Discovery[ edit ] In BCE, Greek philosopher Aristotle suggested that nature abhors a vacuum, a principle that became known as the horror vacui. This concept built upon a 5th-century BCE ontological argument by the Greek philosopher Parmenideswho denied the possible existence of a void in space.
Types[ edit ] Multiple variable bit rate digital bit streams may be transferred efficiently over a single fixed bandwidth channel by means of statistical multiplexing.
This is an asynchronous mode time-domain multiplexing which is a form of time-division multiplexing. Digital bit streams can be transferred over an analog channel by means of code-division multiplexing techniques such as frequency-hopping spread spectrum FHSS and direct-sequence spread spectrum DSSS.
Space-division multiplexing[ edit ] In wired communication, space-division multiplexingalso known as Space-division multiple access is the use of separate point-to-point electrical conductors for each transmitted channel. Examples include an analogue stereo audio cable, with one pair of wires for the left channel and another for the right channel, and a multi-pair telephone cablea switched star network such as a telephone access network, a switched Ethernet network, and a mesh network.
In wireless communication, space-division multiplexing is achieved with multiple antenna elements forming a phased array antenna. Different antennas would give different multi-path propagation echo signatures, making it possible for digital signal processing techniques to separate different signals from each other.
These techniques may also be utilized for space diversity improved robustness to fading or beamforming improved selectivity rather than multiplexing. Frequency-division multiplexing[ edit ] Frequency-division multiplexing FDM: The spectrum of each input signal is shifted to a distinct frequency range.
Frequency-division multiplexing FDM is inherently an analog technology. FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium.
In FDM the signals are electrical signals. One of the most common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television. Only one cable reaches a customer's residential area, but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers without interference.
Receivers must tune to the appropriate frequency channel to access the desired signal. Time-division multiplexing TDM is a digital or in rare cases, analog technology which uses time, instead of space or frequency, to separate the different data streams.
TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver.
If done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path. Consider an application requiring four terminals at an airport to reach a central computer. Each terminal communicated at baudso rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers.
A pair of baud modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed. Orbital angular momentum multiplexing[ edit ] Orbital angular momentum multiplexing is a relatively new and experimental technique for multiplexing multiple channels of signals carried using electromagnetic radiation over a single path.
As of [update] it is still in its early research phase, with small-scale laboratory demonstrations of bandwidths of up to 2. One form is frequency hopping, another is direct sequence spread spectrum. In the latter case, each channel transmits its bits as a coded channel-specific sequence of pulses called chips.
Number of chips per bit, or chips per symbol, is the spreading factor. This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time.
All channels, each with a different code, can be transmitted on the same fiber or radio channel or other medium, and asynchronously demultiplexed.
Advantages over conventional techniques are that variable bandwidth is possible just as in statistical multiplexingthat the wide bandwidth allows poor signal-to-noise ratio according to Shannon-Hartley theoremand that multi-path propagation in wireless communication can be combated by rake receivers.Below is a very interesting read from the FBI Training Division, FBI Academy, Quantico, VA.
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Wenzel designs and builds HI-REL oscillators and systems for Space. Based on MIL-PRF, which defines the requirements for crystal controlled oscillators, Wenzel has shipped space-flight hardware for the Lunar and Mars missions, to visit near-earth asteroids and for tactical military missions.
Time-division multiplexing (TDM) is a digital (or in rare cases, analog) technology which uses time, instead of space or frequency, to separate the different data streams. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver.
Outer space, or just space, is the expanse that exists beyond the Earth and between celestial yunusemremert.com space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic yunusemremert.com .
JUNOS Enterprise Switching is the only detailed technical book on Juniper Networks' new Ethernet-switching EX product platform. With this book, you'll learn all about the hardware and ASIC design prowess of the EX platform, as well as the JUNOS Software that powers it.