Wireless connections explained

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.

Attempts to create more general-purpose wireless interconnects have had a chequered history. There is an official Wireless USB (WUSB) standard, but it was last updated in 2010, and is held back by its speed: theoretically, it should be able to match USB 2’s maximum 480Mbits/sec bandwidth, but in practice you’re unlikely to get anywhere close to that.

A similar concept is the 802.11ad protocol, branded “WiGig” after the Wireless Gigabit Alliance that developed it (now incorporated into the Wi-Fi Alliance). WiGig can use low-frequency communications to talk to a device that’s 10m away on the other side of a wall, or automatically switch up to 60GHz to communicate with a device sitting right next to the transceiver at up to 7Gbits/sec. The technology hasn’t caught on in the mainstream, but the USB Implementers Forum is working on a new approach to wireless USB that will function over WiGig as well as Wi-Fi networks, so the technology could yet have its day.

A more advanced wireless interconnect is Wi-Fi Direct, a standard that lets any number of Wi-Fi-equipped devices exchange files and information directly, rather than having to go through a router. Only one device needs to support Wi-Fi Direct – the others will simply see it as a regular access point – but range and bandwidth will depend on the hardware, and on which Wi-Fi standard is being used (we’ll get into these issues later). Many smartphones and tablets can act as hosts, as can the Xbox One; you can already buy mice, loudspeakers and printers that support Wi-Fi Direct connections.

Wireless display

The Miracast standard lets you transmit video wirelessly to a TV or monitor by sending an H.264-compressed stream over an 802.11n Wi-Fi Direct link. Support is already built into a number of Android devices, and recent Ultrabooks that support Intel’s own WiDi wireless-display technology can talk directly to Miracast-compatible displays. OS support is built into Windows 8.1.

Few TVs are directly compatible with Miracast, but you can buy a receiver for around £60 that plugs into your TV via an HDMI cable. The catch is latency: it takes time for the transmitting device to encode the video stream, and more time for the receiver to decode it again. Officially, Intel’s latest drivers cut this down to 60ms, but we’ve seen external displays lag behind the built-in one by up to a second. That’s fine for presentations and movies, but a disaster for games.

Apple devices don’t currently support Miracast, but the proprietary AirPlay system does a similar job, letting you use a television to mirror the screen of a Mac or iOS device. A selection of receivers is available, with prices starting at around £30; you can also use an Apple TV appliance.

Ad hoc connections

Some wireless technologies aren’t intended for persistent connections, but for ad-hoc data sharing. The extreme example of this is the radio-frequency identification (RFID) tag – a tiny transmitter that shares a single piece of programmed information with any receiver that comes near to it. RFID technology is commonly used for access management, so an automated door might open only when it detects an RFID tag with a valid identity, or a reader on a London bus might use the RFID chip embedded in your Oyster card to log your journey. Contactless payment systems work in the same way, and modern UK passports include an embedded RFID tag detailing the holder’s personal information, since this is quicker to read electronically and harder to falsify than a printed page.

RFID transmitters don’t necessarily need a power source. The chip has such modest power demands that it can run off the current induced by a nearby electromagnet (see below), and this can be built into the reader. Conversely, it’s possible to use powered RFID tags that communicate with passive readers.

RFID supports an extremely wide range of frequencies, from 120kHz up to 10GHz, with different transmission speeds and different ranges; the most powerful active tags can be read at a range of up to 200m. RFID isn’t something you’d necessarily build into a personal computer or smartphone, but it does have consumer applications: for example, you could conceal an RFID tag inside the frame of a bike, and use it as evidence of ownership if the bike is stolen.

A related technology is near-field communication (NFC). This builds on RFID principles by adding two-way communications capabilities: when two NFC devices are close enough to “see” each other (conventionally established by tapping them together, although physical contact isn’t needed), they can take it in turns to send and receive data.

NFC is built into an increasing number of smartphones and tablets. In principle, it could be used to exchange files and information between such devices; in practice, NFC’s useful range of around 4cm (coupled with a slow maximum data rate of 424Kbits/sec) makes this impractical. Implementations such as Android’s “Beam” feature typically use NFC simply to exchange basic device information, which is then used to initiate a faster and more robust link, such as Bluetooth. Similarly, Windows 8.1’s “NFC printing” feature doesn’t actually transmit pages via NFC; the tag embedded in the printer merely broadcasts its network path and driver details to a receiver. This information can then be used to automatically configure the client so that print jobs can be sent to the printer via conventional infrastructure.

Passive NFC tags are also used in retail, enabling customers pay for goods from a digital wallet by tapping their smartphone against an embedded tag. It’s basically the same idea as the RFID-based payment systems mentioned above, but with the roles reversed so that the active reader is provided by the customer. This makes it much cheaper and easier for service providers to implement – all they have to do is program a cheap, robust NFC tag with the appropriate charging details and affix it to a flat surface.

Bluetooth

Bluetooth is the world’s most popular ad hoc wireless connection: today it’s built into most laptops, smartphones and tablets. It works in the 2.4GHz band, like the keyboards and mice mentioned above, but the standard is more sophisticated, including more than 30 “profiles” that allow Bluetooth-enabled devices to work together in different ways. A Bluetooth keyboard, for example, would support the Human Interface Device (HID) profile, so a compatible operating system would know it could receive input from it. A pair of wireless loudspeakers would use the Advanced Audio Distribution Profile (A2DP), meaning the OS would recognise them as an audio output device. Other defined Bluetooth profiles cover remote control for TVs and hi-fi systems, file transfer, printing, speech transmission and network gateway services – so you can, for example, tether a mobile phone to a laptop via Bluetooth and share its 3G connection.

A Bluetooth link is established by instructing two compatible devices to scan for and connect to one another (sometimes a passkey is required to confirm the link, so you can’t simply connect your phone to someone else’s without their consent). The connection range depends on the class – that is, the power rating – of the devices involved: low-power Class 3 devices have an effective range of around 1m; Class 2 works over around 10m; and Class 1 should support a connection distance of 100m.

There have been four major revisions of Bluetooth. They’re all backwards-compatible, and it’s rare these days to see a device using anything older than Bluetooth 2.1, which was released in 2007. This supports all the major profiles and can beam data between devices at up to 2.1Mbits/sec – fast enough to stream high-quality audio to a headset or a car stereo. (This is referred to as Extended Data Rate, or EDR, since it’s around three times faster than the original Bluetooth specification.)

Bluetooth 3, released in 2009, adds support for high-speed transport, which increases the maximum transfer speed between devices to 24Mbits/sec. This works by co-opting 802.11 hardware, rather than relying wholly on the Bluetooth chipset, so not all Bluetooth 3 devices support it. Other upgrades in Bluetooth 3 offer more reliable connections and automatic power management, so the chipset can scale back its consumption when it isn’t transferring large amounts of data.

The current version of Bluetooth is 4.1 – a minor update to Bluetooth 4, which was released in 2010. Both are branded as “Bluetooth Smart”, and the big change is the inclusion of a new operating mode called Bluetooth Low Energy. That name is no joke: it’s been calculated that a Bluetooth Smart device in LE mode can stay connected to a nearby host for almost a year on a single watch battery. The trade-off is a very slow effective data-transfer rate, of around 0.27Mbits/sec, so headsets, speakers and so forth continue to use the standard or high-speed modes; LE is really intended for simple Internet of Things-type devices, such as fitness trackers.

802.11 wireless networking

802.11 wireless – branded as Wi-Fi – is the universal standard for wireless local area networks. Established in 1997, it’s a standard that’s been through numerous revisions, and although backwards-compatibility isn’t built directly into the protocol, it’s normal for hardware to support older and newer Wi-Fi revisions simultaneously.

Although most commonly used in “infrastructure” mode (whereby all clients connect to a wireless access point or router), the 802.11 standards also support an “ad hoc” mode, which can be used to connect devices together directly. In practice, it’s normally easier to connect devices together over a LAN managed by a router, or to use a complementary technology such as Bluetooth.

The revision of the Wi-Fi standard that first came to prominence was 802.11b (ratified in 1999). This wireless system is based, like most of its peers, on the unlicensed 2.4GHz frequency band. To be precise, it uses 13 “channels” ranging from 2.412GHz to 2.472GHz, so if there’s interference on one, you can (manually) switch to another. Each channel is actually 20MHz wide, which means the channels overlap; if you have two routers in close proximity, avoid using adjacent channels as they’ll interfere with one another. Note that in the US only channels 1-11 are licensed for public use, so imported devices may not support the full spread of channels.

802.11b supports a maximum data rate of 11Mbits/sec. In practice, much of this is eaten up by the overhead of the protocol, and by the vagaries of wireless transmission: in real-world use it’s unusual to see a data-transfer rate of more than 5Mbits/sec. If the connection can’t be maintained at full speed, the system can automatically drop down to slower modes (5.5Mbits/sec, 2Mbits/sec and 1Mbit/sec). The theoretical maximum range of 802.11b is 35m indoors and 140m outdoors.

Devices that support 802.11b networking should in theory work with modern routers, but if they’re that old they may not support the WPA2 (Wi-Fi Protected Access) encryption standard, introduced in 2004. Many 802.11b devices were designed to use the older WEP (Wired Equivalent Privacy) security standard; this is now strongly discouraged, after a flaw in the system was discovered in 2001 allowing the encryption to be easily broken by someone eavesdropping on the encrypted signal.

Following 802.11b, 802.11g was introduced in 2003, based on almost identical technology but raising the maximum data rate to 54Mbits/sec. Owing to technical limitations, an 802.11g network will run more slowly when an 802.11b client is connected, owing to the need to manage traffic so as not to swamp the older client. The overall speed isn’t dragged down to 11Mbits/sec, but the drop in throughput for 802.11g devices can be 30% or greater.

802.11n was released in 2009, although “draft-n” hardware based on unfinished drafts of the standard had been on sale for some years beforehand. The first big change in 802.11n is optional support for 5GHz bands as well as 2.4GHz ones, enabling devices to escape the interference from other wireless networks and communications devices (as well as microwave ovens – notoriously “noisy” in the 2.4GHz band). The two signals are completely separate, so a dual-mode router will advertise two SSIDs: the 5GHz one is likely to give a better service.

Another improvement introduced in 802.11n is support for MIMO (multiple-in, multiple-out) designs, allowing routers to combine the transmission strength and bandwidth of multiple antennae. This improves range to approximately double that of previous 802.11 standards, and allows much faster data throughput: across a single Wi-Fi channel, a maximum bandwidth of 300Mbits/sec is theoretically achievable over four antennae. If you’re shopping for an 802.11n-compatible router, look for one with as many aerials as possible. Note that smartphones generally have only a single antenna, so they can’t take full advantage of MIMO speeds: tablets and laptops are more likely to have two or more antennae.

A final innovation in 802.11n is channel bonding, which uses two adjacent Wi-Fi channels at once to transmit over a 40MHz band at twice the data rate of a regular link. By using four-way MIMO hardware in channel-bonding mode, it’s possible to achieve a maximum of 600Mbits/sec – although in the real world you’ll be lucky to sustain half of that.

The most up-to-date Wi-Fi standard is 802.11ac, which was finally approved in January (although many manufacturers jumped the gun last year with hardware based on the pre-approval specification). This standard uses the 5GHz band only, although ac-compatible routers invariably support a fallback 2.4GHz connection for devices using older standards.

802.11ac supports yet higher transmission speeds than 802.11n, with a theoretical maximum of 88Mbits/sec per antenna on a single channel. It’s possible to bond up to eight channels together into an ultra-fat 160MHz band, and to use up to eight antennae for MIMO transmission, yielding a theoretical maximum data rate of 7Gbits/sec. Again, if you want to take advantage of these speeds, make sure your router and client are equipped with as many antennae as possible. The latest generation of 802.11ac hardware (called Wave 2) also adds support for multi-user MIMO, which allows the router to communicate at high speed with several clients at once, increasing the throughput of the network as a whole.

Wireless power

As well as data, it’s perfectly possible to send electrical power across empty space. The technology doesn’t yet work over a long range, but it’s already possible to charge a smartphone wirelessly by placing it onto a charging pad, or directly next to a charging station.

That’s thanks to an open standard called Qi, developed by the Wireless Power Consortium, which uses electromagnets to induce a charge in nearby devices (“nearby” in this case meaning less than 4cm away). Numerous phones from HTC, LG, Motorola, Nokia and Samsung either include built-in Qi charging or support an optional Qi battery.

A rival technology called Rezence is also being worked on an industry group called the Alliance for Wireless Power, whose members include Broadcom, Dell, HTC, Intel, LG and Samsung. This project aims at pushing the development of wireless charging for devices drawing 20W and more – but we haven’t yet seen any hardware, and it remains to be seen whether its range will be any more extensive than Qi’s.