Why is the LLC sublayer not reflected in the packet? What is the specific role of the LLC sublayer? How does Ethernet_II represent the end of a frame?

Introduction: The LLC sublayer read and forget, write in here convenient memory.

Mac won't say it. LLC format Baidu.

LLC Sub-layer

The LLC sublayer in the IEEE802.3 frame adds some other useful features in addition to defining traditional link-layer services. These features are provided by the DSAP, Ssap, and control fields.

For example, the following three types of point-to-point transport services:

• Non-connected packet transport service

  The current Ethernet implementation is such a service.

• Reliable connection-oriented data transfer service

  Pre-establishing the connection to transfer data, the data in the transmission process reliability is guaranteed.

• A non-connected, confirmed data transfer service.

  This type of data transfer service does not need to establish a connection, but it adds a confirmation mechanism to the data transmission, which greatly increases the reliability.

The following example illustrates the application of SSAP and DSAP. Assuming that terminal system A and terminal system B are using a reliable connection-oriented data transfer service, the following process occurs:

1. A sends a data frame to B, requesting a reliable connection for the connection.

2. b after receiving, determine whether your resources are adequate (that is, if you have too many connections), if sufficient, return a confirmation message that contains the SAP value that identifies the connection.

3. When a response is received, it is known that B has established a connection with itself locally. A also creates an SAP value to represent the connection, and one confirms to B, and the connection is established.

4. The LLC Sublayer of a encapsulates the data that it wants to transmit, where Dsap bytes fill in the Sap,ssap bytes returned by B, filling in the sap that you created, and then sending it to the Mac Sublayer.

5. A is sent to the data link after the Mac child layer plus the MAC address and the Length field.

6. After the Mac Sublayer of B receives the data frame, it submits it to the LLC Sublayer, and the LLC sublayer determines the connection to which the data frame belongs according to the Dsap field.

7. b According to the type of the connection of the corresponding checksum confirmation, through these checksum confirmation, only send to the upper layer.

8. After the data transfer is complete, a sends a data frame to B to notify B to remove the connection and end the communication. Pick--huawei Brochure

   
   

These are still not able to answer the doubts in the mind.

At present Mac+ip is not used very well, LLC provides no connection, for connection Dsap and so on, why less he can use.

The LLC role is to specify the upper layer of the agreement, and of course there are said to provide no connectivity and so on.

So DSAP/SSAP is used to sign the upper layer protocol. For example, IPX is 0XE0,

Snap uses org Code+pid to identify protocols, such as APPLE talk 0x809b. Wait a minute.

Look at the packet format

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LLC only appears in the 802.3 Ethernet format, 802.3 of the MAC layer does not have a field indicating the upper protocol field, but indicates the packet length, so the upper layer Protocol requires the LLC to indicate.

In the Ethernet_II frame, change the length field of 802.3 to type. It is not necessary to implement the LLC Sublayer, which contains only a single Mac Sublayer, when the upper layer protocol is distinguished by the Type field.

Only now the manufacturers are using Ethernet_II. The actual effect and 802.3+LLC are the same.

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How to determine the answer is as follows:

Because the Ethernet frame uses 4b/5b encoding, there is a 5bit fixed type of binary, indicating that the end of this packet, the normal transmission is not to appear in this 5bit of a particular sequence, if there is also through the anti-code or complement or something replaced, learned the computer composition principle of the students will know. So there is no need for the protocol to have a length field.

For a 64-byte minimum frame is populated, how to distinguish which paragraph is the content of the fill. The data is inside the IP packet. The IP packet has a length field. You can use this to differentiate.

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Why is the LLC sublayer not reflected in the packet? What is the specific role of the LLC sublayer? How does Ethernet_II represent the end of a frame?