ISDN BRI Interface

The Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) is a method used to bridge the local loop (the last mile) between the Central Office and the premise connection (home). ISDN uses the existing wiring so no new cabling is required. It is a digital service which replaces the analog POTS (plain old telephone set) phone.

ISDN bandwidth

In North America and Europe, ISDN has a bandwidth of 80 khz. In Japan, ISDN has a bandwidth of 320 kHz.

Basic Rate Interface (BRI) consists of 2B + D channels. Which stands for 2 Bearer channels of 64 kbps each for data and one D channel of 16 kbps for handshaking and control. Having a separate channel for handshaking and control is called "out of band" signalling. The 2B channels can be bonded together for a single data channel with a 128 kbps transfer rate.

ISDN lines can be dedicated lines that are always up and connected or they can be dial on demand (DOD) lines. When the line is required the connection is dialed up and made. The connection time for an ISDN line is very quick, in the order of 0.5 second or so. This can result in a substantial cost saving if used over long distance or paying by the minute. The line charges are only for when data is being transferred and not when it is sitting idle.

ISDN - OSI Layers

The D Channel uses up to the OSI Network Layer while the B channel uses both the Data Link and Physical layers.

ISDN - OSI Model

The line encoding is used by ISDN is standard telecommunication 2B1Q which stands for 2 Binary elements encoded in 1 quaternary. A dibit (digital bit) is represents two binary elements for each voltage change. The following table illustrates the 2B1Q encoding used by ISDN:

Dibit Voltage
10 +3
11 +1
01 -1
00 -3


ISDN Premise Connection

The following diagram illustrates a basic ISDN connection from the Central Office to the premise:

ISDN Premise Connection

The Central Office must have ISDN capabilities in the switch in order to connect to an ISDN premise. The connection from the CO to the premise uses the existing analog phone linges. At the BRI premise, a Network Termination 1 (NT-1) device converts the 2 wire analog line to a 4 wire system called the S/T interface. A PRI rate interface is The S/T interface is a bus topology that terminates in a 100W termination.

A total of 8 ISDN Terminal Equipment 1 (TE) devices are allowed to connect to the S/T Interface. If access is required to traditional analog devices such as a plain old telephone set (POTS) which are called Terminal Equipment 2 (TE2) devices, a Terminal Adapter (TA) can be used to provide access.

ISDN Advantages

  • ISDN is a mature technology, it has been around since the late 1980s. It is has been tried, tested and works.
  • It is governed by a world-wide set of standards.
  • It provides symmetrical transfer rates: the transmit rate is the same as the recieve rate.
  • It has consistent transfer rates. If you have a 64kbps bearer channel then that's the speed that you transfer at.
  • It is competitive priced compared to other technologies.

ISDN Disadvantages

  • An external power supply is required. The telco's don't supply power for ISDN lines. If the power fails, the phones won't work.
  • Special digital phones are required or a Terminal Adapter to talk to the existing POTS devices.
  • It is very expensive to upgrade a central office switch ($500,000+) to ISDN
  • If the ISDN fails - the phone fails.

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Copyright July 2013 Eugene Blanchard