《计算机网络 自顶向下方法》读书笔记（三）

R1.What is the difference between a host and an end system? List several different types of end systems. Is a Web server an end system?

姑且认可他的答案的话，不得不说，国外的题目方式还真是别出心裁。这种句式很容易就让人先从二者存在差异出发开始论证。而他的答案是

There is no difference. Throughout this text, the words “host” and “end system” are used interchangeably. End systems include PCs, workstations, Web servers, mail servers, PDAs, Internet-connected game consoles，etc.

不过，我觉得比较奇怪的是web服务器不应该是一种服务而不是指具体的设备吗？

R4. List six access technologies. Classify each one as home access, enterprise access, or wide-area wireless access.

我不清楚这里区分的标准是什么。按照本书的目录来说，就是看主要被使用的场景，比如DSL,WIFI。然而他所给出的答案却是另一番分类：

1.Dial-up modem over telephone line: home;

2.DSL over telephone line: home or small office;

我觉得这里就有些从目前的实际情况出发的意思了。当然，ADSL对于企业来说上行速率太低，所以目前国内有SDSL的服务。

3. Cable to HFC: home;

不知道为何Cable只能为家庭使用？

4. 100 Mbps switched Ethernet: enterprise;

5. Wifi (802.11):  home and enterprise:

6. 3G and 4G: wide-area wireless.

R7. What is the transmission rate of Ethernet LANs?
R8. What are some of the physical media that Ethernet can run over?

IEEE 802.3™: ETHERNET

R11. Suppose there is exactly one packet switch between a sending host and a receiving host. The transmission rates between the sending host and the switch and between the switch and the receiving host are R1 and R2, respectively.Assuming that the switch uses store-and-forward packet switching,what is the total end-to-end delay to send a packet of length L? (Ignore queuing, propagation delay, and processing delay.)

定式思维还是很可怕的。所谓经验就跟刷题一样，以为刷多了任何问题都逃不过自己的掌握，其实自己早已脱离实际。

这里和前面的瓶颈电路并不一样，因为是发送一个分组而不是足够多个。

R12.What advantages does TDM have over FDM in a circuit-switched network?

这里留个坑。提供的解答说，FDM的设备更复杂，从感觉上来说是这样。

R13.Suppose users share a 2 Mbps link. Also suppose each user transmits continuously at 1 Mbps when transmitting, but each user transmits only 20 percent of the time.

这里我第一反应就是FDM和TDM混合使用·····其实别人的意思只是这个user有20%的概率在传数据·····

R18. How long does it take a packet of length 1,000 bytes to propagate over a link of distance 2,500 km, propagation speed 2.5 · 108 m/s, and transmission rate 2 Mbps? More generally, how long does it take a packet of length L to propagate over a link of distance d, propagation speed s, and transmission rate R bps? Does this delay depend on packet length? Does this delay depend on transmission rate?

这里通过粗略计算我们即可看到传播时延明显大于传输时延。所以答案忽略了传输时延也无可厚非。然而延迟不能说和传输时延没有关系吧？结果仔细一看，人家问的就是传播，根本没问时延···

R20.Suppose end system A wants to send a large file to end system B. At a very high level, describe how end system A creates packets from the file. When one of these packets arrives to a packet switch, what information in the packet does the switch use to determine the link onto which the packet is forwarded? Why is packet switching in the Internet analogous to driving from one city to another and asking directions along the way?

这是一个好问题。附上答案：

End system A breaks the large file into chunks. It adds header to each chunk, thereby generating multiple packets from the file. The header in each packet includes the IP address of the destination (end system B). The packet switch uses the destination IP address in the packet to determine the outgoing link. Asking which road to take is analogous to a packet asking which outgoing link it should be forwarded on, given the packet‟s destination address.

当然，从问路的角度，我们就很容易想到，是否这样做造成了（某些时候）效率低下（如果可以第一次问询就得到完整结果的话）？如果任何一个中间人无法被问询（设备错误）或者给出错误的应答，如何完成正确的寻路？如果我们所给的地址已经失效了呢？