Carrier sense multiple access

One random-access method that reduces the chance of collisions is called carrier sense multiple access (CSMA). In this method a node listens to the channel first and delays transmitting when it senses that the channel is busy. Because of delays in channel propagation and node processing, it is possible that a node will erroneously sense a busy channel to be idle and will cause a collision if it transmits. In CSMA, however, the transmitting nodes will recognize that a collision has occurred: the respective destinations will not acknowledge receipt of a valid packet. Each node then waits a random time before sending again (hopefully preventing a second collision). This method is commonly employed in packet networks with radio links, such as the system used by amateur radio operators.

It is important to minimize the time that a communications channel spends in a collision state, since this effectively shuts down the channel. If a node can simultaneously transmit and receive (usually possible on wire and fibre-optic links but not on radio links), then it can stop sending immediately upon detecting the beginning of a collision, thus moving the channel out of the collision state as soon as possible. This process is called carrier sense multiple access with collision detection (CSMA/CD), a feature of the popular wired Ethernet.

Spread-spectrummultiple access

Since collisions are so detrimental to network performance, methods have been developed to allow multiple transmissions on a broadcast network without necessarily causing mutual packet destruction. One of the most successful is called spread-spectrum multiple access (SSMA). In SSMA simultaneous transmissions will cause only a slight increase in bit error probability for each user if the channel is not too heavily loaded. Error-free packets can be obtained by using an appropriate control code. Disadvantages of SSMA include wider signal bandwidth and greater equipment cost and complexity compared with conventional CSMA.

UNIT 11

Open Systems Interconnection

Different communication requirements necessitate different network solutions, and these different network protocols can create significant problems of compatibility when networks are interconnected with one another. In order to overcome some of these interconnection problems, the open systems interconnection (OSI) was approved in 1983 as an international standard for communications architecture by the International Organization for Standardization (ISO) and the International Telegraph and Telephone Consultative Committee (CCITT). The OSI model, as shown in the figure, consists of seven layers, each of which is selected to perform a well-defined function at a different level of abstraction. The bottom three layers provide for the timely and correct transfer of data, and the top four ensure that arriving data are recognizable and useful. While all seven layers are usually necessary at each user location, only the bottom three are normally employed at a network node, since nodes are concerned only with timely and correct data transfer from point to point.

Data recognition and use

The application layer is difficult to generalize, since its content is specific to each user. For example, distributed databases used in the banking and airline industries require several access and security issues to be solved at this level. Network transparency (making the physical distribution of resources irrelevant to the human user) also is handled at this level. The presentation layer, on the other hand, performs functions that are requested sufficiently often that a general solution is warranted. These functions are often placed in a software library that is accessible by several users running different applications. Examples are text conversion, data compression, and data encryption.

User interface with the network is performed by the session layer, which handles the process of connecting to another computer, verifying user authenticity, and establishing a reliable communication process. This layer also ensures that files which can be altered by several network users are kept in order. Data from the session layer are accepted by the transport layer, which separates the data stream into smaller units, if necessary, and ensures that all arrive correctly at the destination. If fast throughput is needed, the transport layer may establish several simultaneous paths in the network and send different parts of the data over each path. Conversely, if low cost is a requirement, then the layer may time-multiplex several users' data over one path through the network. Flow control is also regulated at this level, ensuring that data from a fast source will not overrun a slow destination.

Data transfer

The network layer breaks data into packets and determines how the packets are routed within the network, which nodes (if any) will check packets for errors along the route, and whether congestion control is needed in a heavily loaded network. The data-link layer transforms a raw communications channel into a line that appears essentially free of transmission errors to the network layer. This is done by breaking data up into data frames, transmitting them sequentially, and processing acknowledgment frames sent back to the source by the destination. This layer also establishes frame boundaries and implements recovery procedures from lost, damaged, or duplicated frames. The physical layer is the transmission medium itself, along with various electric and mechanical specifications.

UNIT 12

Telecommunications Media

Telecommunications media is equipment and systems — metal wire, terrestrial and satellite radio and optical fibre—employed in the transmission of electromagnetic signals.Related Content

EHide Notes

very telecommunication system involves the transmission of an information-bearing electromagnetic signal through a physical medium that separates the transmitter from the receiver. All transmitted signals are to some extent degraded by the environment through which they propagate. Signal degradation can take many forms, but generally it falls into three types: noise, distortion, and attenuation. Noise is the presence of random, unpredictable, and undesirable electromagnetic emissions that can mask the intended information signal. Distortion is any undesired change in the amplitude or phase of any component of an information signal that causes a change in the overall waveform of the signal. Both noise and distortion are commonly introduced by all transmission media, and they both result in errors in reception. The relative impact of these factors on reliable communication depends on the rate of information transmission, on the desired fidelity upon reception, and on whether communication must occur in “real time”.

Various modulating and encoding schemes have been devised to provide protection against the errors caused by channel distortion and channel noise. These techniques are described in telecommunication:analog-to-digital conversion, channel encoding, andmodulation. In addition to these signal-processing techniques, protection against reception errors can be provided by boosting the power of the transmitter, thus increasing the signal-to-noise ratio (the ratio of signal power to noise power). However, even powerful signals suffer some degree of attenuation, or reduction in power, as they pass through the transmission medium. The principal cause of power loss is dissipation, the conversion of part of the electromagnetic energy to another form of energy such as heat. In communications media, channel attenuation is typically expressed in decibels (dB) per unit distance. Attenuation of zero decibels means that the signal is passed without loss; three decibels means that the power of the signal decreases by one-half. The plot of channel attenuation as the signal frequency is varied is known as the attenuation spectrum, while the average attenuation over the entire frequency range of a transmitted signal is defined as the attenuation coefficient. Channel attenuation is an important factor in the use of each transmission medium.

UNIT 13

Wire Transmission

In wire transmission an information-bearing electromagnetic wave is guided along a wire conductor to a receiver. Propagation of the wave is always accompanied by a flow of electric current through the conductor. Since all practical conductor materials are characterized by some electrical resistance, part of the electric current is always lost by conversion to heat, which is radiated from the wire. This dissipative loss leads to attenuation of the electromagnetic signal, and the amount of attenuation increases linearly with increasing distance between the transmitter and the receiver.

Wire media

Most modern wire transmission is conducted through the metallic-pair circuit, in which a bundled pair of conductors is used to provide a forward current path and a return current path. The most common conductor is hard-drawn copper wire, which has the benefits of low electrical resistance, high tensile strength, and high resistance to corrosion. The basic types of wire media found in telecommunications are single-wire lines, open-wire pairs, multipair cables, and coaxial cables. They are described below.

Single-wire line

In the early days of telegraphy, a single uninsulated iron wire, strung above ground, was used as a transmission line. Return conduction was provided through an earth ground. This arrangement, known as the single-wire line, was quite satisfactory for the low-frequency transmission requirements of manual telegraph signaling (only about 400 hertz). However, for transmission of higher-frequency signals, such as speech (approximately 3,000 hertz), single-wire lines suffer from high attenuation, radiation losses, and a sensitivity to stray currents induced by random fluctuations in earth ground potentials or by external interference. One common cause of external interference is natural electrical disturbances, such as lightning or auroral activity; another is cross talk, an unwanted transferral of signals from one circuit to another owing to inductive coupling between two or more closely spaced wire lines.

Multipair cable

In multipair cable anywhere from a half-dozen to several thousand twisted-pair circuits are bundled into a common sheath (see Figure 1). The twisted pair was developed in the late 19th century in order to reduce cross talk in multipair cables. In a process similar to that employed with open-wire pairs (described above), the forward and return conductors of each circuit in a multipair cable are braided together, equalizing the relative positions of all the circuits in the cable and thus equalizing currents induced by cross talk.

For many high-speed and high-density applications, such as computer networking, each wire pair is sheathed in metallic foil. Sheathing produces a balanced circuit, called a shielded pair that benefits from greatly reduced radiation losses and immunity to cross talk interference.

Coaxial cable

By enclosing a single conducting wire in a dielectric insulator and an outer conducting shell, an electrically shielded transmission circuit called coaxial cable is obtained. As is shown in Figure 1, in a coaxial cable the electromagnetic field propagates within the dielectric insulator, while the associated current flow is restricted to adjacent surfaces of the inner and outer conductors. As a result, coaxial cable has very low radiation losses and low susceptibility to external interference.

In order to reduce weight and make the cable flexible, tinned copper or aluminum foil is commonly used for the conducting shell. Most coaxial cables employ a lightweight polyethylene or wood pulp insulator; although air would be a more effective dielectric, the solid material serves as a mechanical support for the inner conductor.

Applications of wire

Because of the high signal attenuation inherent in wire, transmission over distances greater than a few kilometres requires the use of regularly spaced repeaters to amplify, restore, and retransmit the signal. Transmission lines also require impedance matching at the transmitter or receiver in order to reduce echo-creating reflections. Impedance matching is accomplished in long-distance telephone cables by attaching a wire coil to each end of the line whose electrical impedance, measured in ohms, is equal to the characteristic impedance of the transmission line. A familiar example of impedance matching is the transformer used on older television sets to match a 75-ohm coaxial cable to antenna terminals made for a 300-ohm twin-lead connection.

Coaxial cable is classified as either flexible or rigid. Standard flexible coaxial cable is manufactured with characteristic impedance ranging from 50 to 92 ohms. The high attenuation of flexible cable restricts its utility to short distances—e.g., spans of less than one kilometer, or approximately a half-mile—unless signal repeaters are used. For high-capacity long-distance transmission, a more efficient wire medium is rigid coaxial cable, which was favoured for telephone transmission until it was supplanted by optical fibres in the 1980s. A state-of-the-art rigid coaxial telephone cable is the transatlantic SG series cable; the third cable in the series, called TAT-6, was laid in 1976 by the American Telephone & Telegraph Company (AT&T) between the east coast of the United States and the west coast of France. Capable of carrying 4,200 two-way voice circuits, the SG system has solid-state repeaters embedded in the cable housing at intervals of 9.5 kilometres (5.75 miles) and has equalizers that can be remotely adjusted to compensate for time-varying transmission characteristics.

Long-distance telephone cable is being phased out in favour of higher-performance optical fibre cable. Nevertheless, the last generation of long-distance telephone cable is still used to carry voice communication as well as broadband audio and video signals for cable television providers. For short-distance applications, where medium bandwidth and low-cost point-to-point communication is required, twisted pair and coaxial cable remain the standard. Voice-grade twisted pair is used for local subscriber loops in the public switched telephone network, and flexible coaxial cable is commonly used for cable television connections from curbside to home. Flexible coaxial cable also has been used for local area network interconnections, but it has largely been replaced with lighter and lower-cost data-grade twisted pair and optical fibre.

UNIT 14

Radio Transmission

In radio transmission a radiating antenna is used to convert a time-varying electric current into an electromagnetic wave or field, which freely propagates through a nonconducting medium such as air or space. In a broadcast radio channel, an omnidirectional antenna radiates a transmitted signal over a wide service area. In a point-to-point radio channel, a directional transmitting antenna is used to focus the wave into a narrow beam, which is directed toward a single receiver site. In either case the transmitted electromagnetic wave is picked up by a remote receiving antenna and reconverted to an electric current.

Radio wave propagation is not constrained by any physical conductor or waveguide. This makes radio ideal for mobile communications, satellite and deep-space communications, broadcast communications, and other applications in which the laying of physical connections may be impossible or very costly. On the other hand, unlike guided channels such as wire or optical fibre, the medium through which radio waves propagate is highly variable, being subject to diurnal, annual, and solar changes in the ionosphere, variations in the density of water droplets in the troposphere, varying moisture gradients, and diverse sources of reflection and diffraction.

Radio-wave propagation

The range of a radio communications link is defined as the farthest distance that the receiver can be from the transmitter and still maintain a sufficiently high signal-to-noise ratio (SNR) for reliable signal reception. The received SNR is degraded by a combination of two factors: beam divergence loss and atmospheric attenuation. Beam divergence loss is caused by the geometric spreading of the electromagnetic field as it travels through space. As the original signal power is spread over a constantly growing area, only a fraction of the transmitted energy reaches a receiving antenna. For an omnidirectional radiating transmitter, which broadcasts its signal as an expanding spherical wave, beam divergence causes the received field strength to decrease by a factor of 1/r2, where r is the radius of the circle, or the distance between transmitter and receiver.

The other cause of SNR degradation, atmospheric attenuation, depends on the propagation mechanism, or the means by which unguided electromagnetic waves travel from transmitter to receiver. Radio waves are propagated by a combination of three mechanisms: atmospheric wave propagation, surface wave propagation, and reflected wave propagation. They are described below.

Atmospheric Propagation

In atmospheric propagation the electromagnetic wave travels through the air along a single path from transmitter to receiver. The propagation path can follow a straight line, or it can curve around edges of objects, such as hills and buildings, by ray diffraction. Diffraction permits cellular telephones to work even when there is no line-of-sight transmission path between the radiotelephone and the base station.

The attenuation spectrum of electromagnetic waves are propagated along horizontal paths over land. A broad range of the attenuation spectrum is shown, from microwave radio to ultraviolet light. Atmospheric attenuation is not significant for radio frequencies below 10 gigahertz. Above 10 gigahertz under clear air conditions, attenuation is caused mainly by atmospheric absorption losses; these become large when the transmitted frequency is of the same order as the resonant frequencies of gaseous constituents of the atmosphere, such as oxygen (O₂), water vapour (H₂O), and carbon dioxide (CO₂) (shown by the spiked curves in Figure 2). The valleys between the peaks of the solid curves are spectral “windows,” which specify frequency bands where transmission occurs with minimal clear-air absorption losses. Additional losses due to scattering occur when airborne particles, such as water droplets or dust, present cross-sectional diameters that are of the same order as the signal wavelengths. Scattering loss due to heavy rainfall (shown by the line labeled “heavy rain 50 mm/hr”) is the dominant form of attenuation for radio frequencies ranging from 10 gigahertz to 500 gigahertz (microwave to submillimetre wavelengths), while scattering loss due to fog (shown by the dotted line) dominates for frequencies ranging from 10³ gigahertz to 10⁶ gigahertz (infrared through visible light range).

Reflected Propagation

Sometimes part of the transmitted wave travels to the receiver by reflection off a smooth boundary whose edge irregularities are only a fraction of the transmitted wavelength. When the reflecting boundary is a perfect conductor, total reflection without loss can occur. However, when the reflecting boundary is a dielectric, or nonconducting material, part of the wave may be reflected while part may be transmitted (refracted) through the medium — leading to a phenomenon known as refractive loss. When the conductivity of the dielectric is less than that of the atmosphere, total reflection can occur if the angle of incidence (that is, the angle relative to the normal, or a line perpendicular to the surface of the reflecting boundary) is less than a certain critical angle.

Common forms of reflected wave propagation are ground reflection, where the wave is reflected off land or water, and ionospheric reflection, where the wave is reflected off an upper layer of the Earth's ionosphere (as in shortwave radio).

Some terrestrial radio links can operate by a combination of atmospheric wave propagation, surface wave propagation, ground reflection, and ionospheric reflection. In some cases this combining of propagation paths can produce severe fading at the receiver. Fading occurs when there are significant variations in received signal amplitude and phase over time or space. Fading can be frequency-selective — that is, different frequency components of a single transmitted signal can undergo different amounts of fading. A particularly severe form of frequency-selective fading is caused by multipath interference, which occurs when parts of the radio wave travel along many different reflected propagation paths to the receiver. Each path delivers a signal with a slightly different time delay, creating “ghosts” of the originally transmitted signal at the receiver. A “deep fade” occurs when these ghosts have equal amplitudes but opposite phases — effectively canceling each other through destructive interference. When the geometry of the reflected propagation path varies rapidly, as for a mobile radio traveling in an urban area with many highly reflective buildings, a phenomenon called fast fading results. Fast fading is especially troublesome at frequencies above one gigahertz, where even a few centimetres of difference in the lengths of the propagation paths can significantly change the relative phases of the multipath signals. Effective compensation for fast fading requires the use of sophisticated diversity combining techniques, such as modulation of the signal onto multiple carrier waves, repeated transmissions over successive time slots, and multiple receiving antennas.

UNIT 15