The method of communication in which transmission can take place in both directions, but happens only in one direction at a time, is called

Correct — B, (b) Half Duplex.

THE STEM IS A DEFINITION, AND IT CONTAINS TWO CONDITIONS THAT MUST BOTH BE MET. Read them separately, because the whole item turns on holding them together:

'transmission can take place in BOTH DIRECTIONS' — the channel is two-way, so this is not simplex; 'but happens only in ONE DIRECTION AT A TIME' — the two directions cannot be used simultaneously, so this is not full duplex.

A channel that is two-way but not simultaneous is HALF DUPLEX, and that is option (b).

THE THREE TRANSMISSION MODES, which is the real content of the question, form a short ladder and are best learned with one everyday example each:

SIMPLEX — one direction only, permanently. The sender can never receive and the receiver can never send. A television or radio broadcast, a keyboard sending keystrokes to a computer, a monitor receiving output. HALF DUPLEX — both directions, but only one at a time. The channel must be 'turned round' between transmissions. A walkie-talkie: one party holds the press-to-talk switch and speaks while the other can only listen, and the exchange is punctuated by 'over' precisely because the participants must signal that the channel is free to reverse. FULL DUPLEX — both directions at the same instant. A telephone call, in which both parties can speak and be heard simultaneously.

WHY HALF DUPLEX EXISTS AT ALL, given that full duplex is plainly better. The reason is that a half-duplex arrangement can work over a single shared medium — one radio frequency, one pair of wires, one shared cable — whereas full duplex needs either two separate paths or a way of separating the two directions in frequency or time. Where the medium is scarce or expensive, sharing it and taking turns is the economical design. The cost is throughput and delay: the channel is idle during the TURNAROUND between one direction and the other, and no participant can interrupt or acknowledge while the other is transmitting.

THE CLASSIC COMPUTING EXAMPLE IS ETHERNET, and it shows the same trade-off inside one technology. Early Ethernet used a shared cable or a hub, so only one station could transmit at a time; collisions were possible and were handled by the CSMA/CD protocol — carrier sense multiple access with collision detection — which is a half-duplex arrangement. Modern switched Ethernet gives every station its own dedicated link with separate paths for each direction, so it runs FULL duplex, collisions cannot occur and CSMA/CD is not needed. The same cable technology, two different modes.

The stem ends on the words 'is called' with no punctuation, and each word of every option is capitalised, as this booklet sets them.

Why the others are wrong

• (a)Duplex — This is a real term, not an invented one, and that is what makes it the sharpest distractor on the list. DUPLEX simply means two-way — a channel over which communication can pass in both directions — and it is the umbrella under which both of the other two-way modes sit. So the option is not false so much as INSUFFICIENT: half duplex is a kind of duplex, and so is full duplex, and the bare word does not say which. The stem, however, does not ask what family the arrangement belongs to; it specifies a distinguishing condition, 'only in one direction at a time', and asks for the mode that condition names. An answer that ignores the condition has answered a question that was not asked. Worse, in ordinary engineering usage 'duplex' unqualified is normally taken to mean FULL duplex, which is the exact arrangement the stem rules out. The habit that protects a candidate here is to identify which words in the stem are doing the discriminating work and to insist that the chosen option accounts for all of them.
• (c)Full Duplex — Full duplex is a genuine mode and the most familiar of the three, but it is the opposite of what the stem describes on the point that matters. In full duplex both directions carry traffic AT THE SAME TIME, which requires either two physical paths or a separation of the directions in frequency or in time slots. A telephone conversation is the standard illustration: both parties can speak at once and each hears the other, so no turn-taking protocol is needed. Modern switched Ethernet, in which each station has a dedicated link with a separate transmit and receive path, is full duplex, and so is a mobile phone call. The stem's clause 'but happens only in one direction at a time' is there precisely to exclude this option, so it functions as the trap for a candidate who reads the first half of the definition, sees 'both directions', and stops.
• (d)Simplex — Simplex is also a real mode, and again it fails on a specific clause of the stem — the first one this time rather than the second. In simplex, transmission is possible in ONE DIRECTION ONLY and never in the other: the sending device can only send and the receiving device can only receive, with no reversal at any point. Broadcast radio and television are simplex, as is the link from a keyboard to a computer or from a computer to a printer that returns no status. The stem explicitly says 'transmission can take place in both directions', which rules simplex out at once. It is useful to note that the three modes differ in exactly two binary properties — is the channel two-way at all, and if so can both directions be used at once — and that simplex answers no to the first while half and full duplex answer yes and then differ on the second.

TRANSMISSION MODE describes the directionality of a communication channel: which way data can flow, and whether both ways can be used at once. Three modes exhaust the possibilities.

SIMPLEX — communication in one direction only. One device is permanently the sender and the other permanently the receiver. Broadcast television and radio, a keyboard feeding a computer, a computer driving a display. The entire capacity of the channel is available to the single direction, and there is no mechanism for the receiver to reply.

HALF DUPLEX — communication in both directions, but only one direction at a time. The devices share the channel and take turns; the changeover from one direction to the other is called the turnaround, and it costs time. Walkie-talkies and citizens' band radio are the classic examples, as is a shared-medium Ethernet segment using a hub. The whole capacity of the channel is available to whichever direction currently holds it.

FULL DUPLEX — communication in both directions simultaneously. This needs either two separate physical paths, or one path with the directions separated by frequency or by time division. A telephone call, a mobile phone call and a switched Ethernet link are full duplex. The capacity is effectively shared between the two directions, but neither has to wait for the other.

THE DESIGN TRADE-OFF. Half duplex is chosen when the medium is single and shared — one radio channel, one cable — because it needs no duplication. Full duplex is chosen when responsiveness matters and the extra path or the extra frequency can be afforded. Simplex is chosen when a reply is meaningless, as in broadcasting.

HOW FULL DUPLEX IS ACHIEVED IN PRACTICE. FREQUENCY DIVISION DUPLEXING gives each direction its own frequency band, which is how most cellular voice systems have worked. TIME DIVISION DUPLEXING alternates the directions so fast that the users perceive them as simultaneous. Separate physical pairs — a transmit pair and a receive pair — are how a twisted-pair Ethernet cable does it.

WHERE THIS SITS IN NETWORKING VOCABULARY, since these terms travel with a cluster of others that examinations mix together. Transmission mode — simplex, half duplex, full duplex — is about DIRECTION. Transmission method — serial or parallel — is about whether bits travel one after another on one line or side by side on several. Synchronisation — synchronous or asynchronous — is about how sender and receiver agree on where one unit of data ends and the next begins. Switching — circuit, packet or message switching — is about how a path through a network is established. Topology — bus, star, ring, mesh — is about the physical or logical arrangement of the links. These are four independent axes, and a given link is described by a choice on each of them.

THE ETHERNET ILLUSTRATION IS WORTH CARRYING because it shows a real technology moving along this axis. Shared-medium Ethernet with a hub is half duplex and needs CSMA/CD — carrier sense multiple access with collision detection — to manage the contention. Switched Ethernet with a dedicated link per station is full duplex, the collision domain has one station in it, and CSMA/CD becomes unnecessary.

ETYMOLOGY, which makes the terms almost self-explaining: SIMPLEX from the Latin for onefold, DUPLEX for twofold. Half duplex is duplex capability used half at a time.

Key facts

• Simplex transmission is one-directional only; the sender can never receive and the receiver can never send.
• Half duplex allows transmission in both directions but only one direction at a time, with a turnaround between them.
• Full duplex allows transmission in both directions simultaneously.
• 'Duplex' by itself means two-way and is the family term covering both half and full duplex; used unqualified it usually implies full duplex.
• A walkie-talkie is the standard example of half duplex; a telephone call is the standard example of full duplex; broadcast radio and television are simplex.
• Half duplex allows a single shared medium to carry both directions, which is why it is chosen when the channel is scarce.
• Shared-medium Ethernet using a hub is half duplex and requires CSMA/CD; switched Ethernet with a dedicated link per station is full duplex and does not.
• Full duplex over one medium is achieved by frequency division duplexing, time division duplexing, or separate transmit and receive paths.
• Transmission mode (direction), transmission method (serial or parallel), synchronisation and switching are independent properties of a link.

Common traps

• Choosing the family term 'Duplex' when the stem specifies which kind of duplex it means.
• Reading only the first half of the definition, seeing 'both directions', and answering full duplex.
• Assuming half duplex means half the speed. It means half the time in each direction; the full channel capacity is available to whichever direction currently holds it.
• Believing all Ethernet is half duplex because of CSMA/CD. A switched Ethernet link is full duplex.
• Confusing transmission mode with transmission method. Direction and serial-versus-parallel are separate questions about the same link.
• Calling a mobile phone call half duplex because only one person tends to speak at a time. The channel supports both directions at once; the turn-taking is a social habit, not a technical limit.

• practice — not a real PYQ

  A walkie-talkie set on which a user must press a switch to speak and release it to listen operates in which transmission mode ?

  • (a)Simplex
  • (b)Half duplex
  • (c)Full duplex
  • (d)Multiplex

  Answer(b) Half duplex — the channel carries traffic in both directions but only one at a time, which is why the press-to-talk switch exists and why users say 'over' to hand the channel back. Simplex would allow no reply at all, full duplex would allow both to speak at once, and multiplexing is the unrelated technique of carrying several signals over one medium.

• practice — not a real PYQ

  Which one of the following pairs of examples correctly matches the transmission mode ?

  • (a)Television broadcast — full duplex
  • (b)Telephone conversation — half duplex
  • (c)Keyboard to computer — simplex
  • (d)Citizens' band radio — simplex

  Answer(c) Keyboard to computer — simplex, because the keyboard only ever sends and the computer only ever receives on that link. A television broadcast is simplex and not full duplex, a telephone conversation is full duplex and not half duplex, and citizens' band radio is half duplex and not simplex.