20CHAPTER 2: ISDN LAN MODEM FUNCTIONALITY DESCRIPTION
Call Routing Protocol This section describes the call routing protocol used by the ISDN LAN Modem and
and IP Address explains how IP addresses are translated.
Translation
Placing a Call to a To illustrate how the ISDN LAN Modem routes outgoing data calls, let us assume Previously Defined that you have set up the following types of service providers.
Destination
■A direct connection to an Internet Service Provider
■A direct connection to a remote office LAN
■A direct connection to a remote office LAN from which you can also access the Internet
If all of those connection types are configured on the ISDN LAN Modem and are associated with your computer, the following algorithm is performed for each of the following scenarios.
Call Routing While No Other Calls Are Connected
If the ISDN LAN Modem has not established any calls to a remote destination and you want to access the Internet from your computer, you simply launch your Web browser (or whichever networking application you like). When the ISDN LAN Modem receives the information packet requesting access to the WAN, it must determine which connection type to use. The ISDN LAN Modem looks at the destination Network ID (which comprises the destination IP address and subnet mask) associated with the packet. If the Network ID of the packet matches the Network ID of the remote LAN, with or without Internet access, then the call is placed to the remote LAN. If it does not match the Network ID of the remote LAN, with or without Internet access, then the call is routed to the direct ISP connection.
Once the connection is established, any authorized user on the LAN can use this connection. The ISDN LAN Modem will translate each individual user’s IP address into a single, shared IP address (assigned by the remote location), thereby allowing up to 25 users to access the same remote location.
The following example shows three users sharing a connection to the Internet and depicts the IP translation as it occurs in the ISDN LAN Modem.
