The A+ Exams Guide: (Exam 220-301), (Exam 220-302) (TestTakers Guides)

Most peripheral devices, such as printers, scanners, and modems, utilize asynchronous transmission methods. With asynchronous transmission, data is not synchronized. Unlike synchronous transmission, data is not sent as a steady stream in a predetermined fashion. Instead, a start bit and a stop bit are placed between each piece, or ‘packet' of information. Asynchronous transmission methods are typically used for devices attached to parallel or serial ports.

Unlike asynchronous transmissions, synchronous transmissions are steady streams of data that are predetermined by a clock or counter. The CPU communicates with internal devices synchronously, basing the transmission of data and instructions on its own internal clock.

Transmission Modes

When two devices connect to each other, they establish and utilize a transmission mode. The transmission mode established between the two devices depends on the technology and configuration of the devices. Three general transmission modes are available that determine whether the transmission of data between two devices will occur only one way, one way at a time, or both ways at the same time. These transmission modes are simplex, half-duplex, and full-duplex.

• Simplex. The simplex form of data transmission goes only one way; data or information can be transmitted in only one direction. A radio and speakers are examples of devices that utilize simplex communication.

• Half-duplex. With half-duplex data transmission, data can be transmitted in both directions, but can only be transmitted in one direction at a time. An example of this transmission method is walkie-talkie: both parties can speak, but only one party can speak at a time.

  Note

  Note: If two devices are set up so that they cannot send and receive data at the same time, they are utilizing half-duplex data transmission.

• Full-duplex. In full-duplex transmission, or simultaneous transmission, data or voice can be transmitted and received at the exactly the same time. Human speech during a regular phone conversation is an example of full-duplex transmission that doesn't work well. Two parties can speak at the same time, although they might not understand each other.