Asynchronous Communication

To understand asynchronous modem communication using RAS, you need to understand the following four compatibility levels in relation to RAS:

• Modem Command Language
• Modem Modulation Standards
• Modem Error Control Standards
• Modem Data Compression Standards and RAS Data Compression

Figure E.1 shows these four compatibility levels being implemented during a WAN connection. The different levels are shown below the section of the asynchronous RAS link to which they apply:

• modem command language affects command compatibility between RAS and the local modem, modem modulation standards affect the telephone line speed;
• compatibility between two modems, modem error control standards affect the local and remote modem level of error control compatibility;
• modem compression standards affect the local and remote modem data compression compatibility; and
• RAS data compression affects data compression between the local and remote computer.

In the diagram, the RAS client is on the right side and the RAS server on the left side, and they are connected over external modems.

Notice that a RAS client or server acts as Data Terminal Equipment (DTE), and a modem acts as Data Communications Equipment (DCE).

Figure E.1 Modem Compatibility Standards