The Logical Link Control (LLC) is a sublayer of the data link layer in the OSI model. It creates frames, standardizes interfaces, controls data flow, and ensures data reaches its destination. LLC1 is connectionless, while LLC2 is connection-oriented and provides better error checking. TCP uses a connectionless method, IP.
Logical link control (LLC) is one of two sublayers of the second layer of the Open Systems Interconnection Model (OSI), the data link layer. It is the LLC that takes the packets and puts them into frames which can then be converted by lower levels into the bits that actually move on a medium. That’s not its only job, however, as the LLC is also responsible for creating a more standardized interface for many different communication protocols, controlling data flow, and ensuring that data gets to its destination.
The way the data link layer works is that the higher 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 Checker, working in concert with its sibling, inserts data into what is known as a frame. The frames are then transferred to the multiplexed analog component (MAC), which assigns specific hardware address destinations to the frames. Once the data link layer has done its job, it passes the data to the lowest layer of the OSI model, the physical layer, which then transforms the data into an electrical signal stream for the wire. In this way, the LLC provides the ability for any upper layer to transmit data without having to know anything about the type of network the data is going over.
There are two types of operations for logical link checking, connectionless and connection-oriented. With a connectionless LLC, also known as logical link control type one, or LLC1, no guaranteed connection is established with the receiving end. LLC type one simply sends and receives streams of data between other connectionless peers without each of them bothering to acknowledge receipt. This is useful for some communication protocols that do not require their transmission to be negotiated in advance, such as the Hypertext Transfer Protocol (HTTP) used on the world wide web and the User Datagram Protocol (UDP) which is used for many other services .
With a logical link type two connection, or LLC2, connection-oriented broadcasting requires each end of the communication to confirm its connection to the other before any data can be sent or received. LLC2 provides better error checking and correction, in case some data is lost along the way. With this pre-established communication link, one of the two ends of the communication is required to acknowledge receipt of the data and ask for anything that might be missing. One of the most well-known and used connection-oriented protocols, the Transmission Control Protocol (TCP), actually uses a connectionless method, the Internet Protocol (IP). It does this by numbering sent packets and rearranging them when received.
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