Chapter 8 Medium Access Methods
Figure 8-1 Medium Access Methods Contention method 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. No station is superior to another station
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: • When should the station access the medium? • What should the station do if the medium is busy? • How should the station determine the success or failure of • the transmission • 4.What should the station do if there is an access conflict?
Figure 8-2 Evolution of Random Access Methods (ALOHA) MA: Multiple Access CS: Carrier Sense CD: Collision Detection CA: Collision Avoidance
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
Figure 8-4 Procedure for Pure ALOHA Protocol RTT: Round Trip Time
Figure 8-5 Vulnerable Time for Pure ALOHA Protocol Tframe = time to transmit a frame (assuming each frame is of fixed length)
Figure 8-6 Procedure for Slotted ALOHA Protocol Procedure for Slotted ALOHA Network Transmit only at the beginning of the time slot. (Time is divided into slots of Tframe)
Figure 8-7 Vulnerable Time for Slotted ALOHA Protocol
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.
Figure 8-8 Collision in CSMA (A finds the medium is idle) (Z finds the medium is idle) t1 < t2 < t3 < t4 < t5) Vulnerable time: the propagation time (the time needed for the signal to propagate from one end of the medium to the other)
Figure 8-9 Persistence Strategies Define what a station should do if, when sensing the medium, it finds the medium busy
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).
Figure 8-10 Nonpersistent Method Reduces the chance of collision Reduces the efficiency of the network
Figure 8-11 1-Persistent Approach Attempts to minimize the idle time Increases the chance of collision
Figure 8-12 p-Persistent Approach M = 20 for p = 0.2 If Rn< 20 send If Rn>= 20 wait (1 <= Rn <= 100) < R Attempts to minimize the idle time, while at the same time minimize the chance of simultaneous transmissions
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
Figure 8-14 CSMA/CA Procedure This procedure avoids collision. See Ch. 15 (pp. 333) IFS: Inter-Frame Space
Figure 8-15 Token Passing Network (Use controlled Access method)
Figure 8-16 Token Passing Procedure