 when data was to be sent, and to be turned "off" when there was no data to be sent (so that the communication link could be used for other computer connec- tions). Here is what engineers came up with. Figure 2.20 represents a packed switched communication network. Let's say the computer marked "A" wants to send data to the computer marked "B." What happens is this. 1. The data from A is broken down into small, equally sized packets of data. Each packet has on it the name of the computer it wants to go to (in our case computer B). 2. The first packet from computer A is sent out to the nearest node. The node, which represents a special processor, basically acts like a traffic policeman. It looks at the data and does two things. 2a. It checks to see if the packet has errors in it that occurred in transmis- sion. If it sees too many errors, it is sent back; if has only a very few errors it is sent on. Figure 2.20 Computer Computer Data communication through a packet-switched network Data packet Data packet Telecommunication Networks 2b. If the packet is to be sent on, the node decides which is the best node to send it to, based on its final destination. 3. This movement from node to node continues until the data packet arrives at computer B. Each packet from computer A to computer B may follow a different path of nodes. And this, my readers, is how you build networks intended only for the transmis- sion of data. 2.5 Mobile Communications One of the fastest growing markets in the world (and the one I am actively research- ing) is the field of wireless communications, also called mobile communications. People on the move (in their car, on their yacht, walking through busy downtown streets) believe that they must be able to reach other people through voice (and possibly data) communications. These people want anytime/anywhere/with-anyone communications, and they are willing to spend their hard-earned dollars to have it. As a result, many engineers work hard to give this to them. Taking a look at Figure 2.21, we see the