USB is the acronym for

Correct — C, (c) Universal Serial Bus.

USB STANDS FOR UNIVERSAL SERIAL BUS, and each of the three words carries a piece of information about what the standard is. Learning it that way is more useful than learning the phrase, because it also explains why the other expansions on the list cannot be right.

UNIVERSAL — one connector in place of many. Before it, a personal computer carried a different port for nearly every class of device: a serial port for a modem, a parallel port for a printer, dedicated round connectors for the keyboard and mouse, a separate game port for a joystick. Each had its own cable, its own speed and its own settings. The standard replaced all of them with a single connector that any peripheral could use, and that universality is the whole reason it was created. SERIAL — the bits of a message travel ONE AFTER ANOTHER along a single data path, rather than side by side along many parallel wires. That seems the slower arrangement and once was; at high speeds it is the faster one, because parallel lines suffer from skew, in which bits sent together arrive at slightly different times, and from interference between adjacent wires. Serial links avoid both, which is why serial has displaced parallel almost everywhere, from disk interfaces to peripheral connections. BUS — a shared pathway to which several devices attach and over which they communicate with the host. A USB installation is a tiered-star arrangement: one host controller at the root, hubs branching from it, and devices on the branches, supporting up to 127 attached devices in all.

WHAT THE STANDARD ALSO SUPPLIES, beyond data, is POWER. A USB port delivers a regulated five volts, and that single design decision is the reason a mouse, a keyboard, a memory stick and a small external drive need no power adapter, and the reason the same connector became the universal charging port for telephones.

THE OTHER PROPERTY WORTH NAMING is hot-swapping. A USB device may be attached or removed while the computer is running, and the operating system detects it, identifies its class and loads the driver without a restart. That is the plug-and-play behaviour this paper asks about separately at question 77, and it is a direct consequence of the standard defining not just a connector but a protocol by which a device announces what it is.

A SHORT HISTORY, since dates are sometimes asked. The specification was developed in the mid-1990s by a group of computing and telecommunications companies and released in 1996; USB 2.0 in 2000 raised the signalling rate to 480 megabits per second; USB 3.0 in 2008 raised it again to five gigabits per second, with later revisions faster still. The connector shapes moved from the rectangular Type-A and squarish Type-B through mini and micro forms to the small reversible Type-C.

All four options end in a real-sounding phrase and only one is the standard's actual name. The stem ends without punctuation, as this booklet sets it.

Why the others are wrong

• (a)Uniform Service Broadcasting — Not a term in computing at all, and the last word is the one that gives it away. BROADCASTING means transmitting from one source to many receivers at once, with no return path and no addressing of individuals — radio and television are the standard examples, and in networking a broadcast is a frame addressed to every station on a segment. A USB connection is the opposite kind of arrangement: a host talks to identified devices over a controlled bus, each transfer directed at a particular endpoint on a particular device, with data flowing back as well as out. 'Uniform', likewise, is not the word the standard uses; the point is that the connector is UNIVERSAL, meaning it serves every class of peripheral, not that anything about it is uniform. An expansion in which none of the three words matches the technology's actual character can be dismissed without knowing the correct answer, simply by asking whether the phrase describes a port on a computer.
• (b)Unique Solution Bus — Only the last word is right. BUS is a real and correct piece of computing vocabulary — a shared pathway over which several devices communicate, and computers contain several of them, including the address bus, the data bus and the control bus inside the processor as well as expansion buses such as PCI. But 'unique solution' is a phrase from mathematics, where a system of equations is said to have a unique solution when exactly one set of values satisfies it, and it has no meaning as the name of a connector standard. The option is constructed on the usual pattern for an acronym distractor: keep the word the candidate is most likely to recognise as correct, and attach to it two words that sound technical without belonging to the subject. Recognising 'bus' as right is therefore no evidence at all that the option is right, since the keyed answer contains the same word.
• (d)Universal Service Broadcasting — This is the near miss, because its first word is the correct one and a candidate who remembers only that the U is for 'universal' will find it satisfying. The second and third words are wrong, and interestingly they are wrong by belonging to a different subject that this very paper also examines. UNIVERSAL SERVICE is a genuine term in telecommunications policy: the obligation to extend telephone and internet access to rural and remote areas that a commercial operator would not otherwise serve. India funds that obligation through the Universal Service Obligation Fund, set up in 2002 and given statutory backing by the Indian Telegraph (Amendment) Act, 2003, financed by a levy on telecom operators' adjusted gross revenue. Broadcasting is a third and separate thing again. So the option assembles a policy term and a media term into a phrase that has nothing to do with a connector on the side of a computer. The lesson is that matching one word of an expansion proves nothing when the keyed option matches that word too.

USB — the UNIVERSAL SERIAL BUS — is the industry standard defining the cables, connectors and protocols by which peripherals connect to a computer, communicate with it and draw power from it. It was published in 1996 by a consortium of computing and telecommunications companies and has become the near-universal peripheral interface.

WHAT EACH WORD MEANS. UNIVERSAL — a single interface for every class of peripheral, replacing the separate serial, parallel, keyboard, mouse and game ports of earlier machines. SERIAL — data are transmitted one bit after another over a single path, in contrast with parallel transmission over many lines at once. BUS — a shared communication pathway with a defined protocol, arranged here as a tiered star with a host controller at the root, hubs beneath it and up to 127 devices in all.

ITS DEFINING FEATURES. HOT-SWAPPING — devices may be connected and disconnected while the system is running. PLUG AND PLAY — the device identifies itself and its class to the host, which loads the appropriate driver automatically. BUS POWER — the port supplies a regulated five volts, so low-power devices need no separate adapter. This is why USB became the standard charging connector for mobile devices. A tiered topology in which hubs extend the number of ports available from a single host connection.

GENERATIONS AND SPEEDS, as usually asked. USB 1.1, 1998 — 12 megabits per second at full speed. USB 2.0, 2000 — 480 megabits per second, called high speed. USB 3.0, 2008 — 5 gigabits per second, called SuperSpeed, with later revisions faster again. Connector families — Type-A and Type-B in full, mini and micro sizes, and the reversible Type-C.

SERIAL VERSUS PARALLEL, which is the concept behind the middle word and is examined in its own right. Parallel transmission sends several bits simultaneously over separate lines and was historically faster for a printer cable. Serial transmission sends one bit at a time over one line. As speeds rose, parallel links ran into SKEW, where bits despatched together arrive at slightly different moments because the lines are not identical, and into CROSSTALK between adjacent conductors. Serial links suffer from neither, can be clocked far faster, and need fewer conductors, so almost every modern high-speed interface — USB, serial ATA, PCI Express, Ethernet — is serial.

WHAT A BUS IS, more generally. Inside a computer, the ADDRESS BUS carries the location being accessed, the DATA BUS carries the contents, and the CONTROL BUS carries the signals that say what operation is to be performed. Expansion buses connect cards and peripherals. The number of lines in a bus — its width — together with its clock rate determines how much it can carry.

RELATED PORTS AND CONNECTORS worth keeping distinct. HDMI carries digital video and audio to a display. Ethernet carries network traffic. VGA carries analogue video. The 3.5 millimetre jack carries analogue audio. Each has its own acronym and its own purpose, and papers of this kind mix them freely.

Key facts

• USB stands for Universal Serial Bus.
• 'Universal' refers to one connector replacing the separate serial, parallel, keyboard, mouse and game ports of earlier computers.
• 'Serial' means the bits travel one after another over a single data path, in contrast with parallel transmission over many lines.
• 'Bus' means a shared pathway with a defined protocol; USB uses a tiered-star topology with a host controller, hubs and up to 127 devices.
• USB supports hot-swapping and plug-and-play: a device may be attached while the system runs and identifies itself so the driver loads automatically.
• A USB port supplies a regulated five volts, which is why low-power peripherals need no adapter and why the connector became the standard charging port.
• The specification was released in 1996; USB 2.0 in 2000 reached 480 megabits per second and USB 3.0 in 2008 reached 5 gigabits per second.
• Connector shapes include Type-A, Type-B, mini and micro forms, and the reversible Type-C.
• Serial links have displaced parallel ones at high speed because parallel lines suffer from skew between bits and from crosstalk between conductors.
• 'Universal service' is a telecommunications policy term, not a computing one: India funds the obligation through the Universal Service Obligation Fund, set up in 2002 and given statutory backing by the Indian Telegraph (Amendment) Act, 2003.

Common traps

• Choosing an option because one of its words is right. Here 'Universal' appears in both the keyed option and a wrong one, and 'Bus' appears in both as well.
• Reading 'bus' as anything other than a shared communication pathway. It is standard computing vocabulary, not a stray word.
• Assuming serial transmission must be slower than parallel. Above modest speeds the reverse is true, because parallel lines suffer skew and crosstalk.
• Confusing 'Universal Serial Bus' with 'universal service', which is a telecommunications policy obligation and has nothing to do with connectors.
• Thinking USB carries only data. It supplies five volts of power, which is why it became the standard charging connector.
• Mixing up the connector generations with the connector shapes. USB 3.0 is a speed specification; Type-C is a plug shape, and the two vary independently.

• practice — not a real PYQ

  In the expansion of USB, the word 'serial' refers to which one of the following ?

  • (a)The devices must be connected in a fixed order
  • (b)Data bits are transmitted one after another over a single path
  • (c)Each device carries a unique serial number
  • (d)The port can be used only by one manufacturer's devices

  Answer(b) Data bits are transmitted one after another over a single path — serial transmission is the alternative to parallel transmission, in which several bits travel simultaneously over separate lines. Devices need not be connected in any fixed order, the serial numbers devices carry are unrelated to the word in the standard's name, and the standard is deliberately open to every manufacturer.

• practice — not a real PYQ

  Which one of the following best explains why high-speed interfaces such as USB and serial ATA use serial rather than parallel transmission ?

  • (a)Serial cables can be made of cheaper metal
  • (b)Parallel lines suffer from skew between bits and from crosstalk, which limit the usable clock rate
  • (c)Serial transmission needs no error checking
  • (d)Parallel transmission cannot carry power alongside data

  Answer(b) Parallel lines suffer from skew between bits and from crosstalk, which limit the usable clock rate — bits despatched together on separate lines arrive at slightly different moments and adjacent conductors interfere with one another, so a parallel link cannot be clocked as fast as a single well-controlled serial line. Serial links still require error checking, and the cost of the metal and the ability to carry power are not the reasons for the change.