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无线与接入网络
无线链路共享空间本身这一介质，按频率和地理区域划分；扩频技术将信号分散到各个频段以增强抗干扰能力。Wi-Fi无法检测冲突，因为发送方自身的信号会淹没其接收机，而隐藏节点又会使载波侦听失效，因此它借助RTS/CTS握手来避免冲突，并通过确认帧来确认传输成功。由分布系统组织的接入点锚定各个小区；通往家庭与个人的最后一公里，则由无源光网络（从中央终端分配传输时隙）和基于授权频谱构建的蜂窝网络共同提供。
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?