Question: Which ISDN Hardware Combination Works Best? I'm planning to upgrade my small office from using PPP via a modem to using ISDN connected to our small Ethernet network. One method includes buying a router/ISDN modem combination unit for $600 or more. I noticed that five-port routers for small Ethernet networks can be had for about $50, and I've seen ISDN modems by themselves for $200 or so. Can I do the separate purchase plan and save big, or am I missing some key element? -Peter Commons <firstname.lastname@example.org>
Answer: The five-port device you're describing is actually an Ethernet hub, which has different function than a router. In this equation, the ISDN device itself is almost incidental. Ethernet, as we discussed in NetBITS-001, Hey, I'm Talking to You, Part 1, is essentially one big electrical system. Every packet on a given Ethernet segment has to reach every machine on that segment. A $50 Ethernet hub is just a way to cross-connect machines on a network using four copper wires (called 10Base-T or twisted-pair Ethernet). For $50, you're not buying much intelligence; it's kind of like an extension cord.
A router, on the other hand, transfers traffic between different networks or protocols. For example, all the machines on a local area network (LAN) that need to reach the rest of the Internet know the address of the router in their TCP/IP configuration. When you send a request for a Web page from a remote server, for instance, your machine figures out it has to send the stream of packets making up the request to the router. The router has at least two interfaces - one interface connects to your LAN using the same kind of network protocol (like Ethernet). The other interface hooks up to the Internet via ISDN, a serial connection leading to a T1 line, or even a modem. The router takes the packets from the LAN interface, removes the LAN packaging, rewraps them in the right format for the outgoing protocol, and then sends them out through the Internet interface. The same process happens in reverse for incoming requests.
It's possible to buy cheap routers with serial interfaces and cheap ISDN serial devices. However, from personal experience, we don't recommend this. It's hard to get dissimilar devices to communicate properly with one another for a reliable connection. An integrated ISDN router includes everything you need in a package that's guaranteed to work with all the different parts it contains. ISDN routers come from Cisco Systems (their new 760 and 770), Farallon (the Netopia line), and many others. Some even include a built-in Ethernet hub, and the Farallon line has an AppleTalk option that lets you combine an Ethernet network, a LocalTalk network, and a link to the rest of the Internet using ISDN. [GF]