INTRODUCTION TO TELEPHONY

Elastix® Certification

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Before telephony

Precarious methods for reaching longer distances, like smoke signals or whistles.

Emissaries or messengers. Sometimes they would die trying.

Postal service

Telegraph

Communications weren't practical before the appearance of telephony.

Brief history:Mid-nineteenth century

In 1849 Antonio Meucci made a demonstration of a device capable of transmitting voice to Havana. In 1854 he made another in New York.

In 1860 the German Johann Phillip Reis builds a kind of telephone based on the original idea by Charles Bourseul.

A couple of years later, Innocenzo Manzetti builds the awaited “talking telegraph” that he himself had envisioned in 1844, but he wasn't interested in patenting it.

Illustration of Reis telephone

Brief history: The patents

In 1871, Meucci filed a document of “patent notice” but wasn't able to finish the process due to his economic condition.

In 1875, Alexander Graham Bell managed to patent a similar device and was the first to do so.

A few hours after Bell, another inventor named Elisha Gray, also tried to patent a similar invention. Bell and Gray enter a legal battle that was ultimately won by Bell.

Bell tries to sell his patent to Western Union, but they aren't interested.

Bell prospers on his own.

In 1886, there were already 150,000 subscribers to the telephone service in the United States.

At first Bell was exclusively the only company to exploit this technology, due to their patents.

Brief history: Bell prospers

Brief history: Development ofThe Technology

In 1891, an “automatic” telephone was invented that allowed users to dial directly.

In 1947, scientists at Bell invented the transistor, which changed the course of human history. In 1948, they won the Nobel Prize for their work.

In the 1960’s, the first communications satellites were launched and communications between continents were made easier.

Principles of voice transmission

Sound waves travel through air at the speed of sound 1244 Km/h (or 340 m/s).

These waves decay quickly and cannot travel great distances.

It's preferable to transport a voice signal through electric waves, whose decay can be controlled through a conductor cable, and can be transported for great distances.

This transformation is made through a device known as microphone.

The human voice (1)

The voice takes up a wide range of frequencies, from the very low to the high approximately from 20Hz to 20kHz.

To transmit an “understandable” voice, it isn't necessary to transmit all the frequencies but only a much lesser range.

Commercial telephones only transmit an approximate range of 300Hz to 3400Hz.

The human voice (2)

The microphone

Transforms the pressure of the mechanical waves that travel through the air into electrical waves.

The carbon microphone was widely used in analog phones. It contained grains of carbon inside a capsule.

The electromagnetic microphone is very widely used today.

The ”electret” microphone is also widely used in telephones.

Schematic of the dynamic microphone

1) voice waves, 2) Diaphragm, 3) Coil, 4) Ferromagnetic Core, 5) Induced current

1.

2.

3. 4.

5.

Bandwidth

It's a measure of the quantity of information that can be transmitted through a medium in a determined amount of time.

A common measurement used to express bandwidth is ”bits per second”. This measurement also is equivalent to bits/s, bps, or baud.

For example, it's used to measure the capacity of data links such as an internet connection.

It can be abbreviated as BW.

Digitizing the voice (1)

Transforming an analog electric wave into a digital signal, of ones and zeros.

In practice, digitizing voice is no more than taking samples of the signal's amplitude at regular intervals.

The frequency of these intervals is calculated using Nyquist's theorem.

The digitized voice signal is less vulnerable to noise. The quality is better.

Digitizing the voice (2)

131 125 123 129 128 125 128 129 125 126 131 126 123

118

120

122

124

126

128

130

132

value

time

10000011 01111101 01111011 01111011

…

Nyquist’s theorem

Establishes the minimum number of frequency samples required to rebuild a wave in its original shape.

Nyquist only determines a minimum frequency. Theoretically, the values that are sampled must be exact, but in practice it's rounded to a defined number of bits.

This minimum frequency is two times the bandwidth that is being sampled. ƒm ≥ 2 BWs

For example, if the telephone transmits voice from 400Hz to 4,000Hz, at a minimum double that will be necessary, or 8,000Hz.

Circuit-oriented networks (1)

A dedicated or exclusive circuit per subscriber is established.

Once the circuit is established, it cannot be used by others.

These types of networks are expensive.

In each circuit the delay is constant, which in a way is an advantage since there is no jitter.

It's the typical kind of network for analog subscribers to traditional telephone companies.

Packet-oriented networks (1)

Through the same medium, different flows of information can be transmitted simultaneously.

The information at the different nodes is divided into packets, these are inserted and sent through the same medium.

The Internet is an example of a packet-oriented network.

On the Internet and IP networks in general, packets may arrive out of order. This can cause problems when voice is being transmitted.

Packet-oriented networks (2)

The PSTN

The Public Switched Telephone Network, or PSTN, is essentially a circuit-based network. In some countries, this is abbreviated RTPC (e.g. Italy calls the PSTN “Rete Telefonica Pubblica Commutata”).

It's the network where all users of traditional telephones are connected.

Originally it was an analog network, but now it's a network that is mostly digital; therefore, there are two kinds of circuits: analog and digital.

Analog Circuits

They're commonly pairs of copper wire that reach subscribers to the telephone service and through which the electric (analog) signal of the voice is transmitted.

The same circuit used for both voice transmission, as well as the necessary signaling to establish, maintain, and end a call.

Power is delivered over analog lines as well, at -48 Volts DC.

Analog Signaling

Analog signals travel through the same conductor as the voice signal.

They serve to establish, supervise, maintain and end a call.

They interchange information between the subscriber and the Central Office (CO)

There are three types: loop start, ground start, and kewlstart

The most common is the loop start.

Analog Signaling in atypical call (1)

There are six distinct states: on-hook, pick-up, dialing, commutation, ringing, conversation.

• On-Hook: The CO provides a voltage of 48 volts DC and the telephone acts as an open circuit. It's also known as on-hook.

• Pick-up: The telephone closes the circuit, putting a low resistance between the telephonic conductors. When the CO realizes, it sends a dial tone.

• Dialing: It can be by pulses or by tones. The tones are a pair of frequencies known as DTMF.

• Commutation: The CO analyzes the dialed number and tries to find the circuit for the destination number

• Ringing: The CO sends a ring signal to the destination. It also notifies the origin with a ring-back signal if it is ringing or a busy signal if the destination is already engaged.

• Conversation: If the destination answers then the telephone circuit is closed.

Analog Signaling in atypical call (2)

Typical tones inAnalog Signaling

Note: These are values for reference, and can differ from reality depending on the city or telephone company that is offering the service, as well as government regulation. In any case, they can be modified locally in the Elastix system.

ToneCharacteristics

USA Europe

Dial tone Two continuous tones of 350 Hz and 440 Hz multiplexed. A single continuous tone at 425 Hz

Busy tone

Two multiplexed tones at 480 Hz and 620 Hz, interspersed in 0.5 seconds of sound and 0.5 seconds of silence.

A single tone of 425 Hz, interspersed with 0.2 seconds of sound and 0.2 seconds of silence. There is also another cadence of 0-5 seconds of sound and 0.5 seconds of silence, but it is less common

Ring tone

Two multiplexed tones at 440 Hz and 480 Hz interspersed in 2 seconds of sound and 4 seconds of silence.

A single tone at 425 Hz, interspersed with 1.5 seconds of sound and 3 seconds of silence. There is also another cadence of 1 second of sound and 4 seconds of silence

Ring-back tone Same as the ring tone Same as the ring tone

DTMFs

DTMF stand for Dual-Tone Multi-Frequency.

They're two simultaneous mixed tones.

They are used to send digits or certain characters through an analog line.

Sending two simultaneous tones is better than using a single tone.

DTMF Frequencies

  1209 Hz 1336 Hz 1477 Hz 1633 Hz

697 Hz1 2 3

A  ABC DEF

770 Hz4 5 6

BGHI JKL MNO

852 Hz7 8 9

CPRS TUV WXYZ

941 Hz *0

# Doperator

The analog telephone

It's not necessary to explain in detail what it is, we've all used it.

A component to which we'll pay special attention is the 2 to 4 wire converter.

This component mixes the audio from the microphone (outgoing signal) with the audio of the speaker (incoming signal.) This is because the telephone layout has two wires, if it were made up of four it wouldn't be necessary.

This component, also called a 2H/4H converter, is sometimes responsible to bring echo into the conversation.

Digital Circuits

Digital circuits are those that carry digital signals. They really transport this digital information through analog carriers.

They carry digital information that has been multiplexed many times, which optimizes resources.

They improve the signal vs. noise ratio. This translates into a better audio quality.

A standard unit is the DS-0, which represents one 64Kbit/s channel. Other units are multiples of a DS-0.

T-carrier andE-carrier circuits (1)

T-carriers were designed as a nomenclature for multiplexed digital circuits.

They were developed by Bell Labs more than fifty years ago.

T-carrier in the USA, E-carrier in Europe, and J-carrier in Japan.

The most known and common are T1 and E1.

T-carrier andE-carrier circuits (2)

A T1 is a digital circuit composed of 24 DS-0´s and has a capacity of 1.544 Mbit/s.

An E1 is composed of 32 DS-0´s and provides 2.048 Mbit/s of capacity.

There are many models of digital line cards that are compatible with Asterisk in E1/T1 formats.

After the T1, there are higher multiples like T2, T3, and T4.

SONET and optical circuits

SONET (Synchronous optical networking) was developed with the objective of having a similar nomenclature to the T-carrier but for fiber optic technology.

SONET uses OC-1 as the base unit, which is equivalent to a T3 in bandwidth

After the OC-1 we have the OC-3, OC-12, OC-24, OC-48, and others.

Digital signaling (1)

Just like in analog communication, it's necessary to use signaling in the call to establish, supervise, and hang up.

The protocols can be grouped into two groups called CAS (Channel Associated Signaling) and CCS (Common Channel Signaling).

The CAS protocols transmit the signaling information along with the data.

The CCS protocols transmit the signaling information in a separate channel from the data.

Digital signaling (2)

CCS protocols offer some advantages over CAS protocols.

In the CAS group there are two that interest us: Robbed bit and R2.

In the CCS group is an important protocol called ISDN and is the one that is most used in Asterisk on digital circuits.

ISDN (1)

ISDN (Integrated Services Digital Network) allows us to transmit voice and data simultaneously through copper telephone pairs with a superior quality to analog telephone lines.

There are two variations called BRI and PRI.

BRI (Basic Rate Interface) was intended for home users, and is composed of 2 data channels of 64Kbit/s each, plus one for signaling of 16Kbit/s, for a total of 144Kbit/s.

The data channels are called B channels and the signaling channels are known as D channels.

ISDN (2)

PRI (Primary Rate Interface) is used for businesses and contains many B channels.

In the USA, PRI has 23 B channels and one D channel (23B+D), all of 64Kbps, which gives us a total of 1,536Kbps.

In Europe, PRI has 30 B channels and one D channel (30B+D), all of 64 Kbps, which gives us a total of 1,984 Kbps.

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