  FIG. 5 , the external device may issue the challenge to the IMD, and the IMD  104  may have the ability to send proximity communications while the external device  102  receives them. As another example, in relation to  FIG. 6 , the IMD  104  may request that the external device  102  issue a challenge that the IMD  104  then responds to via a proximity communication. As yet another example, in relation to  FIG. 7 , the external device  102  may periodically challenge the IMD  104  which responds via a proximity communication. 
     While embodiments have been particularly shown and described, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.