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Fundamentals of Computer Networks ECE 478/578. Lecture #10: Multiple Access in Wireless Networks Instructor: Loukas Lazos Dept of Electrical and Computer Engineering University of Arizona. Wireless Access Protocols. Each node has a fixed range Nodes use omni-directional antennas - PowerPoint PPT Presentation
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Fundamentals of Computer NetworksECE 478/578
Lecture #10: Multiple Access in Wireless NetworksInstructor: Loukas Lazos
Dept of Electrical and Computer EngineeringUniversity of Arizona
Wireless Access ProtocolsEach node has a fixed rangeNodes use omni-directional antennas
Can we use CSMA?What about reservations?Problem: Feedback Mechanism
2
The Hidden Terminal Problem
3
A B C D
The Exposed Terminal Problem
4
A B C D
Multiple Access with Collision Avoidance
5
A B C DRTS
CTS
Addressing the Hidden Terminal Problem
Multiple Access with Collision Avoidance
6
A B C DCTS
RTS
RTS
CTS
Addressing the Exposed Terminal Problem
Distributed Coordination Function
7t
DIFS
data
defer access
otherstations
receiver
senderdata
DIFS
new contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
Senses channel for DIFS (Distributed Interframe Space)
If busy backoff random time, else
Sender sends RTS with NAV (Network Allocation Vector)
Receiver acknowledges via CTS after SIFS (Short interframe space)CTS reserves channel for sender, notifying possibly hidden stations;
any station hearing CTS should be silent for NAV
Sender can now send data at once
Can Collisions be useful?Zig-zag decoding
8
Implementation IssuesFinding the beginning of each packet in a collision
Use the preamble to find the start of each packet
9
Implementation IssuesObtaining matching collisions
Use the correlation to find the start of Pb, then use the header bits to ensure that Pb’ = Pb
Resolving more than two collisions
10
What if Multiple Channels are Available?
11
A B C DCTS (1)
RTS (1, 2, 5, 7)
ACK (1)
D CTS (2)
RTS (2, 5, 7)
ACK (2)
Possible SolutionForce everyone to switch to a common control channel for negotiation of channel assignmentDelay in the channel assignment, wasted bandwidthThroughput of control channel becomes a bottleneck
12