Overview of CT1/PRI Interfaces
A CT1/PRI interface is a physical interface in the T1 system, which can transmit voice, data, and video service packets.
The Pulse Code Modulation (PCM) technology was developed in 1960s and allows the Time Division Multiplexing (TDM) technology to be widely applied in the digital communication system. There are two TDM systems in the digital communication system: E1 system and T1 system. The E1 system is recommended by ITU-T and used in Europe and China. The T1 system is recommended by ANSI and used in North America and Japan. (Actually, Japan uses the J1 system, which is similar to the T1 system.)
A CT1/PRI interface can be used as a CT1 interface or a PRI interface.
- When the CT1/PRI interface functions as a CT1 interface, timeslots 1 to 24 can be bundled into several channel sets. Each channel set functions as an interface, which has the same logical features as a synchronous serial interface and supports link layer protocols (PPP, HDLC, and FR) and the Internet Protocol (IP).
- When the CT1/PRI interface functions as a PRI interface, timeslot 24 functions as the D channel to transmit signals. Therefore, only one group of timeslots except timeslot 24 can be randomly selected to function as a B channel, and then the B channel is bundled with timeslot 24 to form a PRI set. The PRI set functions as an ISDN PRI interface and supports link layer protocols (PPP, and FR) and IP.
This section provides only the physical attribute configuration of ISDN PRI interfaces. For details about ISDN service configuration, see ISDN Configuration in the Configuration Guide - WAN.
Clock Mode
To exchange data correctly, communicating parties must have their clocks synchronized.
- Master clock mode (local clock mode): In this clock mode, a CT1/PRI interface uses the local clock generated by the chip as the reference clock.
- Slave clock mode (line clock mode): In this clock mode, a CT1/PRI interface uses the line clock as the reference clock.
Generally, the interface at one end of a link works in master clock mode, and the interface at the other end works in slave clock mode.
Frame Format
- Super frame (SF): consists of 12 frames that share the same frame alignment information and signaling information. Frame 6 and frame 12 are signaling frames.
- Extended super frame (ESF): consists of 24 frames that share the same frame alignment information and signaling information. Frames 6, 12, 18, and 24 are signaling frames.
Line Idle Code
The line idle code is the code sent over a timeslot that is not bundled to form a channel.
The router supports two line idle codes: 0x7e and 0xff.
Interframe Filling Tag
The interframe filling tag is the code sent when the timeslot bundled to form a logical channel does not send any service data.
The router supports two interframe filling tags: 0x7e and 0xff. On the router, you can set the minimum number of interframe filling tags.
AIS Detection
Alarm indication signal (AIS) alarms are also called upstream alarms, indicating that the peer device or the receive link of the local device is faulty.
If the number of 0s in 512 consecutive bits (two consecutive frames) of a received signal is smaller than 3, an AIS alarm is generated. If the number of 0s in 512 consecutive bits of a received signal is larger than or equal to 3, an AIS alarm is cleared.
RAI Detection
A remote alarm indication (RAI) alarm is sent to an upstream device after the router detects loss of frame (LOF) caused by clock asynchronization or loss of signal (LOS).