3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.9 ISDN components and reference points
ISDN components include terminals, terminal adapters (TAs), network-termination devices, line-termination equipment, and exchange-termination equipment. ISDN terminals come in two types (the figure to the left 1). Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals, such as data terminal equipment (DTE) that predate the ISDN standards, are referred to as terminal equipment type 2 (TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN network through a TA. The ISDN TA can be either a standalone device or a board inside the TE2. If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical-layer interface. Examples include EIA/TIA-232-C (formerly RS-232-C), V.24, and V.35.

Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the Network Termination type 1 (NT1) or the Network Termination type 2 (NT2) device. These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North America, the NT1 is a customer premises equipment (CPE) device. In most other parts of the world, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device; typically found in digital private branch exchanges (PBXs), the NT2 performs Layer 2 and 3 protocol functions and concentration services. An NT1/2 device also exists as a single device that combines the functions of an NT1 and an NT2.

ISDN specifies numerous reference points that define logical interfaces between functional groupings, such as TAs and NT1s. ISDN reference points include the following:

  • R—The reference point between non-ISDN equipment and a TA
  • S—The reference point between user terminals and the NT2
  • T—The reference point between NT1 and NT2 devices
  • U—The reference point between NT1 devices and line-termination equipment in the carrier network; the U reference point is relevant only in North America, where the NT1 function is not provided by the carrier network.

The figure to the left illustrates a sample ISDN configuration and shows three devices attached to an ISDN switch at the central office. Two of these devices are ISDN compatible, so they can be attached through an S reference point to NT2 devices. The third device (a standard, non-ISDN telephone) attaches through the reference point to a TA. Any of these devices also could attach to an NT1/2 device, which would replace both the NT1 and the NT2.

Although ISDN is often described as a circuit-switched facility, it can support circuit-switched, packet-switched, and even semi-permanent connections. The reason why ISDN is capable of such flexibility is relatively simple: the B channels are circuit-switched, whereas the D channel is packet-switched.