TELECOMMUNICATION: long distance comm.- transmitter and receiver are far apart. Telephone is the most simplest device as well as the most complex technology. Telephone system used for-(1)conveying voice info (2)transporting data(with the help of modems). PTNPUBLIC TELEPHONE NETWORK PSTNPUBLIC SWITCHED TELEPHONE NETWORK Anyone who uses a telephone or data modem is a part of this global comm. Network. PTN consists of several large corporations and hundreds of smaller independent companies jointly referred to as TELCO.
SIMPLE ELECTRONIC SET POTS-PLAIN OLD TELEPHONE SERVICE -simplest and most straightforward form of telephone service. -subscribers access the telephone n/w through a pair of wires called the LOCAL SUBSCRIBER LOOP. LOCAL LOOP -most fundamental component. -nothing but just the ordinary telephone cable--2 conductors twisted together and enclosed in polyethylene plastic coating. -connects to ESS of the central office or telephone exchange
The basic telephone set is a simple analog transceiver designed with the primary purpose of converting speech or acoustic s/g electrical s/g The quality of transmission over a telephone connection depends on: the received volume, the relative frequency response of the telephone circuit the degree of interference
Why dc voltage used instead of ac? To prevent power supply hum To allow service to continue in the event of a power out age People are afraid of ac. -48 v is the only voltage required for the operation. Used for supervisory signaling. To provide talk battery for the microphone.
Notify the subscriber when there is an incoming call with an audible or visible signal. Provide a s/g to the telephone network when the call has been answered. Convert speech energy-->electrical energy in Trx and vice versa in Rxr. Incorporate some methods of inputting and sending destination telephone nos. frm telephone set to the central office.(touch-tone key pads) Reduces the cross talk or interference possibilities by regulating the amplitude of speech s/g the calling person outputs onto the telephone line. Incorporate means of notifying the telephone office when a subscriber wishes to place an outgoing call.
Ensures that a small amt of the transmit s/g is fed back to the speaker=>this feed back is called SIDETONE or TALKBACK-prevent the speaker frm talking too loudly. Provide an open ckt(idle condi)to the local loop when the telephone is not in use and a closed ckt (busy condi)when its in use. Provides means of transmitting and receiving call progress s/g b/w central office switch and the subscriber, such as ringing,dial tone etc.
The essential components of a telephone set are RINGER CIRCUIT ON/OFF HOOK CIRCUIT EQUALIZER CIRCUIT SPEAKER MICROPHONE HYBRID NETWORK DIALING CIRCUIT
Ringer Circuit Its an electromagnetic bell Is placed directly across the tip and ring Alerts the destination party of incoming calls On/off hook circuit Just as STDP switch Is placed directly across the tip and ring Connected to the telephone handset so that when the telephone is idle (on hook),the switch is open and when the telephone is in use(off hook),the switch is closed hence completing the electrical path through the microphone.
Equalizer Circuit They are combinations of passive components that are used to regulate amplitude and frequency response of the voice s/g Helps to solve the imp problem of interdependence of the transmitting and receiving efficiencies and the wide range of trx current caused by a variety of local loop cables with diff dc resistances Speaker Receiver for telephone Electrical s/g->acoustic Speaker connected to local loop thru hybrid n/w Usually enclosed in the handset
Microphone Transmitter for the telephone Acoustic->electrical Connected thru hybrid n/w Hybrid Network Also called hybrid coil or duplex coil Special balanced transformer Used to convert a 2-wire ckt to 4-wire and vice versa thus enabling full duplex operation Separates transmitted s/g from the received s/g Another function is that of feed backing- sidetone
Dialing circuit Either a touch-tone key pad or a rotary dialer Enables the subscriber to output s/g representing digits--this enables the caller to enter the destination telephone no.