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Figure 8-1
Medium Access Methods
Contention method
No station is superior to another station
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Stations consult each other to find which station has the right to send.
A station cannot send unless it has been authorized by other stations.
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Random Access
• No scheduled time for a station to transmit. Transmission is random among the stations. ( Random access )
• No rules specify which station should send next. Stations compete with one another to access the medium. ( Contention )
Questions:
1. When should the station access the medium?
2. What should the station do if the medium is busy?
3. How should the station determine the success or failure of the transmission
4. What should the station do if there is an access conflict?
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Figure 8-2
Evolution of Random Access Methods
(ALOHA)
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MA: Multiple Access
CS: Carrier Sense
CD: Collision Detection
CA: Collision Avoidance
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Figure 8-3
ALOHA Network (Wireless LAN, 9600 bps, 1970)
(broadband modulation with a carrier freq. Of 407MHz)
( central controller, acts as a hub )
Simultaneous uploading transmissions will cause collisions.
Rules of the ALOHA protocol :
1. Multiple Access 2. No carrier sense 3. No checking for collision
4. Acknowledgment
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Figure 8-4
Procedure for
Pure ALOHA Protocol
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RTT: Round Trip Time
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Figure 8-5
Vulnerable Time for Pure ALOHA Protocol
T frame = time to transmit a frame (assuming each frame is of fixed length)
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Figure 8-6 Procedure for
Procedure for
Slotted ALOHA Protocol
Slotted ALOHA Network
Transmit only at the beginning of the time slot.
(Time is divided into slots of T frame )
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Figure 8-7
Vulnerable Time for
Slotted ALOHA Protocol
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CSMA
• CSMA: Carrier sense multiple access
• Based on the principle “sense before transmit or “listen before talk”
• Can reduce the possibility of collision
• Cannot eliminate it
Q: Why may there be a collision if each station listens to the medium before transmitting a frame?
A: Possibility of collision still exists because of propagation delay.
When a station sends a frame, it takes time for the first bit to reach every station and for every station to sense it.
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Figure 8-8 Collision in CSMA
(A finds the medium is idle)
(Z finds the medium is idle)
McGraw-Hill t
1
< t
2
< t
3
< t
4
< t
5
)
Vulnerable time: the propagation time (the time needed for the signal to propagate from one end of the medium to the other)
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Figure 8-9
Persistence Strategies
Define what a station should do if , when sensing the medium, it finds the medium busy
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Persistence Strategies
•
Nonpersistent :
A station with a frame to send senses the network.
If the network is busy, it waits a random time before sensing it again .
If not, it sends immediately
• 1-persistent :
A station with a frame to send continues sensing the network until it is idle. At that point, the station sends the frame immediately
(with probability 1).
• p-persistent :
A station with a frame to send continues sensing the network until it is idle. At that point, the station sends with probability p and refrains from sending with probability (1-p).
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Figure 8-10
Nonpersistent Method
Reduces the chance of collision
Reduces the efficiency of the network
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Figure 8-11
1-Persistent Approach
Attempts to minimize the idle time
Increases the chance of collision
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Figure 8-12 p-Persistent
Approach
M = 20 for p = 0.2
If R n < 20
 send
If R n >= 20
 wait
(1 <= Rn <= 100)
<
R
Attempts to minimize the idle time, while at the same time minimize the chance of simultaneous transmissions
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Figure 8-13
CSMA/CD Procedure
Defines what should be done if a collision occurs
Exp. backoff time = (max._prop_time) * (2^N)
N: the # of attempted transmissions
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Figure 8-14
CSMA/CA Procedure
This procedure avoids collision.
See Ch. 15 (pp. 333)
IFS: Inter-Frame Space
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Figure 8-15
Token Passing Network
(Use controlled Access method)
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Figure 8-16
Token
Passing Procedure
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