Introduction to telephony

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.

Brief history: Bell prospers

 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: Development of
The 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 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
2.

5.
1.

3. 4.

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

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)

value
132
131
130
129
128 time
127
126
125
124
123
122
131 125 123 129 128 125 128 129 125 126 131 126 123

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.

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 a
typical 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.

Analog Signaling in a
typical call (2)
• 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.

Typical tones in
Analog Signaling
Characteristics
Tone
USA Europe
Two continuous tones of 350 Hz and 440
Dial tone A single continuous tone at 425 Hz
Hz multiplexed.

A single tone of 425 Hz, interspersed with 0.2
Two multiplexed tones at 480 Hz and 620 seconds of sound and 0.2 seconds of silence.
Busy tone Hz, interspersed in 0.5 seconds of There is also another cadence of 0-5 seconds of
sound and 0.5 seconds of silence. sound and 0.5 seconds of silence, but it is less
common

A single tone at 425 Hz, interspersed with 1.5
Two multiplexed tones at 440 Hz and 480
seconds of sound and 3 seconds of silence. There
Ring tone Hz interspersed in 2 seconds of sound
is also another cadence of 1 second of sound and
and 4 seconds of silence.
4 seconds of silence

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

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.

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
1 2 3
697 Hz A
ABC DEF

4 5 6
770 Hz B
GHI JKL MNO

7 8 9
852 Hz C
PRS TUV WXYZ

0
941 Hz * # D
operator

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 and
E-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 and
E-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.

Introduction to telephony

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Introduction to telephony