| OSI Model | |
|---|---|
| 7 | Application Layer |
| 6 | Presentation Layer |
| 5 | Session Layer |
| 4 | Transport Layer |
| 3 | Network Layer |
| 2 | Data Link Layer
|
| 1 | Physical Layer |
The Data Link Layer is Layer 2 of the seven-layer OSI model. It responds to service requests from the Network Layer and issues service requests to the Physical 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 MAC and 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.
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The uppermost sublayer is Logical Link Control (LLC). This sublayer multiplexes protocols running atop the Data Link Layer, and optionally provides flow control, acknowledgment, and error recovery. The LLC provides addressing and control of the data link. 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 sublayer below it is Media Access Control (MAC). Sometimes this refers to the sublayer that determines who is allowed to access the media at any one time (usually CSMA/CD). Other times it refers to a frame structure with MAC addresses inside. There are generally two forms of media access control: distributed and centralized. Both of these may be compared to communication between people:
The Media Access Control sublayer also determines where one frame of data ends and the next one starts. There are four means of doing that: a time based, character counting, byte stuffing and bit stuffing.
The time based approach simply puts a specified amount of time between frames. The major drawback of this is that new gaps can be introduced or old gaps can be lost due to external influences. Character counting simply notes the count of remaining characters in the frame's header. This method, however, is easily disturbed if this field gets faulty in some way, thus making it hard to keep up synchronization. Byte stuffing precedes the frame with a special byte sequence such as DLE STX and succeeds it with DLE ETX. Appearances of DLE (byte value 0x10) has to be escaped with another DLE. The start and stop marks are detected at the receiver and removed as well as the inserted DLE characters. Similarly, bit stuffing replaces these start and end marks with flag consisting of a special bit pattern (e.g. a 0, six 1 bits and a 0). Occurrences of this bit pattern in the data to be transmitted is avoided by inserting a bit. To use the example where the flag is 01111110, a 0 is inserted after 5 consecutive 1's in the data stream. The flags and the inserted 0's are removed at the receiving end. This makes for arbitrary long frames and easy synchronization for the recipient.
In a snail-mail network, each letter is one frame of data, and one can tell where it begins and ends because it is inside an envelope. One might also specify that a letter will begin with a phrase like "Dear Sir", and ends with a phrase like "Yours faithfully".
The Data Link Layer is often implemented in software as a "network card driver". The operating system will have a defined software interface between the data link and the network transport stack above. This interface is not a layer itself, but rather a definition for interfacing between layers.
In the frame work of the TCP/IP (Internet Protocol Suite) model, OSI's Data Link Layer, in addition to other components, is contained in TCP/IP's lowest layer, the Link Layer. The Internet Protocol's Link Layer only concerns itself with hardware issues to the point of obtaining hardware addresses for locating hosts on a physical network link and transmitting data frames onto the link. Thus, the Link Layer is broader in scope and encompasses all methods that affect the local link, which is the group of connections that are limited in scope to other nodes on the local access network.
The TCP/IP model is not a top/down comprehensive design reference for networks. It was formulated for the purpose of illustrating the logical groups and scopes of functions needed in the design of the suite of internetworking protocols of TCP/IP, as needed for the operation of the Internet. In general, direct or strict comparisons of the OSI and TCP/IP models should be avoided, because the layering in TCP/IP is not a principal design criterion and in general considered to be "harmful" (RFC 3439). In particular, TCP/IP does not dictate a strict hierarchical sequence of encapsulation requirements, as is attributed to OSI protocols.