The North American legacy PSTN is based on the T1 Carrier System using Time Domain Multiplexing (TDM). It is a digital system that digitizes the analog Voice Channel into 8 bit data. This means that there are 2^8 or 256 levels that the 8 bit data can represent.
It samples the analog signal 8000 times a second (2x 4 kHz - makes Nyquist happy!). It is a serial data stream so we transmit the 8 bit data 1 bit at a time. This means that for a digitized Voice Channel, the data rate is:
The basic Carrier used in the T1 Carrier System is called the T1 (sometimes called DS-1) and it carries 24 Voice Channels. The Bit Rate for the T1 Carrier is 1.544 Mbps. If we multiply:
The missing 8 K is used to "frame" the data. It is information used for the Start Frame bytes, End Frame, Error Checking, Routing information, etc..
T1 RJ48 to BNC pair connection
RJ48 Pinout for T1 line
T1 Cross Over Cable
T1 Crossover Cable
The T1 line (and E1) has different variations:
RED: Your T1/E1 port will go into red alarm when it cannot maintain synchronization with the remote switch. A red alarm typically indicates either a physical wiring problem, loss of connectivity, or a framing and/or line-coding mismatch with the remote switch. When your T1/E1 port loses sync, it will transmit a yellow alarm to the remote switch to indicate that it's having a problem receiving signal from the remote switch.
YELLOW: remote alarm indication (RAI): Your T1/E1 port will go into yellow alarm when it receives a signal from the remote switch that the port on that remote switch is in red alarm. This essentially means that the remote switch is not able to maintain sync with you, or is not receiving your transmission.
BLUE: alarm indication signal (AIS): Timing is there but no data or control information is present.
Loopback : Not really an alarm. Indicates that a span is not available, as the port is in either a local or remote loopback mode.
Not Open (NOP) : Something is not connected. The technical explanation example is that "a span that belongs to a device that has been disconnected but is still being used by userspace programs and thus can't be destroyed". When running Asterisk, I've found is that if Dahdi is properly configured and loaded but Asterisk is not running then dahdi_tools or dahdi_scan will indicate the NOP alarm.
Also, for Asterisk situations, you will receive the NOP alarm if your chan_dahdi_trunk.conf (may be named something else on another system) configuration is missing the "signalling = pri_cpe" line. In this case, Asterisk loads but the Dahdi interface in Asterisk doesn't load.
Red/NOP : Combination of alarms and indicates that the span is configured but not physically connected and that Asterisk is not running.
Asterisk Troubleshooting Tip
Dropped Calls - Sometimes if busydetect=yes is set in your /etc/asterisk/chan_dahdi.conf file, it will cause random dropped calls. Busydetect is used for analog calls not digital. Set it for no then restart.
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