In the last century, “wireless” colloquially meant broadcast radio. Today it more often refers to a wireless LAN, operating to one of the various 802.11 standards. However, that’s by no means the only sort of wireless communications technology that’s found in modern computers, smartphones and so forth. It’s worth knowing a bit about the various wireless protocols you’ll come across: the names and numbers involved may be cryptic, but the technologies themselves can be terrifically useful.
Simple broadcast technologies
The simplest sort of wireless communication is where a device transmits an open signal into the ether, which can be received and understood by any listening device. It’s the way traditional broadcast radio works, and the same radio frequency (RF) technology can be used in hobbyist computing projects where there isn’t a need for secure communications.
For example, in recent issues of PC Pro we’ve detailed how to collect data from a Raspberry Pi-based weather station over Wi-Fi. We could alternatively have fitted our Raspberry Pi with a cheap RF transmitter (such as Ciseco’s £10 “Slice of Radio”), and set up a listening PC with an RF receiver to listen to its broadcasts. RF is attractively simple for this type of job: there’s no need to mess around with routers and Wi-Fi passwords, and a tiny radio with a small antenna can broadcast through clear air for up to a kilometre.
The big downside of RF is its susceptibility to interference. If two devices positioned close to one another are broadcasting simultaneously at the same frequency, a listening device probably won’t be able to understand either. In addition, conventional RF modules offer a comparatively slow transmission speed of only 250Kbits/sec.
Another simple broadcast technology is IrDA, named after the Infrared Data Association, which developed the standard. IrDA encodes data in patterns of infrared light, in much the same way as a television remote control. Although the transmitted data can be received by any device in range, the receiver has to be within around 5m of the transmitter. For data transfer, infrared has largely been supplanted by Bluetooth, which doesn’t require line of sight and has much greater bandwidth (offering transmission speeds of 25Mbits/sec versus IrDA’s maximum of 4Mbits/sec).
Wireless interconnects
If you buy a wireless keyboard or mouse, it may use Bluetooth, or it may communicate using radio transmissions in the unregulated 2.4-2.5GHz frequency range. Technically, this is the same type of RF communication as described above, but operating at a much higher frequency. This means that the signal has a shorter range (around 10m is common), especially since the peripherals themselves typically have tiny internal aerials.
That shorter range makes it hard for a would-be spy to get near enough to eavesdrop on your keystrokes. On top of this, modern 2.4GHz-input devices typically use a pairing system, whereby a unique encryption key is shared when you press a physical button on both the keyboard, say, and the receiver. Even if other devices nearby can “hear” your keystrokes, they won’t be able to decode them, and the bidirectional link also means errors can be detected and re-sent. Support for “channel-hopping” – automatically switching between different transmission frequencies – ensures that the keyboard and receiver can communicate even when interference is present.
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