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Device pairing is the mechanism through which an ANT slave device gains knowledge of the complete channel ID of a particular transmitter it intends to communicate with. ANT channel search allows an ANT slave device to search for, and acquire an appropriate transmitting master device according to set search criteria.
If the slave knows the entire channel ID of it master, it means the master and slave are pre-paired, the slave will search and communicate with the master device that shares its exact channel ID.
In many practical applications, the slave may not initially have knowledge of the master’s exact channel ID, or it might be desirable to allow connections to more than one device. For example, the slave may know what device type it wishes to connect to, but not the actual device number or transmission type. In this case, the slave can pair to any ANT master device matching the desired device type by placing a wild card (0) in the device number and transmission type fields. Once communication is established, the slave can obtain the full channel ID of the master and save the information for future communications.
Pairing can be critical in multi-user environments. If the pairing process uses wild cards in any of the channel ID fields, the ANT slave device will connect to the first matching master it finds. For this reason, it is impossible to predict which master the slave will connect to if multiple masters with the same partial channel ID are located within range of the slave. ANT has introduced other pairing techniques which can be used to make sure the slave pairs with the correct device, such as pairing bit, inclusion/exclusion lists, search list, proximity search etc. You can find more details by reading through the "ANT_AN02 Device Pairing" and “ANT_AN12 Proximity Search” application notes.
The device number is a 2 byte value that is part of the channel ID. The purpose of the device number is to identify an individual device. As the device number is a 2 byte value it can be used to distinguish between 2^16 devices.
The device number can be extended by using the top nibble of the transmission type (also part of the channel ID) as the top nibble of the "extended device number". Therefore the extended device number can be used to distinguish between 2^20 devices.
If the device number is 0xF5E4 and the Transmission Type is 0x35, then the extended device number will be 0x3F5E4.
When connecting to a master device for the first time (i.e when the channel ID is unknown) it is common to wildcard the device number and transmission type, and just specify the device type.
Once connected the slave device can store the device number and the transmission type of the master, and use these numbers to connect to the same device in future. If the upper nibble of the transmission type was used to extend the device number then the slave will have a 20 bit "extended device number" that it can use to identify the correct master - which is great news for crowded environments like gyms, marathons, and the ANT+ office!
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, or possibly the device number and transmission type. These display options are all acceptable.