Data communication and network-wired LAN: Ethernet

Document directory

• Ethernet Classification
• Standard Ethernet
• Fast Ethernet
• Gigabit Ethernet

Ethernet Classification

Ethernet has gone through four generations of development:

Standard Ethernet EEE creates a standard Ethernet frame format named 802.3

Lead character: 1 and 0 alternate. The notification receiving system has a frame and synchronizes it with the input clock. It is added to the physical layer and is not part of the frame.

SFD: the start frame delimiter. the next field of the notification recipient is the destination address.

Destination Address: the physical address of the destination.

Source Address: the physical address of the source.

Length/type: defines the upper-layer protocol of the frame or the number of bytes of the data field as the length field.

Data: encapsulated data.

CRC: error detection information.

Frame Length

According to rfc894, the maximum length of the IP packet encapsulated by Ethernet is 1500 bytes, that is to say, the maximum length of the Ethernet frame should be the Ethernet header plus 1500, add a 7-byte leading synchronization code and a 1-byte frame start delimiter, specifically: 7-byte leading synchronization + 1-byte frame start delimiter + 6-byte destination MAC + 6-byte source MAC + 2-byte Frame Type + 1500 + 4-byte FCS.

According to the above, the maximum frame should be 1526 bytes, but the maximum frame we get from Packet Capturing is 1514 bytes. Why not 1526 bytes? The reason is that when the data frame arrives at the NIC, the NIC on the physical layer must first remove the leading synchronization code and the starting delimiters of the frame, and then perform CRC checks on the frame. If the frame checksum is incorrect, the frame is discarded. If the checksum is correct, check whether the target hardware address of the frame meets your receiving conditions (the destination address is your own physical hardware address, broadcast address, and the available multicast hardware address ), if yes, the frame will be handed over to the "device driver" for further processing. At this time, our packet capture software can capture data. Therefore, what the packet capture software captures is to remove the leading synchronization code, the frame start delimiter, and data outside of the FCS, the maximum value is 6 + 6 + 2 + 1500 = 1514.

According to Ethernet rules, the minimum part of the Ethernet frame data domain is 46 bytes, that is, the minimum value of the Ethernet frame is 7 + 1 + 6 + 6 + 2 + 46 + 4 = 72. In addition to the leading synchronization code, frame start separator, and FCS, the packet capture time is 60 bytes. When the length of a data field is less than 46 bytes, the MAC Sub-layer fills in the back of the data field to ensure that the frame length is not less than 72 bytes. The MAC Sub-layer is responsible for filling data, that is, the device driver.

The MAC Sub-layer is responsible for filling data, that is, the device driver. Different Packet Capture programs and device drivers may have different priority levels, and the packet capture program may have a higher priority than the device driver, that is, our package capture program may have captured data before the device driver has not filled 64-byte frames. Therefore, the size of data frames captured by different packet capture tools may be different. (For example, Wireshark may not populate the data segment, while sniffer may capture the data segment)

Addressing

Ethernet uses a 6-byte physical address provided by the NIC, which is usually expressed in hexadecimal notation.

Example: 06: 01: 02: 01: 2C: 4B

The Unicast address, multicast address, and broadcast address source address are always one Unicast address (because the frame only comes from one site). The destination address can be a unicast address, it can also be a multicast address or a broadcast address. If the delimiter of the first byte of the target address field is 0, this address is a unicast address; otherwise, it is a multicast address. A broadcast address is a special case of multicast addresses. The receiver address is all the sites in the LAN. A Broadcast Destination Address 48 is 1, that is, FF: FF is also associated with unicast, multicast, and broadcast addresses in the IP address at the network layer? Yes. In fact, the IP addresses at the network layer must be mapped to the addresses at the data link layer before being sent out. The specific ing will be explained later.
Access Method: standard Ethernet access using CSMA/CD. I have written an article about CSMA/CD: http://blog.csdn.net/todd911/article/details/9275515real-Time Standard changes on the physical layer.
Fast Ethernet IEEE creates Fast Ethernet under the name of 802.3u. Fast Ethernet is backward compatible with standard Ethernet, but its transmission speed is 100 Mbps. The goal of Fast Ethernet is: 1. upgrade the data speed to 100 Mbps. 2. Make it compatible with standard Ethernet. 3. Retain the 48-bit address. 4. retain the same frame format. 5. Retain the maximum and minimum values of the frame length. In the MAC Sub-layer, the bus topology is abandoned in Fast Ethernet and only the star topology is retained. In a star topology, half duplex and Full Duplex are available. Use a Hub Connection in half duplex and a switch connection in full duplex. The half-duplex access method is CSMA/CD. In full-duplex mode, CSMA/CD is unnecessary and CSMA/CD is retained so as to be compatible with standard Ethernet. Automatic negotiation Fast Ethernet adds a new feature: Automatic negotiation, which allows two devices to negotiate their operating mode and transmission speed. The physical layer achieves Gigabit Ethernet. IEEE creates a Gigabit Ethernet under the name of 802.3z. The design goal of Gigabit Ethernet is as follows: 1. upgrade the data rate to 1 Gigabit 2. make it compatible with standard or fast Ethernet 3. use the same 48-bit address. 4. Use the same frame format. 5. Retain the maximum and minimum values of the frame length. 6. Supports automatic negotiation defined in Fast Ethernet. Full Duplex and half duplex (rarely used) Physical Layer implementation in Working Mode

10 Gigabit Ethernet EEE creates a Gigabit Ethernet network under the name of 802.3ae. The design goal of Gigabit Ethernet is as follows: 1. the data rate is upgraded to 10gbps2. make it compatible with standard Ethernet, fast Ethernet, and Gigabit Ethernet. use the same 48-bit address. 4. Use the same frame format. 5. Retain the maximum and minimum values of the frame length. 6. Allow interconnection between existing LAN and man or WAN 7. Make Ethernet compatible with technologies such as frame relay and ATM. Work mode can only work in full duplex mode.