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Do you wanna pass CCIE RS exam？You need make sense these things below
(1) What logical link?
(2) Does the logical link exist?
(3) The difference between bitstream and frame
Sensibility and abstraction
The newborn baby knows the world is perceptual. What kind of sound is heard, what scenery is seen, what items are touched, sounds, scenery, and objects are objectively present. They can be perceived or even touched. The baby slowly knows the world.
For a 1-2-year-old child, if one day you tell her that I am in a bad mood. When children hear this, they will be very confused about what it’s mood?
Can the mood be perceived? No! Does the mood exist? presence!
This is an abstraction. When children grow up to 5 years old, they will slowly understand what is bad. When they see that their parents are not right, they will know that Mom and Dad are not in a good mood.
As children grow older, they will see a sentence like "Government and the people's heart" on TV. How can the heart be connected? This sentence is also an abstraction, which can also be understood as Logic means "when the people suffer disaster, the party and the government will come to help the people."
If the above descriptions are not helpful, then the best way is to use the capture software to capture the incoming and outgoing messages on the network card one by one, and then open the study one by one.
If these messages have never been seen before, it is very inefficient to look at these very abstract textbooks. If you go down to a certain day, the subject will know how difficult it is to retreat.
The reason why computer networks are difficult to learn is that they contain many abstractions. Why do you have so many abstractions?
Because if you don't abstract, the implementation of the network will lose flexibility and scalability.
I remember writing an article about the difference between the application layer HTTP protocol and the TCP/IP protocol.
In fact, the application can completely communicate without using the TCP/IP protocol stack. The application directly sends its own communication message to the network card through the network card interface function, and then the network card can convert the user byte (byte) stream into a bit stream. Go out.
The application needs to ensure reliable communication, and also needs to resolve the URL into an IP address, and also needs to put the message on the IP address. In addition, it needs to tell the network card the MAC address of the next hop device. Isn't this series of actions not exhausting?
And directly call the interface function of TCP/IP protocol, all the above actions TCP/IP can be done for the service, why bother to go with yourself?
So, what is the TCP/IP protocol?
Calculate the abstraction layer and package all the tasks that the application should do, such as reliable transport, IP address, and link layer addressing, into the TCP/IP stack. Make applications such as browsers slim and streamlined.
Then the TCP connection established by TCP on both sides of the communication is naturally an abstract concept, which can be called a logical connection.
With the constant elaboration of the previous article, I finally came to the data link layer.
Why is the data link layer divided into MAC and LLC layers?
MAC, the abbreviation of Media Access Control, mainly downwards to the supplier (physical layer), and the physical layer to deal with this part of the work, that is, to listen to whether the physical line is idle, if the user's frame (bytes) is encoded as 0, 1 The bit stream is then sent to the physical line.
LLC, the abbreviation of Logical Link Control, mainly provides services to distinguished customers (network layer). The main task is to distinguish the identity of customers, such as IPv4, IPv6, ARP, PPPoE and so on.
If you must ask, does the logical link layer LLC exist?
Of course, there is! It exists in the form of software, and although it is invisible, it does exist.
In addition, the user data is not called a message at the data link layer and is called a frame. When it is encoded as 0 and 1 string and transmitted on a physical line, in order to accurately describe the 01 string, it is usually called a bit. flow.
