mail system. In order for the two systems to communicate, a complex set of challenges must first be overcome. Let's examine them a bit more closely. The first and most obvious challenge that must be overcome is the dif- ference between the actual user interfaces on the two systems. The PC- based system's screen presents information to the user in a Graphical User Interface (GUI, pronounced "gooey") format that is carefully designed to make it intuitively easy to use. It eliminates the need to rely on the old command line syntax that was used in DOS environments. The mainframe system was created with intuitive ease of use in mind, but because a different company designed the interface for a mainframe host, under a different design team, it bears minimal resemblance to the PC system's interface. Both are equally capable, but completely different. As a result of these differences, if we were to transmit a screen of information from the PC directly to the mainframe system, it would be unreadable simply because the two interfaces do not share common field names or locations. The next problem that must be addressed is security, illustrated in Figure 2-3. We mentioned earlier that the message that is to be sent from the user in Madrid is confidential, which means that it should probably be encrypted to protect its integrity. Also, because the message is large, the sender will probably compress it to reduce the time it takes to trans- mit it. Compression, which will be discussed in more detail later, is sim- ply the process of eliminating redundant information from a file before it is transmitted or stored to make it easier to manage. Another problem has to do with the manner in which the information being transmitted is represented. The PC-based Eudora message encodes its characters using a seven-bit character set called the Ameri- can Standard Code for Information Interchange (ASCII). A sample of the Figure 2-2 PC to mainframe communica- tions Madrid Marseilles Downloaded from Digital Engineering Library @ McGraw-Hill 