Data Link Layer

The second layer in the OSI Reference Model is the data link layer. Whereas the network layer provides for logical addresses for devices the data link layer provides for physical or hardware addresses. These hardware addresses are commonly called Media Access Control (MAC) addresses. The data link layer also defines how a networking device access the media that it is connected as well as defining the media frame type. This include the fields and components of the data link layer or layer 2 frame. This communication is only for devices on the same data link layer media type (or same piece of wire). To traverse media types Ethernet to Token Ring for instance typically a router is used. The data link layer is also responsible for taking bits (binary 1 and o) from the physically layer and reassembling them into the original data link layer frame. The data link layer dose error detection and will discard bad frames. It typically does not perform error correction as TCP/IP TCP protocol does however some data link layer protocols do support error correction functions. Examples of data link layer protocols and standards for local area  network (LAN) connections include IEEE 802.3 and 802.5 Ethernet II and ANSI FDDI. Example of WAN connections include ATM Frame Relay HDLC (High Level Data Link Control) SLIP (Serial Line Internet Protocol ) and X.25. Bridges switches and network interface controllers or cards (NIC) are the primary networking devices functioning the data link layer which is discussed in more depth in the section Data Link Layer later in this chapter.