Basic telephone service starts at a user phone. A typical phone employs the following circuits: speech, ring, dial, outgoing polarity and transient protection and incoming polarity and transient protection.
Phone line electrical levels are measured in decibels. The decibel has no actual numerical value, but is used only to express a ratio between two voltages, currents, powers, or impedances. The decibel uses logarithms to the base 10 called LOG. To calculate for voltage, current, SPL, distance: 20 Log X1/X2. To calculate for power = 10 Log P1/P2.
A hard two wire circuit connects the basic phone to the service provider referred to as the telco. The wires are known as the tip and ring wires. The tip carries -48 volts DC and the ring is at ground potential. The terms “tip” and “ring” originally referred to the respective parts of the plug which operators manually plugged into a bank of switches. The wire pair is typically 22 gauge twisted which is rated to handle 300 to 3000 Hertz but actually works at much higher frequencies.
The basic phone circuit is assumed to have impedance of 600 ohms. In reality, the impedance is dependent upon the frequency of the signal; therefore the value of impedance varies with the signal. The unaltered line attenuates a 300 Hz signal about 4 dB per 12,000 feet. At 5 kHz, the attenuation jumps to 14 dB per 12,000 feet. This attenuation distortion can present serious problems, especially to digital signals.
Attenuation distortion is limited by the Federal Communications Commission. The FCC defines attenuation distortion as the difference in gain at a frequency with respect to a reference tone of 1004 Hz. To counteract the problem of attenuation of higher frequency signals, inductance is added to the phone line. The term-loaded line refers to this practice. Loaded line is marked H, D or B to denote added inductance every 6000, 4500 or 3000 ft. Standard increments of added inductance are 44, 88 or 135 mH which is also printed on the line.
Delay distortion is another problem resulting from the behavior of the traditional 2 wire pair. The FCC also specifies limits of delay distortion. The value for a basic phone line cannot exceed 1750 microseconds between 800 and 2600 Hz. A special LC filter called a delay equalizer can be utilized to lessen delay distortion. Telcos also offer special lines designated as C2 lines that offer higher standards of attenuation and delay distortion.
Attenuation distortion, delay distortion, bandwidth limits and physical size were all factors of copper wire pairs which have lead to the upgrade of much of the world’s phone systems to fiber optic cable.
Phone systems have used a wide array of signaling schemes. Common types used are pulse, DC Voltage and DC Polarity, Multi-Frequency signaling and Common Channel Inter-Office Signaling. Pulse signaling was common in the days of step by step and crossbar systems. These were used to represent dialed digits, either directly or via pulse senders from a central office switch, to the destination switch. Though not common today, are still used in rare instances.
Voltage changes and polarity changes (as well as pulses) were used in some inter-office and Panel signaling routines. This method is still used today on special leased telco lines to connect radio transmitters to controlling apparatus remotely. The phone line carries a DC control voltage as well as an AC signal which is the transmit and receive audio intelligence.
Multi-Frequency or MF signaling has been used between central office and toll tandem systems. Dual frequency tones are used to send information of the called telephone number throughout the network. These tones have to be within the 300Hz to 3000Hz band of normal voice communications in the telephone system. Most MF toll trunks in the US and Canada use 2600 Hz as supervision and for idle line. Some places used 3750 Hz (out of band) for supervision/idle line control. MF signaling is sometimes referred to as R1 signaling. R2 is an international bi-directional version of MF signaling which is widely used throughout the world. MF signaling is relatively slow and prone to toll fraud.
Common Channel Inter-Office Signaling is used to send more information between tandems and network switches. CCIS works by sending information outside the 300Hz to 3000Hz band on a data circuit parallel to the voice circuit. CCIS is faster and less prone to toll fraud than MF signaling. The modern version of CCIS is called Signaling System 7. SS7 is heavily used in the USA, Canada, UK, Australia and much of the world.
Various transmission methods, which have been used, for telephone service include Open Wire Carrier, Coax Cable, Microwave Tower and Fiber Optic systems.
Open wire carrier carried multiple calls over a pair of copper wires simultaneously by using frequency division multiplexing (FDM). The frequency band was to about100Khz with 4 kHz channel spacing. Open wire carrier use was discontinued in the US many years ago.
Coax cable transmission was used along the same time and as open wire carrier. Coax cable was similar to open wire carrier. It’s application was in urban areas while open wire carrier was used more in rural settings. Frequency division multiplexing was used with coax cable transmission. Today coax cable for telephone transmission is limited to very few areas, mainly as a backup system.
By the 1950’s, microwave transmission became a widespread telephone call transmission medium. Microwave uses frequency division multiplexing. Microwave is still in widespread use in USA as well internationally. Systems that are still in service are often either backup systems or are utilized as part of digital transmission techniques.
Modern phone transmission systems consist of pure glass optical fibers using amplitude modulated infrared light, commonly known as fiber optics. This method revolutionized telecommunications transmission techniques. Fiber optics deliver high bandwidth and high quality transmissions, making their application widespread in North America and much of the industrialized world.
Practically every aspect of telephone service involves research and inovation. While the changes to telephone system have been remarkable in the past century, future gains will likely revolutionize phones beyond our imagination. Surely these changes will be the direct result of research done in electronics, computing and mechanical engineering.
Author: J.C. Banks
Article Source: EzineArticles.com
Provided by: Beading Necklace
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