Electronic Communication Systems
Trade & Technology
Quiz 13 :
Generally most communication systems such as cable television systems, telephone trunks and radio transmission systems share channels to transmit the signals from transmitter to receiver. Two techniques are used to sharing the channels for signal transmission, which are either multiplexing or multiple access systems. The main difference between multiplexing and multiple access scheme is in the multiplexing system all signals are transmitted from a single source and are passing through a single channel for transmission, whereas in multiple access system all signals are originated from different sources and are combined on a single channel for transmission.
In multiple access system all signals are originated from different sources and are combined on a single channel for transmission. There are three multiple access schemes are commonly used to share the channels for signal transmission. They are Frequency division multiple access (FDMA), Time division multiple access (TDMA) and Code-division multiple access (CDMA). In FDMA system the total frequency spectrums is divided into sub parts and each sub part is assigned to different users on full time basis. That means each user can uses their respective sub part spectrum at any time for transmission. In TDMA system the total frequency spectrum can be allocated to each and every user at a particular part of time. That means each user can uses entire frequency spectrum at a regular intervals of time instead of full time basis. The Spread-spectrum communication allow CDMA system, in which each transmitter is assigned with a different Pseudo-noise (PN) code and each pseudo noise sequence of transmitter is given to respective receiver to hop with the transmitter for transmission.
Multiplexing is the technique in which several analog or digital data signals are combined into one signal over a communal channel. In Multiple access method, there is a multipoint physical medium to which various terminals are connected. Now, these terminals can share the capacity of the medium by transmitting over it. (a) Amplitude Modulated broadcast station broadcast the radio signals using Amplitude modulation. In this, it uses multiplexing technique in which multiple radio signals at the same time pass through the air. For example, the frequency division multiplexing technique can be used for AM broadcast station, such that when numbers of channels are combined together and are transmitted over the same medium without interfering with each other. (b) Cable Television systems are used to provide television programs to various subscribers. Cable Television services are provided by transmitting the radio signals through optical fiber cable or coaxial cable. Cable television systems uses multiplexing technique in which on a single cable, multiple television channels can be transmitted. (c) Cable modem is a kind of modem, which acts as a network bridge that helps in providing bi-directional flow of data using radio frequency channels. Cable modem downstream signals are the signals received by the cable modem from the transmitter of the head end of the cable company. Cable modem downstream signals use multiple access technique. That is, consider a cable modem using time division multiple access technique for the downstream signals, then the users share the data on the channel using this technique, that is, by selecting the data which is addressed to them from the data stream. (d) Cable modem upstream signals are the signals transmitted by the cable modem towards the head end of the cable company. Cable modem upstream signals are the signals transmitted by the cable modem using multiplexing technique. It does not use multiple access technique because multiple access technique requires users to sense the transmission of other user.
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