ISDN protocol reference model
ISDN uses the layered protocol model described in ITU-T Recommendation I.320. It distinguishes the User, Control, and Management planes. The U and C planes can each contain stacks of up to seven layers, and some layers may be empty. A protocol block can omit layers that its service does not require. It is a descriptive model and does not prescribe a particular device or network implementation.
User data crosses this plane. B and H channels carry the information with the parameters of the selected service. In a circuit-switched connection, the higher layers (2-7) of the U-plane support transparent transfer between the end-user terminals (TEs).
This plane contains call-control functions. It manages connection setup and release, supports supplementary services, and exchanges information between the terminal and the network. In a typical user-network interface, these signaling messages travel on the D channel.
The M-plane coordinates the system and its layer-management functions. It includes fault detection, performance monitoring, and configuration of functions in the U and C planes.
Physical S/T bus configurations
The S/T interface connects end-user equipment with NT1 or NT2 network-termination functions. ITU-T I.430 describes point-to-point wiring and point-to-multipoint wiring with a passive bus.
- Point-to-point configuration: This is the simplest setup, with one terminal (TE) connected to the NT. The general operational-distance objective is 1 km with a typical 0.6 mm copper conductor. The connection is terminated at both ends with 100 Ω resistors to limit signal reflections.
- Short passive bus: Up to 8 ISDN terminals can be connected at different points on a shared four-wire bus. Different pulse arrival times limit the operational distance to about 100 to 200 meters. I.430 associates 100 meters with a 75 Ω cable and 200 meters with a 150 Ω cable.
- Extended passive bus: The terminal connection points are grouped at the far end of the cable. The typical electrical model includes 4 TEs, a 25 to 50 meter separation between connection points, and a total length of at least 500 meters. Administrations can set the number of terminals and cable length.
Physical-layer transmission on the U interface
The U interface provides bidirectional digital transmission over one copper pair of the local loop. ITU-T Recommendation G.961 describes two methods: echo cancellation (ECH) and time compression multiplex (TCM).
- Time compression multiplex (TCM, or “ping-pong”): The line carries data in only one direction at a time. NT and LT send short bit blocks alternately, and the line rate must be greater than twice the user-stream rate. Buffers keep the input and output streams continuous, while a guard interval between blocks allows for propagation delay and transmitter-receiver turnaround. This method avoids simultaneous transmission, but the line must run at a higher rate.
- : ECH provides simultaneous transmission in both directions. Each transceiver uses a hybrid to separate transmit and receive paths and an adaptive filter that models the local transmitter echo. Subtracting that model leaves the far-end signal. TCM and ECH describe different operating methods for the U line system; the implementation determines the choice.