Network ISDN
Data Link LAPD
Physical 2B1Q

The Integrated Services Digital Network was the first telecommunications service designed specifically for digital data communication. ISDN was designed to run over standard voice digital telephone systems already in place. Consequently ISDN conforms to specifications found in the telecom's digital voice network. However, it took so long for ISDN to be standardized that it was never fully deployed in the telecommunications networks it was intended for.

ISDN Channel Types

ISDN uses two bearer channels and one data channel.

Bearer Channels
The first channel type is a 'bearer' channel or 'B' channel. These are the channels that carry the end-user's actual data. Bearer channels are always 64kbps in the US, and 56kbps elsewhere.
Data Channels
To confuse things, the telcos called the second channel type the 'data' channel or 'D' channel. This channel contains the data THEY consider important such as signalling errors, framing and various control and managment signals. The D channel's speed is 16 kbps for a BRI, and 64 kbps for a PRI.

ISDN Service Types

ISDN provides two types of service, Primary Rate Interface (PRI) and Basic Rate Interface (BRI). These speed and number of channels for these two types of service vary depending upon whether you are in the United States or are elsewhere.

Primary Rate Interface
In the US, a PRI consists of 23, 64kbps 'B' channels and one 64kbps 'D' channel for a total of 1.544 Mbps. Elsewhere in the world a PRI consists of 30 'B' channels running at 56kbps and one 'D' channel also running at 64kbps. The reason for this difference is that overseas systems use a 7-bit word (7 bits x 8000 sample rate = 56,000 bps) and no echo cancellation. The US uses an 8-bit word (8 bits x 8,000 sample/second rate = 64,000 bps) with echo cancellation in the voice network.
Basic Rate Interface
BRI's consist of two bearer channels and one data channel. The Bearer channels are either (US) 64 or 56 kbps (elsewhere) and the data channel is 16 kbps, providing up to 144 kbps of data transmitted and 128 kbps of usable user data.

Digital Calling Services

Calling Line ID

Called Number

Automatic Number Identification


Wiring & Connectors

Primary and basic rate ISDN both use .25 guage copper twisted-pair telephone wire; however, cable TV companies and other organizations have used this protocol as well over coaxial cable. The only difference between PRI and BRI is that time-division multiplexing is used to combine several BRI's into a PRI making the PRI a trunk.

Several Terms used by telecommunications companies and the ITU are used to describe he device, interface and connection points.

ISDN Equipment - Grouped by Function

Terminal Equipment 1 (TE1)
This is any device that understands ISDN signalling standards.
Terminal Equipment 2 (TE2)
This is any device that does NOT understand the ISDN signalling standard.
Network Terminator Type 1 (NT1)
When you connect a TE1 device to the ISDN network, you use an NT1. An NT1 will connect any 2-wire TE1 device to the ISDN network.
Network Terminator Type 2 (NT2)
Devices using 4 wire cables must use an NT2 to convert the 4 wire connector to a 2 wire connector that can be physically connected to the ISDN network through an NT1.
Terminal Adaptor (TA)
A terminal adaptor allows a non ISDN device (a device that is a TE2) to communicate with the ISDN network via an NT1. This is typically needed where the device uses a non-polar electrical signalling system. ISDN's electrical signalling is bipolar, thus a converter is needed.
Local Termination (LT)
This is an NT1 at the ISP's side of the connection.
Exchange Termination (ET)
This is the connection between the customer's last mile (local loop) connection and the service provider's ISDN network. Usually, this is the line card in the ISDN switch at the provider's local exchange office.

ISDN Reference Points

This is the connection reference point designating the connection interface between an ISDN Terminal Adaptor and a non-ISDN device. There really aren't any standards for this reference point as this reference point was designated for devices that allow non-ISDN devices to communicate with the ISDN devices. Clearly there are many proprietary ways to do this, none of which are part of any standard.
This is the connection reference point designating the connection interface between an ISDN capable device and a Network Terminator 1. Reference point 'S' is for user terminals that connect to the ISDN network. Reference point 'T' defines connections between NT1 and NT2 devices. An S/T reference point combines the functions of the S and the T reference points. S/T is governed by the ITU I.430 specification.
This is the connection reference point designating the connection interface between an ISDN NT1 and the ISDN services presented by the ISDN switch.
This is the connection reference point between the line termination equipment and the exchange termination equipment.


The signalling scheme used on ISDN is called '2 Binary, 1 Quaternary' or 2B1Q. This signalling scheme allows twice as many bits to be transmitted in the same time-slice as a normal digital voice line. This is achived by allowing a wider range of voltage states than is used on an ordinary voice line. Link Access Protocol D-channel or LAPD protocol (aka CCITT Q.920/921) runs on top of 2B1Q and provides much of ISDN's 'intelligence' for the reliable transfer of data. LAPD runs in Asynchronous Balanced Mode (ABM) so that both ends of the connection can perform call setup and teardown, bearer channel supervision, error recovery and data transmission. LAPD is also necessary to run ISDN over Frame Relay.


T1 based PRI uses B8ZS for encoding, ESF for bit framing with digital signalling 2B1Q as the initial input sample data. This differs from voice lines, which use PCM encoded data.

E1 based PRI uses HDB3 (high density bipolar order 3); CRC 4 for framing

Link Access Procedure, Data (LAPD)

Link Access Procedure, Data or LAPD is the protocol used to transfer data on an ISDN circuit. LAPD is used on the D channel of ISDN PRI or BRI interfaces.

flag, address, control, information, CRC, flag

Binary value 01111110 - signals the start of the frame.
service access point identifier (SAPI); command-response;Address extension0; Terminal Endpoint Identifer
Signals call control values (11 of them)
This is the payload of the LAPD frame, but contains Q.931 data; information field, Protocol discriminator, Length of call reference value, Call reference value; message type, mandatory and optional information elements
CRC check
This is a check of the data using CRC checking to allow for error correction and recovery of data upon receipt.
011111110 - signals the end of the frame.


ISDN Related Standards

ITU Standards

ITU Q.921

ITU Q.931

SPID - Service Profile Identifer - composed of: Prefix, directory number, suffix. A SPID is sometimes required in order to use ISDN with certain service providers. This is based on the type of ISDN switch used.


Call Setup

  1. SETUP

Call Teardown


Cisco ISDN configuration (IOS)


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