receiver's home. There, the signal travels into the home along a twisted-pair cable and enters into the modem. The modem in this case acts as a demodulator, and it turns the signal that looks like a speech signal (but doesn't sound like one) back into the original data signal. That enters into the receiver, and in this way, the two computers can talk. Data to be sent A signal which looks (a digital signal) like a voice signal Data sent Modem Through POTS, Modem the telephone network acts like acts like a modulator a demodulator Computer source Computer sink Figure 2.19 Data communication via a modem and POTS Currently, because of the internet and the fact that people want data at higher and higher rates (higher rates than you can get using a modem), some alternatives to the modem are now being used. One involves using the coaxial cable that sends TV signals into your house to also send data to and from your computer. A second alternative is to use DSL (digital subscriber line). In this case, the data signal that your computer wants to send is combined with any telephone voice signals that are leaving the house (using FDM) and together these are sent out the telephone line to the Class 5 switching center. I won't get into the details of either system here. Chapter Two Beyond these ways of communicating data, which basically are all about updating existing voice/TV links SO that they also carry computer data, there is another (a better) way. That way is based on using what is called a packet switched network. A packet switched network is a communication network designed specifically for the communication of data. The underlying idea behind the network is this. Data is very "bursty," which means that data sent between computers is usually filled with times when lots of data is sent, followed by times when no data is sent, followed by times when lots of data is sent, and SO on. As a result, people wanted to build a commu- nication network that allowed for the connection between two computers to be turned "on" quickly