nnections. The second type of communication system is called a Token ring network. The Token ring network is shown in Figure 2.24, and it operates as follows: 1. When there is no computer transmitting, the bits 11111111 are sent around the ring connecting the computers. These 8 bits are called the token. 2. When computer A wants to send information, it listens to the ring to make sure the token is on the ring. If it is, it inverts the last bit of the token (putting out 111 11110) and then follows this with its information. Computer A makes sure that it puts the name of the intended computer in its information-for example, if the data is headed to computer B, computer A makes sure it puts "computer B" in its sent data. Telecommunication Networks Computer Coaxial Token cable Figure 2.24 Token ring network Computer 3. Every computer picks up the data sent by computer A, and reads the name to see if it is the intended addressee. If it is not, the computer puts the information back on the ring without reading it. If the data is intended for the computer, the computer reads the data, and then puts it back on the ring. 4. Eventually, the data gets back to the computer who sent it. This computer picks up its data, and puts the token 11111111 back on the ring. And that is how computers communicate using Token ring links. 2.7 Conclusion This chapter introduced you to the world of communication systems. From here on, we'll focus on the details of how these systems work. For example, we will look into a cellular phone. We will describe how the codec (coder/decoder) works in Chapter 4. We will understand how the modem (modulator/demodulator) works in Chapter 5. We will introduce an important part of digital communication systems, called the channel coder/decoder, in Chapter 6. Basically, in what follows, we get into what is called the "physical layer"-how all the insides work. But first is Chapter 3, a review of the statistics and signals and systems details that you'll want to make sure you know before forging ahead. Chapter