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以太网与碰撞检测
以太网是典型的多路访问链路，作为一种共享介质，每个适配器都能听到每一帧，由访问算法决定谁可以发送；其帧携带同步前导码、全球唯一的48位地址、解复用类型字段以及32位CRC；其发送器采用1-坚持策略，并在发送过程中检测碰撞；最坏情况下的碰撞可能需要一个完整的往返时间才会显现，因此帧被填充至512位；重复碰撞通过指数退避化解；现代交换式以太网已淘汰该算法，但其后代仍在无线网络中延续。
1. A Shared Medium
A Shared Medium
2. The Transmitter Algorithm
The Transmitter Algorithm
3. The Worst Case
The Worst Case
4. Exponential Backoff
Exponential Backoff
5. Hearing every frame
Why does an Ethernet adaptor need a destination address check at all, given that frames are delivered point to point on modern links?
6. The 1-persistent choice
An Ethernet adaptor has a frame queued and senses the line busy. What does the 1-persistent algorithm do when the line goes idle?
7. The jamming sequence
Upon detecting a collision, why does an adaptor transmit a 32-bit jamming sequence instead of simply stopping?
8. The worst-case window
Host A transmits at time t on a maximal Ethernet with one-way latency d. Host B starts at t+d, just before A's signal arrives. When does A learn of the collision, and what follows?
9. Why randomize
After two stations collide, why does each pick its retry delay at random from an interval that doubles per collision, rather than waiting a fixed slot?
10. Switched Ethernet today
Why does the collision detection algorithm matter little on a modern wired Ethernet, yet still matter to this course?
11. Minimum frame size
Every Ethernet frame must be at least how many bits long, so that a sender still transmitting can detect the worst-case collision?
12. The jam
How many bits long is the jamming sequence a transmitter sends after detecting a collision?
13. Interframe gap
Adaptors wait a fixed quiet interval after a frame ends before the next may begin. How long is this interframe gap in microseconds?
14. Backoff ceiling
Under exponential backoff, after how many failed retransmission attempts does an adaptor give up and report an error to its host?