Types of Digital Communication Channels

Types of Digital Communication Channels

A communication channel is used in the physical layer to transmit data across a communication network. Additive noise is a typical issue in signal transmission across any medium.

Signal attenuation, amplitude and phase distortion, and multipath distortion are all examples of signal deterioration that may occur over a channel.

The key communication resources available to the designer are Power and Bandwidth. The effects of noise can be mitigated by raising the transmitted signal's strength. However, the power level of the broadcast signal is limited by equipment and other practical limits. There is also a limit on the amount of available channel bandwidth. The physical limits of the medium, as well as the electrical components needed to implement the transmitter and receiver, account for this. These two constraints restrict the quantity of data that may be successfully transferred via any communication route.

We may divide communication channels into two categories based on the mechanism of transmission employed.

Telephone lines, coaxial cables, and optical fibres are examples of directed propagation channels.

Wireless broadcast channels, mobile radio channels, and satellite channels are examples of free space (unguided) propagation channels.

We'll go over some of the key features of three digital communication channels: telephone, optical fibre, and satellite.

1. Telephone channel

Wirelines are widely used in the telephone network for voice signal transmission as well as voice and data transmission. For signal transmission, the telephone channel was formerly formed utilising twisted wire pairs. Twisted pair wirelines are electromagnetic channels that are directed and have a small bandwidth. Now, the telephone network has grown to include a wide range of transmission media (open-wire lines, coaxial cables, optical fibres, microwave radio, and satellites) as well as a complex of switching systems. As a result, the telephone channel is a great alternative for long-distance data transfer.

The bandpass characteristic of the telephone channel covers the frequency range of 300Hz to 3400Hz. It boasts a high signal-to-noise ratio of roughly 30dB and linear response. Over the pass-band, the channel exhibits a flat amplitude response. However, phase delay differences have a significant impact on data and picture transmissions. As a result, an equaliser is built with a flat amplitude response and a linear phase response in mind. Over telephone lines, transmission rates of up to 16.8 kilobits per second (kbps) have been achieved. Electromagnetic interference (EMI) is a natural occurrence in telephone channels, which may be minimised by twisting the lines.

2. Optical fibre channel

A dielectric waveguide that transmits light signals from one location to another is known as an optical fibre. The light signal is concentrated to a centre core. It is encased in a cladding layer with a slightly lower refractive index than the core. Pure silica glass is used for both the core and cladding. A light source (LED or Laser) acts as the transmitter or modulator in an optical fibre communication system. A photodiode, whose output is an electrical signal, detects the light intensity at the receiver. Photodiodes and electronic amplifiers are sources of noise in fibre optical channels.

Optical fibres have distinct properties that make them particularly appealing as a transmission medium. 

• Substantial effective bandwidth due to the utilisation of optical carrier frequencies of roughly 2 x 1014Hz.

• Low transmission losses of as little as 0.1 dB/km

• No cross talk due to immunity to electromagnetic interference.

• Size and weight are both small.

• Highly dependable photonic devices for signal creation and detection are available.

• Ruggedness and flexibility

Due to these distinct properties, optical fibre lines for voice, data, facsimile, and video have been rapidly deployed for telecommunication applications.

3. Satellite channel

A geostationary satellite, an uplink from a ground station, and a downlink to another ground station make up a satellite channel. Typically, microwave frequencies are used for both the uplink and the downlink, with the uplink frequency being higher than the downlink frequency. For satellite communications, the most preferred frequency bands for uplink and downlink are 6GHz and 4GHz. 14/12GHz is also a prominent band. A low-power amplifier is installed onboard the satellite and is often used in a non-linear mode for maximum efficiency. As a result, the satellite channel may be thought of as a strong sky-based repeater. It allows communication over vast distances at high bandwidths and low cost (from one ground station to another). Because of the channel's nonlinear structure, it can only be used with constant envelope modulation methods (i.e, Phase modulation, frequency modulation).

Geostationary communications satellites provide the following system capabilities:

• Broad area coverage.

• Reliable transmission links

• Wide transmission bandwidths.

A typical satellite is given bandwidth of 500MHz in the 6/4GHz range. The satellite's bandwidth is split across 12 transponders. Each transponder may transport at least one colour TV signal, 1200 voice circuits, or digital data at a rate of 50Mbps while requiring roughly 36MHz of satellite bandwidth.

There are several methods to categorise communication routes.

A) A channel can be linear or non-linear (for example, a wireless radio channel) (e.g., satellite channel).

B) A channel can be either time-invariant (e.g., the Optical Fibre Channel) or time variable (e.g., the Optical Fibre Channel) (e.g., mobile radio channel).

C) A channel's bandwidth or capacity may be limited (for example, a telephone channel) (e.g., optical fibre channel and satellite channel).

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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