CSE 313 Data Communication Lec 15 Sabbir Muhammad Saleh 1 CHANNELIZATION Channelization (or channel partition, as it is sometimes called) is a multiple-access
method in which the available bandwidth of a link is shared in time, frequency, or through code, among different stations. In this section, we discuss three channelization protocols: FDMA, TDMA, and CDMA. 2 FDMA In frequency-division multiple access (FDMA),
the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data. In other words, each band is reserved for a specific station, and it belongs to the station all the time. Each station also uses a bandpass filter to confine the transmitter frequencies. To prevent station interferences, the allocated bands are separated from one another by small guard bands. Figure 12.21 shows the idea of FDMA.
3 FDMA 4 FDMA FDMA specifies a predetermined frequency band for the entire period of communication. This means that stream data (a continuous
flow of data that may not be packetized) can easily be used with FDMA. 5 FDMA We need to emphasize that although FDMA and frequency-division multiplexing (FDM) conceptually seem similar, there are
differences between them. FDM is a physical layer technique that combines the loads from lowbandwidth channels and transmits them by using a highbandwidth channel. 6 FDMA The channels that are combined are low-pass.
The multiplexer modulates the signals, combines them, and creates a bandpass signal. The bandwidth of each channel is shifted by the multiplexer. 7 FDMA FDMA, on the other hand, is an access method in the data-link layer. The datalink layer in each
station tells its physical layer to make a bandpass signal from the data passed to it. The signal must be created in the allocated band. There is no physical multiplexer at the physical layer. The signals created at each station are automatically bandpass-filtered. They are mixed when they are sent to the commonchannel. 8 TDMA
In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time. Each station is allocated a time slot during which it can send data. Each station transmits its data in its assigned time slot. Figure 12.22 shows the idea behind TDMA. 9 TDMA
10 TDMA The main problem with TDMA lies in achieving synchronization between the different stations. Each station needs to know the beginning of its slot and the location of its slot. This may be difficult because of propagation delays introduced in the system if the stations are spread over a large area.
To compensate for the delays, we can insert guard times. Synchronization is normally accomplished by having some synchronization bits (normally referred to as preamble bits) at the beginning of each slot. 11 TDMA We also need to emphasize that although TDMA and time-division multiplexing (TDM)
conceptually seem the same, there are differences between them. TDM, as we saw before, is a physical layer technique that combines the data from slower channels and transmits them by using a faster channel. The process uses a physical multiplexer that interleaves data units from each channel. 12 TDMA
TDMA, on the other hand, is an access method in the data-link layer. The data-link layer in each station tells its physical layer to use the allocated time slot. There is no physical multiplexer at the physical layer 13
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