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The logical link control (LLC) is one of the two sub-layers of the open systems interconnection (OSI) model's second layer, the data link layer. It's the LLC that takes packets and puts them into frames that can then be converted by the lower layers into the bits that actually move across a medium. This is not its only job, however, as the LLC is also responsible for establishing a more standardized interface for many different communications protocols, controlling the flow of data, and ensuring that data arrives at its destination.
The way the data link layer works is that the upper layers of the OSI model pass datagrams or packets down the other layers of the model until they reach the data link layer. The logical link control, working in concert with its sibling, inserts the data into what's known as frames. The frames are then handed over to the multiplexed analog component (MAC), which assigns the specific hardware address destinations to the frames. Once the data link layer has completed its job, it passes the data down to the OSI model's lowest layer, the physical layer, which then transforms the data into a stream of electrical signals for the wire. In this manner, the LLC provides the ability for any of the upper layers to transmit data without having to know anything about the type of network the data is going out on.
There are two types of operation for the logical link control, connectionless and connection-oriented. With a connectionless LLC, also known as logical link control type one, or LLC1, there is no guaranteed link established with the receiving end. LLC type one simply sends and receives streams of data amongst other connectionless peers without each of them bothering to confirm receipt. This is useful for some communication protocols that don't need to have their transmission negotiated before hand, such as the hypertext transfer protocol (HTTP) used on the world wide web, and the user datagram protocol (UDP) that is used for numerous other services.
With a logical link connection type two, or LLC2, connection-oriented transmission requires that each end of the communication confirm its link to the other before any data can be sent or received. LLC2 provides better error checking and correction, in the case that some of the data gets lost along the way. With this pre-established communication link, either end of the communication is required to acknowledge receipt of the data and ask for anything that might have come up missing. One of the most well-known and frequently used connection-oriented protocols, the transmission control protocol (TCP), actually rides a connectionless method, the Internet protocol (IP). It does this by numbering the packets sent and putting them back in order when received.
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