Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. The three channelization protocols are FDMA, TDMA, and CDMA

1. COMPUTER NETWORKS
MULTIPLE ACCESS - CHANNELIZATION
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2. OVERVIEW
• Multiple Access
• Random access protocol
• Controlled access protocol
• Channelization protocol
• FDMA
• TDMA
• CDMA
• Reference
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3. Multiple Access
• If there is a dedicated link between the sender and the receiver then
data link control layer is sufficient, however if there is no dedicated
link present then multiple stations can access the channel
simultaneously.
• Decrease collision and avoid crosstalk.
• For example,
In a classroom full of students, when a teacher asks a question
and all the students (or stations) start answering simultaneously
(send data at same time) then a lot of chaos is created( data
overlap or data lost) then it is the job of the teacher (multiple
access protocols) to manage the students and make them
answer one at a time.
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4. TAXONOMY
Fig 1: Taxonomy of Multiple access protocol
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5. RANDOM ACCESS PROTOCOL
• No station is superior to another station
• None is assigned the control over another
• At any instance, a station sends data by following a protocol to make
decision whether to send or not
• It depends on state of medium (idle or busy)
• As there is no scheduled time for station to transmit and as
transmission is among stations, this method is called random access
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6. CONTROL ACCESS PROTOCOL
• The stations consult one another to find station has right to send
• A station cannot send unless it has been authorized by other stations.
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7. CHANNELIZATION
• A multiple access method in which the available bandwidth of a link is
shared in time, frequency or through code, between different stations
• Three Channelization protocol
 Frequency division multiple access
 Time division multiple access
 Code division multiple access
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8. Frequency Division Multiple Access(FDMA)
• Available bandwidth is divided into frequency bands
• Each station is allocated with a band to send its data
• Each band is reserved for specific station all the time
• Each station uses a bandpass filter to confine transmitter frequencies
• In order to avoid station interferences, allocated bands are separated
from one another by small guard bands.
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9. • FDMA specifies a predetermined
frequency band for entire period
of communication
• Stream data (a continuous flow
of data that may not be
packetized) can be easily used
with FDMA
Fig 2: Frequency division multiple access
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10. FDM FDMA
Is a physical layer technique Is a data link layer technique
Combine load from low bandwidth
channels and transmits them by
using a high bandwidth channel
Each station tells its physical layer to
make a bandpass signal from the
data passed to it
The channels that are combined the
low pass
The signal must be created in the
allocation band.
The multiplexer modulates the signal
combines them and creates a
bandpass signal
There is no physical multiplexer at
the physical layer. the signal created
at each station are automatically
bandpass filtered.
The bandwidth of each channel is
shifted by the multiplexer
The bandwidth are mixed when they
are sent to a common channel
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11. TIME DIVISION MULTIPLE ACCESS(TDMA)
• Time division multiple access is a channel access method for
shared medium networks.
• It allows several users to share the same frequency.
• Each station is allocated a time slot during which it can send
data.
• Each station transmits its data in is assigned time slot.
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12. Fig 3:
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13. CONTD,..
• The main problem with TDMA is 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 is difficult because of propagation delays introduced in the
system if the stations are spread over large area.
• To compensate this we insert guard times.
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14. • The process uses a physical multiplexer that interleaves data units
from each channel.
• 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.
CONTD,..
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15. EXAMPLES OF TDMA
• IS-136
• Personal digital cellular(PDC)
• Integrated digital enhanced network
• 2G-second generation 3G based on CDMA
• Universal terrestrial radio access(UTRA)
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16. CODE-DIVISION MULTIPLE ACCESS (CDMA)
• Code-division multiple access (CDMA) was conceived several decades
ago. Recent advances in electronic technology have finally made its
implementation possible.
• CDMA differs from FDMA because only one channel occupies the
entire bandwidth of the link.
• It differs from TDMA because all stations can send data
simultaneously; there is no timesharing

17. ANALOGY
• CDMA simply means communication with different codes.
• For example,
In a large room with many people, two people can talk in
English if nobody else understands English. Another two people
can talk in Chinese if they are the only ones who understand
Chinese, and so on. In other words, the common channel, the
space of the room in this case, can easily allow communication
between several people, but in different languages (codes).

18. Idea
• Let us assume we have four stations 1, 2, 3, and 4 connected to the
same channel. The data from station 1 are dl , from station 2 are d2,
and so on. The code assigned to the first station is cI, to the second is
c2, and so on. We assume that the assigned codes have two
properties
1. If we multiply each code by another, we get O.
2. If we multiply each code by itself, we get 4 (the number of stations).
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19. • Station 1 multiplies its data by its code to get dl .
Cl' Station 2 multiplies its data by its code to get d2
. c2' And so on.
• The data that go on the channel are the sum of all
these term. Any station that wants to receive data
from one of the other three multiplies the data on
the channel by the code of the sender.
• For example, suppose stations 1 and 2 are talking
to each other. Station 2 wants to hear what station
I is saying. It multiplies the data on the channel by
cl' the code of station 1.
• Because (cl . cl) is 4, but (c2 . cI), (c3 . cI), and (c4 .
cl) are all Os, station 2 divides the result by 4 to
get the data from station 1.
Fig 4: Simple idea of communication with code
data =(d) . Cj + dz . Cz +d3 . C3 + d4 . c4) . Cl
=d j . Cl . Cj +dz.Cz . Cl + d3 . C3 . Cl + d4 . C4' CI
=4 X d1
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20. CHIPS
• CDMA is based on coding theory. Each station is assigned a code,
which is a sequence of numbers called chips.
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21. Chip sequences
• I [+1 +1 +1 +11 I 1[+1 -1 +1 -I) I I [+1 + -I - 11 I I [+ -1 -1 +IJ I
• Each sequence is made of N elements, where N is the number of
stations.
• If we multiply a sequence by a number, every element in the
sequence is multiplied by that element. This is called multiplication of
a sequence by a scalar. For example,
[+1 +1-1-1]=[+2+2-2-2]
• If we multiply two equal sequences, element by element, and add the
results, we get N, where N is the number of elements in the each
sequence. This is called the inner product of two equal sequences.
For example,
[+1 +1-1 -n· [+1 +1 -1 -1] = 1 + 1 + 1 + 1 = 4
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22. • If we multiply two different sequences, element by element, and add
the results, we get O. This is called inner product of two different
sequences. For example,
[+1 +1 -1 -1] • [+1 +1 +1 +1] = 1 + 1- 1- 1= 0
• Adding two sequences means adding the corresponding elements.
The result is another sequence. For example,
[+1+1-1-1]+[+1+1+1+1]=[+2+2 00]
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23. DATA REPRESENTATION
Rules of encoding
• If station sends a 0 bit, it encodes it as -1
• If station sends a 1 bit, it encodes it as +1
• When a system is idle, it send no signal i.e. 0
Fig 5: Data representation
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24. ENCODING AND DECODING
Consider station3 is
silent , and its listening
to station2.
Station 3 multiplies the
total data on the
channel by code for
station2
Fig 6: Sharing channel in CDMA
0
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25. SIGNAL LEVEL
• The total data on
channel are multiplied
by the signal
representing station 2
chip code to get a new
signal
• The station then
integrates and adds the
area under signal to get
the value -4, which
divided by 4 and
interpreted as bit0
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26. Multiple Access Control 26

27. SEQUENCE GENERATION
• Walsh table is used, which is two dimensional table (n*n matrix)
• Each row is a sequence of chips
• Values assigned are either +1 or -1
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28. PROBLEM
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29. REFERENCES
• Forouzan A Behrouz, Data Communications and Networking. New
York,2007,pp 363-390
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30. Thank
You!
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