Additional Topics - Application of Transmission Media PDF

Additional Topics APPLICATION OF TRANSMISSION MEDIA TELEPHONY  Origin Greek words: Tele – from afar Phone – sound, voice, voice sound March 10, 1876 - Alexander Graham Bell assisted by Thomas Watson was able to transmit words over the telep...

Additional Topics APPLICATION OF TRANSMISSION MEDIA TELEPHONY  Origin Greek words: Tele – from afar Phone – sound, voice, voice sound March 10, 1876 - Alexander Graham Bell assisted by Thomas Watson was able to transmit words over the telephone; Bell was then issued with patent number 174,465 for his invention US Hres 269: (June 11, 2002)- the contributions of Meucci were recognized but the House of Representatives' resolution did not annul or modify any of Bell's patents for the telephone. Another resolution, Senate Res 223 was introduced in the United States Senate and died, unenacted. The TRUTH “Meucci could not afford the $250 needed for a definitive patent for his "talking telegraph" so in 1871 filed a one-year renewable notice of an impending patent. Three years later he could not even afford the $10 to renew it. He sent a model and technical details to the Western Union telegraph company but failed to win a meeting with executives. When he asked for his materials to be returned, in 1874, he was told they had been lost. Two years later Bell, who shared a laboratory with Meucci, filed a patent for a telephone, became a celebrity and made a lucrative deal with Western Union.”  Brief History: 1877 – there were 6 telephones in the world 1878 – 1st telephone set with combined transmitter and receiver in a single handheld called Butterstamp telephone 1881 – 3,000 telephones; Bell & Watson sold the rights to AT&T 1883 – over133,000 telephones in the US alone 1951 – Western Electric Company introduced the Rotary Dial Telephone called Bell System 500 -type AT & T – American Telephone & Telegraph Company also called Bell Telephone System & Ma Bell Rotary telephone – 500-type DTMF telephone – 2500 – type Parts of Local Loop Main Distribution Frame - termination of the inside plant and the outside plant Manhole – an underground enclosure to facilitate the splicing of two or more cable Cross-connect cabinet / Serving Area Interface (SAI) – distributes the cables to different areas of subscriber and across poles and fixtures Poles and fixtures – carry splices cable aerially Termination box – contains the telephone subscriber’s line and is connected to the distribution plant central office Connector – connects the telephone to the line Telephone set – a subscriber’s device Feeder cable – the largest cable used in local loop, usually 3600 pair of copper wire placed underground or in conduit Distribution cable – smaller version of a feeder cable containing less wire pairs Subscriber or Standard Network Interface (SNI) – a device that serves as the demarcation point between local telephone company responsibility and subscriber responsibility for telephone service Drop Wire – the final length of cable pair that terminates at SNI The telephone set is connected to a central switching office or local exchange through four wires: Tip Green Transmit the signal Ring Red Receive the signal Sleeve / Slave Yellow Spare or for special purpose application Ground Black or White Grounding connection The telephone set is powered by a -42 to -52Vdc (typically -48 Vdc) supplied by the central office on the ring side of the telephone. The common resistance is 600 ohms.  Common Parts of Telephone Set Mouthpiece – the telephone transmitter that converts acoustical energy to a varying electrical energy by means of a carbon microphone that can be transmitted through the transmission system to the receiver of the called telephone Earpiece – the receiver part of the telephone that converts the varying electrical current representing the transmitted speech to variations in air pressure at the called telephone set Dial – used to generate the pulses or tones required for establishing a desired connection Telephone Ringer – converts electrical signal from the central office to an audible signal alerting the subscriber that he is being called Switchhook ( cradle )- it has level spring that permits dc to flow in the loop as a medium for carrying current Transmission Regulation Circuit –consists of a number of capacitors, varistors and resistors that is used to maintain same speech level or suitable strength for normal hearing regardless of strength of received signal. a. Sidetone – sound on the talker’s voice heard on his own receiver used to determine how loudly he should speak b. Telephone Hybrid – as a four-wire device, two-wire is used for transmission and the other two-wire are connected at the receiver The two-wire that connect the subscriber to the central office conduct the following: direct current for off-hook signaling varying direct current for speech alternating current for ringing, dial tone and busy signal pulse data quantized signals or analog voice signals supervision signals undesired electrical interference BORSCHT – the combined interfacing circuit that sorts out voltages and protect circuits against voltage overload Battery feeds from the office to the line to operate station transmitters and DTMF dials. Overvoltage protection is provided to protect the line circuit from damaging external voltages. Ringing connects from a central ringing supply to operate the telephone bell. Supervision refers to monitoring the on-hook/off-hook status of the line. Coding converts the analog signal PCM bit stream in digital line circuits. Hybrids are required in digital line circuits to convert between the four-wire switching and the two-wire cable pair. Testing access is provided so an external test system can obtain access to the cable pair for trouble isolation.  KINDS OF DIALING Pulse Dialing – a sequence of on-hook and off- hook condition that causes loop making and breaking from the telephone set dialer toward the central office 100 ms/pulse Make pulse – 39 ms Break pulse – 61 ms Interdigit delay – 300 ms (minimum) Touch Tone Dialing – uses a combination of two frequencies (low and high) or Dual Tone Multifrequency (DTMF) for sending a telephone number 100 ms/digit Digit tone – 50 ms Delay tone – 50 ms  1209 Hz 1336 Hz 1477 Hz 697 Hz 1 2 3 770 Hz 4 5 6 852 Hz 7 8 9 941 Hz * 0 #  Conditions of Switchhook On-hook condition – the telephone set is not in used – the cradle connection with the mouthpiece is open – the ringer is connected to the central office – no dc current flow through the line – only ac current is allowed to flow to ring the telephone ringer and announce the incoming call  Off-Hook condition – The handset is taken out of the cradle. – The line is converted from the ringer to talking circuit. – The line requires at least 23 mA of current but not to exceed 60 mA because the output will be uncomfortably loud. – When the dialing is completed, a ringing signal is sent to the called subscriber while a ring back is sent from the local exchange of the called subscriber to the calling subscriber. – A DC current from the local exchange is made to pass through the speech circuit during conversation. – The anti-sidetone circuit ensures isolation between the transmitter and the receiver.  Other Parameters of Local Loop Expected Measured Loss Loading Coil Bridge Tap Loop Resistance EXPECTED MEASURED LOSS (EML) EML = Cable Loss + Central Office Loss + Junction Loss EML = 8.55 dB Central Office Loss = 0.8 dB Junction Loss = 0.1 dB PROBLEM: Determine the type of cable to be provided to a subscriber located 4-km away from the central office. TYPE OF CABLE LOSS # 26 2.5 dB / km # 24 2.0 dB / km #22 1.8 dB / km Choices: a. 4km # 26 c. 2km #26 & 2km #22 b. 4km #24 d. 2km #22 & 2km #24  Loading Coils / H88 Loading It is used to reduce attenuation at voice frequency only by canceling the capacitance that inherently builds up between wires with distance. This is accomplished by adding 88 mH every 6000 ft or 1.8 km in series with the wire, but at above the voice frequency range, it greatly increases attenuation. It is used for circuits longer than 18,000 ft. It allows local loops to extend 3 to 4 times their previous length. It can be placed on telephone poles, manholes, or in CCC.  Coil separation Coil separation specification between coil specification between coil in feet in feet A 700 F 2787 B 3000 H 6000 C 929 X 680 D 4500 Y 2130 E 5575 B, D and H type loading coils are the most common because their separations are representative of the distances between manholes. Inductances can be 44 mH, 88 mH and 135 mH but 88 mH is common. Specification reading: 26H88 – 26-gauge wire, 88 mH added every 6000 feet To determine the number of loading coil needed: N = 1 + D - 5000 S Where: N = number of loading coil in whole number D = distance between local exchange and the subscriber (m) S = coil spacing Problem: A subscriber located at 10 km from the central office need to be connected by a 26H88 cable pair. Determine how many loading coils are needed.  Bridge taps It is the unused sections of cable that are connected in shunt to a working cable pair, such as a local loop, that is used for party lines to connect more than one subscriber to the same local loop. It increases the flexibility of a cable by making it easier to reassign a cable to a different subscriber without requiring a person working in the field to cross connect sections of cable. It introduce bridging loss which allow signals to split up and propagate down more than one wire.  Loop Resistance The dc resistance depends on the type of the wire and wire size (often 18 to 26 gauge ) Low wire gauge – large diameter, less resistance, lower attenuation 𝟎. 𝟏𝟎𝟗𝟓 𝑹𝒅𝒄 = 𝒅𝟐 Where: Rdc = dc loop resistance ( ohms per mile) d = wire diameter (inch) 𝟒𝟑. 𝟗𝟎 𝑹𝒅𝒄 = 𝒅𝟐 Where: Rdc = dc loop resistance ( ohms per km ) d = wire diameter ( mm )  PROBLEM Determine the wire gauge to be used for a 10 km loop with the central office resistance for switching equipment of 2.5 kiloohm. Gauge Diam. (mm) Diam. (in) 19 0.911 0.036 20 0.644 0.025 21 0.511 0.020 22 0.405 0.016  BUILDING TELEPHONE FACILITIES Service Entrance – wires or cables entering the building, terminated to the main cross connecting point within the building from the nearest telephone facility General Recommendations for Aerial Service Entrance 1. Size of entrance conduit shall not be less than 50mm diameter 2. Recommended initial conduit should be capable of handling the ultimate telephone requirements of the building. 3. Generally, the maximum span from telephone pole to building should be 30m. 4. Minimum recommended separation: a. Minimum clearances between telephone cable and power entrance must be 1m radial distance. b. Minimum clearance from ground level when crossing the street must be 5.5m. c. Minimum clearance over areas accessible to pedestrian only, must be 3.5m.  General Recommendations for Underground Entrance Conduit 1. Use corrosion-resistant material or PVC duct. 2. Should not include more than two 90 degree bends. 3. Minimum depth is 600mm,under areas used for vehicular traffic inside private property. 4. Conduits placed on private property must not be terminated in joint-use manholes with electrical cables. 5. Conduits should be separated from power conduits by not less than: – 300 mm in well-tamped earth – 50 mm of concrete 6. The size of underground entrance conduit within the building shall be the same size as the service conduit outside the building and in no case smaller than 75mm in diameter.  CABLE COLOR CODES American National Standards Institute (ANSI) Recommendations ANSI is not a governmental organization and has no enforcement abilities like OSHA does. However, ANSI’s standard is generally followed and recognized as among the best in the world. In fact, many of OSHA’s color standards are drawn from the ANSI system. ANSI CABLE COLOR CODES