When
we talk into our phone using analog communication it is using the properties of
electricity for voice transmission. This is where we can dive fully into the
properties of electricity or just keep it simple. For the time being we would
keep it simple. We don’t really need to fully understand the properties of
electricity to understand analog voice going over phone lines.
Electricity
has all kind of properties such as voltage and frequencies that are sent over
the wire. When you are speaking into a
phone it is the job of that phone to convey the word that come out of your
mouth in such a way that could be represented by electricity. When you are
talking to somebody over the phone, you are speaking into a handset which is
converting voice into waveform that is shown above.
This electric form can travel any length of
distance over wire and received on the other side, and then the other side
takes that electricity and plays back the sound represented by it. So all these
properties of electricity are used to convey the properties of your voice as
you speak into an analog handset. That is how we have communicated over
decades. Let’s
get deeper into that electric signaling in the phone.
Anytime
you have anything electrical, you have to think about electricity flowing
through that device. For example, if we have a flash light, then to light it up
we need a battery just like the one shown above. Now we would connect an
electric wire from the negative end of the battery to the light bulb and also
from the positive end to the light bulb, (light bulb is known as load) load
sucks energy from the battery.
With
the nature of electricity we have all these atoms that desire to balance
themselves out. The negatively charged atoms (when the battery is not plugged
in) keep trying to find a positively charged source in the air that is why the
batteries have expiration date. So when we connect the wire the electrons start
moving to the positively charged atoms through wire. And when these electrons
pass through the load, then energy is releases in the form of light and the
bulb lights up.
When
we have a phone line in our homes, which is always a pair of wires one is
called RING and other TIP. "Tip" and "Ring"
are common terms in the telephone service industry referring to the two wires
or sides of an ordinary telephone
line.
Tip is the ground side (positive)
and Ring is the battery
(negative)
side of a phone circuit. In the UK these are referred to as the 'A' (earth) and
'B' (battery) wires. The ground side is common with the central office
of the telephone company
(Telco); the battery side carries −48 volts of DC voltage when in an
"idle" or "on hook" state (nominally -50 volts in the UK).
The combination of tip and ring, then, makes up a normal phone line circuit,
just as a car's battery needs both connections leads to have a complete
electrical circuit. To ring the phone to alert to an incoming call, about 90
volts of 20 Hz AC current is superimposed over the DC voltage already present
on the idle line. The reason behind is that DC current is the one that has to
travel through the phones to be received back by the telephone company. Inside
of out phone, there is an AC transistor/ AC receiving chip, so when the phone company
needs to ring your phone, they send something that is receivable by that AC
chip.
When our
telephone handset is on-hook, the wires are disconnected.
The telephone switch, such as a PBX, prevents current from flowing through the
telephone handset. When in an on-hook state, the circuit is said to be open,
thus preventing the current from flowing through the telephone. In this state,
only the ringer is active.
When the
telephone handset is removed from the cradle, the circuit transitions to an off-hook
state and the switch hook toggles to a closed state. This results in current
flowing through the electrical loop. The flowing current informs the telephone
switch that the subscriber is requesting to place a telephone call. When the
telephone network senses the off-hook state via the current flow, it provides a
signal in the form of dial-tone that it is ready to accept the call.
This
whole signaling method is known as loop start. So when the receiver is
picked up, the loop gets connected between the wires and electric signals pass
down the circuit. Loop start signaling works pretty well for the home phones. The
reason it works well is because it is just a single line. When I pick up the
phone to make a call, I m going to signal to the other side, it’s not a high
volume line means there are not tones of calls coming in and out and all kind
of things. So our homes phone lines work as we are accustomed to. When we get
to larger environment then there is a problem with the loop start process
because we have higher class volumes. Take this example, We have a PBX system
inside of a company that has 100s of employees there.
PBX systems allow them to run their internal
phone systems, if we are using analog signaling for the outside world, we might
have a bunch phone lines coming in from the central office because more than
one person from the company might be talking out to the PSTN world at a time.
If we had the loop start signaling method in case of above figure, then it
would cause the problem of GLARE. What it is?
When
you pick up the phone, it signals to the PBX system that you want to grab
outside lines to make calls to the central office. Remember this PBX is
handling high volume of calls going out from the internal network and coming in
all the time. So if somebody picks up the line in the central office at the
exact same time when a call happened to be coming in to the central office then
those calls are bridged. You may have experienced it at home where when you
pick up the phone and you feel that there is no dial tone and somebody there is
on the phone.
In
PBX environment GLARE is evil because the person coming in the central office
wanted to talk to manager but gets connected with the information office. So
GALRE became a very serious problem in the PBX world. What they created to
solve GLARE is a signaling method known as Ground Start.
What that
does is when somebody picks up the line; PBX doesn’t automatically connect the
circuit, it grounds that line (both Tip and Ring) temporarily and sends a
signal in the form of dial-tone that it is ready to accept
the call. When a subscriber makes a call, the telephone switch sends voltage to
the ringer to notify the recipient of an incoming call. The caller also
receives a ring back tone from the telephone switch, which alerts the caller
that the telephone switch is sending ringing voltage to the called party.
It is important to know that only the
ringing that the recipient (i.e. the called party) hears is the supervisory
signal; the ring back tone that the
caller hears is simply a call-progress indicator and is not a supervisory
signal.
This
signaling method was used in phone booth for quite some time in which when we
insert a coin, that coin touches both TIP and RING wires and grounds them, when
they get grounded, the telephone switch give us the dial tone. Today we
first dial and then deposit the coin.
In
today’s world, the loop start signaling is used for home phones where there is
a very little chance of GLARE. Ground
start signaling is usually used for businesses and PBX (Private
Branch Exchange) systems. There are also other types of
signals that we need to communicate to convey different messages across the
line, there are three main categories of signaling that we are concerned with
and they are;
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