Analog transmission involves sending information through a transmission medium in the form of an analog signal.
Transmission medium
Transmission media can support the propagation of sound waves, electromagnetic waves, and optical signals. These media typically confine these signals within their metallic wires or fiber-optic cables. Radio transmission is delivered via line-of-sight microwave and satellite communications. The type of medium, its shielding properties, and even the number of twisted pairs in copper wire cables all determine the potential data transmission rate over that cable.
Transmission lines can be either "balanced" or "unbalanced." Balanced cables typically consist of two conductors, such as twisted-pair or bifilar cables. Unbalanced cables, on the other hand, usually take the form of coaxial cables. In a balanced line, both wires are connected to the generator (transmitter) and the receiver, carrying identical currents—but in opposite directions. In an unbalanced cable, the current flows through one conductor and returns via the ground.
Cable Type
Here are the commonly used cable types in network communication systems.
Straight cables—also known as copper wires enclosed in insulation—can come either as individual strands or bundled together, like flat cables, and are commonly used to connect short-distance or low-bit-rate peripheral devices. Serial cables, such as those used for modems or serial printers, rely on this type of wiring. While crosstalk can occur between adjacent wires, twisting the wires together helps minimize interference, as explained below.
Twisted-pair cable consists of a pair of copper wires wrapped together and encased in an insulator, forming a balanced circuit. This twisting arrangement effectively minimizes interference issues. Twisted-pair cables are commonly used in audio telephone systems and computer networks. By employing advanced cables—such as Category 5—and ensuring proper connectivity across all twisted pairs leading to the connection point, high data transmission rates (up to 100 Mbps) can be achieved. If necessary, multi-strand twisted-pair cables with shielding can also be utilized.
Coaxial cable is a solid copper-core wire encased in an insulator, surrounded by a grounded, shielded mesh, and covered with an outer jacket. While coaxial cables enable high data transmission rates, recent advancements in rated transmission technologies for twisted-pair cables now offer data speeds that match or even approach those of coaxial cables. Nevertheless, coaxial cables still maintain the advantage of supporting longer transmission distances compared to twisted-pair cables.
Fiber optic cables consist of a central glass core designed to guide the propagation of light waves. This core is encased in a cladding layer that reflects light back into the core, ensuring efficient transmission. Surrounding the core and cladding are a thick plastic jacket and specialized fibers that enhance the cable's strength. For applications requiring extremely long-distance transmission, fiber optic cables with metal cores—designed to boost durability—can be used. Fiber optic cables transmit photons through pristine glass, offering unparalleled signal integrity: there’s no interference, and no signal leakage occurs. Additionally, fiber optic cables can carry data over significantly greater distances compared to copper-based cables. For more information, see "Fiber Optic Cables."
Broadband transmission: This type of transmission utilizes infrared and radio waves to achieve connectivity.
See Microware Communication, Satellite Telecommunication, Wireless LAN Communication, and Wireless Mobile Communication.
Transmission method
Transmission Methods　
Signals are generated using electricity, radio waves, or light to transmit encoded information. There are two primary methods for transmitting signals over a medium: analog signal transmission and digital signal transmission.
Analog signals represent infinitely continuous variations in voltage or waves within a device that generates, measures, records, or transmits these signals. For example, the waveform produced by a telephone in a phone booth is an analog signal.
Digital signals are transmitted by switching the channel's electrical levels between high and low states. Their primary task is to encode information into a transmission signal represented by binary digits—1s and 0s.
Simulated Transmission
Analog signals are a way of transmitting energy—for example, sound waves travel by vibrating air. A sound wave with varying pitch and volume can be mapped onto a piece of paper.
A telephone is a transducer device that converts analog sound waves into corresponding electrical signals. At the receiving end of a telephone or audio system, the amplitude (volume) and pitch of the speaker's vibrating signal are captured. Pitch reflects the frequency of the wave's oscillation, which is typically measured in cycles per second (cps) or Hertz (Hz). One Hertz equals one cps. For instance, 1 kHz (kilohertz) equals 1,000 Hz, 1 MHz (megahertz) equals 1,000 kHz, and 1 GHz (gigahertz) equals 1,000 MHz. The range of frequencies humans can hear typically spans from 20 Hz to 20,000 Hz—and this is precisely the operational range of high-fidelity audio equipment as well.
Simulated waveforms can be either very simple or highly complex. A single sound wave produced by a single piano tuning fork contains only one distinct frequency. In contrast, a complex waveform—such as the human voice or the sound of an organ—consists of multiple frequencies combined together, as illustrated in Figure T-16.
The telephone system uses analog switching lines for audio communication. In general, data communication over analog lines faces several limitations that restrict their practicality. To transmit digital signals as analog ones, modulation is required. Additionally, because audio lines have a relatively narrow frequency band, this limits the transmission rate. Moreover, analog signals often need to be amplified step by step over long distances, which can inadvertently amplify any distortions present in the signal as well. For applications requiring the transmission of large volumes of information—such as graphics processing or voice handling—there’s a growing need for significantly wider bandwidths beyond what analog systems can provide.
Digital Transmission
Audio, video, data, and other forms of "information" can be encoded into binary digits, enabling efficient transmission—these digits are sent as electrical pulses. Within the cable, voltage fluctuates between high and low states. Specifically, binary '1' is transmitted by generating a positive voltage, while binary '0' is conveyed via a negative voltage. As a result, digital services typically offer greater reliability than analog services, especially over long distances. If the signal needs amplification, digital signals can simply be regenerated. In contrast, analog signals must be amplified step-by-step over long distances, inevitably picking up noise along the way that gets amplified right alongside the original signal.
Analog-to-digital conversion, also known as digitization, is the process of transforming continuously varying waveforms from the real world into digital signals that can be stored and processed by computers. This analog-to-digital conversion process—commonly referred to as Pulse Code Modulation (PCM)—involves sampling the signal at fixed time intervals and capturing its amplitude and frequency information in binary form. The precision of these numerical values depends on the number of bits used to represent them. For instance, if an analog waveform is sampled 1,000 times per second, it yields 1,000 discrete digital values ready for storage or transmission. When speech is converted for transmission over digital lines, it is sampled 8,000 times per second, with each sample being encoded into an 8-bit binary value. As a result, a digitized audio channel requires a bandwidth of 64,000 bits per second.
Transmission Mode
Transmission Modes
Data can be transmitted either bit-oriented or character-oriented (also known as byte-oriented). In bit-oriented transmission, these bits represent a continuous stream of data—such as image data—and, aside from a specific flag indicating the start of a frame, carry no special meaning for either the sender or the receiver. In contrast, in a typical character-oriented protocol, sequences of 8 bits represent control codes and alphanumeric characters. Character-oriented protocols encompass various transmission modes, which will be discussed below.
Simplex Circuit: A one-way connection, much like radio broadcasting, where the receiver cannot send a response. In data communication, simplex circuits are used in master-slave configurations, with one device controlling the others—and no acknowledgment is required from the controlled devices. Typically, simplex circuits are not employed when human interaction is needed at the controlled station.
Half-duplex circuitry refers to a type of duplex transmission where communication can only occur in one direction at a time. A prime example is CB (CITIZEN BAND) radio communication, where only one operator can speak at any given moment. Once an operator finishes speaking, they say "over," allowing the other operator to take their turn. Similarly, in half-duplex circuits, a signaling system indicates when a device has completed sending or receiving data, enabling the next device to access the line. Half-duplex communication can be implemented using single-wire, two-wire cables, or twisted-pair wiring.
A full-duplex circuit enables two-way, simultaneous communication. In digital networks, this type of circuit typically requires two pairs of wires to function properly. However, analog circuits connected via a modem only need a single pair of wires. The circuit’s bandwidth is divided into two distinct frequency bands, allowing data to flow in both directions at the same time.
When connected to a terminal in half-duplex mode, characters typed at the terminal are both displayed and transmitted immediately. However, in full-duplex mode, the terminal only shows the characters after receiving an "echo" back from the receiving system. If the communication systems aren’t using the same mode, issues can arise. For instance, if a terminal is in half-duplex mode, it immediately displays the characters entered by the user. But if the receiving system or host is operating in full-duplex mode, it will also send a character back to the terminal, resulting in two characters appearing on the screen for every single keypress. On the other hand, if the terminal is set to full-duplex mode while the host remains in half-duplex mode, the terminal won’t display any characters at all. This happens because its full-duplex capability requires waiting for an echo from the host—but since the host is in half-duplex mode, it doesn’t send any echoes back to the terminal.
Data Communication Equipment (DCE)
Many data communication devices are named after the DCE component in the DTE/DCE pairing (see "Data Communication Devices"). The DCE sits between the Data Terminal Equipment (DTE) and the transmission circuit or channel, providing two types of connections: one for linking the DTE to the communication network, and another for connecting the communication network back to the DTE. Additionally, it typically terminates a circuit and supplies the clock signal required for that circuit. Below, we’ll discuss several types of transmission equipment.
The modem—short for modulator/demodulator—converts digital signals into analog ones, enabling these signals to travel over the audio telephone network. Digital electrical pulses representing binary 0s and 1s are transformed into analog waveforms at one end of the transmission, and then reconverted back into digital signals by another modem at the receiving end. This modem is controlled by commands from data communication protocols, handling tasks such as dialing and answering phone calls. It also manages the transmission rate, which can range from 300 bps up to 9,600 bps. With compression techniques and adherence to the latest standards, speeds as high as 28,800 bps are already achievable, while even faster rates are currently under development.
The Channel Service Unit (CSU) is a device used to connect digital communication lines, such as T1. Not only is it affordable, but it also provides an endpoint for digital signals. The CSU divides the line into separate circuits, ensuring that even if other connected communication devices fail, the line itself remains intact. Typically, the CSU is used in conjunction with a Data Service Unit (DSU).
The Data Service Unit (DSU) is the hardware device used when transmitting digital data over a digital channel. This equipment converts data from bridges, routers, and multiplexers into bipolar digital signals suitable for use on digital lines.
Repeater and circuit driver signals can become distorted over long-distance lines, potentially rendering digital signals unrecognizable. To address this issue and extend transmission distances, repeaters can be introduced into the line to detect these distorted pulses and regenerate them. In telephone systems, a repeater is typically required every 1,818 meters (6,000 feet). Meanwhile, for serial connections between computers and peripheral devices, a line driver is needed roughly every 15 meters (50 feet). Some line drivers even allow this distance to be extended up to 5,000 feet.
A bridge connects two similar or dissimilar packet-oriented local area networks. It operates at the data link layer of the Open Systems Interconnection (OSI) reference model. The bridge can forward packets to other LANs or filter them to reduce network traffic across the interconnected networks.
The router connects a Local Area Network (LAN) to other LANs, linking them to the backbone cable or to a Wide Area Network. Like a bridge, a router offers advanced functionality—it can determine the destination of each node and identify the optimal route to reach it, then efficiently forward data packets toward their intended destination.