Editing Digital Signal 1 (section)

==Inband T1 versus T1 PRI==
Additionally, for voice T1s there are two main types: so-called "plain" or Inband T1s and PRI ([[Primary rate interface|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 (known as ''B'' channels for ''bearer'') but also a 24th line (known as the ''D'' channel for ''data''<ref>Versadial, [https://www.versadial.com/faqs/definitions-common-call-recording-terms/ Call Recording Terms/Definitions], last accessed 8 June 2015</ref>) that carries [[line signaling]] information. This special ''D'' channel carries: [[Caller ID]] (CID) and [[automatic number identification]] (ANI) data, required channel type (usually a B, or bearer, channel), call handle, [[Dialed Number Identification Service]] (DNIS) info, requested channel number and a request for response.<ref>Newton, H: "Newton's telecom dictionary", page 225. CMP books, 2004</ref>

Inband T1s are also capable of carrying CID and ANI information if they are configured by the carrier by sending DTMF *ANI*DNIS*. However, PRIs handle this more efficiently. 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 its own setup and tear-down 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). Although an inband T1 has 24 channels, the 23 channel PRI can set up more calls faster due to the dedicated 24th signalling channel (D Channel).

Before T1 PRI existed there was T1 CAS. T1 CAS is not common today but it still exists. CAS is Channel Associated Signaling. It is also referred to as Robbed Bit Signaling. CAS is a technology with roots in the 60's and before.