ISDN and Linux—Surfing at Warp Speed

This article presents a detailed tutorial on setting up an ISDN link to the Internet with Linux.
Getting an ISDN Account with Your ISP

ISDN dial-up accounts tend to be more expensive than a 28.8K connection. This is changing as more and more ISPs convert to access routers supporting both ISDN and 28.8K connections off the same ISDN line. UUNET Technologies (in the U.S.) and UUNET Canada offer ISDN access using this type of technology.

When you call an ISP to get an ISDN account, you need to learn a bit about their service. Ask them what ISDN router they are using. This information can be helpful in tracking down problems should they occur. Not every ISDN router will talk to all other ISDN routers. The Ascend MAX is popular among ISPs. If they permit a 128K 2B account, make sure they support MPPP (Multilink PPP).

Also consider the type of service you need. Most ISPs offer either dedicated or dial-up ISDN. Dedicated services give you a line on the ISP's router for your use only. They typically also allocate you a block of IP numbers with the service. You need a fixed IP number if you intend to pick up SMTP mail over the link. Dial-up situations tend to use dynamic IP assignments, which means you are assigned a different IP number on each call. A few ISPs assign you a single IP number for your use. While this is rare, it is a method allowing pickup of SMTP mail over a dial-up account without using UUCP. How you intend to access the net and for what purpose will determine the type of account you need.

Network Address Translation, or IP Masquarade as it is called by the Linux kernel, is required by a router to provide access to multiple systems to a single dial-up ISDN account. In the case of an ISDN modem or an ISDN Network Adapter, the Linux kernel deals with the translation. If you are assigned a class C block or a point-to-point number by your ISP, address translation is not required.

To Compress or Not to Compress

The most recent ISDN devices feature some form of compression. Their marketing departments will say that compression will lead to increased throughput on the order of two to four times. They claim a 128K line will seem to run at 256K. Don't be fooled. In real live situations, you will be lucky to perceive an increase of 10%. Consider most information the device will be downloading is already compressed, and therefore, cannot be compressed further (or at least very little). JPEG and GIF files from the web are already compressed, and most ftp archives are either zipped or gzipped, tar files. Only text benefits greatly from compression, and text is usually only about 20% of the data on a web page.

Thus, when shopping for an ISDN device, don't place compression high on your priority list.

Using an ISDN Router

An ISDN router is the easy way to get connected—enter in a few bits of information into the router's configuration, and you are on the Net. A router is the best method to get a LAN talking to the Internet, as this is a dedicated computer designed to handle the load of passing packets. The only drawback can be the price. For most individual users, an ISDN router will be beyond the reach of their pocketbook. For business, a router is essential for reliability.

To get started, you need to obtain a network card and install it; refer to the Ethernet Mini-HOWTO for details.

The best Ethernet card to use is one with a memory buffer, as it gives the best performance. Most routers come with a twisted 10baseT cable to connect your router directly to your Ethernet card. If you intend to connect more than one computer, an Ethernet hub is required. Avoid 10base2 (coax) cabling, if at all possible, to avoid hassles. It may be cheaper initially, but it can lead to diagnostic and cabling nightmares.

Two ISDN routers I have worked with and found reliable are the Ascend Pipeline 50 and the Farallon Netopia.

The Ascend Pipeline series is well suited for dedicated ISDN applications, as it was originally designed for routing a netblock or a static IP number. The Ascent router supports address translation; something Ascend calls NAT (Network Address Translation). If POTS jacks are required, then consider the Pipeline 75 which is basically the Pipeline 50 with POTS jacks.

The Farallon Netopia is, by far, one of the easiest routers to configure. The Netopia supports dedicated connections and supports Farallon's version of address translation, called SmartIP, for use with dial-up applications. The Netopia comes in a 12-user version as well as a full routing version. Two POTS jacks are available as an option. What makes this router unique is its PC Card (PCMCIA) slot. Insert a PC Card modem, and the router can be dialed into even when the ISDN connection is down. This is very handy for those users who require remote configuration and troubleshooting abilities. The Netopia also includes a scheduler, so that connections can be scheduled for variable times during the week. No other ISDN router I've used has this feature.

The Ascend Pipeline series is the most popular, and most ISPs are either currently using them or have worked with them in the past. The Netopia is newer, and its feature set makes it worth noting. Of the many routers I have worked with, the Ascend and Farallon products have most impressed me.