Preface For the transfer of information, we need transport facilities dimensioned for the maximum information flow between a user and a network node, or between network nodes. These transport facilities employ a variety of what we have traditionally called transmission techniques. The original technique was optimised to handle voice transport in its basic analog form. Today, however, development is moving in the direction of completely digital transport systems capable of meeting the different requirements that are imposed by voice, data and video.
Change of the transport load from analog to digital
Existing network digitised in stages, coming closer to the subscriber
Bandwidth The usable frequency range of a connection is called bandwidth. For telephony, the ITU-T recommends connections that can handle frequencies between 300 and 3,400 Hz; that is, a bandwidth of 3.1 kHz. Normally, the ear can detect sound with frequencies in the interval 15 to (approximately) 15,000 Hz, but measurements show that the frequency range 300-3,400 Hz is fully adequate for speech to be heard clearly, and for us to be able to recognise the voice of the person speaking.
Contd.. Therefore, the microphone in our telephone must react to frequencies in the range 300-3,400 Hz and transform them into electrical voltages with an acceptable amplitude (strength) in the entire frequency band. The same requirements for bandwidth apply to the loudspeaker in our telephone
B/W contd. This analog bandwidth can (somewhat incongruously) be said to have a corresponding digital "bandwidth": analog digital PSTN 300-3,400 Hz 64 kbit/s ( after PCM coding ) GSM 300-3,400 Hz 13 kbit/s ( after special coding in and between exchanges, the bit rate is converted into 64 kbit/s )
Transmission media copper, which is used in two main types of cable: paired cable and coaxial cable; glass fibre, which is used in optical fibre cable; and radio waves, which are used in terrestrial point-to-point systems or area coverage systems (such as mobile telephony), and for point-to-point or area coverage communication via satellite. In principle, all media can be used for point-to-point communication, but only radio technology can handle communication with mobile terminals.
Contd.. Optical systems feature advantages of capacity, quality and economy. With respect to economy, copper-based systems are able to keep up only over the very last section - to the residential subscriber - where the need for capacity is small. Copper is still used to a large extent between subscribers and the exchange, which is mainly due to past practices. For the same reason, coaxial cables are still used between exchanges - although in recent years their main use has been for cable TV.
Carriers Carriers are exclusively analog in nature; that is, they carry waves of some kind: light waves or electromagnetic waves. In a purely physical sense, light too is made up of electromagnetic waves but - owing to the special characteristics of light - we see optical fibres as carriers of their own type of signal. On the other hand, the information to be transported is digital in many cases, at least the output signal from voice coders, video coders and computers. The GSM system is representative of the combination of digital information on an analog (radio) carrier, in so far as the voice coder is placed in the mobile telephone. (In ordinary fixed telephony, the voice coder is usually located in the local exchange or in the access node.)
Modulation and baseband transmission By allowing the transferred information to manipulate the carrier in some way - for example, by turning on and off light waves - the information can be perceived by a receiving exchange or terminal. This manipulation of the carrier is called modulation. The original transmission technique for telephony over copper pairs (baseband transmission) is still the most commonly used technique between a fixed telephone and the switching node. It applies the principle of analog information without carrier between a telephone and the voice coder.
Simplex - duplex; asymmetrical transmission In TV broadcasts it is enough to send information in one direction - a technique called simplex. Duplex means that information is sent simultaneously in both directions between two points. The technique of sending video information with a large bandwidth in one direction, and considerably less information (such as control signals) in the opposite direction, is called asymmetrical transmission. This technique has been brought to the forefront for video-on-demand and for high-quality information services, such as the wideband Internet. Half-duplex means alternative receive/send
Amplification Owing to the phenomenon of attenuation, special equipment must be used between nodes when the distance exceeds certain values (which are different for copper-based transmission, optical fibre and radio link systems). The points at which such equipment is found are called intermediate repeaters. Repeaters can be used purely for amplification (when the analog carrier has become too weak) or for a combination of amplification and regeneration (next page), when the digital baseband signals need to be "refreshed "
Regeneration It means that distorted information signals are read and interpreted, and recreated and amplified to their original appearance before they are forwarded. Noise and other disturbances completely disappear. This is not the case with analog transmission where disturbances are amplified as well.
Multiplexing Implementing and maintaining the transmission links in telecommunications networks is an expensive undertaking for network operators. Much is gained by transmitting several calls on the same physical connection (such as a wire pair). The technique used for multichannel systems, both in analog and digital networks, is called multiplexing. It can be divided into three groups: A) frequency division multiplexing (FDM); B) time division multiplexing (TDM); and C) wavelength division multiplexing (WDM).
Frequency division multiplexing Frequency division multiplexing (FDM) is used for transmitting analog information. Multiplexing is comparable to the technique that makes it possible to select the desired transmitting station on a radio. Each transmitter has been assigned a particular frequency on which information is superimposed and sent to the listener. By turning the frequency selector we can easily change to another transmitter.
Time division multiplexing
But.. All methods are not necessary for all types of communication. For example, in terms of transmission, a local telephone call can be illustrated as shown in following figure.
Transmission media Copper :There are two main types of metallic cable: paired cable and coaxial cable. In addition, open wire (metal wire without insulation) is also used in rural areas. Paired cable and coaxial cable are used both for analog and digital transmission; open wire as a rule is only used for analog 3.1 kHz connections. Paired cable :The simplest form of paired cable is found in our homes. This cable has only two conductors that connect the telephone to the wall socket. Operators have more to choose from in their store rooms: 2-, 10-, 50-, 100- and 500-pair cables, to name a few examples. Paired cable is used mainly in the access network between subscribers and the exchange, and - to some extent - in the trunk network between exchanges.
paired cable The paired cable was originally developed for analog connections. A large part of the existing cable network still consists of paper- insulated cables. Plastic is a better insulation material, because it is insensitive to moisture and has lower attenuation at higher frequencies, and that is why newly manufactured paired cable is plastic-insulated. The major part of the paired-cable network is buried under ground.
CONTD.. Normally, the conducting material is copper, and the conducting wires are manufactured in a number of standardised diameters - the most common diameters are 0.4, 0.5, 0.6 and 0.7 mm. The wires in the cable are twisted together to form pairs (two conductors) or quads (four conductors). The attenuation per kilometre depends on the diameter of the wire and on the frequency.
Coaxial cable Coaxial cable is used in analog (FDM) and digital (TDM) multichannel systems - in local data networks, cable-TV networks, and as a feeder for radio antennas. The cable consists of one or more coaxial tubes, each of which has an inner conductor surrounded by a tube-shaped outer conductor. The coaxial tube has very high transmission capacity (10,800 voice channels in analog multichannel systems). In the trunk network, the tubes are used in pairs, one for each direction of transmission. Today, coaxial cable is no longer installed in the trunk part of the telecommunications network. Instead it is being replaced by optical fibre cable. Nonetheless - apart from being used for cable TV - coaxial cable may come into use in the access part of future broadband networks.
The radio spectrum The radio spectrum, from 3 kHz to 300 GHz, is one range of the electromagnetic spectrum (infrared, visible and ultraviolet light, and X-ray frequencies are other ranges). The radio spectrum is divided into eight frequency bands as shown by Figure from VLF (very low frequency) to EHF (extremely high frequency).Figure
Radio wave (type 1) The propagation of a radio wave depends on its frequency. Radio waves with frequencies below 30 MHz are reflected against different layers of the atmosphere and against the ground, allowing them to be used for maritime radio, telegraphy and telex traffic. The capacity is limited to some tens or hundreds of bit/s.
Radio wave (type 2) Above 30 MHz, the frequencies are too high to be reflected by the ionised layers in the atmosphere. The VHF and UHF frequency bands, which are used for TV, broadcasting and mobile telephony, belong to this group. Frequencies above 3 GHz suffer severe attenuation caused by objects (such as buildings) and therefore require a free "line of sight" between the transmitter and the receiver. Radio link systems use frequencies between 2 and 40 GHz, and satellite systems normally use frequencies between 2 and 14 GHz. The capacity is in the magnitude of Mbit/s.
Radio link Each radio link needs two radio channels: one for each direction. A few MHz spacing is needed between the transmitter frequency and the receiver frequency The higher the carrier frequency, the shorter the range. For example, a 2 GHz system has a range of approximately 50 kilometres, and an 18 GHz system has a range of 5-10 km. At regular intervals, the signal is received and forwarded to the next link station. The link station may be either active or passive. An active link station amplifies or regenerates the signal. A passive link station generally consists of two directly interconnected parabolic antennas without any amplifying electronics between them.
Satellite Satellite systems are quite similar to radio link systems; the only real difference being that the intermediate link station is in orbit around earth instead of being set up on the ground Satellites describe either a polar or a geostationary orbit. Those with a polar orbit pass over the poles at an altitude of about 1,000 km and are used for meteorological and military purposes
Geostationary Satellites Satellites used for telecommunications are placed in geostationary orbits in the equatorial plane 35,800 km above the earth's surface. They have an orbiting time of 24 hours which, because of the earth's rotation, gives them the appearance of being stationary. Approximately one third of the earth's surface is covered by an antenna with global radiation. Satellite links are used in national as well as international telecommunications networks. Intercontinental use has decreased in favour of optical submarine cables.
The transmission properties of satellite links are excellent and problems are few. However, the long distance between terrestrial stations via the satellite does cause a 240 ms delay, which in itself is troublesome to voice communication and which may give echoes with a propagation time of about 0.5 seconds. Intelsat (International Telecommunications Satellite Organization) was founded with the aim of financing, developing and running worldwide commercial telecommunication satellite systems. One of Intelsat's satellites is Intelsat VI, which has 80,000 voice channels
Optical fibre In the 1870s, an Englishman, Tyndall, showed that light can be conducted through a bent jet of water. At the end of the 19th century, Graham Bell designed an optical telephone. The difficulty in finding appropriate light sources, however, made it necessary to wait 100 years before this technology could be used in practice. The first field trials with optical cable were carried out in 1975, and in 1980, the first commercial systems were opened for telephone traffic.
It has an enormous transmission capacity. Today, there are systems for several Gbit/s Gbit/s approximately corresponds to 32,000 simultaneous telephone calls at 64 kbit/s. The limitations are in the terminal equipment. The interface between electrical and optical transmission requires E/O converters.
advantages of optical fibre The advantages of optical fibre systems can be summarised in the following points: very high capacity; long repeater spacing; small cable dimensions; low weight; small bending radius; no crosstalk; and immunity to electromagnetic interference
An optical fibre optical cable - a slotted core cable with 36 fibres
structure The glass fibre has a glass core with a surrounding glass cladding. The core consists of doped glass with a somewhat higher refractive index than the cladding, which is made of pure quartz glass. Normally, the diameter of the cladding is 125µm. The diameter of the core is different for different types of fibre - 8, 10 or 50 µm. The fibre has a primary coating (as a rule consisting of cured acrylate) to provide protection against moisture and chemicals, and an outer - fixed or loose - secondary coating
Structure/contd.. The optical cable is provided with a strength member made of steel or plastic that gives the cable the strength necessary to withstand tensile stress and bending. In addition, the cable contains filling that fixes the fibres and protects them from excessive bending and moisture. The cable cladding is made of plastic, as a rule polyethylene. The number of fibres in a cable varies depending on the field of application - there are, in fact, cables with thousands of fibres.
Properties of light
Optical communication This total reflection is utilised in optical fibre communication. If the angle of incidence is sufficiently large, then the light in the fibre will reflect repeatedly in the interface between the materials. The fibre need not be straight but can conduct light even when bent
Wavelength division multiplexing (WDM) Wavelength division multiplexing enables a number of channels to be sent at different wavelengths in the same fibre, in the same direction or in both directions. Although it has been given a name of its own, WDM is an optical fibre variant of frequency division multiplexing.
WDM/contd.. Two WDM systems currently being developed are coarse WDM and dense WDM. In coarse WDM, 2 to 10 wavelength channels are used with large wavelength separation between the channels (5-50 nm). In dense WDM, 5 to 100 wavelength channels are used with less wavelength separation between the channels (0.1-5 nm). A simple variant of WDM already in use employs 1,310 and 1,500 nm at the same time in the same fibre. This makes it possible to double the capacity, or to achieve duplex transmission with one wavelength per direction.
Analog transmission Bandwidth: The bandwidth requirements for analog transmission equipment in access and interexchange networks vary depending on the utilisation of the transmission link. In the case of analog single-channel connections, a bandwidth of 3.1 kHz (frequency range 300-3,400 kHz) is required. Other requirements apply to analog multichannel connections. A frequency-multiplexed FDM connection with 2,700 channels requires a bandwidth of 12 MHz, and a connection with 10,800 channels requires 60 MHz. Accordingly, the possible number of channels is proportional to the bandwidth of the connection
Analog transmission Attenuation: Attenuation as well as its opposite, amplification, are measured in decibels. By measuring the transmitted power, P1, and the received power, P2, the attenuation, A, can be calculated. The formula is: Attenuation: A = 10·log (P1/P2) Example: A = 10log(400mW/10mW) = 16 dB. By using the 10-logarithm, the total attenuation of a connection can be calculated as the sum of the attenuation/amplification of the parts. Note: measurements in decibel solely indicate a relationship between two quantities. As a rule of thumb, the doubling or halving of power corresponds to an increase or decrease by about 3 dB. The attenuation value obtained in no way indicates the real strength of the received signal.
Fading Fading, a phenomenon that arises when radio signals are reflected against different atmospheric layers and the ground, is one of the most difficult problems to master in radio communication CASE 1: The refractive index of air for radio waves is normally inversely proportional to height. It affects radio waves, which gradually bend towards the earth and reach a bit beyond the horizon. Therefore, radio-optical sight is somewhat longer than optical sight. The refractive index is affected by the air temperature, pressure and humidity Deflection caused by refraction
Fading… Case2: At frequencies below 10 GHz, fading is mainly caused by the interference that arises between direct signals and those reflected from the earth, or between different signal paths through the atmosphere. This type of multi-path propagation is most common during summer nights and in early autumn Interference due to multipath propagation
Fading… Case 3: The attenuating effect of rain becomes noticeable at frequencies above 10 GHz when the wavelength gets down to the size of a rain drop. Signals are heavily attenuated due to scattering and absorption of energy. The higher the frequency, the more importance attenuation due to rain assumes, compared with other causes of fading.
Noise Noise is generated in all types of electronic circuit. No system can be made totally free of noise - the best we can do is set limits to how much noise we are willing to permit. Limit values have been recommended by the ITU-T. The absolute noise level is not the most interesting phenomenon; what determines audibility is the relationship between the level of the transmitted signal and the noise. That is why we use the term signal-to- noise ratio. When the attenuation on the line makes it necessary to use amplifiers, the signal-to-noise-ratio will decrease, because the incoming noise is amplified just as much as the signal. In the end, the noise level may be so high that we have to select a lower signal level (fewer amplifiers) to maintain an acceptable signal-to-noise ratio.
Noise (External interference) In metallic systems, external interference caused by power lines and radio transmitters can lead to a noticeable degradation of transmission quality. Interference can also be due to lightning strikes in the vicinity of a metallic cable. Should this occur, an "energy-rich" pulse will propagate along the line. In a worst-case scenario - if adequate lightning arresters are not provided - this pulse might destroy cables and electronic equipment.
Noise (Crosstalk ) The phenomenon of a connection being disturbed by a conversation on another line is called crosstalk. The problem is particularly noticeable on connections with paired-cable transmission As far as telephony is concerned, we distinguish between two types of crosstalk: intelligible crosstalk, where we can understand the content of another call between two subscribers; and unintelligible crosstalk, which means that the conversation on an adjacent channel disturbs us, although we do not understand what is being said. The interference may have the character of noise but is more disturbing because of its rhythm and intonation. Of these two, intelligible crosstalk is most severe since it jeopardises integrity. The ITU-T recommends a level spacing of at least 52 dB between the received test tone level and crosstalk level.
Noise (Sidetone ) When we talk on the phone we are supposed to hear our own voice in the receiver. This is called sidetone. The level of the sidetone should not be too high or too low. Modern telephone sets contain functions that balance the sidetone effect, while taking line impedance into account. For this reason, older telephone sets ought to be exchanged for newer ones whenever the network is being moder nised.
Noise in Digital transmission
Frequency spectrum of Noise By means of the frequency spectrum of noise, we can distinguish between two main types of noise: white noise and 1/f noise Examples of white noise include thermal noise, shot noise and partition noise.
Other noises.. Thermal noise occurs because the charge carriers in a material are in constant motion. This motion becomes more vivid as temperatures increase and vice versa. Shot noise arises in semiconductors and is caused by the individual charge carriers (electrons or holes) that make up the electric current. In calculations for semiconductors - diodes or transistors, for example - we assume that the current is constant. However, in reality only the time average is constant. We disregard the fact that the individual charge carriers cause current variations, what we call shot noise. for 1/f noise the power is greatest at low frequencies and decreases with increasing frequency.
Multiplexing Why? Advanced time division multiplexing makes it possible to produce multiplexed signals up to 10 Gbit/s commercially. The capacity will be further increased by means of wavelength division multiplexing in optical fibre cable.
Multiplexing is used to reduce transmission costs - several channels along the same route share the same transmission medium, such as optical fibre. We will now familiarise ourselves with three standardised multiplexing hierarchies, each of which is based on time division multiplexing: PDH, plesiochronous digital hierarchy; SDH, synchronous digital hierarchy; and SONET, synchronous optical network.
Plesiochronous digital hierarchy The level of the first order (PCM frame )
The 125 microseconds are the result of sampling each voice channel 8,000 times per second (8,000 Hz); that is, the cyclic time is 1/8,000 = seconds. After first- order multiplexing, the voice channels share this time. In the European variant, the first channel - time slot 0 - is used for frame synchronisation. Time slot 16 can be used for signalling or traffic, and all other time slots are used for traffic, usually voice communication. Signalling system No. 7 (SS7) can use any time slot except TS0. In the American PCM frame, with 24 time slots, no special time slot has been allocated for signalling. Instead, one bit in each time slot in every sixth frame is replaced by signalling information. The replaced bit corresponds to the least significant bit in the sample. As a consequence, only seven out of eight bits can be used transparently through the network
Multiplexing and demultiplexing into and from PDH level two
The PDH structure is "rigid". To rearrange and drop tributaries (for example, 2, 8 or 34 Mbit/s) from a main flow (such as 140 Mbit/s), the main flow must be demultiplexed step by step down to the desired tributary level and then multiplexed back to the desired main flow level. Rearrangement is done manually in a digital distribution frame (DDF).