If two servers are not on the same LAN or WAN, they must connect to each other by modem. Even if they are on the same WAN (the Internet, for example), you might want to provide them with a modem connection for backup purposes. This can be a "direct dialup" connection, a "pass-thru" connection, or a "network dialup" connection.
Direct dialup connections can occur when two servers both have modems connected to telephone lines. (The calling server could actually have access to a modem pool instead of having its own modem.) Either server can dial up the other directly. Figure 4.5 illustrates a direct dialup connection.