Physical Layer

Abstract

Every output, transfer and reception of information takes place by way of a physical medium, meaning a carrier of this information. Also on the Internet, the transfer of information is conducted via a medium of physical communication. These range from traditional electric conductors, simple cables and fiber-optic cables (fiberglass cables) to conductor-independent forms, whereby electromagnetic waves in different frequencies serve as information carriers. But how does the information to be transferred, digitally coded as a series of “zeros” and “ones,” actually arrive at the physical communications medium? Before taking a detailed look in this chapter at the different wired and wireless media of transfer, the theoretical foundation of physical data transfer will be explained, whereby so-called modulation procedures and multiplex procedures stand at the forefront of how this binary information is put into a physically transferable “form,” thus enabling an efficient data transfer, also over shared media.

“The visible comprises the basis for the knowledge of that which is invisible.”

– Anaxagoras, (499–427 B.C.)

Glossary

Bandwidth

A given measurement in hertz (1 Hz = 1/s) used in physics, telecommunications and computer science with different meanings. Seen physically, bandwidth B designates the difference between two frequencies (lower and upper cutoff frequency), which form a continuously connected frequency range. In analog telecommunication technology, bandwidth designates the frequency range in which electrical signals with an amplitude decrease of up to 3 dB are transmitted. The larger the bandwidth, the more information can theoretically be transmitted in one unit of time. In computer science the term “bandwidth” means the data transmission rate (also called transmission rate or data rate) of digital signals, used as a measurement for speed in which data can be transmitted over a transmission medium in the form of bits per time unit

Channel capacity

Channel capacity defines the upper limit of an error-free transmitted information ow (number of transmitted bits) in a specific transmission medium based on the signal-to-noise ratio and in relation to the frequency spectrum to be transmitted

Chirp Impulse

A signal whose sinusoidal signal waveform continuously rises, or falls, over time in frequency is called a chirp impulse. Among other technical applications, chirp impulses are used in spread spectrum modulation methods, e.g., the Chirp Spread Spectrum (CSS). This method has proven resilient to interference caused by the Doppler effect as only the frequency change of a chirp impulse over time is of importance. In nature the chirp impulse is used by bats for tracking

Coaxial cable

A bipolar cable with a concentric structure, normally containing a core of solid copper wire (inner conductor), surrounded by an insulation layer (dielectrium, that is usually made up of synthetic materials or gases). Flexible coaxial cables can also have an inner conductor of thin, braided or stranded copper wire. The isolation jacket itself is encased in a hollow cylindrical outer conductor that mostly takes the form of a woven metallic braid. This outer conductor is once again encased in a protective shield that is normally made of synthetic material and has insulating properties, as well as being noncorrosive and waterproof. Signal transmission over coaxial cables proceeds via the inner conductor, while the outer conductor serves as the reference ground and is used at the same time for signal feedback

Communication

Communication is the process of a one-way or reciprocal exchange, conveyance and reception of information by humans or technical systems

Communications protocol

A communications protocol (also simply protocol) is a collection of rules and regulations that determines the data format of the information to be transmitted as well as establishing the guidelines for its transmission. Protocols contain agreements about the to-be-transmitted data packets the establishing and terminating of a connection between communication partners, as well as the manner of data transmission

Digital

(digitus = [lat.]finger), The designation for a technology/method that only uses discrete, discontinuous, i.e. cascading mathematical values. The basis for digital technology is the binary (two value) number system, that can only distinguish between the conditions “true” and “false,” or the number values “1” and “0”. These binary numerical values are called bits (binary digit) and represent the smallest units of information

Digital communication

Digital communication refers to the exchange of digital information over specialized digital communication channels. The data format of the information determines the respective media (text, image, audio, video, etc.). The information is transmitted according to the conditions of the implemented communication protocols via a digital communication channel (e.g. Internet or WWW)

Direct wave propagation

Electromagnetic waves in the frequency range of 30 MHz and 30 GHz are no longer reflected in the ionosphere and propagate as direct waves. Ground waves in this frequency are damped so strongly that in order for even a small area to be covered, line of sight propagation between the antennas is necessary

Doppler effect

A phenomenon named after the Austrian physicist Christian Doppler (1803–1853), the Doppler effect describes the change in perceived or measured frequencies of all wave types in which the source and the observer approach each other or recede from one another. As the observer and source move closer to each other the frequency perceived by the observer increases, when they move farther apart the frequency decreases. The best-known example of this is the siren of a passing ambulance. The Doppler effect plays an important role in wireless telecommunication with mobile terminals and is necessary to take into account when modulation and multiplexing methods are applied

Electromagnetic wave

The designation for a wave that consists of interlinked electrical and magnetic fields. A time-varying electrical field always generates a magnetic field, while at the same time a time-varying magnetic field always generates an electrical field. One does not occur without the other, even if there is no carrier. Electromagnetic waves are used for the transmission of analog and digital signals

Fiber optic cable

The term for fibers made out of a transparent, light-permeable, material (usually glass or plastics) that serve to transport light or infrared radiation. Fiber optic cables are comparable to coaxial cables as both have a glass core (coaxial cables a copper core) where the light used for information transmission propagates. Based on their diameter, a distinction is made between multi-modal optical fibers (thick fiber optic cable, in which during light transmission runtime interference occurs) and mono-modal optical fibers

Fourier analysis

Any periodic signal g(t) can be decomposed into a seemingly endless number of simple sine und cosine functions. This decomposition is called Fourier analysis after the French mathematician and physicist Jean-Baptiste Fourier (1768–1830)

Ground wave propagation

Electomagnetic waves in the frequency range of 3 kHz up to approx. 30 kHz propagate as ground waves along the earth’s surface. The range of a ground wave is dependent on both the frequency and the conditions of the ground

Internet

The Internet is the world’s largest virtual computer network made up of computer networks linked over the standard Internet Protocol Suite (TCP/IP). The most important offers of the Internet—also called “services”—include electronic mail (email), hypermedia documents (WWW), file transfer (FTP) and discussion forums (usenet/-newsgroup). Its huge popularity has been achieved through the introduction of the World Wide Web (WWW). Often thought of as synonymous with the Internet, the WWW is in fact only one of the many services the Internet offers

Internetworking

The implementation of several different individual networks to an internet of connected networks. To enable this suitable linking network devices (routers) are necessary. They direct the path of a data package through the network system and, with the help of the Internet protocol, enable a secure delivery. The network system appears to the user as a homogeneous, virtual network (Internet)

Intersymbol interference

In intersymbol interference (also called crosstalk) there disturbances appear between temporal successive, transmission symbols in a digital data transmission on a single transmission channel

Ionosphere

The layer of the earth’s atmosphere where a significant amount of charged particles (electrons and ions) are found. In radio transmission this atmospheric layer is used for the transfer of shortwave signals (HF frequency range). Here is where signals in this frequency range are reflected back to the earth’s surface and in this way can be transmitted worldwide. If a radio wave strikes the ionosphere the charged particles are induced to oscillate. The energy thus spent in transmitting energy is not lost as the oscillating electrons are also transmitters, and through their vibrations emit the same frequency in phase-shifted form again

ISO/OSI Reference Model

A specification of the ISO that was designed and published as the basis for the development of communication standards. The core of this standard is an international reference model consisting of seven layers. The goal of the ISO/OSI Reference Model is to enable the communication of different computer and protocol worlds over uniform interfaces. In contrast to the TCP/IP Reference Model which forms the very fabric of the Internet, the ISO/OSI Reference Model has decreased significantly in importance

Line coding

In digital telecommunications a line code determines how a signal is to be transmitted to the physical layer. Specific level sequences, such as light intensity in fiber optics and voltage or currency on electrical lines are assigned bit sequences. A distinction is made between line codes and other forms of coding, such as channel coding or source coding. With the help of supplementary redundancy, channel coding enables the recognition, or correction of data transmission and memory errors. Source coding, by contrast, removes redundancy from the information of a data source and is used for data compression

Medium

The form of a transportation channel for the transmission of information between sender and receiver. In order to transmit information there must be an appropriate transmission medium for exchange between sender and receiver. A distinction is made between wired and wireless media

Modulation

In information technology modulation is described as a process in which the to-be-transmitted (data) signal changes (modulates) a carrier signal, thereby enabling transmission of the original signal over the normally higher frequency of the carrier signal

Multiplexing

A designation for methods of signal and information transmission in which multiple signals are combined (bundled) and at the same time transmitted over a medium

Noise ratio

This describes the relation between the amplitude of the output signal and the amplitude of interference signals. The noise ratio is also called the signal-to-noise ratio (SNR) and is indicated in decibels (dB)

Physical layer

Generally speaking, all of the physical and technical characteristics of a physical medium used for data transmission are defined in the physical layer. The protocol in the individual layers of the TCP/IP Reference Model of the Internet are all directed toward the physical transmission medium. The physical layer, which is itself not a part of the TCP/IP Reference Model, defines the reciprocal action between the network hardware and the physical transmission medium. Determined therein are: the layout of the plug connections and their electrical or optical parameters, the specifications of the physical nature of cables (electrical and optical), as well as the specifications of amplifying elements, network adapters and, additionally the data transmission method implemented

Refractive index

The refractive index (also index of refraction, refraction) is a fundamental parameter of geometric optics and describes the refraction (change of direction) and the reflection behavior (reflection and total reflection) of electromagnetic waves upon impacting an interface from one medium to the other

Skywave propagation

So-called skywave propagation occurs in electromagnetic waves within the frequency range of 300 kHz and 30 MHz. Here electromagnetic waves from the sender propagate in space and through reflection in the ionosphere return to earth to be reflected again. In this way, skywaves achieve an extremely long range and, in the ideal case, can even be spread around the entire globe

Signal

A signal is understood as the information-bearing temporal process of a measurable quantity within a physical system

Signal noise

No signal can be transmitted without a loss of energy. The distortion (interference) of a signal through outer influences is called signal noise

Signal-to-noise ratio

This describes the relation between the amplitude of the output signal and the amplitude of interference signals. The signal-to-noise ratio is given in decibels (dB). It is also designated as the dynamic and is an important standard for the quality of a signal

Soliton

A soliton is a wave packet that can spread out over long distances without being disturbed by other waves. A wave packet is made up of different individual elements all having different (frequency dependent) propagation velocities in a propagation medium (dispersive nonlinear). Due to non-linear effects, it is possible that the individual frequencies that make up a wave packet can be transformed into one another thus achieving a dynamic balance. This results in a soliton, which propagates without changing its form

Spread spectrum

With spread spectrum or frequency spreading a technique is defined in which a narrow band signal is converted into a signal with a larger bandwidth than is necessary for information transmission. In the process an electromagnetic transmission energy, which had previously been concentrated in a smaller frequency range, is now distributed over a larger frequency area. The power density of the signal can thereby be lower than in a narrow band without the data getting lost during data transmission. An advantage of this method is the greater robustness of the signal to narrow band disturbances

Total reflection

Total reflection is a wave phenomenon of light and manifests itself on the interface between two media, e.g, air and water. For example, here light is not refracted on the interface but reflected completely, i.e., it is reflected in the output medium. The transmission of optical signals in fiber optic cables is facilitated by total reflection

Twisted pair cable

Designation for a type of copper cable in which both strands of a wire pair are twisted together. Normally a simple twisted pair consists of two insulated approx. 1 mm thick copper wires, which are twisted spirally around each other. Often different stranded pairs with different strengths of twisting (lay lengths) are stranded into a cable