The second-lowest layer (layer 2) in the OSI Reference Model stack is the data link layer, often abbreviated “DLL” (though that abbreviation has other meanings as well in the computer world).The data link layer, also sometimes just called the link layer, is where many wired and wireless local area networking (LAN) technologies primarily function. For example, Ethernet, Token Ring, FDDI and 802.11 (“wireless Ethernet” or “Wi-Fi’) are all sometimes called “data link layer technologies”. The set of devices connected at the data link layer is what is commonly considered a simple “network”, as opposed to an internetwork.
The data link layer is often conceptually divided into two sublayers: logical link control (LLC) and media access control (MAC). This split is based on the architecture used in the IEEE 802 Project, which is the IEEE working group responsible for creating the standards that define many networking technologies (including all of the ones I mentioned above except FDDI). By separating LLC and MAC functions, interoperability of different network technologies is made easier, as explained in our earlier discussion of networking model concepts.
The following are the key tasks performed at the data link layer:
- Logical Link Control (LLC): Logical link control refers to the functions required for the establishment and control of logical links between local devices on a network. As mentioned above, this is usually considered a DLL sublayer; it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.
- Media Access Control (MAC): This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts. For example. Ethernet uses the CSMA/CD method of media access control, while Token Ring uses token passing.
- Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer.
- Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labeling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.
- Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack. For example, a cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if it was received correctly.
Physical Layer Requirements Definition and Network Interconnection Device Layers
As I mentioned in the topic discussing the physical layer, that layer and the data link layer are very closely related. The requirements for the physical layer of a network are often part of the data link layer definition of a particular technology. Certain physical layer hardware and encoding aspects are specified by the DLL technology being used. The best example of this is the Ethernet standard, IEEE 802.3, which specifies not just how Ethernet works at the data link layer, but also its various physical layers.
Since the data link layer and physical layer are so closely related, many types of hardware are associated with the data link layer. Network interface cards (NICs) typically implement a specific data link layer technology, so they are often called “Ethernet cards”, “Token Ring cards”, and so on. There are also a number of network interconnection devices that are said to “operate at layer 2”, in whole or in part, because they make decisions about what to do with data they receive by looking at data link layer frames. These devices include most bridges, switches and barters, though the latter two also encompass functions performed by layer three.
Some of the most popular technologies and protocols generally associated with layer 2 are Ethernet, Token Ring, FDDI (plus CDDI), HomePNA, IEEE 802.11, ATM, and TCP/IP's Serial Link Interface Protocol (SLIP) and Point-To-Point Protocol (PPP).