CSMA/CD  载波侦听，多路访问，冲突，冲突检测(回退算法)

CSMA/CD stands for Carrier Sense Multiple Access/Collision Detection. It is a media access control method used in Ethernet networks. In CSMA/CD, multiple devices share a common communication channel, and each device listens to the channel before transmitting data. If a device detects another transmission on the channel, it waits for a random period of time before attempting to transmit again.

If two devices transmit data simultaneously and a collision occurs, the devices detect the collision and stop transmitting. They then wait for a random period of time before attempting to transmit again. This process of detecting collisions and retransmitting data is known as collision detection.

CSMA/CD is used in half-duplex Ethernet networks where multiple devices share a single communication channel. In full-duplex Ethernet networks, collision detection is not required, as each device has its own dedicated transmit and receive channels.

Ethernet is a widely used networking technology that uses frames to transmit data over a network. The frame structure of Ethernet consists of the following components:

1. Preamble: The preamble is a sequence of 7 bytes (56 bits) that precedes the frame. It is used to signal the start of the frame and to synchronize the clocks of the sender and receiver.

2. Start Frame Delimiter (SFD): The SFD is a single byte (8 bits) that marks the end of the preamble and the beginning of the frame.

3. Destination Address: The destination address field is a 6-byte (48-bit) field that specifies the MAC address of the destination device.

4. Source Address: The source address field is a 6-byte (48-bit) field that specifies the MAC address of the sender.

5. Length/Type: The Length/Type field is a 2-byte (16-bit) field that specifies either the length of the data field or the type of protocol being used.

6. Data: The Data field is a variable length field that contains the actual data being transmitted.

7. Frame Check Sequence (FCS): The FCS is a 4-byte (32-bit) field that is used to verify the integrity of the data being transmitted.

The Ethernet frame structure can vary depending on the specific type of Ethernet being used. For example, in the case of IEEE 802.3 Ethernet, the Length/Type field is used to indicate the length of the data field, while in the case of IEEE 802.2 Ethernet, the Length/Type field is used to indicate the type of protocol being used.

communication types among LAN

单播，组播，广播

There are three main types of communication that can occur among devices on a Local Area Network (LAN):

1. Unicast: Unicast communication involves a single sender and a single receiver. In this type of communication, the sender sends a message to a specific device on the network, and only that device receives and processes the message.

2. Multicast: Multicast communication involves a single sender and multiple receivers. In this type of communication, the sender sends a message to a specific group of devices on the network, and all devices that are part of that group receive and process the message.

3. Broadcast: Broadcast communication involves a single sender and all devices on the network. In this type of communication, the sender sends a message to every device on the network, and all devices receive and process the message.

The choice of communication type depends on the specific requirements of the application or service being used on the network. For example, unicast communication is commonly used for peer-to-peer file sharing or for sending messages to a specific device, while multicast communication is often used for streaming video or audio content to a group of devices. Broadcast communication is typically used for network discovery or for sending important notifications to all devices on the network.