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 Can we use CSMA? What about reservations? Problem: Feedback Mechanism

The Hidden Terminal Problem C D

The Exposed Terminal Problem C D

Multiple Access with Collision Avoidance RTS A B C D CTS Addressing the Hidden Terminal Problem

Multiple Access with Collision Avoidance RTS CTS A B C D CTS RTS Addressing the Exposed Terminal Problem

Distributed Coordination Function 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 DIFS RTS data sender SIFS CTS SIFS receiver NAV (RTS) DIFS data other stations NAV (CTS) t defer access new contention

Can Collisions be useful? Zig-zag decoding

Implementation Issues Finding the beginning of each packet in a collision Use the preamble to find the start of each packet

Implementation Issues Obtaining 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

What if Multiple Channels are Available? ACK (1) ACK (2) D A B C D CTS (1) CTS (2) RTS (1, 2, 5, 7) RTS (2, 5, 7)

Possible Solution Force everyone to switch to a common control channel for negotiation of channel assignment Delay in the channel assignment, wasted bandwidth Throughput of control channel becomes a bottleneck