Which wireless technology requires a clear line of sight between the two devices and cannot pass through a wall?

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.