The telephone system in this country has been around for more than a hundred years . And when you think about the actual physical infrastructure that makes up the POTS, you can see why it made sense and still makes sense to take advantage of the miles and miles of copper wire to connect computers at different sites.
The telephone system is a switched network. This means that as a connection is established on the telephone system's network, it can be switched along different paths. For example, when you make a phone call, your voice communication does not necessarily always move along the same route, even though you dial the same phone number when you call a particular person.
As far as the transfer of data is concerned , the telephone system offers more than one possibility for moving information from source to destination. This includes dial-up connections, dedicated leased lines, and different packet-switching technologies. One of the newest technologies for data transfer over the POTS is Digital Subscriber Line (DSL). This technology allows for digital communication over existing phone lines and is fast becoming a viable choice for data transfer by home users (such as for high-speed connection to the Internet) and companies who can't justify the cost of some of the other data communication possibilities such as Frame Relay or ATM (both discussed later in this chapter).
Before concentrating on the different possibilities for data movement on the POTS, we need to take a look at some terminology related to the physical infrastructure of the telephone system. If you have ever dealt with your local phone company, just in terms of voice telephone service, you know that you're typically responsible for the wiring and equipment inside your house (unless you purchase some kind of special service contract from your local phone company). The lines outside your house are the phone company's responsibility.
In the business world, the phone or LAN equipment inside the company would be called the customer premise equipment (CPE) . The place where the CPE ends and the phone company's line begins is called the demarcation . The actual physical wires that connect the business to the nearest phone company switching station is called the local loop . Figure 13.1 provides a diagram showing these different areas.
Figure 13.1. The CPE is connected to the phone system by the local loop.