Communications Technology Blog

In this transmission technology, light waves are modified in intensity to transmit data. Digital pulses are converted to ON and OFF pulses of light which are used to transmit data. Fibre optic systems can only transmit digital data using this technique. Analog signals must be converted to digital signals before they can be transmitted over fibre optic facilities. A single fibre optic cable resembles a human hair. Lightwaves travel through the core of a fibre which is made up of very pure glass. The layer around the core is called cladding and is made up of less pure glass th~n the core.

The basic component of a fibre optic transmission system is the Transmitter. There are two primary devices that serve as lightwave sources. These are the Injection Laser Diode (ILD) and the Light Emitting Diode (LED). The fibre optic cable is the transmission medium. A photodetector or photodiode is used as the receiver which receives the lightwave pulses and converts them into electrical signals. Both types of devices emit and propagate light in a conical manner. However, they differ in the angle of emission and intensity.

Satellite microwave systems use a satellite as a repeater. A satellite microwave system has only one transmitter, one repeater and one receiver. Satellite microwave systems have receivers and transmitters located in ground stations. The repeater is the satellite that orbits in space. A communications satellite orbits the earth at the same speed as the earth. This is known as geo-synchronous orbit. The satellite always remains directly in line with the same point on earth and functions as a fixed microwave repeater.

In the orbiting satellite, the equipment used to receive, amplify and retransmit signals is called a transponder. satellite Impairments
As there is only one repeater in a satellite system, there are fewer transmission impairments. The only major impairment that occurs with satellite transmission is delay. Delay can be as much as one-fourth of a second. In a satellite signal, delay takes the forms of echoes. Echo control devices, such as echo cancellers, are used to reduce this impairment.

There are several impairments that occur with microwave transmission. The most noted are fading, absorption and obstruction. Fading is caused by atmospheric conditions, which tend to occur mostly at night and at sunrise during summer. This reduces the strength of the primary signal. Absorption is a problem primarily at frequencies of 11 GHz and higher. Signal loss occurs due to rain and water vapor in the air. Absorption is an unpredictable impairment and is taken into account when transmission systems are designed. Obstruction is a major problem since the terrestrial transmission towers are stationary, any physical object between the towers can block the signal. Airplanes and birds are examples of obstructing objects.

When serious impairment problems arise, equipment within the microwave systems can maintain continuous transmission by switching the signal to the alternate standby paths. Even with so many Impairments, Terrestrial Microwave is the most commonly used form of communication even today.

Microwaves are electromagnetic radiations with very short wavelengths. As they have short wavelengths, they have high frequencies. Frequencies used for commercial microwave transmission typically range between 2 GHz and 23 GHz. The frequency at which microwave radio signals are transmitted is commonly called Radio Frequency (RF). Radio frequencies that are used for commercial microwave transmission are 2, 4, 6, 11, 18 and 23 GHz.

Microwave signals are transmitted using a combination of transmitters and repeaters. The distance that a microwave signal travels from a transmitter to a repeater is called a hop. A hop may also cover the distance from one repeater to another, or from a repeater to a receiver. The number of miles within one hop depends on the terrain. Microwave transmissions require very low power. Only 10 watts or less are needed to radiate the highly focused signal from one antenna to another.

The input signal to a microwave facility is often a composite carrier signal. The microwave facility uses the input signal to modulate an RF carrier signal generated by the transmitter. Then the transmitter emits the ElectroMagnetic Radiation (EMR) signal towards. the receiver. This EMR signal is always an analog signal.

Coaxial cables have been used since 1936, but their use was limited because of high installation costs. Today, the use of coaxial cables has significantly improved due to their ability to carry high frequency analog and digital signals. Coaxial cable systems are highly reliable. The cost of maintenance and expansion is relatively low. As with open wire pairs and twisted-wire pairs, a coaxial cable has two metal conductors, usually made of copper. Each coaxial cable consists of an inner conductor that is made of solid metal. An insulating material separates the conductor from an outer cylindrical conductor. An outer sheath protects the coaxial cable against the elements of weather.

There are several advantages of using coaxial cables. The main advantages are - Coaxial cable connection permits the transmission of high frequency signals, including carrier frequencies. Further, at high frequencies, the coaxial outer conductor shields the inner conductor from extraneous electromagnetic radiation signals. such as crosstalk. Also, attenuation at high frequencies is less severe with coaxial cables than with twisted pair cables. Therefore. coaxial cables are better suited for high frequency transmission, as used in high speed data and video applications. Coaxial cables are the optimal metallic medium of transmission for frequencies above 1 MHz and below 1 GHz. However, for frequencies upto 1 MHz, twisted pair cables are more suitable.

Terrestrial microwave systems follows a line-of-sight path during transmission. A narrowly focused beam of EMR is generated by the transmitter. It is then sent from one microwave tower to the next, until it reaches the final destination. There are two types of terrestrial microwave transmission systems-analog and digital. However, the RF signal used for transmission is always analog.
Both analog and digital systems begin with an input signal, and modulate it to an Intermediate Frequency (IF) signal. This is modulated to produce the Radio Frequency (RF) output signal. The RF signal travels through space between microwave towers.
A waveguide, usually a metallic tube, is used to confine and guide the microwave signal from the base of the microwave tower to the antenna.

At the receiving end, the signal is broken in the reverse order. First the RF signals are separated by a filter. Next, each RF channel is demodulated to the JF frequency. Finally, the IF signal is demodulated again into the composite signal. The composite signal is demultiplexed into the source signals. Microwave radio signals can be sent either over short or long distances. A short distance route, called a short-haul microwave route, usually consists of ten or fewer hops. Each hop is 16 or more kilometers, depending on the terrain. Microwave frequencies of 18 and 23 GHz are used for short-haul routes. A long-haul system refers to a route that carries a microwave signal over long distances. long-haul routes may span 6400 kms using as many as 150 hops.

Wires are twisted together for the same reason that open wires are transposed (frogged) on poles. It cancels some of the induced noise, such as crosstalk. The twisted pairs within cables are separated and wrapped into bundles called binder groups. Binder groups and pairs within binder groups are color-coded so that each pair in a cable can be identified. Every circuit, including the subscribers loops, tends to impede (or disrupt) signal flow. The extend of this impedance depends on two characteristics of the circuit namely resistance and reactance to a certain extent. Resistance is the tendency of a circuit to impede the passage of DC current. The longer the loop, the greater will be the resistance. Reactance, on the other hand is the tendency of a circuit to impede the flow of AC current. Reactance has two components namely Inductance and Capacitance

The tendency of the local loop to impede the flow of the signal depends on a combination of several factors: resistance, inductive reactance and capacitative reactance. This combination equals what is known as Impedance. To correct this situation, load coils are inserted in the loop.

Wire was the first medium used in telecommunications. Today, wire transmission facilities are used in both the local loop and inter-office trunks. Wire transmission facilities include the signal being transmitted, the equipment located in central offices, and the equipment along the transmission route. Open, unshielded wires also called open wire lines was the first type of transmission media used for telephones. To reduce crosstalk and power-line noise, open wire pairs are transposed, or frogged, at regular intervals. Transposition enables each wire in the pair to be equally exposed to power lines and to other wire pairs. Noise is reduced through a cancellation effect.

Open wires have an advantage over twisted pair cables. There is less resistance in open wire lines because they are made of thicker wires. This helps to reduce signal loss. Twisted pair cables are used for local loops, and for analog and digital carrier systems that cover relatively short distances, that is 200 miles or less. A twisted pair cable gets its name from the arrangement of wires inside the cable. One twisted pair is made up of two individual wire conductors usually made up of copper. Many twisted pairs are packed into a cable. A plastic insulated cable can carry from 6 to 1800 pairs of cables. Cables have an outer protective sheath that may be made of aluminum, steel or polyethylene.

Source signals can be one of these four types - Voice signals, Program signals, Video signals, and Data signals. Voice signals produce the sounds you hear through the receiver in your telephone handset. Voice signals are analog, with a bandwidth of 4,000 Hz. Program signals transmit audio signals and wired music over telephone lines. Program signals are analog signals with a bandwidth of up to 15,000 Hz and are carried by special service facilities.

Video signals carry picture information from a television broadcast. Like program signals, video signals are carried only by special service facilities. These analog signals have a bandwidth of 4.5 MHz. Data signals are analog or digital signals that carry information. A modem is required for analog data transmission. Carrier systems are a means for multiplexing, transporting and de-multiplexing signals from one end of a transmission path to the other end. As all the carrier system networks must work together, the signals that their transmission facilities produce must conform to a set of standards called the transmission protocol.

It is not safe to assume that you will have more than 10 hours overnight to achieve all the processing, even if the user day is only 8 or 9 hours long. Delays in data arrival can cause a company significant problems, and one must make sure that he/she will be able to complete the overnight processing without running over into the business day. Naturally this will vary from data warehouse to data warehouse, but of the 10 hours allowed to complete the overnight processing, it is normal to have about 60% available to complete the post processing of the data and the aggregation and index creation. That gives about 6 hours to complete all these tasks.