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).