The frame structure and the meaning of each field, describing the structure of the Ethernet frame an

1. Typical frame structure: Ethernet II

Fields included in Ethernet II:

Preamble: includes two parts: synchronization code (used to synchronize all nodes in the local area network, 7 bytes long) and detection flag (7,1 byte of the start symbol of the frame);

(i.e., the type of the upper-layer protocol), 2 bytes long;

Data: Encapsulated packets, 46 1500 bytes long;

Checksum: error check, 4 bytes long.

The main feature of Ethernet II is that the protocol used to encapsulate the packets in the frame is identified by the type leak domain, which is a valid pointer through which the data link layer is used.

It can host multiple upper layers (network layers.

Agreement. However, the disadvantage of Ethernet II is that there is no field to identify the frame length.

II. Primitive.

The original frame is an early novell

Default encapsulation for NetWare networks. It makes the sail with a hole with the frame type, but without the LLC domain. Differences with Ethernet II:

Changing the type field to a length field solves the original problem. However, because the type domain is defaulted, it is not possible to distinguish between different upper-layer protocols.

3. To distinguish data frames.

The type of data encapsulated.

The IEEE introduced the SNAP standard. They work at the LLC (Logical Link Control) sublayer of the data link layer. The problem of identifying the upper-layer protocol is solved by demarcating a new area called a service access point (SAP) in the data field of the frame, that is, the standard includes two service access points, the source service access point (SSAP) and the target service access point (DSAP).

Each SAP is only 1 byte long, and only 6 bits are reserved to identify the upper-layer protocol, and the number of protocols that can be identified is limited. Therefore, another solution was developed to add a 2-byte type field (with the value of SAP set to AA) on top of it, so that it could identify more upper-layer protocol types, and that is.

Generally, several consecutive images are encoded into three frame types: P, B, and I.

The p-frame is an encoded image that compresses the amount of transmitted data by fully reducing the time redundancy information with the previous encoded frame in the image sequence, also known as ** frame. The p-frame is derived from the p-frame or i-frame** that precedes it, and compresses the data of the current frame according to the difference between the current frame and the adjacent previous frame or frames. The method of joint compression of P frame and I frame can achieve higher compression without obvious compression traces.

P-frames belong to the forward ** inter-frame encoding. It only refers to the i-frame or p-frame that is close to it in front.

When compressing a frame into a B frame, it compresses the frame according to the different points of the data of the adjacent first few frames, this frame and the next few frames, that is, only the difference between the current frame and the previous and subsequent frames is recorded. Ultra-high compression ratios can only be achieved by using B-frame compression. In general, I frames have the lowest compression efficiency, P frames are higher, and B frames are the highest.

Keyframes - Any animation to show motion or change, at least two different key states before and after, and the change of intermediate state and the connection computer can be done automatically, in flash, the frame that represents the key state is called keyframe.

Transition Frame - The frame in which the computer automatically completes the transition between two keyframes is called a transition frame.

Definition: A frame is made up of several parts that perform different functions to facilitate transmission.

Frames define the format of data and control information in a bitstream. In asynchronous communication, each frame is separated by a start and stop bit, and in synchronous communication, each frame is separated by time. The domains in an Ethernet frame are described below:

Flag This domain contains a string of bits (usually 01111110) that identifies the start and end of a frame.

Address This domain can specify both source and destination addresses.

Control This domain contains information about synchronous communication or indicates the size of the data in the next domain.

Data This field contains the data that is carried by the frame.

Frame Check Sequence (FCS) This field contains error detection values that verify the correctness of the grouping.

Note: The reason why the data is transmitted in the form of frames is that when the line misoperates, the error can be recovered in time. Only corrupted frames require retransmission.

The data in a frame can be either variable-length or fixed-length. If a frame defines a variable-length data domain, then its size can vary in the range of a few thousand bytes. Most LANs, such as Frame Relay, employ variable-length data domains.

Networks that use fixed-length frames (called cells), such as ATMs, have predefined transmission rates, which are useful when transmitting time-sensitive information such as images. Fixed-length cells do not cause latency at network switching devices, while variable-length frames may prevent other frames from being transmitted. However, frames with variable-length data fields can deliver more user data at a time.

The larger the data domain, the less information about the frame-related frame format and header information will be. For example, in Fibre Channel, a complete transmission may be a frame with only one head, and conversely, the same transmission may require many ATM cells, each with its own head.