don't feel that the spe- cifics 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 seven 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 knowl- edge 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 (Digital Signal 3) is the logical carrier sent over a physical T3 circuit. In practice, the terms are pretty interchangeable; most people will under- stand 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, and not the physical medium. You'll encounter two flavors of DS3s: channelized and clear-channel. A channelized DS3 is one in which there are 672 DSOs, 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. Framing When I stated earlier that a DS3 is actually a group of seven DS2s multiplexed together, I was referring to a channelized DS3. When DS3s were designed in the 1960s, there really wasn't a need for data circuits like those we have today. DS3s were designed to handle phone calls, which is why they are multiplexed the way they are. DS3s require framing for the same reasons that DS1s do. The difference is that there can be multiple DS1s multiplexed within a DS3. Each of those DS1s has its own clocking, framing, and encoding that must be maintained within the DS3. The DS3 must also have its own clocking, framing, and encoding, which must not interfere with the multiple