Computer network Learning Notes

Data Link Layer

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The data link layer belongs to the lower level of the computer network. There are two main types of channels used by the data link layer:

(1) Point-to-point channel. This channel uses a one-to-one point-to-point communication method.

(2) broadcast channel. This channel uses a one-to-many broadcast communication method, so the process is more complex. There are many hosts connected on the broadcast channel, so a dedicated shared channel protocol must be used to coordinate data transmission for these hosts.

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The most important elements of this chapter are:

(1) The characteristics of the point-to-point channel and broadcast channel of the data link layer, and the Protocol (PPP protocol and CSMA/CD protocol) used by the two channels.

(2) The Data Link layer three basic problems: package framing, transparent transmission and error detection.

(3) Hardware address of the Ethernet MAC layer.

(4) adapter, transponder, hub, bridge, Ethernet switch and the role of the use of the occasion.

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1. Data link layer using point-to-point channel

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1. Data links and Frames

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" Data Link " and " link " are not the same thing.

A linkis a physical line from a node to an adjacent node , and there is no other Exchange node in the middle. In the case of data communication, the communication path between the two computers often goes through many segments of such links. Visible links are just a part of a path.

Data Link: When data needs to be transmitted on a single line, in addition to a physical circuit, there must be some communication protocols necessary to control the transmission of the data, and the hardware and software that implements these protocols are added to the link (the most common method is to use Network adapters (both hardware and software) make up the data link.

--The adapter is also called the network card , generally contains the data link layer and the physical layer of these two layers of functionality.

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Frame -The protocol data unit of the data link layer of the point-to-point channel.

The data link layer sends the data that the network layer submits to the link, and the data in the received frame is taken out and handed over to the network layer. That is, the data link layer transmits frames .

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Earlier data communication protocols were known as communication procedures (procedure). In the data link layer, procedures and protocols are therefore synonyms.

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The main steps of the data link layer for the point-to-point channel when communicating are as follows:

1. The data link layer of Node A adds the header and trailer to the network layer to enclose the data in frames.
2. Node A sends the encapsulated frame to the data link layer of Node B.
3. If the data link layer of Node B receives the frame error-free, then extracts the data from the received frame to the network layer above, otherwise discards the frame.

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3.1.23 Basic Questions

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There are many kinds of data link layer protocols, but there are three basic problems that are common:

• Encapsulating into frames
• Transparent transmission
• Error detection

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1. Encapsulating into Frames

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Encapsulating Frames is the addition of headers and tails before and after a piece of data, which makes up a frame.

An important function of the first and the tail is the frame delimitation (that is, determining the bounds of the frame).

Each link layer protocol specifies the maximum length of the data portion of the frame that can be transmitted-the MTU of the largest transmission unit(Maximum Transfer unit).

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When the data is a text file consisting of printable ASCII code, a special frame delimiter can be used for frame delimitation.

(ASCII code is 7-bit encoding, can be combined into 128 different ASCII code, which can be printed 95, and non-printable control characters are 33.) ）

A control character SOH(start of Header) is at the front of the frame anyway, representing the beginning of the frame's header. Another control character, EOT(end of transmission), represents the end of the frame.

(Soh and EoT are the names of control characters, their decimal codes are 01 (binary 00000001) and 04 (binary 00000100) respectively. is not a three (s,o,h) character. )

2. Transparent transmission

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Computer network Learning Notes