NRZ, MDT, MDF, ISDN, and Passive S0 Bus Explained
NRZ, MDT, MDF, ISDN, and Passive S0 Bus
NRZ (Non-Return-to-Zero): One of the drawbacks of NRZ is that the signal must be accompanied by synchronization pulses, i.e., the clock signal. Without this, it’s impossible to determine bit synchrony. Therefore, a separate channel is required exclusively for the clock signal, leading to resource wastage.
Symbol Distortion: Bandwidth limitation and the transmission of digital information require significant bandwidth. Because the signal is square, it necessitates a large bandwidth, and since lines have limited bandwidth, symbol distortion occurs. This causes what are known as “queues.” This problem occurs on all lines.
MDT (Time-Division Multiplexing): MDT is used to multiplex different digital signals. The principle is to exploit the delay between signals, merging parts of other signals and sending the complete sequence through the same transmission line. At the receiving end, the different signals are separated in time. Using this technique, it’s possible to combine different digitized voice conversations and send them through the same channel.
MDF (Frequency-Division Multiplexing): MDF is based on the modulation of different signals to be transmitted, thus occupying different frequency bands. These are then sent together through the same transmission channel without interference. At the receiving end, separate bandpass filters, each tuned to the frequencies of the different signals, are used to separate the signals. These are then demodulated, thus recovering the original information.
The ISDN (Integrated Services Digital Network) System: ISDN is based on the idea of a digital channel, which is simply a digital path through which bits flow from the customer to the telephone operators and vice versa. There are two different standards based on different speeds for the bit pipe: one designed for users with low transmission needs and another for users requiring high bandwidth.
To structure the possible functions at the user level and simultaneously provide an idea of the physical configuration, functional groups (TR1, TR2, NT1, NT2, and TA) are defined. These are sets of functions that may be needed for user access to ISDN. These functions can be performed by one or more parts of a computer. Reference points (R, S, T, U, and V) are conceptual points that divide the functional groups. In a given access, a reference point may correspond to a physical interface between different computers, or this physical interface may not exist.
The Passive S0 Bus: A key feature of the reference point S in basic access is the ability to connect up to eight devices, each with its own phone number. Only two can use the line simultaneously, as the two channels are multiplexed on the bus, each with 64 kbit/s independent. There are several types of passive buses:
- Short Passive Bus: A cable of up to 200 meters can be used, with up to 10 rosettes installed, allowing connection of up to 8 terminals.
- Extended Passive Bus: The bus can reach 500 to 1,000 meters, and only 4 terminals can be connected, all within the last 50 meters of the bus length.
- Point-to-Point Bus: This can reach 1,000 meters and allows only one terminal to be connected. A termination resistor must be installed at the rosette.