Channelization
This is another method to share available bandwidth of a channel
among multiple stations. Here we discuss three channelization protocols: FDMA,
TDMA, and CDMA.
Frequency Division Multiple Access
In frequency division multiplexing the available bandwidth
is divided into different frequency bands. And each station allocate a specific
frequency band for send data. Guard bands are used to avoid station
interferences between adjacent stations. FDMA allows continuous stream of data
that data may not be packetized. There is some difference between Frequency
division multiplexing (FDM) and Frequency Division Multiple Access (FDMA); FDM
is a physical layer method to combine low-bandwidth channels, but FDMA is an
access method in the data link layer.
Time Division Multiple Access
In TDMA, bandwidth of the single channel is time shared
among multiple stations. Each station can only send data at their allocated
time slot. As above, Time Division Multiplexing is different from Time division
multiple
access; in TDM collect data from slower channels and combine then send
through a faster channel. TDM is a physical layer techniques and it use
multiplexers. TDMA is a data link technique, and it tell to the physical layer
to use a specific allocated time slot. Code Division Multiple Access
CDMA allows to send all channels simultaneously. In CDMA
each station is assigned a code. The code is a sequence of numbers and calls
them chips. Codes allocated to
different stations are unique, and the codes are assigned in an orthogonal
sequence manner with the following properties; to generate chip sequences we
use Walsh table.
- Each code contains N elements, where N is the number of stations that use the channel. For example, consider a channel with four stations, we can assign the following code to a station; [+1 +1 +1 +1]
- If we multiply, a number in a sequence for some reason then must need to multiply all numbers in that sequence by that number. This process is known as multiplication of a sequence by a scalar.
2*[+1 +1 +1 +1] = [+2 +2 +2 +2]
- If we multiply same sequences, element by element and add the results (called inner product of two equal sequence) then we get total number of stations in the network,
[+1 +1 +1 +1] * [+1 +1 +1 +1] = 1 1
1 1 add the result 1+1+1+1=4
- If we multiply two different sequences, element by element and add the results (called inner product of two different sequence) then we get 0.
[+1 +1 +1 +1] * [+1 +1 -1 -1] = 1 1
-1 -1 add the results 1+1-1-1 = 0
- If add two sequences the get another code
[+1 +1 +1 +1] + [+1 +1 -1 -1] = [+2 +2 0 0]
To send data by using CDMA, the
data must be encoded as follow: 0 is encode to -1,
1 is encode to +1 and if the
station is idle then it represent by a 0.
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I am not very clear about the difference between FDMA and FDM and similarly TDMA and TDM. How is operation at the physical layer different from operation at the data link layer? Is it possible for u to explain a bit more?
ReplyDeleteFDM, is a physical layer technique that combines the loads from low-bandwidth channels and transmits them by using a high-bandwidth channel. 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. FDMA, 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 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 common channel.
DeleteTDM 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.
DeleteTDMA, 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.
I think your doubts are clear now, If not please comment here or wait for our advanced networks and communication course. There we will discuss all topics in a problem oriented manner. I think that the problem oriented study is not necessary for UGC NET but it need for GATE related exams.
DeleteAnyway Thanks for your comment.
Thank you. I understand what you were saying better now.
ReplyDelete