 remember: physical layer first! Most problems are caused by a bad cable or piece of equipment. Chapter 21: T1 CHAPTER 22 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 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