Bluetooth technology allows

Correct — C, (c) wireless connection between various devices/equipments over short distances. Bluetooth is a short-range radio standard whose whole purpose is to let nearby devices talk to one another without a cable. A handset and a headset, a laptop and a mouse, a phone and a car audio system, a computer and a printer: each pair is joined by a low-power radio link operating in the 2.4 GHz industrial, scientific and medical band, which needs no licence anywhere in the world. Because that band is crowded, the classic version of the standard hops rapidly between seventy-nine one-megahertz channels, so that interference on any one channel damages only a fraction of the traffic. Devices that have been paired keep a shared key and recognise each other afterwards, and a group of them forms a piconet in which one device acts as master and up to seven others are active alongside it.

Distance is the property the standard is built around, and it is set by the transmitter's power class: the class most handsets use reaches roughly ten metres, the highest class about a hundred, and the lowest about one. That is the personal area — the space around one person and the devices on and near them — which is why Bluetooth is described as a personal area network technology, in the same series as the local, metropolitan and wide area networks. Option (c) states exactly this: a wireless connection between various devices over short distances. The technology was developed at Ericsson from 1994, the Bluetooth Special Interest Group was formed in 1998, and the name is taken from Harald Gormsson, the tenth-century Danish king called Bluetooth, with a logo that binds together the runes for his initials.

Why the others are wrong

• (a)sending of files within the range of 10 km — The number is right and the unit is wrong, which is what makes this the sharpest distractor in the set. Ten is indeed the figure associated with Bluetooth, but it is ten metres, not ten kilometres — the nominal reach of the ordinary power class used in handsets and headsets. Even the highest-powered class of the standard is quoted at about a hundred metres, and that is in the open with nothing in the way. A ten-kilometre link is a different engineering problem altogether: it needs far more transmitted power, directional antennas or licensed spectrum, and it belongs to cellular networks, point-to-point microwave links or satellite. A candidate who reads the option quickly sees a familiar number and stops. The lesson is to read the unit as carefully as the figure, because an examiner who wants to punish half-knowledge will keep the number a candidate remembers and change what it measures.
• (b)sending an e-mail — Electronic mail is an application service, not a property of a radio link. Sending a message requires a mail client, an account on a mail service and a route to that service across the internet, and it works in exactly the same way whether the last few metres of that route are copper, fibre, Wi-Fi or a cellular connection. Bluetooth is one such last link and carries whatever bytes are handed to it, so nothing here is impossible; but naming e-mail as what Bluetooth allows describes the wrong layer of the system, in the way that describing a road by saying it allows one to buy vegetables describes the market rather than the road. The question asks what the technology itself makes possible, and the answer has to be stated in terms of the connection it provides — between which devices, and over what distance.
• (d)downloading of movies from Internet — Downloading films is a job for a broadband connection, whether fixed or cellular, and the phrase 'from Internet' in the option is the giveaway: Bluetooth does not connect anything to the internet. It connects one device to another device close by, and any internet access those devices enjoy comes from a different network — the fixed line behind a router, or the cellular link a handset holds. A phone can share that cellular connection with a laptop over a Bluetooth link, but the internet connection in that arrangement is still the cellular one and the Bluetooth link is only the short hop at the end. The capacity of the classic radio, a few megabits per second at best, is also matched to small transfers and audio streams rather than to bulk video, which is precisely why Wi-Fi and not Bluetooth is the technology people use for large downloads.

Networks are classified by the area they cover, and Bluetooth occupies the smallest class. A personal area network spans the few metres around one user and the equipment they carry; a local area network covers a building or a campus; a metropolitan area network a city; a wide area network a country or the world. Bluetooth was designed for the first of these. It works in the unlicensed 2.4 GHz band, hops between channels many times a second so that a busy band does not destroy the link, keeps transmitted power low so that batteries last, and organises devices into a piconet with one master and up to seven active companions. What a pair of connected devices may then do with each other is decided by profiles — the hands-free profile for a headset, the advanced audio distribution profile for stereo sound, the human interface device profile for a keyboard or a mouse, the object push profile for sending a file. The later low energy variant, introduced with version 4.0, cut power consumption far enough for sensors, fitness bands and beacons to run for months on a coin cell, and it is the reason the standard has outlived the headset era it was invented for.

Key facts

• Bluetooth is a short-range wireless standard for connecting devices to each other, forming what is called a personal area network.
• It operates in the unlicensed 2.4 GHz ISM band and, in its classic form, hops between seventy-nine channels of one megahertz each.
• Range depends on the power class: about ten metres for the class used in handsets, about a hundred metres for the highest class and about one metre for the lowest.
• Connected devices form a piconet with one master and up to seven active companion devices; pairing gives them a shared key so they recognise each other later.
• The technology was developed at Ericsson from 1994 and the Bluetooth Special Interest Group was formed in 1998.
• The name comes from Harald Gormsson, the tenth-century Danish king known as Bluetooth, and the logo joins the runes of his initials.
• Profiles decide what a link may be used for — hands-free calling, stereo audio, keyboards and mice, and file transfer each have their own.
• Bluetooth Low Energy, introduced with version 4.0, cut power consumption enough for sensors, fitness bands and beacons to run on a coin cell.

Common traps

• Keeping the number and losing the unit — ten metres becomes ten kilometres, and the option looks familiar enough to pass.
• Describing a technology by an application it can carry rather than by the connection it provides.
• Assuming any wireless technology gives access to the internet; Bluetooth links devices to each other, not to a network operator.
• Confusing Bluetooth with Wi-Fi because both use the 2.4 GHz band; the band is shared but the purpose and the range are not.
• Forgetting that range varies by power class, so a single quoted figure is a typical value and not a limit of the standard.

• practice — not a real PYQ

  Bluetooth technology is best described as a standard for which one of the following types of network?

  • (a)Personal area network
  • (b)Local area network
  • (c)Metropolitan area network
  • (d)Wide area network

  Answer(a) Personal area network

• practice — not a real PYQ

  In which frequency band does Bluetooth operate?

  • (a)900 MHz cellular band
  • (b)1.8 GHz licensed band
  • (c)2.4 GHz unlicensed ISM band
  • (d)5.8 GHz satellite downlink band

  Answer(c) 2.4 GHz unlicensed ISM band