Subscriber’s telephone set
Contents
2.1 Sequence of operations during a simple telephone call. 2.2 Basic functions of subscriber's telephone set 2.3 Types of telephone sets 2.4 Main circuits of of subscriber's telephone set 2.5 On/Off switch hook 2.6 Transmitter 2.7 Receiver 2.8 Hybrid network 2.9 Side tone 2.10 Equalizer circuit
2.1 Sequence of operations during a simple telephone call
Let’s consider a simple telephone call between two subscribers, say A and B, whose lines terminated in the same exchange. Suppose that subscriber A initiates the call. i.e. the calling customer is A and Called customer is B. The following sequence of operations will occur during the call.
1. Initially A’s handset is in the ‘on-hook’ condition. i.e. the line is in the idle state waiting for the calls to be originated or received. The exchange is monitoring the state of each line. 2. If A lifts the handset, a call request signal goes to the exchange by flowing a current through the line. 3. The exchange takes A’s signal and then makes a connection to A’s line ready to call. 4.A now dials B’s telephone number by using DTMF signalling. 5.Then the telephone exchange identifies this address signal and selects the required outgoing line. 6.Now the exchange checks the status of B to see whether B’s line is busy, free or out of service. 7. If B’s line is engaged or out of service that is informed to A by sending an engaged tone. 8.If B’s line is free the exchange makes a connection to B’s line 9.The exchange sends a signal (by sending a ringing current) to B for alerting him to receive the call. At the same time a ringing tone is sent back to A as a call progress signal.
10.When B answers by lifting the handset the line is looped and current flows. This provides an answer signal to the exchange causing it to stop sending the alerting signal and ringing tone from A’s line. This provides the answer signal to A. If B does not answer, A replaces the handset causing the exchange to clear the connections. 11.Receipt of the answer signal from B causes the exchange to complete the connection between A and B and now they start the conversation. At the same time the exchange starts to supervise the call to detect the call duration for charging purposes. 12.At the end of the conversation when each customer replaces the handset, line current ceases and so provides a clear signal to the exchange. 13. The exchange then clears the connection between A and B and their lines become back to idle state.
2.2 Basic functions of subscriber’s telephone instrument
To carryout the operations described above in section 2.1, we need A’s telephone instrument, B’s telephone instrument, telephone switching office (or exchange) which connects A and B and interface to the switching office. Now, we could say that the subscriber’s telephone instrument plays a major role in this regard and we can identify the following as the main functions of the subscriber’s telephone instrument:
1. Requesting the use of the telephone system when the handset is lifted. 2. Indicating that the system is ready to use by receiving the dial tone. 3. Sending the dialed telephone number to the system. 4. Indicating the status of a call by receiving tones such as ringing, busy etc. 5. Converting speech signals into electrical signals and vice versa when transmitting and receiving calls. 6. Automatic adjustment of power supplied to it. 7. Sending clear signals when a call is finished.
To perform above functions the subscriber’s telephone instrument is made up with several circuits and we will discuss about these circuits from the section 3.2.
2.3 Types of telephone sets
Depending on the power system used, the telephone set is divided into two main groups
2.3.1 Local battery telephone system
Figure 2.1 illustrates a very simple telephone circuit for conversation in one direction where telephone user 'A' (A subscriber) talk to telephone user 'B' (B subscriber).
The battery is to provide direct current (d.c.) to energise the transmitter of A. When A talks, alternating current of speech frequency produced by his transmitter, superimposed on the direct current flows through the external line and the distant receiver of B.
An arrangement like this where each transmitter is energized by individual batteries connected locally in the transmitter circuit is called 'local battery system'.
2.3.2 Central battery telephone system
If a battery is centrally placed to provide transmitter-energizing current for many telephone circuits. To be shared by many telephone users is called 'Central Battery Telephone System' (Fig. 2.2).
2.4 Main circuits of the subscriber’s telephone set
The basic block diagram of a subscriber’s telephone set is shown in Fig. 2.3. below.
As you see in this block diagram, each subscriber’s telephone is connected to the nearest switching office (telephone exchange) using a local loop. This local loop is provided by the telephone company and it contains 2 wires, one is called as tip and the other is called as ring.
The batteries at the switching office output a dc voltage of -48V on the ring and connects tip to the ground. The only voltage necessary for the operation of a subscriber’s telephone instrument is this -48Vdc supplied by the switching office. The subscriber’s telephone instrument is connected to the local loop using a RJ-11 connector. We can identify the main circuits of the subscriber’s telephone instrument as given below:
1. ON/OFF hook switch 2. Transmitter (Microphone)
3. Receiver (Speaker)
4. Dialling circuit
5.Ringing circuit
6. Hybrid network
7. Equalizer circuit
2.4.1 ON/OFF Switch hook
This is a cradle switch which is used to connect/disconnect the ringing circuit and the speech circuit to the line. When we keep the telephone hand set in its cradle, the weight of the handset holds the switch hook buttons down and the switches are open. This is called ‘ON Hook’ condition. When we lift the handset), the spring loaded buttons come up and the switch hook closes. This is called ‘OFF Hook’, condition. This OFF hook signal indicates the exchange that a subscriber requests a call The exchange returns a dial tone to the called telephone indicating that it is ready to receive the dialled telephone number.
2.4.2 Transmitter
This is the part of the telephone into which we talk. The main function of the transmitter is to convert a speech signal into an electrical signal in terms of the variations in an electric current that can be transmitted through the transmission system to the receiver of the called telephone.
2.7 Receiver
The main function of the receiver or the speaker is to convert the varying electrical current representing the transmitted speech signal to variations in air pressure usable by the human ear. Electromagnetic receivers were used in early telephone systems. As you see in the Fig. 2.3 the transmitter and the receiver are connected to the local loop through a circuit called hybrid network. Let’s see what this network is and how it operates.
2.8 Hybrid network
The function of a hybrid is to interface a 2-wire circuit to a 4-wire circuit to allow full-duplex operation. In communications we use the word full duplex for the circuits which can transmit signals in both directions. 4-wire circuits are full duplex as they have 2 wires for each direction while 2-wire circuits are half duplex. Almost all of the subscriber’s local loops are 2-wire circuits while 2-wire circuits are used for the other parts/circuits of the network. The telephone handset is also a 4-wire circuit with 2-wires used for the transmitter connection and two for the receiver connection.
The hybrid used in the telephone set is a special balanced transformer to carryout this purpose. A hybrid network is also used in the telephone switching office to interface local loops to trunk lines. Another function of the hybrid network is to allow a small portion of the transmit signal to be returned to the receiver in the form of a side tone.
2.9 Side Tone
As a portion of speech currents produced by A's transmitter due to A's speech flows through his own receiver. 'A' could hear his own speech in his receiver. This reproduction of one's speech in his own receiver is called 'side tone'. If the side tone is eliminated completely, the telephone appears to be 'dead' to the speaker. If the level of the side tone is very low, the speaker (Psychologically) tends to feel that his speech is not carried and increase his voice.
A very high level of side tone will be cumbersome for the speaker and he tends to lower his voice, thereby lowering the speech level in the distant receiver. What do you think the most comfortable level of the side tone? The most comfortable level of side tone for the speaker occurs when its volume in his receiver is approximately equal to the volume heard in free ear when speaking normally.
2.10 Equalizer (balancing) circuit
Equalizer circuit is a combination of passive components such as resistors, capacitors etc. and it is used to provide impedance matching between the line and the telephone handset under different conditions such as receiving/sending calls.
Subscriber signalling devices
The main signaling devices in the subscriber’s telephone equipment are the dialing circuit and the ringing circuit.
3.1 Dialler (Dialling circuit)
The basic function of the dialer is to provide a telephone user (subscriber) with a means of indicating to the telephone exchange of his desired destination. This destination (called party) may be another subscriber, a human operator or some recorded announcement such as weather forecast, time etc. There are two main types of dialing mechanisms pulse dialing and tone dialing.
3.1.1 Pulse Dialling (Rotary Dial)
This method is used in old fashioned telephone sets.
Most probably, you may have seen a telephone instrument with a rotary dial where 10 numerals (0 to 9) are marked on a circular ring and, over which a circular plate with 10 slots is fixed. Now, we will see how the dialling function is achieved with a rotary dial.
When the handset of a telephone is removed from its normal position (i.e. on -hook to off hook), a metallic connection is made within the telephone, there by completing an electrical circuit between the telephone instrument and the exchange. This initial completion of electrical path is commonly known as 'making a d.c. loop' and it results in a direct current flow along the external line through the telephone instrument.
When we dial a number, a particular number of pulses are generated (by disconnecting and reconnecting the d.c. loop) and this number of pulses is determined by how far the dial is rotated before releasing it. This action winds up a spring that rotates the dial back to the rest position when it is released. A small governer inside the dial causes it to return at a constant rate of rotation.
The pulse dialling produces a series of current pulses in the loop circuit by disconnecting and connecting the circuit. For example, when the subscriber dials digit 5, the d.c. path established through the telephone is opened and closed 5 times, thereby transmitting a series of pluses to the exchange. This process is called 'loop-disconnect impulsing' and is illustrated in Figure 4.1.
Terms used in impulsing
The time between the corresponding points (similar points) of two successive impulses is known as an 'Impulse Period.' An impulse period consists of a break period followed by a short make period.
∴ Impulse period = break period + make period.
The ratio of break period to the total impulse period is maintained at a standard value of 2/3 and is called the 'Impulse Ratio'.
The number of impulses per second is referred to as the 'Impulse frequency'. The most common impulse frequency is 10 impulses per second (ips). With new type of telephone exchanges, telephone instruments with 20-ips dials can also be used.
E.g. In a dial with an impulse frequency of 10 ips, what are the duration of make and break periods (Impulse period)? Determine the make and break period for this frequency.
When the subscriber continues dialling several digits, trains of impulses corresponding to each digit dialled are transmitted along the line to the exchange. The exchange equipment should be able to recognise one train of from the other, to correctly identify each digit dialled by the subscriber.To achieve this, minimum time duration is always ensured between two successive impulse trains. This is known as the 'Minimum Inter digit pause' and is about 400ms.
To achieve this, minimum time duration is always ensured between two successive impulse trains. This is known as the 'Minimum Inter digit pause' and is about 400ms. Approximately, half of this time is taken up by the subscriber in rotating the finger plate (dialling), and the impulse sending mechanism of the dial is arranged to introduce a period of about 200ms before each trains of impulses. Figure 4.2 illustrates the Inter digit pause.
E.g: When the subscriber dials '253' what is the minimum total time taken for the trains of impulses, if the impulse frequency of the dial is 10 ips?