The Data Link Layer

The Data Link Layer is the protocol layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. The Data Link Layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Physical Layer. Examples of data link protocols are Ethernet for local area networks (multi-node) and PPP, HDLC and ADCCP for point-to-point (dual-node) connections.

The data link is all about getting information from one place to a selection of other close, local places. At this layer one does not need to be able to go everywhere globally, just able to go somewhere else locally. In the OSI model protocol stack the Network Layer, which is on top of the Data Link Layer, is analogous to the postal office making a best effort to delivering international mail. If a parcel is to be delivered from London to New York it can be sent via a variety of means: it can travel across the Atlantic by air or by sea, for which the exact route itself can also vary. The postal office (the Network Layer) only needs to try to get the parcel from the source to the correct destination regardless of the exact path it takes. The Data Link Layer in this analogy will be more akin to the role of a truck driver: the driver needs to know the local route to get from the post office to the airport/port. In fact, the driver would not need to know that the parcel he/she is delivering is ultimately bound for New York.

The Data Link Layer also serves the function of media access control. An example would be in an apartment building there is an WLAN access point (AP) in each of two neighboring apartments. A client can request access to one of the APs (say, AP A) by sending radio-frequency signals from his/her laptop. Since the two APs are in close proximity they may both be able to receive the request signals sent out by the client. It is the job of the Data Link Layer protocol to let AP B know that when it receives the client's signals they are not intended for it but for another AP. For AP A the decision as to whether the client is permitted access can also occur on the Data Link Layer.

The data link thus provides data transfer across the physical link. That transfer might or might not be reliable; many data link protocols do not have acknowledgments of successful frame reception and acceptance, and some data link protocols might not even have any form of checksum to check for transmission errors. In those cases, higher-level protocols must provide flow control, error checking, and acknowledgments and retransmission.

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. 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.