Data communication systems (3.13.6)
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A data communication system is defined as a method of communicating information sent from a source to a receiver. The data may be analogue or digital.
The information is sent along a transmission path (communication channel) from the source to the receiver. Channels are physical (copper, optical fibre, air/water as a medium) and carry various signal types (electromagnetic waves, sound, etc.).
The communication channel directionality can be one of three types:
- Simplex: information is sent in one direction only (i.e. sender to receiver). Examples include broadcast TV, radio, and GPS signals.
- Duplex: information is sent between the source and the receiver and flows in either direction. Examples include phone calls and wifi signals.
- Half-duplex: information can be sent in either direction. However, it can only be sent in one direction at a time. Walkie-talkies are one example.
A communication system involves multiple distinct system blocks through which the information passes before reaching the receiver. The image below shows the different system blocks in a communication system.
The first part of a communication system is the input of information. The information can exist in many forms, such as images, sound, digital information, or analogue information.
The information flows into an input transducer. The purpose of the transducer is to convert the type of information into a form that is suitable for transmission. The most common form suitable for transmission is electrical. Energy is converted from one form into another by the transducer. Examples of input transducers are microphones, cameras, and computers.
The modulator superimposes the signal’s information onto a carrier wave (e.g. a radio or infrared wave) in a form suitable for transmission. For example, Morse code uses a two-state digital code to encode information by modulating light or radio signals.
The amplifier increases the voltage, current, or power of the modulated signal to minimise noise and extend the signal’s range during transmission. As a signal travels farther, it becomes weaker and is attenuated. By amplifying the signal, the range can increase to hundreds of miles.
A transmitter is a type of transducer that converts an amplified signal into a suitable form and transmits it. For example, in radio transmission, the transmitter is an aerial that converts an amplified electrical signal into radio waves.
The transmission path is the physical medium through which the signal propagates. The transmission path can be via a variety of media, such as copper cables, fibre-optic cables, or electromagnetic waves.
The receiver acquires energy from the transmitted signal and converts it back into an electrical signal. The signal strength tends to be weak when it reaches the receiver. Therefore, the receiver must be large enough to acquire enough energy from the signal. For example, the TV aerial is a receiver of radio waves.
Similar to the transmitter, the receiver also requires an amplifier. This is because the signals are weak; they need to be amplified to be identified and interpreted correctly, without any background noise.
It is important to note that the demodulator separates the original information from the modulated carrier signal used by the transmitter to send it.
After the demodulator has recovered the original information from the carrier signal, it passes it to the output transducer. It converts the electrical signal to the necessary output required for reading. For example, speakers are output transducers because they convert electrical signals into sound.
In electronic communication systems, the most commonly used transmission media are metal wires, optical fibres, and electromagnetic waves.
A metal wire, such as a copper cable, carries electrical current to transmit information. The copper cable can be either coaxial or twisted-pair.
Coaxial cables consist of a central copper core surrounded by an insulating material, with a braided metal shield to isolate the core from the shield. The purpose of the metal shield is to protect the transmitted data from any electrical interference. The outside of the cable consists of a tough protective jacket to protect it from the environment.
Twisted-pair cables consist of pairs of insulated copper strands twisted together. Bundles of these woven pairs are then covered with an outer plastic insulator. The twisting of the pairs of copper strands minimises electrical interference from nearby pairs or other electrical equipment. The bundles are then covered with an overall shield for additional protection against interference, followed by a tough protective plastic jacket to shield the cable from the environment.
Both coaxial and twisted-pair cables are susceptible to attenuation. This can be due to resistive losses in the wires or at the junctions where they are joined.
The metal in the wires is also susceptible to corrosion and oxidation, which degrades the performance over time.
Optical fibres are another type of transmission medium that utilises total internal reflection to guide light pulses along a glass fibre. They contain a long, thin strand of glass that transmits light, commonly infrared, over long distances with very little energy loss.
Optical fibres contain a pure glass core, with a high refractive index, that is surrounded by a glass cladding of lower refractive index. This is then surrounded by a strengthening material and a protective jacket, providing durability to the cable. Since optical fibres are made of glass, they are not susceptible to corrosion.
Optical fibre cables can transmit more information at once than coaxial cables. The light pulses are digitally coded and have high frequencies; they are totally internally reflected thousands of times per metre along the cable.
Optical fibres are less susceptible to attenuation than metal cables. However, the signal can be distorted by pulse broadening caused by material and modal dispersion. The effects from dispersion can be minimised by using monochromatic (i.e. single-frequency) laser light to send the signals or by using optical fibre repeaters.
Communication systems use optical fibres in a variety of ways. Examples are:
- Internet uses optical fibres to get high bit rates and, as a result, high download speeds on computers.
- Landline uses optical fibres to transmit digitised voice signals over long distances, providing faster, clearer, and more reliable communication.
- Television uses optical fibre to provide high-capacity, high-quality transmission of digital television signals. This results in faster, clearer, and more reliable service than coaxial or satellite methods.
Apart from metal wires and optical fibres, electromagnetic waves are also used to transmit data.
Radio waves and microwaves are the most common wavelengths used for transmission. Because they are waves, there is no physical link between the source and the receiver.
The waves propagate through the air and are picked up by antennas tuned to their frequency. The long wavelengths of radio and microwaves mean they can easily penetrate solid objects, and they propagate in different ways according to their frequency.
The different ranges of electromagnetic waves that can be used in transmission are:
- Ground waves: Low-frequency radio waves (150 kHz to 300 kHz),
- Sky waves: High-frequency radio waves (3 MHz to 30 MHz), and
- Space waves: High-frequency microwaves (2 GHz to 100 GHz).
Ground waves (low-frequency radio waves) propagate near the Earth’s surface. They diffract around hills and large obstacles, closely following the curvature of the Earth by having their wavefronts angled down towards the surface.
Ground waves can reach receivers beyond the horizon and are therefore received from long distances away from the transmitter. Examples of communication using ground waves are AM radio broadcasts and maritime communication.
Sky waves (high-frequency radio waves) can penetrate and be refracted by the atmosphere.
These waves can also be reflected by the ionosphere, which is an electrically charged layer in the atmosphere.
The ionosphere is located in the upper atmosphere, approximately 80–100 km above the Earth’s surface. Here, neutral atoms of air are ionised by radiation from the Sun and cosmic rays. Sky waves are totally internally reflected by the ionosphere and travel back towards Earth at distances far from the transmitter, enabling cross-continent communications.
The range of sky waves is dependent on the frequency, location, time of day, and the angle at which they strike the ionosphere. Sky waves are the preferred mode of propagation for military communications and some radio stations.
Space wave (high-frequency microwaves) can be used for communication along a direct line of sight.
The waves travel in straight lines and do not diffract around obstacles. Due to the small wavelengths, parabolic transmitters are used to focus the waves into a narrow beam to communicate between distinct points. For example, microwave communication is used in mobile phones, satellite TV, and satellite navigation systems.
Microwaves can transmit large amounts of information. For example, mobile phone networks that utilise 4G allow for download speeds of over 15 Mbps.
Satellites in orbit around Earth serve as relay stations that can send communication signals to any point on Earth. The signals received and transmitted by the satellites use space waves (i.e. microwaves).
The signal is transmitted from the ground to the satellite. This is known as the uplink. The signal is then amplified and sent back down to the desired point on the ground where it is received. This is known as the downlink.
The image below shows the process of satellite communication.
Microwave radiation is used in the uplink and downlink signals in satellite communication.
The table below shows the different frequency bands and their uses for satellite communication.
Satellites used in communication are usually in a geostationary orbit – one where the orbital period is equal to the period of rotation of the Earth. As a result, the satellite’s position relative to the ground remains fixed, and neither the transmitting nor the receiving station needs to move to track it.
Satellites have specific frequencies for the uplink and downlink signals.
- Uplinks are stronger signals because ground-based transmitting stations have more powerful transmitters than those on satellites. If there is overlap between the uplink and downlink frequency bands, the uplink channels may interfere with the weaker downlink channels, thereby desensitising the transmissions.
- The downlink signal frequency is lower than the uplink signal frequency to reduce downlink signal attenuation. Higher frequencies result in greater attenuation, which is why lower frequencies are needed.
Guided and unguided communications have different levels of security. Guided communications (ones that use physical wires) are more secure.
Unguided:
- Radio and satellite communications are unguided. Therefore, they can be picked up by a receiver that is within range; the information is not secure.
- Wifi networks can be insecure because data downloaded from the network onto a computer can be read by another device using the same link. Wifi networks prevent this by using a pre-shared key (PSK) – an encrypted code known only by the computer and network so the two can communicate.
Guided:
- Signals are confined to the cable and do not radiate into the environment. Information cannot be easily read unless there is a physical breach of the cable. Metal wires can be buried underground, making them difficult for hackers to access.
- Optical fibres are secure because physical breaches are easily detected due to the immediate signal loss.
Different transmission media have different advantages and disadvantages. The table below shows key aspects of different transmission media.
Both metal cables and radio waves can transmit analogue and digital signals. Digital signals can transmit more data than analogue signals over the same transmission medium. Multiplexing is a technique where multiple streams of analogue or digital data are combined into a single signal over the same communication channel.
- A single cable can transmit multiple digital signals simultaneously using a technique known as time-division multiplexing (TDM).
- TDM involves combining (or multiplexing) multiple digital signals into a single continuous signal. Each signal is assigned its own time slot in a repeating cycle so that none overlap. The combined signal is then transmitted and split up (demultiplexed) into its constituent signals at the receiver.
TDM is an efficient method of using the transmission medium and is compatible with high-speed digital networks.
The image below shows an example of time-division multiplexing in a simple three-input system.
If there are three inputs that are sending signals over a common transmission path, then each input is assigned a fixed-duration time slot, typically in the order of The total transmission time is divided between the three inputs, and these time slots are grouped into frames.
- Each frame contains one slot for each input, so for three inputs, there will be three slots per frame. The frames are transmitted one after another and during their assigned slot each input dictates the transmission path bandwidth in turn.
- At the receiving end, the frames are separated (demultiplexed) and the slots for each input are separated and sent to the respective receiver.
It is important to note that in TDM, the transmission medium’s data rate must be greater than the data rates of the devices used to send and receive signals.
TDM efficiently uses the available frequency band by transmitting an entire frame over a single channel frequency. Each user during their time slot can access the full digital bandwidth of the transmission medium.
Although analogue signals can also be used in TDM, digital signals are more common because they are better suited. A limitation is that if no data is sent by any one of the inputs during its slot, the time slot is still reserved and transmitted anyway. This wastes the transmission-path bandwidth.
A modulator combines the information from a signal wave with a carrier wave in a form suitable for transmission.
There are two methods of modulating an analogue carrier wave: amplitude modulation (AM) and frequency modulation (FM).
The image below shows an example of amplitude modulation.
Amplitude modulation is a process by which the wave signal is transmitted by modulating the amplitude of the carrier wave.
Carrier waves, such as radio waves, have frequencies of the order of MHz and GHz. Typical audio signals have frequencies of the order of Hz to kHz. Therefore, to transmit a typical audio signal, the amplitude of the higher-frequency carrier wave can be modulated to match the amplitude variations of the lower-frequency signal.
It is important to note that the bandwidth of an analogue signal is the range between the highest frequency and the lowest frequency in a signal.
The amplitude modulation process produces a band of frequencies on either side of the carrier frequency, that are higher and lower in frequency. These are known as sidebands. The result is two sidebands that are mirror images of each other, centred on the carrier wave’s frequency.
- The highest audio frequencies, , in the audio bandwidth will be furthest away from the carrier frequency on either side.
- The lowest audio frequencies, , in the audio bandwidth will be closest to the carrier frequency on either side.
The amplitude-modulated signal consists of the carrier frequency and two frequency sidebands: an upper and a lower sideband.
The bandwidth of the modulated signal spans from to (f_C + f_H\). This is equal to twice the highest audio frequency, , transmitted:
The transmission medium needs to be able to carry a minimum frequency of .
The AM bandwidth is, therefore, defined as:
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Data communication systems
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Frequency modulation (FM) is used in many radio communications as it provides very high-quality analogue audio signals.
The carrier signal’s amplitude in FM is kept constant, whereas in AM it is non-constant. While the carrier signal’s frequency is modulated to vary in accordance with the modulation signal.
The FM signal has a range of different frequencies. The maximum instantaneous difference between the FM signal frequency and the carrier frequency is known as the frequency deviation, .
The FM bandwidth is defined as:
Where
- is the highest frequency in the audio you want to transmit.
Radio stations that utilise FM pay close attention to the frequency deviation, , since a smaller deviation allows more channels to be fitted into the available frequency spectrum.
- In FM broadcasting, transmission channels are spaced 200 kHz apart, with a maximum frequency deviation of 75 kHz. This leaves a 25 kHz buffer above and below each channel to minimise overlap and interference.
- In AM broadcasting, the channel spacing is only 10 kHz. However, since signals are amplitude modulated, the frequency deviation is irrelevant.
Frequency modulation (FM) characteristics:
- Noise resistance: FM broadcasts are highly resistant to noise because the carrier signal’s amplitude remains constant. FM receivers primarily process frequency variations, ignoring amplitude variations caused by electromagnetic interference or static noise.
- Audio quality and bandwidth: The quality of an FM audio signal increases as the frequency deviation increases. A larger deviation generally provides a better signal-to-noise ratio but requires more spectrum space (bandwidth).
FM systems are categorised by their bandwidth, which relates to their maximum frequency deviation:
- Wide-band FM (WBFM) has a large frequency deviation. It is used by FM broadcasters for high-fidelity audio transmissions, such as music, which require a wider frequency range for better quality.
- Narrow-band FM (NBFM) has a smaller frequency deviation. It is used for two-way speech and data transmissions where bandwidth efficiency is critical.
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Data communication systems
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The data capacity of a channel, also known as channel capacity, is the maximum theoretical rate at which information can be reliably transmitted through that channel, measured in bits per second (bps).
The maximum data rate of a channel equals twice the maximum available bandwidth of the communications channel.
Where
- maximum bit rate is in bits per second (bps), and
- bandwidth is in hertz (Hz).
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Data communication systems
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