Forouzan _ Multiple access protocols.ppt

12.1
Chapter 12
Multiple Access
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

12.2
Figure 12.1 Data link layer divided into two functionality-oriented sublayers

12.3
IEEE 802 LAN ARCHITECTURE

12.4
IEEE 802 LAN
• Physical layer
•Physical layer deals with actual transmission and
reception of bits over the transmission medium
•MAC (Medium Access Control
•Its concerned with the sharing of physical
connection among several computers. MAC is a
protocol for accessing high speed internet links and
for transferring data frames from one station to
another.

12.5
IEEE 802 LAN
LLC( Logical Link Control)
•Provides a service interface
•Protocols on network layer uses the services
provided by LLC layer.
•Concerned with managing traffic(flow and error
control)

12.6
Figure 12.2 Taxonomy of multiple-access protocols discussed in this chapter

12.7
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.
ALOHA
Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance
Topics discussed in this section:

12.8
Two features:
• First, there is no schedules time for a station to
transmit. Transmission is random. Hence they are
called ransom access methods.
•Second, no rules specify which station should send
next. Stations compete with one another to access the
medium. That is why these methods are called
contention methods.

12.9
ALOHA
• Earliest random access method developed in 1970’s.
•When a station sends data, another station may
attempt to do so at the same time. The data from the
two stations collide and become garbled.

12.10
PURE ALOHA
• The original ALOHA is called Pure ALOHA.
•The idea is each station sends a frame, whenever it has
a frame to send.
•However, since there is only one channel to share,
there is a possibility of collision.
.

12.11
Figure 12.3 Frames in a pure ALOHA network

12.12
PURE ALOHA
• We need to resend the frames that have been destroyed
during transmission.
•Pure ALOHA relies in acknowledgement from the
receiver.
•When a station sends a frame, it expects the receiver to
send back an acknowledgment.
•If the acknowledgment does not arrive after a time-out
period, the station assumes that the frame has been
destroyed and resends the frame.

PURE ALOHA
• Pure ALOHA dictates that when the time-out period
passes, each station waits for a random amount of time
before resending the frame. This time is called back-off
time Tb.
•Pure ALOHA has a second method to prevent
congesting the channel with transmitted frames.
•After a maximum number of retransmission attempts
Kmax, a station must give up and try later.
•The time-out period is equal to the maximum possible
round-trip propagation delay, which is twice the amount
of time required to send a frame between two most
widely seperated stations(2*Tp)

12.14
Figure 12.4 Procedure for pure ALOHA protocol

12.15
Figure 12.5 Vulnerable time for pure ALOHA protocol

12.16
The throughput for pure ALOHA is
S = G × e −2G .
The maximum throughput
Smax = 0.184 when G= (1/2).
Note

SLOTTED ALOHA
• Pure ALOHA has a vulnerable time of 2Tfr. This is
because there’s no rule that defines when a station can
send. Slotted ALOHA improves the efficiency of Pure
ALOHA.
•In Slotted ALOHA, we divide the time into slots of Tfr
seconds and force the station to send only at the
beginning of the time slot.
•Because the station is allowed to send only at the
beginning of a slot, if the station misses that slot, then it
must wait until the beginning of the next slot.

12.18
Figure 12.6 Frames in a slotted ALOHA network

SLOTTED ALOHA
• There’s still a possibility of collision, is two stations try
to send at the beginning of the same slot. However, the
vulnerable time is now reduced to half , equal to Tfr.
.

12.20
The throughput for slotted ALOHA is
S = G × e−G .
The maximum throughput
Smax = 0.368 when G = 1.
Note

12.21
Figure 12.7 Vulnerable time for slotted ALOHA protocol

CSMA
• To minimize the chance of collision and to increase the
performance, the CSMA method was developed.
•Carrier Sense Multiple Access(CSMA) requires that
each station first listen to the medium (check the state of
the medium) before sending. This is based on the
principle “sense before transmit "or “listen before talk”.
•The possibility of collision still exist because of
propagation delay.
.

CSMA
• A station may sense the medium and find it idle, only
because the first bit send by another station has not yet
been received.
•When a station sends a frame or any other station tries
to send a frame during this time, a collison will result.
But if the first bit of the frame, reaches the end of the
medium, every station will already have sensed the bit
and will refrain from sending.
.

12.24
Figure 12.9 Vulnerable time in CSMA

Persistence Methods
• What should a station do if the channel is busy?
•What should a station do if the channel is idle?
•3 methods have been devised to answer these questions:
•The 1-persistent method
•The non-persistent method
•The p-persistent method
.

1-persistent method
•In this method, after the station finds the line idle, it
sends its frame immediately (with probability 1). This
method has the highest chance of collision because two
or more stations may find the line idle and send their
frames immediately.
.

Non persistent method
•In this method, a station that has a frame to send senses
the line. If the line is idle, it sends immediately.
•If not, it waits a random amount of time and then senses
the line again.
•This method reduces the chance of collision because its
unlikely that two or more stations will wait the same
amount of time and retry simultaneously.
•Reduces the efficiency of the network because the
medium remains idle while there maybe stations to send.

12.28
Figure 12.10 Behavior of three persistence methods

P- persistent method
•This method is used if the channel has time slots with a
slot duration equal to or greater than the maximum
propagation time. It reduces the chances of collision and
improves efficiency.
•In this method, after a station finds the line idle, it does
the following:
•With probability p, the station sends its frame.
•With probability q=1-p, the station waits for the
beginning of next time slot and checks the line again.

12.30
Figure 12.12 Collision of the first bit in CSMA/CD

12.31
Figure 12.13 Collision and abortion in CSMA/CD

12.32
Figure 12.14 Flow diagram for the CSMA/CD

12.33
Figure 12.15 Energy level during transmission, idleness, or collision

12.34
Figure 12.16 Timing in CSMA/CA

12.35
In CSMA/CA, the IFS can also be used to
define the priority of a station or a frame.
Note

12.36
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.
Note

12.37
Figure 12.17 Flow diagram for CSMA/CA

12.38
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.
Reservation
Polling
Token Passing
Topics discussed in this section:

12.39
Figure 12.18 Reservation access method

12.40
Figure 12.19 Select and poll functions in polling access method

12.41
Figure 12.20 Logical ring and physical topology in token-passing access method

Forouzan _ Multiple access protocols.ppt

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