Nó cũng xác định định dạng dữ liệu, bao gồm cả khung của dữ liệu trong thông điệp truyền đi, thủ tục kiểm soát lỗi, và các hoạt động điều khiển liên kết khác. Bởi vì nó xác định định dạng dữ liệu, bao gồm cả thủ tục để sửa lỗi truyền dẫn, lớp này sẽ trở thành trách nhiệm cho việc cung cấp đáng tin cậy của thông tin. | 44 chapter two It also defines data formats including the framing of data within transmitted messages error control procedures and other link control activities. Because it defines data formats including procedures to correct transmission errors this layer becomes responsible for the reliable delivery of information. An example of a data link control protocol that can reside at this layer is the ITU s High-Level Data Link Control HDLC . Because the development of OSI layers was originally targeted toward wide area networking its applicability to local area networks required a degree of modification. Under the IEEE 802 standards the data link layer was initially divided into two sublayers logical link control LLC and media access control MAC . The LLC layer is responsible for generating and interpreting commands that control the flow of data and perform recovery operations in the event of errors. In comparison the MAC layer is responsible for providing access to the local area network which enables a station on the network to transmit information. With the development of high-speed local area networks designed to operate on a variety of different types of media an additional degree of OSI layer subdivision was required. First the data link layer required the addition of a reconciliation layer RL to reconcile a medium-independent interface MII signal added to a version of high-speed Ethernet commonly referred to as Fast Ethernet. Next the physical layer used for Fast Ethernet required a subdivision into three sublayers. One sublayer known as the physical coding sublayer PCS performs data encoding. A physical medium attachment sublayer PMA maps messages from the physical coding sublayer to the transmission media while a medium-dependent interface MDI specifies the connector for the media used. Similarly Gigabit Ethernet implements a gigabit media-independent interface GMII which enables different encoding and decoding methods to be supported that are used with .
