ISDN Architecture
ISDN network architecture: functional groups, reference points, and network termination equipment.
The ISDN Protocol Reference Model
While ISDN predates the finalization of the full OSI model, its architecture is best understood using a similar layered approach. The ISDN model is structured into three distinct vertical planes, each responsible for a different aspect of communication, along with seven horizontal layers that mirror the OSI model's functions.
This is where the user's actual data flows. It handles the transmission of information through B and H channels. In a circuit-switched connection, the higher layers (2-7) of the U-plane represent a direct, transparent pipe between the end-user devices (TEs).
This plane contains the functions for call control. It manages the setup and teardown of connections between users, controls supplementary services, and handles information exchange between the user and the network's control center. All this signaling information travels on the D channel.
This plane oversees the entire system. Each layer within the M-plane manages its corresponding layer in the U and C planes, handling functions like fault detection, performance monitoring, and configuration.
Physical Bus Configurations (S/T Interface)
The ISDN standard provides flexibility in how end-user devices are physically connected to the network termination unit (NT1/NT2) at the S/T reference point.
- Point-to-Point Configuration: This is the simplest setup where a single terminal (TE) is connected to the NT. Because timing is straightforward, this configuration allows for the greatest distance, typically up to 1 km using standard 0.6mm copper cable. The bus must be terminated at both ends with 100 惟 resistors to prevent signal reflections.
- Short Passive Bus: This configuration allows up to 8 ISDN devices to be connected anywhere along a shared 4-wire bus. However, the timing constraints caused by signals arriving from different points on the bus limit its maximum length to a range of 100 to 200 meters.
- Extended Passive Bus: A special configuration where up to 8 devices are clustered together at the far end of the bus. This arrangement minimizes timing differences between the terminals, allowing the bus length to be extended to about 500 meters.
Physical Layer Transmission (U Interface)
A major technical challenge for ISDN was to achieve full-duplex digital transmission over the existing single copper pair of the local loop (the U interface). Two primary methods were developed to accomplish this.
- Time-Division Method ("Ping-Pong"): In this method, the line is used for transmission in only one direction at a time. The NT and LT devices take turns sending short bursts of data at a rate at least double the required user data rate. A guard time is inserted between bursts to allow for signal propagation delay. This method is simpler to implement but requires a higher transmission speed, which can increase sensitivity to noise.
- : This more advanced technique allows for simultaneous transmission in both directions (full duplex). Each device is equipped with a hybrid coil (rozga艂臋藕nik) and an adaptive filter. The filter constantly models the echo of the device's own transmitted signal and subtracts it from the incoming signal, leaving only the signal from the remote end. This is the more commonly used method in public networks due to its efficiency.
