The data link layer is the second layer in the OSI reference model, located between the physical layer and the network layer. It is responsible for establishing, maintaining, and releasing data link connections between network nodes, and achieves reliable data transmission between adjacent nodes through frame transmission. Its core functions include framing (using byte counting method, character padding, bit padding, and illegal encoding method to achieve frame delimitation) [2] [6], error control (CRC verification) [1] [4], flow control (sliding window protocol) [4] [6], and link management [5].
The protocol system of this layer is divided into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC) [1] [5]. The former uses the stop-and-wait ARQ protocol to achieve error control [1], while the latter solves the problem of channel competition and performs MAC addressing [5-6]. The main protocols include High-Level Data Link Control Protocol (HDLC) and Point-to-Point Protocol (PPP). Among them, the HDLC frame structure consists of a flag field, an address field, a control field, an information field, and a check field, supporting three operation modes: normal response, asynchronous response, and asynchronous balance [2-4]. The Ethernet protocol uses the CSMA/CD mechanism to achieve media access control, and addresses physical devices through MAC addresses [5-6].
The data link layer realizes the encapsulation and decapsulation of network layer data through transparent transmission technology, and the frame check sequence field can verify the integrity of the frame content [2] [4]. The MAC layer subsystem includes time slot processing and time synchronization modules, ensuring the effective allocation of channel resources [3].