1. To is to impress the characteristics (intelligence) of one waveform onto a second waveform by varying the amplitude, frequency, phase, or other characteristics of the second waveform. 2.... is the means whereby a signal of some type is entered into and carried by an electronic signal carrier. 3. modulation describes a broadcast situation in which the level of voltage that is carried over the medium vary noticeably over time. 4. A signal in a communications channel modulated to carry analog or digital signal information is called a . 5. Communications inside a computer are examples of pure... communications.
TEXT 2. Multiplexing
9. :
multiplexing - ( ); (. ); (. c)
multiplexer - ; (); -
channel ['ʧæn(ə)l] - ( , - )
subchannel -
enable [ɪ'neɪbl ], [en-] - ; ,
frequency-division multiplexing (FDM) -
time -division multiplexing (TDM) -
subscriber [səb'skraɪbə] - , ;
tune [t(j)uːn ], [ʧuːn] - ; ;
access ['ækses] (n) ; (v)
wave-division multiplexing (WDM) - ;
slot - , ;
timeslot - ( , )
route [ruːt] - ( ; , . .- , )
Statistical Time Division Multiplexing (STDM) - ( )
statistical multiplexer - (, - (timeslot) )
Space division multiplexing (SDM) - ; -
phased-array antenna - ,
Code Division Multiplexing (CDM) -
code division multiple access (CDMA) -
spread-spectrum technology -
Universal Mobile Telecommunications System (UMTS) -
3G (third-generation) - ( )
Global Positioning System (GPS) - -
Time Division Multiple Access (TDMA) [] ,
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Frequency-Division Multiple Access (FDMA) -
10. :
Apart from modulation, another method to increase the transmission speed and convey more information is to divide the bandwidth of a signal carrier so that more than one modulated signal can be sent on the same carrier. This is known as multiplexing (or muxing). The aim is to reduce network costs by minimizing the number of communications links needed between two points.
In multiplexing, the carrier is sometimes referred to as a channel and each separate signal carried on it is called a subchannel. The device that puts the separate signals on the carrier and takes them off of received transmissions is a multiplexer, or a muxer.
Among various techniques that enable multiple channels to coexist on one link the most common are FDM (frequency-division multiplexing) and TDM (time -division multiplexing).
FDM is inherently an analog technology. It achieves the combining of several digital signals into one medium by sending signals in several distinct frequency ranges over that medium. Today's environment doesn't make great use of FDM, but it is still used extensively in cable TV and radio. Only one cable reaches a customer's home but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers. Receivers must tune to the appropriate frequency (channel) to access the desired signal. A variant technology, called wavelength-division multiplexing (WDM) is used in optical communications.
In digital transmission, signals are commonly multiplexed using TDM, in which two or more signals are transferred simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. A problem with TDM is that there is a one-to-one correlation between each port and time slot, so if the device attached to, for example, port 2 is out for the day, nobody else can make use of time slot 2. Hence, there is a tendency to waste bandwidth when vacant slots occur because of idle stations. However, this type of TDM is more efficient than standard FDM because more subchannels can be derived.
Statistical Time Division Multiplexing (STDM) is an advanced version of TDM in which both the address of the terminal and the data itself are transmitted together for better routing. A statistical multiplexer, or a stat mux, has more memory than other muxes, so if all the time slots are busy, excess data goes into a buffer. If the buffer fills up, the additional access data gets lost, so it is important to think about how much traffic to put through a stat mux in order to maintain performance variables. Dynamically allocating the time slots enables you to make the most efficient use of bandwidth.
Space division multiplexing (SDM) is nothing more than the provision of multiple fixed bandwidth channels by multiple physical paths (i.e., pairs of wires or optical fibers). A good example of SDM is the use of a 25-pair cable to carry the conversations of 25 individual users from the customer's premises to the local telephone companys central office location. Wired space-division multiplexing is typically not considered as multiplexing. In wireless communication, space-division multiplexing is achieved by multiple antenna elements forming a phased array antenna. Different antennas make it possible for digital signal processing techniques to separate different signals from each other.
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Code Division Multiplexing (CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. One of the basic concepts in data communication is the idea of multiple access allowing several transmitters to send information simultaneously over a single communication channel. To accomplish this CDM employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code). All channels, each with a different code, can be transmitted on the same fiber and asynchronously demultiplexed. CDM techniques as an access technology, namely Code Division Multiple Access (CDMA), are used in Universal Mobile Telecommunications System (UMTS) standard for the third generation (3G) mobile communication. Another important application of the CDMA is the Global Positioning System (GPS). Other widely used multiple access techniques are Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA).
11. :
coexist [ˌkəuɪg'zɪst] -
inherently [ɪn'her(ə)ntlɪ] - ; ;
take turns - -. , ,
recurrent [rɪkʌ̱rənt, AM -kɜ͟ːr-] -
excess [ɪk'ses ], [ek-] - (n) ; ;
(adj) ;
buffer ['bʌfə] -
allocate ['æləkeɪt] - ;
premises ['preməsəz] -
accomplish [ə'kɔmplɪʃ] - ,
12. :
1. What are the two methods to increase the transmission speed and convey more information? 2. Are the words the carrier and the channel interchangeable in multiplexing? 3. Name the most common techniques that enable multiple channels to coexist on one link. 4. Where is FDM still extensively used? 5. How are STDM and TDM connected? 6. What enables to use the bandwidth in case of STDM? 7. How is space-division multiplexing achieved in wireless communication?
13. , :
, , , , , , , ; ; , ; () ; () .
14. :
Communications link; frequency range; service provider; one-to-one correlation; waste bandwidth; performance variables;coded channel-specific sequence of pulses; digital signal processing techniques; assign a code.
15. :
1. One of the basic concepts... data communication is the idea of multiple access allowing several transmitters to send information simultaneously... a single communication channel. 2. various techniques that enable multiple channels to be sent one medium the most common are FDM and TDM. 3. FDM is still used extensively cable TV and radio. 4. Service providers can send multiple television channels only one cable all subsribers. 5. To access a desired channel you have to tune the appropriate frequency. 6. With TDM, the time domain is divided several recurrent timeslots... fixed length.
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