Since graduation and other reasons, drag the video to learn now, deeply inadequate capacity of their own schedule. Closer to home, back to study for this week.
Introduction/The Link Layer
60-80 years, the most used network to store and forward network (store and forward networking): the message will go into a computer, and stay for some time to find their next stop in the network, continue forward until you find the end . Therefore, fewer forwarding is one of the characteristics of the network, however, this network also has a "do not have much to share," this feature, only once a message is sent through these links, and all other information just waiting time , in a row.
From the 1960s to the 1980s, this gap has finally been improved in ARPnet, and that is to become the packet network (Packet Network). Packet network is based on the "shared network in order to quickly" put forward, it wants the user to address more quickly the fault, the data transmitted can be returned within half a second, but not the same as spending ten minutes before, or even a two days. In order to realize this possibility, researchers have paid nearly 20 years of efforts to continuously discarded and rewritten architecture, we can see in the video, in the late 1970s, and more than 100 computers have been added to this new network system.
So the practical application of packet switching, how is it? Professor postcard again cited the case before the course mentioned. Assuming that a postcard can handle only 10 characters, 30 characters if you want to send the information, the natural need three postcards, they will be numbered and placed in the mailbox. Different postcards can be sent different postman may also go through different routes, but in the end, they can reach their destination and makes up a complete information, you do not need as before, like a train, like them connected together. Therefore, store and forward network computers, from powerful with a large disk drives, computer functions, the development is only a very small forwards the packet destination computer, not a long time to store messages.
In fact, packet switching still need to continue forward, to reach the final destination. But compared to traditional forwarding mechanism, packet switching is no longer forwarded to the whole message, but a smaller packets. At the University of Michigan and Stanford University, for example, they will have a variety of local network in the campus, there are computers, servers, and there are all sorts of things. They will all packets, sends all the data to the Internet, then these packets would find their way on the Internet.
In these jumps forward in the process, there are still many problems to be solved in order to simplify the solution or solutions will be broken down into simpler, more manageable components. They came up with a four-layer network model, broken down into a number of subsets by a big problem, working separately, to come up with a way to make them part of the interaction, which is known as the TCP / IP protocol suite, in addition, there are OSI seven-layer model with a network of. And to make these layers to complete their tasks, you need to write the corresponding statement, this is the Internet Standards.
Link layer connection is considered a problem between two computers, for example, when you want to send mail, the mail from your home to take people, put the mail on his shoulders, it's a link, they put the mail on the truck, then onto a truck-mail, and send it to another place, then took it out on the train. Trucks, trains, or the postman postman is these small link layer. This is the link layer, it is only concerned about what defined in this line, not on other things on the other end. Its job is to get the data at one end, and these data how do we share? It does not care. We see a common link technologies such as Ethernet or WiFi or cable modem, DSL, satellite or optical fibers, which are various forms of link layer. In addition to wireless, Ethernet is probably the most common link layer.
Therefore, based on the characteristics of the link layer, when the manufacturers to build an Ethernet or wireless adapter, they will set a serial number in it, to indicate the physical address for this computer and which packets belong to it in a line know .
After the completion of the connection between the devices, how to solve the plurality of communication devices, it has become a new problem. Researchers have come up with a way to avoid confusion, Ethernet to achieve this is to use a technique called "CSMA / CD" is. When we are ready to send datagrams, there may be other hosts also wants to send a datagram, so we need to listen, wait until there is no movement on the link, and then start sending information, then continue listening sent information, when conflicting information, they will return it. After a series of waiting and sending the data for each computer can reach their destinations safely. It can be said that the technology just like traffic lights, can effectively guarantee the data transfer between the conflict occurs on a computer.
Bob Metcalfe-The Ethernet Story
The previous paragraph is intended to outline the development of the network, so we have a general idea of the development of the network architecture, this section is the introduction of Ethernet inventor Bob Metcalfe.
After Bob completed a study of Arpanet, he received the invitation of Signet research projects Xerox, the purpose of this project is to build a PC connected everywhere architecture. Then the connection method, there are still many problems, one of which is like a home run as the line, from the core of the base, the haphazard extended to the rest, like a rat's nest; and the other problem is that their research team another project - the first laser printers, laser printers work they want to perform file transfer between computers, which requires transmission rates of up to 20M / s, but this printer even though the rate of increase, can only reach 144K / s.
Prior to this, Charles Simony research team has developed to a certain stage Signet, Bob took over his job. In the video, Bob praised Simory computer network idea, because a lot of LAN module translates into a part of Signet is after, and LAN in 1990 was only invented, we can say, Signet is a product ahead of its time. To be beside the point here, after research Signet exit, Simony developed a text editor called Bravo, and Bravo finally evolved into the current Microsoft Office, the Charles Simony has become a billionaire, but also to the trips to the space station.
在如何组织这个局域网的过程中，Bob偶然间遇到了Hawaiian大学的一个分组无线电网络，叫做ALOHA网。ALOHA网络的美妙之处在于它解决了一个分布式问题。他们能够很轻松地共享无线电频道。
而实现方法又是什么呢？Hawaii 大学的 Norm Abramson 设计了这一种非常简单的随机重传程序，操作者可以输入卡片信息，之后通过终端发送至主机上，接着等待一段时间，如果有收到回执，说明发送成功，但是如果没有收到，这可能意味着有两台终端同时决定发送信息。所以，当许多终端参与进来的时候，信息传递过程中的碰撞发生之后，他们将会设置一个随机数，来尽可能地实现在不同的时间段进行重新发送。在某一时刻发生的信息碰撞在未来不易发生，因为这两个终端在未来会在不同的时间段重新发送。ALOHA实现了通过分布式的方法来对一条电缆进行共享，能够解决“老鼠窝”问题。
接下来就是实践了，Bob先是买了一条一千米长的电缆，并且拿到了一台脉冲发电机，通过电缆将示波器与脉冲发动机相连，然后开始沿着电缆发射方波，并从示波器观察发送情况，一开始显示的不是方波，而是一种相对不稳定的波形，但在设定了数字门限（digital threshold）后，就能发现方波，这让Bob有了一些信心，能够实现之后的数据发送。
在发送数据的过程中，Bob团队则会对发送的比特数据包进行编码，编码也很简单，就是现在广为人知的曼彻斯特编码。曼彻斯特编码的美妙之处在于，当您发送数据包时，会不断地进行转换，不需要听很长时间，就能知道包裹是否经过。所以以太网和ALOHA网络之间的最早的区别之一，就是载波感觉，在ALOHA中，你不能判断其他人是否和你同时在传输，但在以太网上，你可以。这样做的好处是，如果你在发送，而其他人同时发送，你最好还是放弃吧，因为你们已经破坏了对方的数据包，所以不要那么轻举妄动，之后则会恢复原本会丢失的带宽，继续发送损坏的数据包。
Bob还回忆起当时做了一些刀工，把铜线上的绝缘材料去除。但是看起来他并不擅长做这工作。与他同屋的有一个学生，曾经做过电视线路的安装，所以非常擅长做这些活，这名学生叫David Boggs，协助了Bob促成了以太网的诞生。
回到正题，Bob仍需要继续解决“老鼠窝”的问题，希望只通过一条电缆就能实现网络接入，但面临着一些技术问题，需要找到一个不破坏网络但能利用网络的方法。在与David一起研究的过程中，他们找到了一种设备叫gerald tap，而这个设备就解决了他们所面临的技术难题，“老鼠窝”问题也随之解决。
本周的课程相较于之前增加了许多，所以分为上下两章，理解起来比之前轻松了一些，但部分内容还是有疑惑，加上英语水平有限，也无法解决，比如Bob一开始提出的打印机问题之后是怎么解决的？也许是我理解上有偏差，希望之后重温的时候能明白过来。