The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer.

Beneath the LLC sublayer is the Media Access Control ( MAC ) sublayer, which is dependent on the particular medium being used ( Ethernet, token ring, FDDI, 802. 11, etc.

In the seven-layer OSI model of computer networking, the logical link control ( LLC ) data communication protocol layer is the upper sublayer of the data link layer, which is itself layer 2.

The LLC sublayer acts as an interface between the media access control ( MAC ) sublayer and the network layer.

The LLC sublayer is primarily concerned with:

This implies that the need for LLC sublayer flow control and error management has reduced.

An LLC sublayer was a key component in early packet switching networks such as X. 25 networks with the LAPB data link layer protocol, where flow control and error management were carried out in a node-to-node fashion, meaning that if an error was detected in a frame, the frame was retransmitted from one switch to next instead.

The IEEE 802. 2 standard specifies LLC sublayer for all IEEE 802 local area networks, such as IEEE 802. 3 / Ethernet ( if the EtherType field is used ), IEEE 802. 5, and IEEE 802. 11, and in some non-IEEE 802 networks such as FDDI.

However, If Ethernet is used without EtherType field, Ethernet is considered as lacking LLC sublayer.

The LLC sublayer follows the IEEE 802. 2 standard.

The uppermost sublayer, LLC, multiplexes protocols running atop the data link layer, and optionally provides flow control, acknowledgment, and error notification.

* Logical link control ( LLC ) sublayer:

The MAC sublayer acts as an interface between the logical link control ( LLC ) sublayer and the network's physical layer.

This standard describes mainly the data link layer ( composed of the logical link control ( LLC ) sublayer and the media access control ( MAC ) sublayer ) and some aspects of the physical layer of the OSI reference model.

The NDIS forms the Logical Link Control ( LLC ) sublayer, which is the upper sublayer of the OSI data link layer ( layer 2 ).

There is also a form of CMIS that is developed to operate directly on top of the LLC sublayer.

The LLC sublayer takes the network protocol data, which is an IP packet, and adds control information to help deliver the packet to the destination node.

Those protocols operate directly on top of 802. 2 LLC, which provides both datagram and connection-oriented network services.

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Another example of a data link layer which is split between LLC ( for flow and error control ) and MAC ( for multiple access ) is the ITU-T G. hn standard, which provides high-speed local area networking over existing home wiring ( power lines, phone lines and coaxial cables ).

The LLC provides addressing and control of the data link.

* IEEE 802. 2 ( provides LLC functions to IEEE 802 MAC layers )

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It performs the multiple PDP Context PDU transfers and it also ensures that NPDUs from each PDP Context are transmitted to the LLC layer in sufficient time to maintain the QoS. SNDCP provides services to the higher layers which may include connectionless and connection-oriented mode, compression, multiplexing and segmentation.

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LLC is consequently only a multiplexing feature in today's link layer protocols.

For example, while HDLC specifies both MAC functions ( framing of packets ) and LLC functions ( protocol multiplexing, flow control, detection, and error control through a retransmission of dropped packets when indicated ), some protocols such as Cisco HDLC can use HDLC-like packet framing and their own LLC protocol.

Over telephone network modems, PPP link layer protocols can be considered as a LLC protocol, providing multiplexing, but it does not provide flow control and error management.

The Subnetwork Access Protocol ( SNAP ) is a mechanism for multiplexing, on networks using IEEE 802. 2 LLC, more protocols than can be distinguished by the 8-bit 802. 2 Service Access Point ( SAP ) fields.