Understanding Signals: Phase, Frequency, and Modulation
Signal: A sequence of voltage changes characterized by phase, amplitude, and frequency.
Phase: A measure of the relative position in time within a single period of a signal.
Amplitude: The instantaneous value of the signal at a given time.
Frequency: The number of repetitions per unit of time, measured in Hertz (Hz). It is the inverse of the period.
Hertz (Hz): Equivalent to one cycle per second.
Signal Spectrum: The range of signal values from minimum to maximum frequency.
Digital Signals: Discrete signals that take specific values.
Analog Signals: Continuous signals that can take an infinite number of values within an upper and lower bound.
Modulation: The process of changing the signal spectrum so it can be transmitted through the bandwidth of a transmission medium.
Bandwidth: The capacity of a channel to transmit a certain frequency spectrum, measured in bits per second (bps).
Multiplexing: The simultaneous transmission of multiple data streams over a single communications channel.
Modulation Techniques and Multiplexing
There are various techniques for modulating signals, including wave modulation and pulse modulation. Multiplexing involves sharing a physical channel to create multiple logical channels.
Types of Multiplexing:
- Frequency Division Multiplexing (FDM): Logical channels share a single physical channel by dividing the frequency spectrum.
- Time Division Multiplexing (TDM): Logical channels are assigned by allocating time slots for using the physical channel among different transmitters.
ISDN Reference Points
TR1: The boundary between the public and private network, isolating the user from the subscriber loop. It typically uses an RJ45 connector for connecting to a passive bus.
TR2: Performs functions such as switching, routing, or concentration. TR1 and TR2 refer to subscriber equipment used in ISDN networks.
AT: Provides compatibility for non-ISDN equipment.
Reference Points:
- R: The interface between non-ISDN computers and AT devices, or analog terminals.
- S: Defines the communication between a computer and ET1 (ISDN TR2). It is a 4-wire interface.
- T: Similar to S, representing the separation between user facilities and online processing equipment.
- U: Interfaces with signals on the local loop.
- V: Represents the separation between switching and transmission functions within the ISDN local exchange.
Passive Bus Configurations:
- Short Passive Bus: Up to 200m cable with 10 rosettes, connecting up to 8 terminals.
- Extended Passive Bus: Reaches 500m to 1000m, connecting up to 4 terminals within the last 50m.
TMA System Elements
CTM: Base stations that connect to the central fixed telephone network for establishing fixed telephone conversations. It includes a PC and a transmitter/antenna, connected to the CTM through a dedicated circuit.
EM: Mobile terminal stations.
RF Module: Handles all signals entering or leaving the mobile telephone channel, selected by the logic module.
AF Module: Converts the intermediate frequency signal from the RF module to a voice signal.
Logic Module: The base of the cellular telephone, using EPROM memory for storing information. The CPU directly controls the AF and RF modules, as well as the DTMF generator.