Sunday 2 October 2011

Telecom landline resistance facts

The telecommunications industry has experienced tremendous changes during the last 25 years on both legislative and technical fronts. Landline is near extinction and mobile telecommunications dominate the system. Telecommunications is communication over a distance. It encompasses all forms of communications and includes communications by voice; video or data that we wish to transmit from one point to another. People often separate the communication of voice and data into two catagories, using the term telecommunications to denote the transmission of voice signals and data communication to refer transmission of data signals.
            In public switched telephone network (PSTN), voice signals emerge from a telephone as analog signals. The word analog is related to analogous, electrical signals caused by a voice wave hitting the diaphragm of a telephone set’s transmitter are analogous (similar) to the shape of the voice wave. All telephones connect to a local central office, which have an automated switching system that connects callers to their desired destination. The local central office contains a line circuit for every telephone connected to it. The line circuit in turn contains a device called codec (coder/decoder) that converts analog voice signals received from the telephone into digital signals. These digital voice signals are then carried over the digital circuits that comprise the PSTN.
            Data is converted into digital codes when it is stored inside computers. The transmission of these digital codes results in a digital signal. Digital data can be transmitted over the PSTN by using modems(MOdulator-DEModulator)or can be transmitted over packet data network(also known as public data network-PDN).The PDN is a wide area network which uses facilities in the PSTN that have been reserved for data transmission. All circuits in PSTN are digital circuits and can carry digitized voice data and video. It is appropriate today, therefore, to consider telecom as including voice data and video.
           Though telecommunications began with telegraph in 1837, In 1876 Gray and Graham bell filed papers with patent office for an invention called telephone. Modern telecoms have VLSI devices to convert analog signals to digital and carry them between central offices. Many signals from many different conversations or data, inputs into one very signal stream. These devices use a technique called multiplexing to place many signals over one transmission medium. The multiplexing devices used to place many signals over glass fiber have developed to a point where they change the light signals used transmit signals over fiber at a rate of almost ten billion times a second.
            Most large businesses use a private voice switching system called a private branch exchange (PBX).The use of a PC to serve as aPBX also has the benefit of allowing the computer to provide information about the call on the screen. Customer information can be kept in a data base file on the PC, and the PC can use that database to get information that helps with handling the call. Several software programs are available that enhance the PCs ability to interact with incoming call according to information associated with the call.
            For data applications (internet) all computers in a building are connected together by a LAN, these LANs will be connected together with a Metropolitan Area Network (MAN) or Wide Area Network (WAN).LANs use a technology called Ethernet where PCs are connected via two twisted pair copper cables with RJ45 connecters at both ends.
            The voltage for the transmitter in telephone instrument and the dual tone multi frequency (DTMF) circuit of the touch tone dial is supplied from the central office. The voltage at central office battery is 52Volt DC.If the phone is on hook, no electric current will flow and we would also measure 52Volt at the telephone. When the telephone is taken off-hook to make or answer a call, electric current will flow. As the electric energy travels over the wire pair between the telephone and the central office, some of the voltage is lost as heat in the wire pair. We refer to the electric energy as a voltage drop ,The amount of voltage drop in the wires depend on the resistance of the wire and amount of current flowing in the wire pair. The resistance of the wire depends on the size (gauge) and length of wire used. Resistance (R) of a conductor, twisted copper wire of length’l’ cross sectional area ’a’ and resistivity ’ρ’ is given as R= (ρ*l)/a resistivity of copper (ρ) is 17.2*10^-9 ohm/meter. And that of silver and gold are 16.29*10^-9 and 24.4*10^-9 respectively.
            The length is variable and depend on the distance the phone is at from the central office(telephone exchange).Every wire pair used to connect telephone to a central office will connect through an electronic circuit(called the line circuit)to the central office battery. The amount of voltage used up by the line circuit and wire pair will leave a voltage between 8 and 24 volts for the telephone. The variance in voltage at the phone is because longer wire pair will have more voltage drop (use up more voltage) than shorter wire pairs. For the longest wire pair, the telephone will get about 8 volts. On a very short wire pair the phone will get about 24 volts.
            A 19 gauge wire 1000ft long has a resistance of 8.051ohms and a 16 gauge wire of same length has a resistance of 4.016ohms.Thus a difference in gauge size of 3 results in a resistance change by a factor of 2.In telecommunications the gauges of wire used are 26,24,22,and 19 guages,with 26 gauge predominating. As the gauge number increases the cross sectional area dicreaseesThe cables that connect our telephone to the local telephone exchange switching system must not contain mire than1800ohms of resistance, when the local switching system is computer controlled switch. With older electromechanical switching system, the resistance of wires connecting a telephone to a switch was limited to 1200ohms.
Phone requires 0.02 amp of current flow through the instrument to provide adequate power for the transmitter and touch tone dialing pad. Computer controlled switching system use a DC power supply of 52volts.All telephones attach to switching system one pair of wires. At the switching system this pair of wire connect to a device called a line circuit the job of the line circuit is to interface a telephone to a switching system. The switching system connects the negative terminal of a 52volts battery through the line circuit to one of the wires connected to a telephone. This wire is called the ring .The positive terminal of 52volts battery is connected through the line circuit to the second wire connected to a telephone. This wire is called the tip. The line circuit has the resistance of 200ohms between the -52v DC and the wire going to the telephone. The line circuit also has the resistance of 200ohms between the positive terminal of the battery and the other wire going to the telephone. The positive terminal of the battery is connected to the earth ground, thus the positive terminal is grounded and the negative terminal is -52 v with respect to ground.
  Since telephone needs at least 20 mile amps (0.02amps) of current flowing through it to operate properly, the resistance of the wires attaching a telephone to the central switch must be selected to ensure this criterion is met. Notice in the circuit of battery supply, that the resistance of each wire is 900ohms.The resistance of the telephone is400ohms and resistance of each side of the line circuit is 200ohms, for each connection to the wire pair. Ohms law dictates that with 52volt applied to the circuit, 20mA of current will flow through the telephone (and every where in the series circuit) I=V/R=52/ (200+900+400+900+200) =52/2600=0.02A
            Now that we know 1800Ω is the maximum resistance allowed in wires connecting a telephone to a local switch. We can select the gauge of wire needed to ensure that this value is not exceeded. Resistance per 1000ft of 26 gauge wire is 40.81 Ω.We need two wires to connect a telephone to the local switch. Electric current will flow out over one wire from the switch to telephone and will return to the switch over the second wire. Thus the ring wire, the telephone, and the tip wire are electrically in series. The resistance for 2000ft of wire is 81-62 Ω.We refer to the two wires as a loop because they form a loop [in connecting a telephone to a central switch. The resistance per 1000ft loop using 26 gauge wire is 81.62 Ω.If we divide 1800 by 81.62 we will find that maximum allowable loop using 26 gauge wire is 22,000ft or a little more than4 miles. If the telephone is located more than 4mi from the central switching system, a larger diameter wire will be needed. The gauge of wire to use in the will be governed by the length of the loop needed to connect a telephone to a switch (the distance from the switch to telephone)
            Since we know that the resistance can be substituted for actual components in a laboratory environment to perform test or measurements, let’s do that for a telephone. We stated earlier that telephones have a resistance of 400 Ω.This is referred to as the nominal resistance value of the telephone. Its actual resistance varies depending on the voltage applied to it. The resistance of a telephone cannot be measured accurately with an ohmmeter because the ohmmeter does not place a high voltage on the telephone. The ohmmeter will provide us with the resistance of a telephone under a very low voltage condition. We can calculate resistance of a telephone in a line circuit indirectly by measuring voltage and current in a line circuit and using Ohm’s law to find resistance. Using Ohm’s law V=IR we can calculate the various volatages dropped across each components of the circuit
1.For one half of the line relay in the central switch:E=0.02*200=4V
2.For one wire connecting the telephone to the switch:E=0.02*900=18V
3.For the telephone:E=0.02*400=8V
4.For the return wire telephone to the switch:E=0.02*900=18V
5.For one half of the line relay in the central switch:E=0.02*200=4V

Total of all voltage drops:52V

Kirchoff's voltage law is satisfied. The sum of the voltages dropped in the circuit equals the sum of the voltage applied to the circuit, we can also see that the voltage dropped across the telephone in the circuit is 8V.