#21 Transmitting is Cheaper than Receiving
An invisible fact of radio communications, but one that impacts the design of all of the devices that we use is that receiving data is typically a lot more power hungry than transmitting it. This is in large parts because devices often need to keep their receiver turned on for much longer than the actual message they want to receive, because they don’t know ahead of time exactly when it is going to arrive.
This is why there are (transmit-only) beacons that have a battery life of years. These get received by phones that have big batteries, but only with an expected battery life of days at best.
This is why your Bluetooth headphones will periodically advertise their presence and only listen for connection requests for an extremely brief interval thereafter, putting the onus of the expensive Bluetooth ‘scan’ (i.e. listening continuously for several seconds) on the device that wants to connect to them.
This is also why most mesh networks don’t lend themselves to battery powered infrastructure. Bluetooth Mesh and Zigbee can have battery powered end-devices, so long as they don’t route traffic as part of the mesh. Wirepas is the only technology that has a mode optimised for battery-powered infrastructure.
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