The Rules of Conversation: Direction of Data Flow

In any communication between two devices, the direction in which information can flow is one of the most fundamental properties of the connection. This directionality determines whether the conversation can be one-way, two-way taking turns, or a simultaneous two-way exchange. There are three main modes of transmission.

1. Simplex Transmission

Simplex is the simplest mode of transmission, functioning like a one-way street. Information can only travel from the sending device to the receiving device, never the other way around. The roles of transmitter and receiver are permanently fixed.

Examples of Simplex Communication

• Radio and TV Broadcasting: A central tower sends out signals, and home radios or televisions receive them. You cannot send a signal back to the radio station through your radio.
• Computer to Monitor: A computer sends visual information to a monitor, but the monitor does not send data back to the computer (excluding modern protocols with metadata).
• GPS: Satellites transmit signals down to GPS receivers on Earth, which listen passively. Your car's GPS does not transmit back to the satellites.

2. Half-Duplex Transmission

Half-duplex mode is like a single-lane bridge where traffic can go in both directions, but cars must wait for the bridge to be clear before crossing. Both devices on the link can transmit and receive, but never simultaneously.

Examples of Half-Duplex Communication

• Walkie-talkies (CB Radio): One person must press a button to talk, and the other person can only listen. To respond, the second person must wait for the first to finish and release the button. Saying "Over" signifies the end of a transmission to let the other person know it's their turn to speak.

3. Full-Duplex Transmission

Full-duplex mode is the most efficient form of communication, akin to a two-lane highway where traffic flows in both directions at once. This allows for a natural, interactive exchange of information.

Examples of Full-Duplex Communication

• Telephone Conversation: Both parties can speak and hear at the same time, allowing for interruptions and a natural flow of conversation.
• Modern Ethernet Networks: A computer connected to a switch can send and receive data packets simultaneously, dramatically increasing the effective throughput of the network connection.

How Full-Duplex is Achieved

Achieving simultaneous two-way communication over a single medium requires special techniques:

• Two Physical Channels: The simplest method is to use separate physical paths for each direction, such as using two pairs of wires in an Ethernet cable-one pair for sending, one for receiving.
• Frequency-Division Duplex (FDD): The available frequency band is split into two separate sub-bands. One sub-band is used for transmitting, and the other is for receiving. This is common in mobile phone systems.
• Time-Division Duplex (TDD): Transmission time is divided into very short, alternating time slots. One slot is for transmitting, the next is for receiving. The switching is so rapid that it creates the illusion of a continuous, simultaneous connection.
• Echo Cancellation: An advanced technique used on single-pair wires (like DSL lines). The device transmits and receives on the same frequency at the same time. A sophisticated digital signal processor (DSP) calculates the "echo" of its own transmitted signal and subtracts it from the received signal, leaving only the signal from the other end.