In just about every country in the world, stringing private transmission lines across (or underneath) public property is illegal. The network designers must rely on the existing telecommunication facilities like PSTN.

                          Typical computer-computer connection via a local cable and via a dial-up telephone line. A cable running between two computers can transfer data at 10^9 bps, maybe more. In contrast, a dial-up line has a maximum data rate of 56 kbps.

Structure of Telephone System:

1-Fully-interconnected network

  If a telephone owner wanted to talk to n other telephone owners, separate wires had to be strung to all n houses. It became immediately obvious that the model of connecting every telephone to every other telephone was not going to work.

2-Centralized switch

Then The company ran a wire to each customer’s house or office. To make a call, the customer would crank the phone to make a ringing sound in the telephone company office to attract the attention of an operator, who would then manually connect the caller to the calling party by using a jumper cable.

3-Two-level hierarchy

To make long-distance calls between cities, so system began to connect the switching offices. so second-level switching offices were invented. After a while, multiple second-level offices were needed

By 1890, the three major parts of the telephone system were in place: the switching offices, the wires between the customers and the switching offices, and the long-distance connections between the switching offices.

Structure of the Telephone System

1.Each telephone has two copper wires coming out of it that go directly to the telephone company’s nearest end office(within 1 to 1o km) (also called a local central office).

2.The two-wire connections between each subscriber’s telephone and the end office are known as the local loop.

3.If a subscriber attached to a given end office calls another subscriber attached to the same end office, the switching mechanism within the office sets up a direct electrical connection between the two local loops.

4.If the called telephone is attached to another end office, a different procedure has to be used. Each end office has a number of outgoing lines to one or more nearby switching centers, called toll offices.

5.If do not have a toll office in common, the path will have to be established somewhere higher up in the hierarchy. Regional offices form a network by which the toll offices are connected. The toll exchanges communicate with each other via high-bandwidth intertoll trunks (also called interoffice trunks).

Major Components of the Telephone System

1.Local loops

                Analog twisted pairs going to houses and businesses


                Digital fiber optics connecting the switching offices

3.Switching offices

                Where calls are moved from one trunk to another

An end office has up to 10,000 local loops.

Area code + exchange indicated the end office

so (423) 586-xxxx

When a computer wishes to send digital data over an analog dial-up line, the data must first be converted to analog form for transmission over the local loop. This conversion is done by a device called a modem. At the telephone company end office the data are converted to digital form for transmission over the long-haul trunks.

Digital Subscriber Lines (DSL)
  • New digital services over the local loop. Services with more bandwidth than standard telephone service are sometimes called broadband
  • DSL is also called an always on connection because it uses existing 2-wire copper telephone line connected to the premise and will not tie up your phone as a dial-up connection does. There is no need to dial in to your ISP as DSL is always on.
  • The two main categories of DSL for home subscribers are called ADSL and SDSL
  • Dividing the spectrum available on the local loop into three frequency bands: POTS (Plain Old Telephone Service), upstream (user to end office) and downstream (end office to user)
  • Available 1.1 MHz spectrum on the local loop into 256 independent channels of 4 KHz each. Channel 0 is used for POTS. Channels 1-5 are not used, to keep the voice signal and data signals from interfering with each other. Of the remaining 250 channels for upstream and for downstream.
  • A common split is 32 channels for upstream and the rest downstream. 1 Mbps downstream and 256 kbps upstream (premium service).
  • 4 KHz +312.5 Hz Guard Band