[已注销]对《计算机网络》的笔记(12)

计算机网络
  • 书名: 计算机网络
  • 作者: 特南鲍姆
  • 副标题: 第4版
  • 页数: 891
  • 出版社: 清华大学出版社
  • 出版年: 2004
  • 第2页
    The distinction between a computer network and a distributed system:
    In a distributed system, the independent computers look like a single coherent system. They are organized by a layer of software (called Middleware) on the top of different operating system. So the how the machine works is transparent to usrs. One example is the WWW, in which everything is a document.
    but in a computer network, the coherence is absent. Users are exposed to the machines. If a user want to run a program on a remote machine, he has to log onto the machine fisrt.
    2011-03-09 20:27:35 回应
  • 第10页
    in these two scenaries, 'wireless' != 'mobile'
    1.you buy a macbook air but cannot afford a wireless router.(don't ask me why)
    so you have to be 'wired' surf the net though your notebook has the ability to be 'mobile'
    2.this time you buy a desktop because you want to save some money(both in the mac and wireless router) but just find your apartment lacking of network cabling. Unforturnatly, you have to purchase both a wireless router and a wireless network card to get online. Although you can't walk around with your desktop in hand , but you are still 'wireless'.
    2011-03-09 20:54:00 回应
  • 第17页
    Broadcast networks(opposite tho the ring) can be divided into static and dynamic:
    static:
    Divde the time into slices and use a round-robin[1] algorithim. Each machine broadcast in its own time slice.The disadvantage is obvious: if one machine have nothing to speak, the others still have to wait until it time 'belongs' to it is over.
    dynamic:
    1.centralized: there is arbitration to determines who goes next
    2.discentralized:
    each machine must dicide for itself whether to transit
    (automatically)
    [1]
    The term round-robin was originally used to describe a document signed by multiple parties in a circle to make it more difficult to determine the order in which it was signed.
    from wikipedia
    2011-03-09 21:24:07 回应
  • 第23页
     wireless networks (3 catagories)
    1.System interconnection
    conncect the components of a computer using short-range radio (so without wire) such as Bluetooth
    it often uses the master-slave paradigm: the computer is the master and the keybord and mouse are the slaves.
    2.Wireless LANs
    it enables one computer to communicate with others.(2 types)
    1) a central radio modem communicating all computers
    2) one computer communicating with another
    a standard for wireless LANs called IEEE 802.11 (all the notebooks around me use this standard. and most of them are using 802.11b or 802.11g)
    3.Wireless WANs (3 generations)
    1) analog for voice only
    2) dital for voice only
    3) digital for both voice and data
    ************************
    Bob Metclfe's 's metaphor of wireless is quite interesting (a portable potty~).
    Why he hates wireless?
    He is the inventor of the Ethernet.
    2011-03-09 22:16:42 回应
  • 第32页
    both Connection-oriented and the Connectionless Services can be reliable and unreliable. So it is hard to say there is a link between the connection and the reliability. Whether the service is reliable depends on whether the system requires a acknowledge before next sending.
    For example, VoIP is unreliable because of its demand for synchronization, but it is still a connection service.
    Register mail is quite reliable(used in the registeration of many sites), and it is connectionless.
    2011-03-13 23:02:22 回应
  • 第36页
    Service vs. Protocol
    Service is someting like the object in programming. It often provides other object(in this case, layer) some method/function to use but not showing how they are implemented.
    Protocol is more like a rule. Sometimes we delivery information in text. If we don't make a rule how to define the format. The one who receives it will be confused.
    Supposing I leave a message to my girl friend:
    message={3,13,124883418,Annie}
    It has several meanings if don't send it with a specification like:
    first digit=Month
    second digit=Day
    third digit=Tel number
    string=your friend's name
    So my girl friend knows there is someone named Annie called in March 13th and her number is blablabla
    2011-03-13 23:26:16 回应
  • 第50页
    Using the connectionless subnets or the connect-oriented subnet is the 'To be or not to be question' in networking.
    An analog (Tanenbaum uses this word frequently) is that I want to tell my girl friend how much I love her. One way I can just call to say I love you.(name of a famous song) The other way is write several love letters and send them to her. In first situation, the conversation is continuous. My every word will arrive in sequence. But there is always a annoying time before the connection is built.
    In second situation, the post office is the key. They are in charge of sending every letter to my girl friends. Suppose the letters will arrive in deferent ways(shipping, air or the pidgin). In result, the word will in random order. Anyway she still gets the meaning as long as there's no letter missing in the transmission.
    ATM (the abbreviation of Asynchronous Transfer Mode) is connection-oriented though it uses the packet switching subnet. Because it requires to build a temporary connection, and there is always a virtual circuit in the subnet. When the connection is set up, the data is transmitted in packages.
    2011-03-19 20:09:25 回应
  • 第65页
    The technique used in the Ethernet to avoid conflict in transmission is just like a discussion in our daily life:
    Three people are sitting around and gossiping about the things happened recently. If someone of them, like Jim want to say a funny story. He will look around first to see if Mary and Peter is speaking. It is rude to interrupt other people's speech. If no one else have topics to say and there is a silence, he will begin his story. And if Mary is holding her microphone, Jim will wait a random time then retry again.
    Air is the only Ether in the real life, so there should be one person making his speech each time.
    In the case of wireless, the problem arouses. It also has a reflection in our life:
    Jim, Mary and Peter are sitting in a row, Mary can hear both Jim and Peter, but Peter and Jim cannot hear each other. So if Jim want to talk to Peter, he isn't sure whether Peter is talking to her, too. So it is likely to collide.
    2011-03-20 18:44:53 回应
  • 第90页
    Bandwidth & Data Rate
    People always get confused in the difference between the data rate and bandwidth. Some people thought they are two sides of one thing. Actually data rate comes after the bandwidth.
    Tanenbaum points out:
    The range of frequencies transmitted without being wrongly attenuated is called the bandwidth.
    The range is from 0 to one number and the number is the so called bandwidth.
    So it exists a question why high frequency signal is easy to be attenuated?
    One explanation is the higher frequency means the shorter wave length. It is hard for a short wave to get through the relatively long channel.
    But the bandwidth always effects the data rate.
    Henry Nyquist derived an equation expressing the maximum data rate for a finite bandwidth [noiseless] channel.
    max data rate = 2 H log2V bits/sec
    Claude Shannon make a further work and extended it to the case of a channel subject to random noise:
    max data rate = H log2(1+S/N) bits/sec
    2011-03-20 18:44:01 回应
  • 第126页
    Modem vs. codec (A good explanation)
    http://cboard.cprogramming.com/brief-history-cprogramming-com/20772-difference-codec-modem.html
    The 'Modem" MOdulates and DEModulates a digital signal to stepped (analog) ADPCM (Adaptive Differential Pulse Code Modulation) frequencies. This allows for higher transfer rates on a single DS0 circuit.
    This is a hardware conversion.
    A "Codec" on the other hand, COmpresses and DECompresses a digital data signal only. No analog involved. It is nothing more than a compression/decompression algorithm tweaked for optimal use with Video or Audio data.
    This is a software conversion.
    ----------------------------------------------------------------------------------------------
    A modem is a piece of hardware. It is an electronic device that comunicates over a digital or analog medium (line) to transmit data. This data is coded (digitalized) and then converted to fit the medium it is transmited through (analog or digital) so the information is recieved on the other end by another modem and is then interpreted.
    A codec is software, it is a program 'key' of sorts that tell the computer how to 'translate' the digital information of a file and present it in an understandable manner. It does not necesarily mean that it is a security measure, but it is the 'dictionary' with which the information is written. Some codecs are contained in chips but that nonetheless is a small 'computer' that contain the instructions so it is still a piece of programing.
    So the modem is like your mouth and ears and the codec is the part of the brain that 'understands' what you are hearing.
    2011-03-21 11:21:45 回应
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