Before the jump in Internet traffic in the mid 1990s, DS1s were found almost exclusively in telephone company central offices as a means to transport voice traffic between locations. DS1s have been and still are the primary way phone carriers connect their central office switches to the cell sites deployed throughout a city.
Companies today, often use an entire DS1 for Internet traffic, providing 1.544 Mbps of connectivity (actually, it is 1.536 Mbit/s; the other 8 Kbit/s goes to framing overhead). However, DS1 can be ordered as a channeled circuit, and any number of channels can be reserved for non-data (for example, voice) traffic. Additionally, for voice T1s there are two types: so-called "plain" or Inband T1s and PRI (Primary Rate Interface). While both carry voice telephone calls in similar fashion, PRIs are commonly used in call centers and provide not only the 23 actual usable telephone lines (the 24th line carries signaling information) but also Caller ID (CID) and Automatic Number Identification (ANI) data, commonly referred to in industry parlance as 'signaling data'.
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Inband T1s are also capable of carrying CID and ANI information if they are configured by the carrier to do so but PRI's handle this as a standard and thus the PRI's CID and ANI information has a much better chance of getting through to the destination. While an Inband T1 seemingly has a slight advantage due to 24 lines being available to make calls (as opposed to a PRI that has 23), each channel in an Inband T1 must perform it's own set up and teardown of each call. A PRI uses the 24th channel as a data channel to perform all the overhead operations of the other 23 channels (including CID and ANI). So even though an Inband T1 has 24 channels, the PRI can actually dial more calls faster because of the dedicated data (also called "D" channel.)