Telecommunications Fundamentals, Chapter 1A: Network Overview

Components for Transmission

Three components of any transmission system are the: (Figure 1.10):

The transmitter

The receiver

The communication path

In its simplest form, the CPE or customer premises equipment, is the transmitter and receiver. The media (twisted pair copper, coaxial cable, optical fiber, radio waves) that connects the CPE is the path.

How A Telephone Works

The telephone (Figure 1.11) is typically located on the customer's premises. It serves as the customer's network access device. Basic parts of the telephone set are:

Ringer

- Always on line

- Alerting device (bell, buzzer) for incoming calls

Switch Hook

- Completes the loop (path) when lifted off-hook

Dual-Tone Multi-Frequency Pad (DTMF) or Rotary Dial

- Signaling device that generates the pulses or tone required to identify the called number and billing information

Handset

- Contains the transmitter and receiver

Transmitter

Converts speech energy (acoustical energy) into electrical energy that can be transmitted over the path to the central office and on to the target destination.

Receiver

- Converts the analog electrical signals back into acoustical energy.

The transmitter (Figure 1.12) consists of three parts:

Diaphragm with a dome

Chamber

Carbon granules or a conductor

This is how the transmitter works:

Vibrations of the voice sound waves cause the diaphragm to vibrate.

The attached dome causes the carbon granules to vibrate (compress or decompress) within the chamber.

A current flows from the dome through the carbon granules in the chamber. The amount of current that flows depends on how tightly the carbon granules are packed.

Thus, the voice sound wave energy is converted to an electrical energy wave for transmission over the network.

The electrical signal is an analogous representation of your voice, hence the term "analog signal."

The most common telephone receiver is the electromagnetic receiver. It also consists of three parts (Figure 1.13):

Diaphragm

Electromagnetic

Permanent magnet

When a varying electrical current flows through the electromagnet, the resulting magnetic field either attracts or opposes the magnetic field of the permanent magnet.

This causes the diaphragm to move closer or further away from the permanent magnet (vibrate), in step with the electrical waveform.

The electrical waveform is converted back to an acoustical waveform, resulting in a lifelike representation of the original transmission signal. To summarize, the major functions performed by the telephone set are (Figure 1.14):

Requests use of the telephone switching system when the handset is lifted off-hook.

Indicates the switching system is ready for use by receiving a dial tone.

Generates and sends the telephone number of the address (by dialing the number or by means of a touch-tone keypad).

Indicates the status of a call by receiving tones, such as audible ringing, busy tone, or recorded message.

Indicates an incoming call by ringing a bell or some other device.

Converts acoustical energy into an analogous electrical signal for transmission to a distant party.

Converts electrical energy into an analogous acoustical signal representing the sounds of the sender's voice.

Informs the system a call is finished when the handset is placed back on the switch hook.

[Editor's note. The telephone is an electrical instrument. Electricity works the phone itself: operates the keypad, makes it ring. Electric current does not convey the voice, however, sound simply varies that current. It's these electrical variations, analogs of the acoustic pressure originally spoken into the microphone, that represent voice. We have two different points here: the first, about the current itself, and, secondly, about how that current is altered.

To sum up, electricity is indeed conveyed to the phone, whereupon 1) the current operates the telephone and 2) the current is varied by the voice to communicate. Got it? Tom]

Telephone Connection to the Central Office

Many customers' telephones are connected to the central office by a pair of wires within a cable (Figure 1.15). Why two wires?

Because your telephone is an electro-mechanical instrument, it requires a battery source and a ground source.

The battery source is supplied from the central office equipment to your telephone set by a wire called the ring lead. The ground source is transmitted from the central office by a wire called the tip lead. Together, the tip and ring of the telephone set are commonly referred to as a cable pair.

Electrical Current in the Local Loop

Local telephone companies purchase their power from the same source as local residential customers (Figure 1.16).

The power is received into the central office in an area called the power plant.

Alternating current is received by the rectifier which converts it to direct current.

Direct current is then used to power the equipment used in the net work.

Direct current is used in the local loop because:

AC and analog are both represented by a sine wave

[Editor's note. This is an odd point. Would anyone care to tell me what they might have been thinking? -- Ken Solomon adds the following;

"As far as I know DC was used to prevent hum, and to allow service to continue in the event of a power outage and because people were afraid of AC. It is used, after all, in the electric chair. Also, Bell used wet cells and probably would have killed himself using an AC generator. And, even though AC would have induced hum the interference and cross talk using DC was terrible anyway until a two-wire loops were introduced."

"In the early days iron wires were used and they were strung from roof top to roof top (this is pre poles). Since iron is a fairly poor conductor it picked all kinds of squawks, screeches, buzzing, parts of conversations (from adjacent lines) and other noises. Also, a single strand grounded wire will act as an antenna picking up atmospheric noises as well. Later, pure copper was used but it proved too soft so they used iron coated with copper which worked much better since it had many times the conductivity of iron alone. Of course if you hook up two modern phones in your house with modern conductors and a solid power supply you shouldn't have any interference. But plug in some cheap intercoms and you'll pick up (or induce) noise. BTW, did you know that the best place to ground the phones was in the soft earth surrounding the outhouse?"]

In most central offices the power is sent from the rectifier through the batteries and then out into the local loop via the central office equipment.

During a power outage batteries are used as the main source of power for the local loop.

A generator is started to provide power to the CO equipment.

Underground tanks are used to store the fuel.

In the event of generator failure, the batteries in the CO will power the CO equipment. Most battery back-up lasts 6-8 hours.

Voice Frequency Range

The tones produced by the human vocal cords range from about 30 vibrations per second to 20,000 vibrations per second.

[Ed note. Or is this the range of human hearing?] A voice signal is characterized as an analog signal, or a sine wave, with the following characteristics:

Continuously changing value

No abrupt discontinuities

All values between the extremes are allowed

Sine waves are measured in hertz, or cycles per second (Figure 1.17).

One hertz (Hz) = one cycle per second (CPS)

All are a measure of the frequency of a waveform (Figure 1.18)

Continuous

All values allowed

In the telecommunications industry, a voice frequency range of 300-3400 hertz is used (Figure 1.19).

This is known as the voice band of frequencies.

Anything above or below the voice frequency range is filtered out.

The electrical signals produced and transmitted by the telephone network rise and fall, just as the human voice does (Figure 1.20).

In common terminology: Voice Signal = Electrical Signal = Analog Signal

Review

1. Another name for a B-Box.

2. One of 3 components of any transmission system.

3. What a telecommunications network must be able to do.

4. The end _______ contains a switching system that provides dial tone and ringing.

5. The battery source from the C.O. is supplied on this wire.

6. The _______loop provides customer access to the network.

7. Another name for I.O.F.

8. Voice and sound waves cause this part of the transmitter to vibrate.

9. One of the major disadvantages of a direct connect network.

10. This switch is used as a hub to connect switches.

11. The type of current used in the local loop.

12. The ground source from the C.O. is supplied on this wire.

13. Bell 's first networks were of this type.

14. A voice signal is characterized as an _______ signal.

15. A system of elements linked by facilities.

16. A piece of CPE equipment used with computers.

17. Part of the central office power plant.

18. Historically, copper was the ________ used.

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