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A telemetry system for radio-frequency communications between an implantable medical device and an external device providing improved noise immunity is disclosed. Multiple communications channels are used to enable establishment and re-establishment of communications between a particular pair of dev

A telemetry system for radio-frequency communications between an implantable medical device and an external device providing improved noise immunity is disclosed. Multiple communications channels are used to enable establishment and re-establishment of communications between a particular pair of devices in a multiple device environment.

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What is claimed is: 1. A telemetry system for enabling radio-frequency (RF) communications between an implantable medical device and an external device over a wireless medium, comprising: an antenna, an RF transmitter/receiver, and a controller incorporated into each of the implantable and external

What is claimed is: 1. A telemetry system for enabling radio-frequency (RF) communications between an implantable medical device and an external device over a wireless medium, comprising: an antenna, an RF transmitter/receiver, and a controller incorporated into each of the implantable and external devices, wherein the RF transmitter/receiver of each device are tunable and may be switched among a plurality of different RF communications channels that differ in frequency; an inductive link transmitter/receiver incorporated into each of the implantable and external devices enabling communication over an inductive link; wherein the controller is interfaced to the RF transmitter/receiver and inductive link transmitter/receiver in each device; wherein the external device controller is programmed to select a particular one of the plurality of RF channels as a designated first RF channel for use in an RF communications session with the implantable device and communicate the choice of the designated first RF channel to the implantable device using the inductive link; wherein the external device controller is programmed to monitor the designated first RF channel for activity until the channel is found to be not busy and then establish a communications session with the implantable device over the designated first RF channel; and, wherein the external device and implantable device controllers are programmed to revert to a previously agreed upon fall-back RF channel upon a degradation in the quality of the designated first RF channel to such an extent so as to be non-operational. 2. The system of claim 1 wherein the external device controller is programmed to monitor the quality of the designated first RF channel by determining if the channel throughput over a certain time falls below a certain level. 3. The system of claim 1 wherein the external device controller is programmed to monitor the quality of the designated first RF channel by determining if a specified number of data frames are unacknowledged. 4. The system of claim 1 wherein the external device controller is programmed to monitor the quality of the designated first RF channel by determining if the frame error rate exceeds a threshold value. 5. The system of claim 1 wherein the implantable device and external device controllers are programmed such that, if the previously agreed upon fall-back RF channel is not available, either because of interference or because it is busy, a plurality of previously agreed upon fall-back RF channels are reverted to in a specified sequence. 6. The system of claim 5 wherein the external device and implantable device controllers are programmed with a specified timeout duration for each of the plurality of fall-back RF channels so that if no communications are established within the timeout duration, the devices move to the next fall-back RF channel in the sequence. 7. The system of claim 1 wherein the external device controller is programmed such that, if it is determined that the designated first RF channel is degraded to an unacceptable level but is still operational, the external device is programmed to: cease transferring data; monitoring other RF channels for activity in order to find and select another non-busy RF channel for use as an alternate RF channel; and, communicate the selection of the alternate RF channel to the implantable device over the designated first RF channel; and, establish communications with the implantable device over the alternate RF channel. 8. The system of claim 1 wherein the external device controller is programmed to monitor an RF channel for activity by listening for a valid preamble known to be transmitted by all devices during the transmission of data frames and programmed to regard the RF channel as unused if no such preambles are detected over a specified time period. 9. The system of claim 1 wherein the external device controller is programmed to monitor an RF channel for activity by measuring the signal strength of the RF channel over a specified period of time and programmed to regard the channel as unused if the peak signal strength over specified period of time is less than a specified value. 10. The system of claim 1 wherein the external device controller is programmed to monitor an RF channel for activity by: listening for a valid preamble known to be transmitted by all devices during the transmission of data frames; measuring the signal strength of the channel over a specified period of time; and, regarding the RF channel as unused if no such preambles are detected over a specified time period and if the peak signal strength over specified period of time is less than a specified value. 11. A method for enabling radio-frequency (RF) communications between an implantable medical device and an external device over a wireless medium, comprising: providing an RF communications link between each of the implantable and external devices, wherein the RF communications link may be switched among a plurality of different RF communications channels that differ in frequency; providing an inductive communications between each of the implantable and external devices; programming the external device controller to select a particular one of the plurality of RF channels as a designated first RF channel for use in an RF communications session with the implantable device and to communicate the choice of the designated first RF channel to the implantable device using the inductive link; programming the external device to monitor the designated first RF channel for activity until the channel is found to be not busy and then establish a communications session with the implantable device over the designated first RF channel; and, programming the external device and implantable device to revert to a previously agreed upon fall-back RF channel upon a degradation in the quality of the designated first RF channel to such an extent so as to be non-operational. 12. The method of claim 11 further comprising programming the external device to monitor the quality of the designated first RF channel by determining if the channel throughput over a certain time falls below a certain level. 13. The method of claim 11 further comprising programming the external device to monitor the quality of the designated first RF channel by determining if a specified number of data frames are unacknowledged. 14. The method of claim 11 further comprising programming the external device to monitor the quality of the designated first RF channel by determining if the frame error rate exceeds a threshold value. 15. The method of claim 11 further comprising programming the implantable device and external device such that, if the previously agreed upon fall-back RF channel is not available, either because of interference or because it is busy, a plurality of previously agreed upon fall-back RF channels are reverted to in a specified sequence. 16. The method of claim 15 further comprising programming the external device and implantable device controllers with a specified timeout duration for each of the plurality of fall-back RF channels so that if no communications are established within the timeout duration, the devices move to the next fall-back RF channel in the sequence. 17. The method of claim 11 further comprising programming the external device such that, if it is determined that the designated first RF channel is degraded to an unacceptable level but is still operational, the external device is programmed to: cease transferring data; monitoring other RF channels for activity in order to find and select another non-busy RF channel for use as an alternate RF channel; and, communicate the selection of the alternate RF channel to the implantable device over the designated first RF channel; and, establish communications with the implantable device over the alternate RF channel. 18. The method of claim 11 further comprising programming the external device to monitor an RF channel for activity by listening for a valid preamble known to be transmitted by all devices during the transmission of data frames and programmed to regard the RF channel as unused if no such preambles are detected over a specified time period. 19. The method of claim 11 further comprising programming the external device to monitor an RF channel for activity by measuring the signal strength of the RF channel over a specified period of time and programmed to regard the channel as unused if the peak signal strength over specified period of time is less than a specified value. 20. The method of claim 11 further comprising programming the external device to monitor an RF channel for activity by: listening for a valid preamble known to be transmitted by all devices during the transmission of data frames; measuring the signal strength of the channel over a specified period of time; and, regarding the RF channel as unused if no such preambles are detected over a specified time period and if the peak signal strength over specified period of time is less than a specified value.