Telecommunications Signals and Transmission Media
Mathematical Concepts
A signal is only the manifestation of a physical quantity. In the case of signals used in telecommunications, this signal has a characteristic: it can spread through various means and channels. This suggests the idea of mathematically representing the signal as a function variable over time.
- Frequency: The frequency reports on the number of sections of the horizontal axis function per unit time, measured in hertz (Hz).
- Period: The period reports the time needed for a repetition of the periodic structure of the signal and is measured in seconds.
- Phase: Phase shift is a shift of the signal in time. It can vary depending on the time origin taken to represent the function and is measured in radians.
- Maximum Size: The maximum value reached by the function. It should not be confused with the value the signal takes at any given time.
Multiplexing
Multiplexing is a technique used to combine, in a single channel, signals from different issuers and send them to one or more recipients. Ultimately, it comes to sharing a physical channel, establishing several logical channels on it.
Transmitting information from various issuers on the same channel is possible by waiting systems. Therefore, data to be sent should alternate.
For multiplexing to be performed, it is necessary that the transmission capacity of the channel is greater than the transmission capacity of the issuers.
Types of Multiplexing
Frequency Division Multiplexing (FDM): In FDM, the logical channels share a single physical channel. Each logical channel is assigned a frequency band centered on a carrier signal on which the message is modulated. Between two consecutive frequency bands, a security measure shall be established in order to avoid interference.
Time Division Multiplexing (TDM): In TDM, the logical channels are assigned by distributing the time to use the physical channel between different issuers, providing slots or time slots.
Wavelength Division Multiplexing: Used on fiber optics.
Capacity to Carry Data
Bandwidth: Amount of information that can flow from one place to another over a period of time.
Throughput: Throughput is the measure of the transfer of bits across the media during a period of time.
Transfer Capability Area: The measure of usable data transferred during a specified time period. It is therefore of far greater interest to the user of the network.
The Means of Transmission
The transmission medium is the hardware that facilitates the transport of information. Transport can be mechanical, electrical, optical, electromagnetic, etc.
The aim of the physical layer is to create the optical, electrical, or microwave signal representing the bits in each frame. Then, these signals are sent by the media one at a time.
Copper Media
Copper is the simplest and most economical means of guided transmission. It presents the inconvenience of electrical resistance. This resistance is affected by the maximum length. When it exceeds a certain length, it is necessary to make use of repeaters to restore the electrical signal level.
If a combination of zeros and ones is transmitted, the destination will interpret a combination other than that originally sent.
RJ45: It is the most commonly used cable in telephony and fax. It is used in local area networks (LANs). RJ11 is used in traditional telephony.
UTP Cable: No external metallic coating. Twisted pair cable, simple and cheap. 100-ohm impedance.
STP: Metallic coated. It is less flexible. 150-ohm impedance.
Coaxial Cable: It is more immune to interference, so the bandwidth can be increased. Its structure is formed by a solid central conductor surrounded by an insulating dielectric, an outer mesh, and an insulating material to protect the whole.
Fiber Optic
Fiber optic allows the transmission of light signals. It is insensitive to external electromagnetic interference. Light signals can be transmitted through laser sources, which are semiconductors that produce light when electrically excited.
Fiber Optic Problems
It is more expensive, special equipment is required, and it is more fragile than copper media.
Singlemode Fiber: It allows the transmission of signals with bandwidth up to 2 GHz. Light travels along a single path. It uses laser sources. The dimension of the core is 3.3 microns.
Multimode Fiber: In multimode fibers, light can travel in several ways. It uses LED light sources. Cores tend to be 50 or 62.5 microns in diameter.
Wireless Medium
Wireless media are used for convenience and flexibility. They have a low transmission rate.
Features
They use electromagnetic signals, using open environments. They are sensitive to interference, and the transmission can be intercepted.
Radioterrestrial Systems: The transmission medium is free space, through electromagnetic waves that propagate at the speed of light. It can cause interference due to adverse weather. The greater the frequency of the signal issued, the more sensitive it is to interference. Shortwave frequencies in the environment are in the tens of MHz, using the ionosphere as a mirror reflector between sender and receiver.
Microwave: With frequencies in the range of GHz.
Artificial Satellite: Outside the atmosphere, these are the most reliable wireless transmissions, enabling very high frequency, reaching 100 GHz transmissions.
Drawbacks: High cost of placing a satellite in space and maintenance, and slow transmissions.
Modulation
Modulation encompasses all the techniques used to carry information on a carrier wave.
Basic Modulation Techniques
Amplitude Shift Keying (ASK) or Amplitude Modulation
Frequency Shift Keying (FSK) or Frequency Modulation
Phase Shift Keying (PSK) or Phase Modulation
Advanced Modulation Techniques
Quadrature Modulation (QM) is a system through which two carrier signals are sent to the line.
Quadrature Modulation Types
Quadrature Amplitude Modulation (QAM): Both carriers are modulated in amplitude.
Quadrature Phase Modulation (QPM): Both carriers are modulated in phase.
Amplitude and Phase Modulation (HMWQ): A combination of amplitude modulation with phase modulation.
Combined Modulation
Combined modulation is sending more bits per baud by combining any of the basic techniques in a single carrier modulation.
Trellis-Coded Modulation (TCM)
With combined modulation, any noise or distortion in the line may eventually change the signal enough.