Data transmission

Data transmission, also known as digital transmission, is the transfer of data between digital devices through a transmission medium. Two different modes of data transmission exist: parallel transmission and serial transmission. Both modes transfer binary data, or binary bits. Parallel transmission uses data lines to send groups of bits simultaneously. In serial transmission, bits are sent one after the other. Serial transmission may be asynchronous, synchronous, or isochronous. Asynchronous transmission uses start bits and stop bits and also has gaps between the different bytes. With synchronous transmission, timing the transmission of each bit is required. Isochronous ensures data arrives at guaranteed, regular intervals for real-time applications. Different transmission media are used to transfer data. Transmission media also include wireless systems such as Wi-Fi and cellular networks, as well as satellite communication.

Overview

In both parallel transmission and serial transmission, bits are represented by a series of 0s and 1s. In parallel transmission, the bits of data are grouped together in a certain length and are sent from the sender to the receiver. Data lines, or wires, connect the sender and receiver. Each data line sends one bit, and all the bits are sent at the same time. This means that all the groups of bits are sent simultaneously on separate data lines. Typically used for short distances, parallel transmission is useful because it is a quick way of transferring data all at once. However, this type of data transmission is expensive because of the large number of wires that are required.

Serial transmission differs from parallel transmission because bits are sent serially, or one after the other. Unlike parallel transmission, only one data line is needed between the sender and receiver. Therefore, all the bits are sent on the same line. Serial transmission is typically used over long distances and is less expensive than parallel transmission because only one line is needed. However, serial transmission sends bits one at a time on one line, while parallel transmission sends multiple bits at once on separate lines. Many systems use serial interfaces such as USB (Universal Serial Bus), SATA (Serial Advanced Technology Attachment), PCIe (Peripheral Component Interconnect Express), and Thunderbolt because they reduce wiring and can support high data rates.

Asynchronous transmission and synchronous transmission involve bit synchronization, which determines the beginning and end of the data transmission and offers timing control. With asynchronous transmission, one byte of data (either a number or a letter of the alphabet) is sent at a time. This type of serial transmission uses start bits, stop bits, and sometimes a parity bit, which helps detect errors. A start bit, usually a 0, tells the receiver that a group of bits has arrived, while a stop bit, usually a 1, indicates that transmission has ended. Gaps, or idle time, exist between the transmission of different bytes.

With synchronous transmission, the bit stream consists of longer frames of multiple bytes. In synchronous transmission, no start or stop bits or gaps exist between the different bytes. Therefore, timing the transmission of each bit is required. Both the sender and receiver must operate at the same clock frequency, and the receiver must recognize and separate the bit stream into bytes to reconstruct the original information. Synchronous transmission is faster and more efficient than asynchronous transmission, mainly because of the absence of the extra start and stop bits and gaps. Asynchronous transmission is less expensive than synchronous transmission.

Data is transferred through transmission media, including magnetic media, twisted pair cables, coaxial cables, fiber optic cables, and power line communication (PLC). Magnetic media uses either magnetic tapes or magnetic discs. Data backup is stored on tapes or discs and then physically transferred. Magnetic media is typically used when the size of the data is very large.

A twisted pair cable consists of two insulated copper wires that are twisted together. One of the wires carries the signal, and the other wire usually serves as a ground, which provides a reference point for the signal. The reason the wires are twisted together is to reduce noise, which is electromagnetic interference, and crosstalk, which is unwanted signals in the channel. Twisted pair cables come in two types: shielded twisted pair (STP) cable and unshielded twisted pair (UTP). The STP cable is covered in metal foil, while the UTP cable is not. STP cables are less susceptible to noise and crosstalk than UTP cables.

The coaxial cable consists of two copper wires. The core wire is covered in an insulating sheath. The other wire is wrapped around the core wire and is also covered by a sheath. Both wires are enclosed in a plastic cover. Coaxial cables offer protection against noise and crosstalk. They come in three categories—RG-59 for cable television, RG-58 for some older network (thin Ethernet), and radio uses, and RG-11 for higher-frequency signal distribution.

A fiber optic cable uses the properties of light. It has a core made of glass or plastic. Light is emitted at one end of the wire, travels through the wire, and is converted into electric data at the other end. Fiber optic cables are available in two modes: single-mode fiber and multimode fiber. As their names imply, single-mode fiber carries one ray of light, while multimode fiber carries multiple rays of light.

Power line communication (PLC) transmits data signals over power cables. PLC modulates the data and sends it over the cables, and then the receiver demodulates the data and interprets it. PLC can control and monitor powered devices. Narrowband PLC and broadband PLC are two types of PLC. Broadband PLC offers higher data rates than narrowband PLC. PLC is also used in systems that monitor and control electrical devices, such as smart grids and connected home systems.

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