I’m going to treat DS3 (Digital Signal 3) a little differently than I did T1s. While I believe knowledge of T1 framing and signaling is useful for a network engineer, I don’t feel that the specifics are all that important when dealing with DS3s. For example, contrary to what many people will tell you (often adamantly), a DS3 is not defined as 28 DS1s. A DS3 is actually the result of multiplexing 7 DS2s. If you’re saying to yourself, “There’s no such thing as a DS2,” you’re not alone. A DS2 is a group of four DS1s, and is not seen outside of multiplexing.
While it may be interesting to know that a DS3 is composed of DS2s, that knowledge won’t help you build or troubleshoot a network today. In this chapter, I’ll explain what you do need to know about DS3s: simple theory, error conditions, how to configure them, and how to troubleshoot them.
A DS3 is not a T3. DS3 is the logical carrier sent over a physical T3 circuit. In practice, the terms are pretty interchangeable; most people will understand what you mean if you use either. However, from this point on, I’ll refer to the circuit simply as a DS3, as we’re really interested in the circuit, not the physical medium.
You’ll encounter two flavors of DS3s: channelized and clear-channel. A channelized DS3 has 672 DS0s, each capable of supporting a single POTS-line phone call. When a DS3 is channelized, Cisco will often refer to it as a “channelized T3.” A clear-channel DS3 has no channels and is used for pure data.