Data link layer of Computer Network

Data Link Layer 1 in computer networks as mentioned earlier: when data is transferred between two adjacent nodes, the data link layer assembles the IP data packets handed over by the network layer into frames, and transmits the data in frames transparently on the links between two adjacent nodes. Three basic problems of the data link layer protocol are encapsulation into frames, transparent transmission, and error detection. CRC (Cyclic RedundancyCheck) is widely used in error detection ). In fact, CRC can only ensure that the received frames are free of errors, but it cannot solve Frame loss, frame duplication, or frame out-of-order. The data link layer designed by OSI is reliably transmitted. It incorporates frame numbers, validation, and retransmission mechanisms to solve these three problems. However, the current data link layer does not actually adopt these mechanisms, but simply uses CRC. Reliable transmission is implemented by the transport layer. Ii. Ethernet is synonymous with a non-strict "Lan". The services provided are unreliable (best effort for delivery ). All data sent uses Manchester encoding. At the same time, only one computer can send messages. Therefore, Ethernet adopts the Cattier Sense Access with Collision Detection protocol (CSMA/CD. CSMA/CD Protocol is the most important protocol in the data link layer. It is relatively basic and is not detailed here. One of its important concepts is the contentionperiod, also known as the collision window ). It is the end-to-end round-trip time of Ethernet. Only after the competition period has not detected a collision, can we be sure that this transmission will not have a collision. Therefore, the shortest valid frame length is specified to ensure that the transmission time of a single data frame is not less than the contention period. The shortest valid frame length of Ethernet is 64 bytes. Ethernet channel utilization. Is the sending time of a single frame, which is the end-to-end one-way time. A bridge is required for the expansion of Ethernet at the data link layer. A multi-interface Bridge is a layer 2 switch ). 3. MAC address, also known as hardware address and physical address, refers to the adapter (NIC) Address, 6 bytes in total. The first three bytes are company identifiers, which are assigned by RA. The last three bytes are extension identifiers, which are assigned by the manufacturer. The bitwise of the first byte of the MAC address is I/G (Individual/Group). If it is 0, it indicates a single site address. If it is 1, it indicates a Group address (Multicast ). The minimum and second bits of the first byte are G/L (Global/Local) bits. If it is 0, it is Global management (unique). If it is 1, it is Local management (User allocation ). The format of the MAC frame is as follows. It can be seen that the MAC frame header has 14 bytes and the tail has 4 bytes. The data part length is 46 ~ 1500 bytes. The tail end is the frame inspection sequence of the CRC test. When delivered to the physical layer, an 8-byte synchronization code must be added before the frame to synchronize the clock frequency.