Introduction to Telephony
and POTS lines
The first patent for a telegraph was issued in 1840
Voice technology started with the telegraph in 1844
The basic phone was developed in 1876
By 1881 there were 54,000 phones in use
Since telegraph lines followed by telephone lines were
first on the poles, today we still call them telephone
Although today, they are dominated with power, CCTV,
and other services
This infrastructure combined with central offices (CO)
make up the public switched telephone network
This network was dominated by one company…..
AT&T (American Telephone & Telegraph) first
established in 1885 as a subsidiary of Bell.
• Some of the first phones
Telephones connected to the PSTN use a single pair
Telephones are connected in parallel and operate on a
current loop back to the central office.
Telephones receive their power from the phone
company known as the central office (CO).
Telephone wiring is connected to battery banks in the
CO which is why they’re sometimes referred to as
This is why phones still work even when the power
At the residential dwelling the phone drop from the
telephone pole (aerial) to the house is terminated on a
network interface device (NID)
The NID sometimes referred to as the NI is the
Demarcation (D-marc) point.
The D-marc is the physical boundary between the
customer’s premises wiring and the public utility
The NID is typically located on the outside of the house
and provides primary protection from lightning strikes
NEC (National Electric Code) article 800.90 (B); states
that the primary protector shall be located in, on or
immediately adjacent to the structure or building served
and as close as practicable to the point of entrance.
In commercial applications the NID is usually referred
to as the D-marc and is located in the premises and is
designed for 25 pair to 200 pair cables and sometimes
up to 900 pair or more.
Why do you think the NID is always located on the
outside of the building in residential environments?
The telephone drop to the NID consists of 4 wires,
though the phone only requires 2 wires (1 pair)
From the NID a cable was normally run to your kitchen
phone and then these conductors were “daisy chained”
thru the house from jack to jack
The 2 wires are called tip and ring and make up what is
referred to as a pair and what is referred as the
subscriber loop by the phone company
Tip is the ground side (positive) and Ring is the battery
(negative) side of a phone circuit.
1 = SLEEVE
2 = RING
3 = TIP
4 = INSULATOR
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"
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
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.
When you pick up the receiver you go into the offhook state.
On the original switchboards the tip was connected to
ground to prevent the operator from being electrocuted.
The bandwidth of an analog telephone lines is fairly
low, generally around 3000 Hz
This signal is sent over a pair of wires on a cable that is
called UTP (Unshielded Twisted Pair)
UTP cable is a generic term used to describe all types
of voice and data cables from 2 pr. To 600 pr.
In the communications industry UTP cable is
categorized by a number system
POTS cable is consider to be the lowest category of
cable and is not recognized by the industry any more
POTS is category 1
Currently category 7 is the highest category of cabling
which is used for high speed data networks
The inside wiring for phones is called JK, quad or star
cable, or as technicians refer to as POTS line.
This 4 conductor cable is 22 AWG (American wire
POTS: Plain Old Telephone Service.
The POTS color code is as follows:
• This color code is no longer used though it is still
prevalent through out the country.
Today the minimum cable standard for residential
wiring is 4 pair CAT 3 cable.
CAT 3 cable is also UTP and is 24 AWG, 100 Ω with a
subtle twist amongst the pairs
The 4 pair color code is:
It will be necessary to know how to interface a 4 pair
CAT 3 cable with a quad cable in order to support the
many small business and residential customers that still
have POTS lines on their premises