Layer-1 protocols are used between the terminal or the host DTE and the DCE. The protocols are used between the DCE equipment and the system port, and inside the system.

The following Layer-1 protocols are used between the DTE equipment and the DCE equipment. DCE equipment can be data modules, modems, or Data Service Units (DSUs). A DSU is a device that transmits digital data to a particular digital endpoint over the public network without processing the data through any intervening private network switches.

• RS-232: A common physical interface used to connect DTE to DCE. This protocol is typically used for communicating up to 19.2 kbps.

• RS-449: Designed to overcome the RS-232 distance and speed restrictions and lack of modem control

• V.35: A physical interface used to connect DTE to a DCE. This protocol is typically used for transmissions at 56 or 64 kbps.

The following protocols are used at Layer 1 to govern communication between the DCE equipment and the port. These protocols consist of codes inserted at the originating DCE and stripped at the port. The DS1 protocol can be inserted at the originating, outgoing trunk port and stripped at the destination port.

• Digital Communications Protocol (DCP): A standard for a 3-channel link. This protocol sends digitized voice and digital data in frames at 160 kbps. The channel structure consists of two information (I) channels and one signaling (S) channel. Each I channel provides 64 kbps of voice and/or data communication, and the S channel provides 8 kbps of signaling communication between the system and DTE equipment. DCP is similar to ISDN BRI.

• Basic Rate Interface (BRI): An ISDN standard for a 3-channel link, consisting of two 64-kbps bearer (B) channels and one 16-kbps signaling (D) channel.

• Primary Rate Interface (PRI): An ISDN standard that sends digitized voice and digital data in T1 frames at 1.544-Mbps or, for countries outside the United States, in E1 frames at 2.048-Mbps. Layer 1 (physical), Layer 2 (link), and Layer 3 (network) ISDN-PRI protocols are defined in DEFINITY Communications System and System 75/85 DSE/DMI/ISDN PRI Reference Manual. At 1.544 Mbps, each frame consists of 24 64-kbps channels plus 8 kbps for framing. This represents 23 B channels plus 1 D channel. The maximum user rate is 64 kbps for voice and data. The maximum distances are based on T1 limitations. At 2.048 Mbps, each E1 frame consists of 32 64-kbps channels.

• Analog: A modulated voice-frequency carrier signal.

• ADU Proprietary: A signal generated by an ADU. The signal is for communication over limited distances and can be understood only by a destination ADU or destination system port with a built-in ADU.

• Digital Signal Level 1 (DS1): A protocol defining the line coding, signaling, and framing used on a 24-channel line. Many types of trunk protocols (for example, PRI and 24-channel signaling) use DS1 protocol at Layer 1.

• European Conference of Postal and Telecommunications rate 1 (CEPT1): A protocol defining the line coding, signaling, and framing used on a 32-channel line. Countries outside the United States use CEPT1 protocol.