ISDN over the copper subscriber loop
Before broadband internet became widespread, users relied on the . In the PSTN access network, voice was commonly transmitted in analog form, although the core network could be digital. Computers required digital data transfer, so modems converted data into audible tones. This method limited the bit rate and occupied the line for the duration of the call.
ISDN, or the Integrated Services Digital Network, introduced a digital user-network interface for voice and data transmission. It used existing access infrastructure, including copper pairs, and allowed voice, data, and video services to share one line. The available services depended on the selected interface and bearer service.
ISDN reference configuration
ITU-T Recommendation I.411 defines the ISDN reference configuration. The model distinguishes functional groups, which describe device roles, and reference points, which describe the interfaces between those roles.
Functional device groups
- TE1 (Terminal Equipment 1): An ISDN-compatible device, such as an ISDN phone or video-conferencing terminal.
- TE2 (Terminal Equipment 2): A device without an ISDN interface, such as an analog phone or a computer with a standard modem or serial port.
- TA (Terminal Adapter): An adapter that gives a TE2 device access to the ISDN interface. It is sometimes described as an ISDN modem.
- NT1 (Network Termination 1): Terminates the two-wire subscriber loop and connects it to the user-side interface. It provides physical-layer functions.
- NT2 (Network Termination 2): A device such as a digital PBX or LAN router that can perform switching, concentration, and management for multiple internal ISDN devices.
- LT/ET (Line/Exchange Termination): Functional terminations on the telecommunications provider's exchange side.
Reference points and the U interface
- R: The interface between a non-ISDN device (TE2) and its Terminal Adapter (TA).
- S: The four-wire bus interface connecting ISDN terminal equipment, TE1 or TA, to the NT2. Up to 8 devices can be connected on the bus.
- T: The interface between NT2 and NT1. When no NT2 is present, the S and T points coincide at the S/T interface on the NT1.
- U: The two-wire physical interface between NT1 and the line termination at the exchange. It denotes the subscriber-loop interface. The I.411 reference-point model lists R, S, and T.
ISDN channel types
ISDN organizes access capacity into logical digital channels. Each channel has a defined role for user information, signaling, or higher-rate transmission.
- B channel, bearer channel: This is the user-information channel. A circuit-switched bearer service provides and can carry digitized voice, computer data, or video.
- D channel, signaling channel: This is the signaling and control channel. Depending on the access type, it operates at or and carries information needed to set up, manage, and tear down B-channel calls, including dialing digits and caller ID. It can also carry low-rate packet data.
- H channels, higher-rate channels: These were defined for applications requiring more capacity than a single B channel. They provide bit rates of (H0), (H11), and (H12).
ISDN access: BRI and PRI
These channel structures supported two main ISDN access variants: BRI with fewer channels and PRI with more channels. Each variant addressed a different range of applications.
Basic Rate Interface (BRI)
BRI was intended for residential users and small businesses. Its two B channels allowed data or voice transmission without being limited to a single modem call.
2B + D16
- Two 64 kbit/s B channels for voice or data.
- One 16 kbit/s D channel for signaling.
- Total user-data bit rate: 128 kbit/s.
Primary Rate Interface (PRI)
PRI was intended for larger organizations. It bundles many B channels and connects digital PBXs to the public network.
North America/Japan: 23B + D64
Europe/Rest of World: 30B + D64
- 23 or 30 64 kbps B channels.
- One 64 kbit/s D channel for signaling.
- Total bit rate: 1.544 Mbit/s (T1) or 2.048 Mbit/s (E1).