Wireless Technologies: Wi-Fi, Bluetooth, NFC and RFID

The correct answer is D, 5 GHz. Wi-Fi operates in the unlicensed industrial, scientific and medical bands, and the two classic bands are 2.4 GHz and 5 GHz, with 6 GHz added in the latest equipment. Of the four choices only 5 GHz is an actual Wi-Fi band, so it is the answer. Option A, 2.4 MHz, looks tempting because 2.4 is the familiar number, but the unit is wrong by a factor of a thousand; 2.4 MHz lies in the medium and shortwave radio range used for broadcasting, not for wireless networking. Option B, 5 MHz, is likewise a shortwave frequency and is far too low to carry the bandwidth a wireless LAN needs. Option C, 10 GHz, lies in the microwave X band used in radar and satellite links; no Wi-Fi standard operates there. The lesson for the exam is to read the unit as carefully as the number.

The correct answer is C, IEEE 802.15.1. The 802.15 group of standards covers wireless personal area networks, and 802.15.1 is the one that was drawn from the Bluetooth specification; the technology itself is now developed by the Bluetooth Special Interest Group. Option A, IEEE 802.3, is Ethernet, the wired local area network standard that uses twisted pair or fibre cable, so it is not wireless at all. Option B, IEEE 802.11, is Wi-Fi, a wireless local area network reaching tens of metres through an access point, a different scale of network from Bluetooth's personal area. Option D, IEEE 802.16, is WiMAX, intended for wireless broadband across a metropolitan area. A quick way to remember the set is 802.3 wired LAN, 802.11 Wi-Fi, 802.15.1 Bluetooth, 802.15.4 Zigbee and 802.16 WiMAX.

The correct answer is B, seven. In a piconet one device acts as the master and sets the clock and the hopping sequence, and up to seven other devices can be active slaves at the same time, which makes eight devices in the piconet in all. Addressing in classic Bluetooth uses a three bit active member address, and three bits give eight values, one of which is reserved for the master; that is the arithmetic behind the number. Further devices can be held in a parked state and brought in when a slot falls free. Option A, three, is simply too small. Option C, eight, is the favourite trap: eight is the total size of the piconet including the master, not the number of slaves. Option D, sixteen, exceeds what the three bit address allows. When two piconets are bridged by a shared device the combination is called a scatternet.

The correct answer is A, 13.56 MHz. NFC is built on the high frequency band of radio frequency identification and uses magnetic induction between two loop antennas at 13.56 MHz, which is why the devices must be brought within about four centimetres of each other. Option B, 125 kHz, is the low frequency RFID band used for animal tags and older access cards, which have a wider read distance but much slower data transfer. Option C, 2.4 GHz, is the band shared by Wi-Fi, Bluetooth and microwave ovens; its reach is metres, far too long for a technology whose security rests on being readable only at a touch. Option D, 865 MHz, is the ultra high frequency RFID band allotted in India for logistics and for FASTag on the windscreen. Remembering 13.56 MHz as the number common to NFC and high frequency RFID cards settles several questions at once.

The correct answer is B, the radio signal of the reader. A passive tag is only a chip and an antenna. When it enters the field of a reader, the incoming radio waves induce a current in the antenna, that current powers the chip for an instant, and the chip replies by modulating the reflected signal, a method called backscatter. Because there is no battery the tag costs very little, can be printed as a sticker, and lasts for years, but its read range is short and it cannot start a conversation of its own. Option A describes an active tag, which carries a battery and transmits on its own over tens of metres, and is used on shipping containers and vehicles. Option C is wrong because a solar cell would need light, which is useless for a tag inside a carton or on a library book. Option D is wrong because the earth's magnetic field is far too weak and steady to power or carry data.

The correct answer is B, passive ultra high frequency RFID. A FASTag is a battery free RFID sticker pasted on the inside of the windscreen. An overhead reader at the toll plaza energises it, reads the identity number, matches it with the linked prepaid wallet or bank account and debits the toll while the vehicle keeps moving. The ultra high frequency band is chosen precisely because it can be read from several metres at speed. Option A, Bluetooth Low Energy, needs pairing and is meant for personal devices, not for a stranger's reader at a plaza. Option C, an infrared link, requires a clear line of sight and fails in rain, dust and sunlight, so it is unsuited to a highway. Option D is wrong because satellite positioning tells where a vehicle is but cannot by itself identify and charge it at a gantry, though satellite based tolling is being tested as the next step.

The correct answer is A, a Danish king who united warring tribes. The technology is named after Harald Gormsson, the tenth century king of Denmark nicknamed Bluetooth, who brought the Danish tribes under one rule. The engineers at Ericsson who were building a radio link to replace cables chose the name because their aim was the same, to unite devices that could not talk to one another, and the familiar logo is a binding of the runes for the king's initials. Option B is wrong because no inventor of that name exists; the engineer usually credited with the radio design is Jaap Haartsen. Option C is a folk explanation with no basis, since the antenna has no such shape or colour. Option D is wrong because the work was done at Ericsson in Lund in Sweden, but the name honours the Danish king and is not a place name at all.

The correct answer is D, WPA3. WPA3 is the newest of the four and the strongest; it replaces the old pre shared key handshake with simultaneous authentication of equals, which resists offline guessing of the password, and it encrypts traffic even on an open network. Option A, WEP, or wired equivalent privacy, is the oldest and is completely broken; its short initialisation vector lets an attacker recover the key from captured traffic in minutes, and it should never be used. Option B, WPA, was a stopgap that added the TKIP key mixing scheme to old hardware and has since been found weak. Option C, WPA2, brought in AES based encryption and is still widely used and reasonably safe, but it is vulnerable to the key reinstallation style of attack and to offline password guessing, which is exactly what WPA3 was designed to stop. The order of strength is WEP, then WPA, then WPA2, then WPA3.

The correct answer is B, Infrared. Infrared communication, the old IrDA link of television remote controls and early mobile phones, uses light just beyond the red end of the visible spectrum. Light of that wavelength is stopped by any opaque object, so the transmitter and the receiver must point at each other with nothing in between, and the range is only a metre or two. Option A, Bluetooth, uses 2.4 GHz radio waves that bend around and pass through ordinary walls, which is why a headset still works with a phone in a pocket. Option C, Wi-Fi, also uses radio and is in fact chosen in the 2.4 GHz band precisely because it penetrates walls better than 5 GHz. Option D, Zigbee, is a low power radio mesh on 802.15.4 that is designed to relay signals from node to node around obstacles. Infrared's need for line of sight is the reason Bluetooth replaced it for device links.

The correct answer is B, frequency hopping spread spectrum. A Bluetooth link does not stay on one frequency. Master and slave agree on a pseudo random sequence and jump together between the narrow channels of the 2.4 GHz band many hundreds of times a second. If one channel is occupied by a Wi-Fi network or a microwave oven, only the packets that landed there are lost and are resent on the next hop, and an eavesdropper who does not know the sequence cannot follow the conversation. Option A is wrong because a single fixed channel is exactly what hopping avoids. Option C, wavelength division multiplexing, is a technique of optical fibre in which several colours of laser light share one strand; it has no place in radio. Option D is wrong on both counts: Bluetooth uses digital modulation, not plain amplitude modulation, and it works in an unlicensed band that needs no spectrum licence.

The correct answer is B, it can be read without line of sight and many tags at a time. A barcode must be presented squarely to a scanner, one item at a time, and a torn or dirty label fails. An RFID tag answers a radio signal through cardboard, cloth and plastic, so a whole carton or a shelf of library books can be counted in one sweep, and the vehicle need not stop at a toll gantry. Option A is wrong because a printed barcode costs almost nothing while even a cheap passive tag costs several rupees, and that cost is the main reason barcodes survive on low value goods. Option C is wrong and is in fact the opposite of a strength: many RFID tags are rewritable, so the data on them can be updated during the journey of the goods. Option D is wrong because reading a tag on a moving object is one of the technology's defining uses, as FASTag shows.