.jpg "the pp header and trailers used to create a ppp frame")
MAC may refer to the sublayer that determines who is allowed to access the media at any one time (e.g. It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the originator and recipient machines. The LLC provides addressing and control of the data link. The uppermost sublayer, LLC, multiplexes protocols running at the top of the data link layer, and optionally provides flow control, acknowledgment, and error notification. The data link layer is often divided into two sublayers: logical link control (LLC) and media access control (MAC). In the ITU-T G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) local area network using existing home wiring ( power lines, phone lines and coaxial cables), the data link layer is divided into three sub-layers (application protocol convergence, logical link control and media access control). Other data-link-layer protocols, such as HDLC, are specified to include both sublayers, although some other protocols, such as Cisco HDLC, use HDLC's low-level framing as a MAC layer in combination with a different LLC layer. In some networks, such as IEEE 802 local area networks, the data link layer is described in more detail with media access control (MAC) and logical link control (LLC) sublayers this means that the IEEE 802.2 LLC protocol can be used with all of the IEEE 802 MAC layers, such as Ethernet, Token Ring, IEEE 802.11, etc., as well as with some non-802 MAC layers such as FDDI. In contrast to the hierarchical and routable addresses of the network layer, layer 2 addresses are flat, meaning that no part of the address can be used to identify the logical or physical group to which the address belongs. The frame header contains the source and destination addresses that indicate which device originated the frame and which device is expected to receive and process it. In those cases, higher-level protocols must provide flow control, error checking, acknowledgments, and retransmission.
That transfer can be reliable or unreliable many data link protocols do not have acknowledgments of successful frame reception and acceptance, and some data link protocols might not even perform any check for transmission errors. Within the semantics of the OSI network architecture, the protocols of the data link layer respond to service requests from the network layer, and perform their function by issuing service requests to the physical layer. The data link provides for the transfer of data frames between hosts connected to the physical link. In the Internet Protocol Suite (TCP/IP), the data link layer functionality is contained within the link layer, the lowest layer of the descriptive model, which is assumed to be independent of physical infrastructure. Data-link protocols specify how devices detect and recover from such collisions, and may provide mechanisms to reduce or prevent them.Įxamples of data link protocols are Ethernet, Point-to-Point Protocol (PPP), HDLC and ADCCP. When devices attempt to use a medium simultaneously, frame collisions occur. In this way, the data link layer is analogous to a neighborhood traffic cop it endeavors to arbitrate between parties contending for access to a medium, without concern for their ultimate destination. Inter-network routing and global addressing are higher-layer functions, allowing data-link protocols to focus on local delivery, addressing, and media arbitration. Data-link frames, as these protocol data units are called, do not cross the boundaries of a local area network. The data link layer is concerned with local delivery of frames between nodes on the same level of the network. The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking.
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