T1 Line

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:

8 bits x 8000 samples = 64 Kbps

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:

24 Voice Channels x 64 Kbps per Voice Channel = 1.536 Mbps

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..

Digital CircuitVoice ChannelsBit Rate# of T1 Circuits
T1 (DS-1)
1.544 Mbps
T2 (DS-2)
6.312 Mbps
T3 (DS-3)
44.736 Mbps
T4 (DS-4)
274.176 Mbps

T1 RJ48 to BNC pair connection

Typical Connectors:

  • T1 - Twisted Pair or Coax Cable
  • T2 - Coax Cable
  • T3 - Coax, Fibre Optics or Light Route Radio
  • T4 - Coax or Fibre Optics

RJ48 Pinout for T1 line

A T1 line is wired according to the RJ48 standard. Typically, RJ48 uses STP (shielded twisted pair). RJ45 refers to the UTP wiring standard used for LAN protocols like Ethernet. Both Ethernet and E1/T1 use the same modular connector 8P8C.

T1 Cross Over Cable

T1 Crossover Cable

You can rent any quantity of a T1 line, you don't have to rent the complete circuit. You basically rent a time-slot on the line based on 64 kbps channels. This is called Fractional T-1.

The T1 line (and E1) has different variations:

  • primary-4ess Lucent 4ESS switch type for the U.S.
  • primary-5ess Lucent 5ESS switch type for the U.S.
  • primary-dms100 Northern Telecom DMS-100 switch type for the U.S.
  • primary-dpnss DPNSS switch type for Europe
  • primary-net5 NET5 switch type for UK, Europe, Asia and Australia
  • primary-ni National ISDN Switch type for the U.S.
  • primary-ni2c The Cisco NAS-SC switchtype based on NI2C
  • primary-qsig QSIG switch type
  • primary-ts014 TS014 switch type for Australia (obsolete)
  • primary 30 Channels
Alarms that can occur on a T1:

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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Copyright July 2013 Eugene Blanchard