783, would be written as follows in base two: 1101001111111 From right to left that's one 1, one 2, one 4, one 8, one 16, one 32, one 64, no 128s, no 256s, one 512, no 1,024s, one 1,048s, and one 4,096. Add them all up (1 + 2 + 4 + 8 + 16 + 32 + 64 + 512 + 2,048 + 4,096) and you should get 6,783. That's binary arithmetic. Most PCs today use the seven-bit ASCII character set shown in Table 2-1. The mainframe, however (remember the mainframe?), uses an eight-bit code called EBCDIC. What happens when a seven-bit ASCII PC sends information to an EBCDIC mainframe system that only understands eight-bit characters? Clearly, problems would result. Something, therefore, has to take on the responsibility of translating between the two systems SO that they can intelligibly trans- fer data between each other. Logical Layering Another problem that arises has to do with the logical relationship between the applications running in the two systems. While the PC most likely supports the e-mail account of a single user, the mainframe undoubtedly hosts hundreds, perhaps thousands of accounts, and must therefore ensure that each user receives his or her mail and only his or her mail. Some kind of user-by-user and process-by-process differentia- tion is required to maintain the integrity of the system and its applica- tions. This is illustrated graphically in Figure 2-7. Figure 2-7 Logical session management Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols The next major issue has to do with the network over which the infor- mation is to be transmitted from Madrid to Marseilles. In the past, infor- mation was either transmitted via a dedicated and very expensive point-to-point circuit or over the relatively slow public switched tele- phone network, or PSTN. Today, however, most modern networks are packet-based, meaning that messages are broken