Basic knowledge of data link layer

Basic knowledge of the data link layer the channels used by the data link layer are divided into the following two types:
Point-to-point channel: This channel is a one-to-one communication method ---------- ppp protocol
Broadcast Channel: This channel uses a one-to-many broadcast communication method. For this method, you need to follow the dedicated shared channel protocol to coordinate the transmission of host data; www.2cto.com link: A physical line from a node to an adjacent node. There are no other exchange nodes in the middle. Also known as: physical link data link: When you need to transmit data on a line, in addition to using a physical device, you also need some necessary communication protocols to control data transmission, if you add hardware and software that implement these protocols to the link, it is called a data link. The most commonly used is a network adapter. Generally, the adapter includes the data link layer and physical layer functions-also known as logical link point-to-point communication.
1. The main steps for point-to-point channel communication at the data link layer are as follows: 1> the data link layer of node A adds the IP datagram handed over by the network layer to the header and tail. 2> node A sends the encapsulated frame to the data link layer of Node B. 3> If node B's data link layer no error occurred when receiving the so-called frame, the IP datagram is extracted from the received frame and sent to the network layer.
Ii. Common Problems at the data link layer: encapsulation into frames, transparent transmission, and Error Control 1> encapsulation into frames: add the first and end of a piece of data respectively, thus forming a frame. The important role of the header and the tail is to define frames. In addition, it also contains many necessary control information. Each data link layer protocol specifies the maximum length of the Data part of the frame-the maximum transmission unit MTU. Www.2cto.com can define frames with special frame delimiters: EOF and SOH 2> transparent transmission: Because the frame boundary is fixed, therefore, no 8-bit combination is allowed in the transmitted data part, which is the same as the ASCII character of the delimiter. Otherwise, a boundary error occurs. If there is an escape character in the transmitted data part, add an escape character 'esc 'before the escape character, and remove the escape character at the data link layer of the acceptor. If the escape character appears in the data section, add "ESC" before it. If the receiving end finds that there are two ECSS, delete one. 3> Error Detection: bits may cause errors during transmission. If 1 is changed to 0, 0 is changed to ------ error control. The common error control used in data link testing is CRC verification.
Iii. Characteristics of PPP 1> requirements: simple, encapsulated into a frame, transparent, multiple network layer protocols, multiple types of links, error control, detection of connection status, the largest transmission unit, network layer address negotiation, Data Compression negotiation 2> Not Required features: error Correction, serial number, multi-point line, half duplex or single duplex
4. the PPP protocol consists of three parts: 1. A method to encapsulate IP datagram into a serial link. 2> one LCP for establishing, configuring, and testing data link connections 3> one set of network control protocols NCP
5. The format of the PPP protocol the header and tail of a PPP frame are divided into four fields, the first field of the three fields, and the second field of the tail, respectively, it must be 0x7E. It indicates that the address field A in the beginning or end of A frame is set to 0FF, and the control field C is set to 0x03. The fourth field in the first part is A two-byte protocol field. When the value is 0x0021, the PPP frame information is reported by the IP address. If the value is 0xC021, the information field is the LCP data of the PPP link control protocol, 0x8021 indicates that this is the first field (2 bytes) at the end of the control data at the network layer. It uses the CRC frame verification sequence FCSCSMA/CD: CSMA/CD to adopt the IEEE_802.3 standard.
Its main purpose is to provide addressing and media access control methods so that nodes on different devices or networks can communicate over multiple networks without conflict. Www.2cto.com
Bridge: expand the data link layer to use a bridge. The bridge works on the data link layer. It forwards and filters the first received Frames Based on the target address of the MAC frame. Benefits of a bridge: filtering communications, increasing throughput, expanding the physical range, improving reliability, and connecting different physical layers and MAC layers at different speeds. Virtual Network VLAN: is a logical group composed of some lan cidr blocks that are not related to physical addresses. Author glinuxi