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Wireless and Access Networks
Wireless links share the medium of space itself, divided by frequency and geography, with spread spectrum techniques spreading signals across bands for resilience, Wi-Fi cannot detect collisions because a transmitter's own signal swamps its receiver and hidden nodes defeat carrier sense, so it avoids collisions with RTS/CTS handshakes and confirms success with acknowledgments, access points organized by a distribution system anchor the cells, and the last mile to homes and pockets is served by passive optical networks granting transmission slots from a central terminal and by cellular networks built on licensed spectrum.
1. Sharing the Airwaves
Sharing the Airwaves
2. Why Collision Detection Fails
Why Collision Detection Fails
3. Access Points and Mobility
Access Points and Mobility
4. The Last Mile
The Last Mile
5. The hidden node
Nodes A and C are both within range of base station B but not of each other. Both transmit to B at overlapping times. What happens, and why did carrier sense fail to prevent it?
6. Why RTS and CTS
What problem does the RTS/CTS handshake solve that carrier sense alone cannot?
7. The exposed node
Node B is transmitting to node A, and node C, which can hear B, wants to transmit to node D, which is out of B's reach. What does the exposed node problem say about C?
8. Following the mobile node
A laptop moves from one access point's cell to another's mid-session. Which mechanism keeps frames reaching it?
9. Who schedules the PON
In a passive optical network, why can't home units simply transmit upstream whenever they sense the fiber idle?
10. Licensed versus unlicensed
What fundamentally distinguishes a cellular network's spectrum use from Wi-Fi's?
11. The chipping code
In direct-sequence spread spectrum, each data bit is exclusive-ORed with an n-bit pseudorandom sequence known by what two-word name?
12. The acknowledgment rule
In 802.11, what does the receiver send back after a frame decodes cleanly and passes its CRC, substituting for the collision detection radios cannot perform?
13. PON fan-out
A passive optical network's tree of splitters can fan a single fiber out to as many as how many homes?
14. Frame address count
An 802.11 frame carries how many 48-bit addresses, reflecting its journey through access points and the distribution system?
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