Feb 01, 2013
Pairing to Devices with Extended Device Numbers
So you probably already know the basic concept of ANT connections: an ANT master transmits at regular intervals (e.g. 4Hz) and an ANT slave can then search for the master. Once the slave has found the master, it can switch it's radio on and off at the right times to be able to receive the master's transmissions. The slave can also send messages back to the master.
You probably also know that the slave will only find masters that have channels configured in a way that matches the slaves configuration - i.e. that are on the same network, that use the same RF frequency, and that have matching channel IDs.
The channel ID is a 4 byte value that defines the:
1 byte Transmission type
1 byte Device type
2 byte Device number
The ANT master always sets these values on the ANT+ network:
The transmission type is used to define whether the data is being sent using page numbers, and whether shared or independent channels are used. The top nibble of the transmission type can either be set to zero, or used to extend the device number. If used, then the top nibble of the transmission type becomes the top nibble of the device number.
The device type is used to define the what kind of device is transmitting. On the ANT+ network, this value is used to distinguish between heart rate monitors, foot pods, glucose monitors etc.
The device number is used to identify individual ANT master devices (i.e. to distinguish between two heart rate monitors). If the top nibble of the transmission type is used to extend the device number, then the resulting "extended device number" consists of 20 bits, allowing the 2^20 devices to be separately identified - that's 1 048 576 devices. For comparison, devices using the non-extended two byte device number, 2^16, or 65 536 individual devices can be identified. So if large numbers of sensors are expected, consider using extended device numbers to make the channel ID more uniquely identifiable.
So, how does the ANT slave match it's channel ID to that of the master it wishes to find?
Typically the slave knows what kind of device it is looking for and can set its device type accordingly. But, unless the correct transmission type and device number of the ANT master have been programmed into the ANT slave (either at the factory or because the device has paired to the master before and has stored the information) then these values will need to be wildcarded (i.e. set to zero for the search).
However, once connected, the ANT slave learns the full channel ID of the master, and can store this value for use in future searches. Then when the user turns on the device it will automatically connect to the same sensor.
In crowded environments, for example gyms, or large race events, the user is likely to switch on their device when surrounded by many possible slaves. But if the full channel ID is used for the search then it will still only connect with matching masters.
If the master used the extended device number: this means that there is a (number of nearby devices)/ (1 048 576) chance that another nearby device is found instead of the one that the user owns.
If the master did not use the extended device number: this means that there is a (number of nearby devices)/ (65 536) chance that another nearby device is found instead of the one that the user owns.
This difference is the reason that the extended device number is used. It's all about pairing reliably.
Please note that if a display shows the device number to the user, it may show either the 2 byte device number, or the 20 bit extended device number (if the upper nibble of transmission type is used), or possibly the device number and transmission type. These display options are all acceptable. If the display allows the user to key in the device number from the display keyboard, the display should consider there are two possible "device numbers" that could be keyed in.
To make slave devices interoperable with both the standard and extended device numbers, the display device typically needs to wildcard both the device number field and transmission field to perform the first pairing search.
In conclusion: The extended device number is not intended as a number that must be displayed - it is intended to increase a device's chance of pairing to the right device every time - even in crowded environments.
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