From a few posts ago there was a number of requests as to what a DS3 Hub was and how they could use it. A DS3 Hub is a Channelized DS3 or an M13 Circuit. I need to explain a DS3 then I will delve into this.
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. I’ll explain what you do need to know about DS3s.

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 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, 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 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.

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 multiplexed circuits within it. There are a couple of different framing methods that can be used. Your choice should be dictated by the DS3′s intended use.
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