Tip

WAN interface confusion

One of the most confusing things for beginning network administrators is trying to figure out the different types of WAN circuits. There's a good reason for this, of course. The WAN technologies we have today are the product of lots of different standards bodies, both foreign and domestic, and the heritage of our 100-year-old voice technology. It's a confusing mess of circuit-switched, cell-switched, packet-switched and now "label-switched" technologies. So here's a tip to help you sort things out when you're trying to order the right hardware for your router to support the service you are buying from a provider.

Once you understand two things the picture should get clearer. First is the difference between physical protocols and encapsulated. Second is that there are lots of "interfaces" in some protocols and the one you get may depend on the provider or country you're in.

As an example, a typical "serial" interface on a router can be configured with any of the following WAN protocols: PPP, Frame-Relay, HDLC, etc. But, if you want to use ATM or ISDN, you usually need to buy hardware that only does ATM or ISDN. In the first example,

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frame relay is really just a Layer 2 encapsulation. ATM and ISDN however, specify physical components as well. Clear as mud? The point is, if you're searching through some vendor's hardware list, you're unlikely to find a module named "Frame-Relay interface", but you would probably find "ATM card" and an "ISDN card."

The second issue is that to make most WAN connections, there are a series of components required and the names and order vary by technology. Some of these components you can provide yourself, and some are provided by vendors. Often, these are different in different countries, or for different service providers.

One example is the CSU/DSU. If you order a T1, you probably need one of these. But sometimes the service provider will provide and configure it, and sometimes you'll have to do it. In other words, the "demarc" could be before or after the CSU. Also, with most hardware vendors you have the option of providing this functionality in your router, or as a separate piece of hardware. Another example is ISDN's ST and U interfaces. Depending on what country you live in, you may get one or the other, and in turn have to provide the "NT1" yourself or not.

Unfortunately, I don't have the space to go through an exhaustive list of the technologies out there, but hopefully these examples will help you understand some of the types of options there are, so you can ask your service provider the right questions.

If you're managing your own router, always be sure to ask

  1. What interface you need, and if they know of any specific version numbers or compatibility problems.
  2. Where and what the demarc is. This should include what type of cable, as many technologies have several options.
  3. What special parameters you need to configure in software.

Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.


This was first published in June 2007

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