| 1.05 Public Switched Telephone Network |
| Whether you're interested in voice, data or networking, it is important to have an understanding of the structure and operation of the telephone network. |
| We begin with a basic model for the telephone network in this slide, and build on it in subsequent discussions. At the top of the diagram, we have a telephone and a telephone switch. The telephone is located in a building called a Customer Premise (CP), and the telephone switch is located in a building called a Central Office (CO). One could refer to the telephone as Customer Premise Equipment (CPE). |
| The telephone is connected to the telephone switch with two copper wires, often called a local loop or a subscriber loop, or simply a loop. This a dedicated access circuit from the customer premise into the network. We usually have the same arrangement at the other end, with the far-end telephone in a different customer premise and the far-end telephone switch usually in a different central office. |
| There is a maximum length for the local loop, which is usually 18,000 feet - about three miles or five kilometers. Copper is a good conductor of electricity - but not perfect: it has some resistance to the flow of electricity through it. Because of this, the signals on the loop diminish in intensity or attenuate with distance… if the loop was too long, you wouldn't be able to hear what the other person was saying. Thus, COs traditionally had a serving area of three miles radius around them, about 27 square miles or 75 km2. |
| With suburban sprawl, we can't build COs every five miles, so in practice, new subdivisions are served from remote switches, which are low-capacity switches in small huts or underground controlled environment vaults. The remote provides telephone service locally on the loops in the subdivision. The remote and the loops are connected back to the nearest CO via a loop carrier system that uses fiber or radio. |
| Telephone switches are connected with trunks. While subscriber loops are dedicated access circuits, trunks are shared connections between COs. To establish a connection between one customer premise and another, the desired network address (telephone number) is signaled to the network over the loop, then the network selects an unused trunk circuit going in that direction and the switch connects the loop to that trunk - for the duration of the call. When one end or the other hangs up, the trunk is released for someone else to connect between those two COs. This method for sharing the trunks is known as circuit switching. |
| It is important to note that even though today there may be digital switching and digital transmission, the last 3 mi. / 5 km of the network, the subscriber loop, most often still has its original characteristics, which date back to the late 1800s (!). |
| Voice and data equipment which connects to the PSTN over regular telephone lines must work within the characteristics of the local loop, so an understanding of the characteristics and limitations of the local loop is primordial to understanding of most kinds of communications. |
Source: Teracom Course 101, Telecom, Datacom and Networking for Non-Engineering Professionals,
Module 1: Fundamentals of Telecommunications, slide 1.05 |
