lecture 1

Why packet switching is chosen for the Internet?

Circuit switching, the predecessor of ~

Circuit switching is all manually connected. The line is dedicated for the end hosts during the call.

Every phone call has its 64Kb/s circuit that it doesn’t have to share with anybody else.

Problems

Time of silence 当用户不在操作的时候, e.g. reading a webpage.

最后一个问题，Switches需要manage connection states between any two hosts. 太麻烦了。

Packet switching

Data + Header = Packet

Difference between Routers and Switches will be discussed in later lectures.

Animation of Packets in the network（大概在11分钟）

Packet switches have buffers

• The routers maintain no per-communication state. Just maintain the forwarding table.
• 对应了end-to-end principle。路由器只负责转发。

为什么要用packet switching？

Regarding the resilience, since each packet is individually routed along the path, if something breaks (a link breaks or a router breaks), becasue we have no state in all of the switches for this particular flow, we could simply send the packet through a different path, and it will find its way eventually. Packets can be re-routed.

— Lecture 2 —

End-to-end Delay: is composed of

1. propagation delay = l/c （光速的传播速度）
2. packetization delay = p/r （packet的上传速度）p: length of the packet r: the rate on which the packet runs (bits/s). Packet由比特流组成，所以要等待整个packet 到达（例外：cut-through packet swithcing, 拿到header就开始转发）.这里的例子是store and send形式的，等待全部接收完成。
   
   can be calculated as
   
   
3. Queue delay (also called as packet buffer, FIFO queue) . 下图的红圈部分。
   The bigger the packet is, the less likely we are to drop a packet along the link. Or when two packets show up at the same time, one of the two will be dropped.

The complete end-to-end dealy now becomes:

Note here, everything except for Queueing delay is deterministic. Queueing delay is unpredicted.

Stanford to Tsinghua has larger variation. Comes from the queueing delay, more other users, encountering more other users’ traffic.

Summary