CH 12
Uploaded by
Susanta SarangiCH 12
Uploaded by
Susanta SarangiChapter 12
Multiple Access
12.1
Figure 12.1 Data link layer divided into two functionality-oriented sublayers
12.2
Figure 12.2 Taxonomy of multiple-access protocols discussed in this chapter
12.3
12-1 RANDOM ACCESS
In random access or contention methods, no station is
superior to another station and none is assigned the
control over another. No station permits, or does not
permit, another station to send. At each instance, a
station that has data to send uses a procedure defined
by the protocol to make a decision on whether or not to
send.
12.4
Figure 12.3 Frames in a pure ALOHA network
12.5
Figure 12.6 Frames in a slotted ALOHA network
12.6
Figure 12.8 Space/time model of the collision in CSMA
12.7
Figure 12.10 Behavior of three persistence methods
12.8
Figure 12.12 Collision of the first bit in CSMA/CD
12.9
Figure 12.16 Timing in CSMA/CA
12.10
Note
In CSMA/CA, if the station finds the
channel busy, it does not restart the
timer of the contention window;
it stops the timer and restarts it when
the channel becomes idle.
12.11
12-2 CONTROLLED ACCESS
In controlled access, the stations consult one another
to find which station has the right to send. A station
cannot send unless it has been authorized by other
stations. We discuss three popular controlled-access
methods.
12.12
Figure 12.19 Select and poll functions in polling access method
12.13
12-3 CHANNELIZATION
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.
In this section, we discuss three channelization
protocols.
12.14
Note
We see the application of all these
methods in Chapter 16 when
we discuss cellular phone systems.
12.15
Figure 12.21 Frequency-division multiple access (FDMA)
12.16
Note
In FDMA, the available bandwidth
of the common channel is divided into
bands that are separated by guard
bands.
12.17
Figure 12.22 Time-division multiple access (TDMA)
12.18
Note
In TDMA, the bandwidth is just one
channel that is timeshared between
different stations.
12.19
Note
In CDMA, one channel carries all
transmissions simultaneously.
12.20