Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 137
Remote Networks

There are times when networks must be connected when they are geographically separated. This means that networks cannot be connected by the conventional means of a LAN interconnect. Imagine trying to cable a network together with one subnet in California and another in Virginia. Ethernet cannot stretch that far. The only feasible way of doing this is by using some type of WAN service from the telephone company. AT&T, MCI, and Sprint all provide WAN services for data networks. They come in many forms, but again, for simplicity, this book will explain point-to-point serial lines. The choices available are Frame Relay, Switched Multimegabit Data Service (SMDS, primarily used in metropolitan areas and not cross country), Integrated Service Digital Network (ISDN), or leased lines. For simplicity reasons, leased lines will be explained here. The slide shows networks connected via serial lines.

Just as the router has physical connectors to connect to a LAN, the router has a connection that enables this type of connection. Instead of the LAN interface on the router, the router will have a serial line interface. The connector for this is usually a V.35, EIA-232 (formerly RS-232-D), or an RS-449 connector. The connection will then be connected to a device known as a Data Service Unit/Customer Service Unit (DSU/CSU). This is a box that receives the serial signal from the router and repeats it to the telephone switching office.

The leased line is a specially conditioned line that is provided by the phone company. This line has been conditioned to handle high-speed digital traffic. It is not the normal line that is used with voice switching. This line is permanently switched to provide a connection between two points. Therefore, it is sometimes called a point-to-point link. It is analogous to dialing a number, receiving a connection, and never hanging up.

Remote Networks

The router at the remote end will also be attached to a DSU/CSU. It will be able to receive the signals generated at the remote end. The typical speeds at which these lines run vary. The most common are 56 Kbps and T1 (1 .544 Mbps) lines, called leased lines because the customer does not own the line. It is leased from the phone company and the rates vary depending on the length of the line. Rates are usually cheaper for short runs (the other point of the network is a few miles away) and more expensive for longer runs. Rates also vary depending on the speed of the line.

The serial line provides a simple interconnect between two routers that cannot be connected directly by a LAN. The real problem in using them in an IP internet is that they consume a full network number or a subnet number. There have been methods to overcome this using variable-length subnet masking, which is available with the routing algorithms of OSPF and RIPv2. Otherwise, they generally act as a full network even when there are only two points connected.