Last Updated on July 9, 2026

Integrated Services Digital Network (ISDN) consists of data-transport and digital telephony services provided by local exchange carriers. ISDN enables the digitalization of the Public Switched Telephone Network (PSTN) which allows voice, text, music, data, graphics, video, and other forms of data to be transmitted over traditional telephone lines. ISDN was designed to create an end-to-end digital communication. This would facilitate high speed data transfer, video conferencing, digital voice communication, and other digital communications applications.

This article will summarize the technology, services associated to ISDN, and why businesses choose to record ISDN calls.

What are the ISDN devices?

Point-to-Point Protocol (PPP) is a data link layer network communications protocol used to establish a direct connection between two devices. PPP is utilized to encapsulate IP packets to transmit over dedicated lines between two endpoints (point-to-point links). PPP also established a standard for data encapsulation, link configuration, network layer address negotiation, network authentication, error detection, and network protocol multiplexing. PPP facilitates these functions by providing Network Control Protocols (NCPs) and Link Control Protocol (LCP) to establish configuration parameters.

What are Point-to-Point Protocol Components?

ISDN devices are the hardware components used to connect phones, computers, and networks to an ISDN digital telephone system. These devices include:

• Terminals
• Terminal Adapters (TA)
• Network-Termination Devices (NT1 and NT2)
• Line-Termination Equipment
• Exchange-Termination Equipment

There are two types of ISDN terminals. Terminal equipment type 1 (TE1) are specialized terminals such as phones, video conferencing systems, routers, etc. that directly connect to ISDN interfaces. Terminal equipment type 2 (TE2) are non-ISDN terminals that predate ISDN standards such as Data Terminal Equipment (DTE). TE1 equipment connect directly to ISDN networks via twisted pair four wire digital links. TE2 are non-ISDN equipment and require a TA to convert the interface to work on ISDN networks. The TA can be a board installed inside a TE2 or a stand-alone device. A TA that is a stand-alone device connects to the TE2 through a physical interface.

The next connection point beyond the TE1 and TE2 on the ISDN network are the NT1 and NT2 devices. These devices connect the four-wire subscriber wiring to the standard two wire local loop. The NT1 is a customer premise equipment device in North America, whereas; the NT1 is provided by the carrier and is part of the network in other parts of the world. The NT2 devices are typically private branch exchanges (PBXs) and perform functions on ISDN’s layers 2 and 3 along with combining traffic from multiple terminals into fewer network channels. NT1+NT2 devices also exist which combines the functions of both devices.

ISDN has reference points that indicate the logical reference between groups such as NT1s and TAs. The ISDN reference points include the following:

• R – Connects TE2 to TA. used when a non-ISDN device requires a TA to access ISDN resources.
• S – Connects TE1 or TA to NT2, used inside customer premises. Multiple ISDN devices may share this interface.
• T – Connects NT2 to NT1, similar to the S interface, separates consumer owned devices (eg, PBX) from the NT1 on the network.
• U – Connects NT1 to ISDN provider, it is the physical line connecting the customer premise to the telephone company’s ISDN.

The image below is a sample configuration of three devices connected to an ISDN switch to the Central Office. Two of these devices are ISDN compatible and are connected to the NT2 via S reference points. The third device is not ISDN compatible and requires a TA via an R reference point. Any device could attach to an NT1+NT2 device and would replace both NT1 and NT2.

ISDN Services

Basic Rate Interface (BRI) and Primary Rate Interface (PRI) are the two services associated with ISDN.

ISDN BRI

ISDN BRI service provides one Delta (D) channel and two Bearer (B) channels. The B channel transports user data at a rate of 64 kbps and the D channel carries control and signaling information and operates at 16 kbps. The D channel may provide user data transmission under certain circumstances. The D channel consists of Layer 1 – 3 of the OSI reference model. BRI also provides an extra 48 kbps for overhead and framing bringing a 192-kbps total bit rate. The BRI physical layer specification is International Telecommunication Union-Telecommunications Standards Section (ITU-T) (formerly the Consultative Committee for International Telegraph and Telephone [CCITT]) I.430.

ISDN PRI

ISDN PRI service provides one D channel and 23 B channels in Japan and North America. This yields a 1.544 Mbps total bit rate (PRI D channel operates at 64 kbps). ISDN PRI in Australia, Europe, and other parts of the world provide one 64 kbps D channel and 30 B channels for a 2.048 Mbps total rate. The PRI physical layer specification is ITU-T I.431.

ISDN Media Layers Specifications

There are three media layers in ISDN to facilitate the physical transportation of data.

ISDN Physical Layer 1

The ISDN Physical layer differs depending on whether the frame is inbound, from network to terminal, or outbound, from terminal to network. The image below shows both physical layer interfaces. These are 48 bits long frames and 36 bits of these frames are data. These are used as follows:

• A – Activation Bit, activates devices
• B1, B2 – Channel bits, handles user data
• D – D channel (4 bits x 4,000 frames/sec = 16 kps), handles user data
• E – Echo of previous D bit, provides resolution for contention when multiple terminals on a passive bus content for a channel
• F – Framing bit used for synchronization
• L – Load balancing, adjusts the average bit value
• S – Spare bit, unassigned

A single circuit can support multiple ISDN user devices physically attached. Collisions may occur when two terminals transmit simultaneously in this configuration. Thus, ISDN features are provided to prevent these collisions. The TE echoes a D bit back to the NT when the TE receives it in the next E bit position. The next E bit is expected by the TE to match the last D bit it transmitted.

A continuous stream of binary 1s on the D channel means the channel is idle. Terminals cannot transmit unless they detect this continuous stream of 1s corresponding to a priority that is pre-established. The TE will stop transmitting if it detects a bit in the E channel that is different from the D channel. This ensures only one terminal transmits at a time. The terminal priority is reduced by requiring more continuous detection of 1s before transmitting after a successful D-message transmission. Until all devices have an opportunity to send a D message, the terminal will not be able to increase its priority. Signaling information has a higher priority than non-signaling information and telephone connections have a higher priority than any other service.

ISDN Data Link Layer 2

Layer 2 of the ISDN is Link Access Protocol on D channel (LAPD) of this signaling protocol. This layer ensures signaling and control information flows and is received correctly across the D channel. The LAPD frame format uses information, unnumbered, and supervisory frames. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921.

• SAPI – Service access point identifier (6 bits)
• C/R – Command/Response bit
• EA – Extended addressing bits
• TEI – Terminal endpoint identifier

The length of the LAPD address can either be one or two bytes. The address becomes two bytes when the extended address bit of the first byte is not set; the address is one byte if it is set. The SAPI identifies the portal where LAPD services are provided to the Network Layer (3) is contained in the first Address field byte. A response or a command is indicated within the C/R bit frame. The TEI identifies either multiple or a single terminal and a broadcast is indicated when the TEI is set to all ones.

ISDN Network Layer 3

ITU-T (CCITT) originally published ITU-T I.451 (ITU-T Q.931) and I.450 (ITU-T Q.930) are two Layer 3 specifications used in signaling for ISDN. These protocols support circuit-switched, packet-switched, and user to user connections. Call termination, miscellaneous messages, call establishment, and information are specified including CONNECT, USER INFORMATION, STATUS, DISCONNECT, CANCEL, RELEASE, and SETUP. The image below is an example of the stages of an ISDN circuit switched connection.

ISDN in Summary

ISDN consists of data transport and digital telephony services provided by local telephone companies. ISDN facilitates the digitalization of the PSTN to transmit data, music, voice, graphics, video, and other materials.

ISDN devices include:

• Terminals
• Terminal Adapters (TA)
• Network-Termination Devices
• Line-Termination Equipment
• Exchange-Termination Equipment

ISDN utilizes two services BRI and PRI. BRI provides two B channels and one D channel. PRI provides 23 B channels and on D channel in North American and Japan.  PRI provides 30 B channels and one D channel in Europe and Australia.

Why do businesses record phone calls on ISDN environments?

The use of ISDN is widespread throughout the world particularly in businesses due to its ability to transmit voice, text, music, data, graphics, video, and other forms of data. An added benefit to utilizing ISDN is connecting call recording capabilities for businesses that utilize the phone calling technology. Call recording provides businesses the opportunity to reduce corporate and personal liability, increase customer and employee satisfaction, train and run performance reviews, and reduce errors and misunderstandings.

Contact us to learn more about call recording solutions for your business.

Previous Call Recording Articles:

Last Updated on July 9, 2026