1

Introduction to Telephony and POTS lines

BASIC TELECOMMUNICATIONS

2

The first patent for a telegraph was issued in 1840Voice technology started with the telegraph in 1844The basic phone was developed in 1876By 1881 there were 54,000 phones in use

BASIC TELECOMMUNICATIONS

3

Since telegraph lines followed by telephone lines were first on the poles, today we still call them telephone polesAlthough today, they are dominated with power, CCTV, and other servicesThis infrastructure combined with central offices (CO) make up the public switched telephone network (PSTN).This network was dominated by one company….. AT&T (American Telephone & Telegraph) first established in 1885 as a subsidiary of Bell.

BASIC TELECOMMUNICATIONS

4

• Some of the first phones

BASIC TELECOMMUNICATIONS

5

Telephones connected to the PSTN use a single pair of wiresTelephones are connected in parallel and operate on a current loop back to the central office.

BASIC TELECOMMUNICATIONS

6

Telephones receive their power from the phone company known as the central office (CO).

BASIC TELECOMMUNICATIONS

7

Telephone wiring is connected to battery banks in the CO which is why they’re sometimes referred to as battery circuits.This is why phones still work even when the power goes out.

BASIC TELECOMMUNICATIONS

8

At the residential dwelling the phone drop from the telephone pole (aerial) to the house is terminated on a network interface device (NID)

BASIC TELECOMMUNICATIONS

9

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

BASIC TELECOMMUNICATIONS

10

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.

BASIC TELECOMMUNICATIONS

11

Why do you think the NID is always located on the outside of the building in residential environments?

BASIC TELECOMMUNICATIONS

12

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

BASIC TELECOMMUNICATIONS

13

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

BASIC TELECOMMUNICATIONS

14

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.

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.

BASIC TELECOMMUNICATIONS

15

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 off-hook state.

BASIC TELECOMMUNICATIONS

16

On the original switchboards the tip was connected to ground to prevent the operator from being electrocuted.

BASIC TELECOMMUNICATIONS

17

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.

BASIC TELECOMMUNICATIONS

18

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

BASIC TELECOMMUNICATIONS

19

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 gauge)POTS: Plain Old Telephone Service.

BASIC TELECOMMUNICATIONS

20

The POTS color code is as follows:

• GREEN Tip 1 Positive• RED Ring 1 Negative• BLACK Tip 2 Line 2• YELLOW Ring 2 Line 2

• This color code is no longer used though it is still prevalent through out the country.

BASIC TELECOMMUNICATIONS

21

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

BASIC TELECOMMUNICATIONS

22

The 4 pair color code is:

TIP

RING

TIP

RING

TIP

RING

TIP

RING

PAIR 1

PAIR 2

PAIR 3

PAIR 4

BASIC TELECOMMUNICATIONS

23

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

POTS CAT 3

GREEN WHITE/BLUERED BLUE/WHITEBLACK WHITE/ORANGEYELLOW ORANGE/WHITE

BASIC TELECOMMUNICATIONS