The telephone network was originally created to provide analog voice communication through a system called POTS. It has since evolved to carry digital data in addition to voices. The modern network uses both analog and digital technologies, with local loops to homes and businesses using analog twisted pair lines while trunks between switching offices use digital fiber optics. The network is made up of local end offices, toll offices, and higher level primary, sectional and regional exchanges all connected via interoffice trunks to establish connections between subscribers.

1. Telephone Networks

2. Telephone Network
• originally created to provide voice communication.
• Need in communication resulted in invention of dial up modem
• Need of high speed resulted in invention of DSL Digital subscriber line.

3. Telephone Network
• Uses circuit switching
• In late 1800s plain old telephone system (POTS), was originally an
analog system using analog signals to transmit voice.
• in the 1980s, began to carry data in addition to voice.
• The network is now digital as well as analog.

4. Telephone Network
• Local loops (analog twisted pairs going into houses and businesses).
• Trunks (digital fiber optics connecting the switching offices).
• Switching offices (where calls are moved from one trunk to another).

5. Telephone Network
• Each telephone has two copper wires coming out of it that go directly to the
telephone company's nearest end office (also called a local central office).
• The distance is typically 1 to 10 km, being shorter in cities than in rural
areas.
• In the United States alone there are about 22,000 end offices. The two-wire
connections between each subscriber's telephone and the end office are
known in the trade as the local loop.
• Each end office has a number of outgoing lines to one or more nearby
switching centers, called toll offices.
• These lines are called toll connecting trunks.

6. Telephone Network
• If both the caller's and callee's end offices happen to have a toll connecting
trunk to the same toll office (a likely occurrence if they are relatively close
by), the connection may be established within the toll office.
• Primary, sectional, and regional offices form a network by which the toll
offices are connected.
• The toll, primary, sectional, and regional exchanges communicate with each
other via high-bandwidth intertoll trunks (also called interoffice trunks).

7. Telephone Networks
• Local loops consist of category 3 twisted pairs nowadays, although in the
early days of telephony, uninsulated wires spaced 25 cm apart on telephone
poles were common.
• Between switching offices, coaxial cables, microwaves, and especially fiber
optics are widely used.
• In early days communication was only analog.
• Now everything is digital expect local loop.