ISDN PRI Interface

In North America, the ISDN Primary Rate Interface (PRI) consists of 23B + D channels and runs over the T1 line.

Which stands for 23 Bearer channels of 64 kbps each for data and one D channel of 64 kbps for handshaking and control. Typically, channels 1-23 are used for bearer and channel 24 for the D channel.

In Non North American countries, the ISDN Primary Rate Interface (PRI) consists of 29B + D channels and runs over the E1 line.

Which stands for 29 Bearer channels of 64 kbps each for data and one D channel of 64 kbps for handshaking and control. Typically, channels 1-14 and 16-30 are used for bearer and channel 15 for the D channel.

In both standards, the Bearer channels can be bonded in any combination as required and used for voice or data communications.

OSI Layers

ISDN PRI resides in both the Network (Q.931) and Data Link layer (Q.921) of the OSI model. T1 resides in the Physical layer.

Unfortunately, E1/T1 has many different variations, which makes it interesting to get equipment from different manufacturers to work together:

  • 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

The Physical Connection

You will be connecting to a Telco service provider who will provide the Network Termination (NT) connection. Your T1 line will act as the Customer Premise Equipment (CPE). These terms are used to indicate who provides the master clock. The NT provides the master synchronization clock for the connection and it is always the telco. The configuration shown here may not match what your Telco expects. The number one problem with T1/ISDN PRI lines is matching your configuration with what the Telco expects. Contact the Telco for their expected parameters.

There are many different connectors used for T1 lines and the latest is the 8P8C connector which is the same connector used by Ethernet. It is often called an RJ45 connector but that is actually the wiring standard for Ethernet using an 8P8C connector. For a T1 line, the wiring standard is RJ48 using an 8P8C connector.

8P8C Modular Connector.

RJ48 Wiring Standard - T1 lines uses Pins 1/2 and 4/5

T1 RJ48 to BNC pair connection

If this page has helped you, please consider donating $1.00 to support the cost of hosting this site, thanks.

Return to

TelecomWorld 101

Copyright July 2013 Eugene Blanchard