a time. You'd be surprised how common this misconception is. Types of T1 Terminology is important when discussing any technology, and T1 is no exception. Many terms are commonly misused, even by people who have been in the industry for years. The terms T1 and DS1 are often thrown around interchangeably, although doing this can get you into trouble if you're talking with people who have a long history in telecommunications. You may also hear some people refer to a Primary Rate Interface (PRI) as a "digital T1," which is not strictly correct. All T1s are digital. The difference with PRI is that it uses digital signaling within the data channel as opposed to analog signaling within each voice channel. Even with an "analog" T1, each channel's audio must be converted to digital to be sent over the T1. You may encounter a lot of conflicting information when learning about T1s. While there is a lot to learn, there are only a few basic types of T1s: Chapter 21: T1 Channelized T1 A channelized T1 is a voice circuit that has 24 voice channels. Each channel con- tains its own signaling information, which is inserted into the data stream of the digitized voice. This is called in-band signaling. Provided the circuit has been pro- visioned correctly (see the upcoming sections "Encoding" on page 357 and "Framing" on page 359), with the use of an Add/Drop CSU/DSU, a channelized T1 can be used for data. A PRI is a voice circuit that has 24 channels, one of which is dedicated to signaling. Thus, the number of available voice channels is 23. The voice channels are called bearer channels, and the signaling channel is called the data channel. This type of signaling is called out-of-band signaling. Clear-channel T1 A clear-channel T1 is not framed in any way. There are no channels and no or- ganization of the bits flowing through the link. Clear-channel T1s are a rarity, as most data links are provisioned with Extended Super Frame framing. You can think of in-band signaling like pushing buttons on your phone during a call. The tones a