Logic Link Control (LLC) is the IEEE 802.2 LAN protocol that specifies an implementation of the LLC sublayer of the data link layer. IEEE 802.2 LLC is used in IEEE802.3 (Ethernet) and IEEE802.5 (Token Ring) LANs to perform these functions:
- Managing the data-link communication
- Link Addressing
- Defining Service Access Points (SAPs)
- Sequencing
The LLC provides a way for the upper layers to deal with any type of MAC layer (e.g. Ethernet - IEEE 802.3 CSMA/CD or Token Ring IEEE 802.5 Token Passing).
LLC was originated from the High-Level Data-Link Control (HDLC) and it uses a subclass of the HDLC specification. LLC defines three types of operation for data communication:
- Type 1: Connectionless, the connectionless operation is basically sending but no guarantee of receiving.
- Type 2: Connection Oriented. The connection-Oriented operation for the LLC layer provides these 4 services: Connection establishment; Confirmation and acknowledgement that data has been received; Error recovery by requesting received bad data to be resent; Sliding Windows (Modulus: 128), which is a method of increasing the rate of data transfer.
- Type 3: Acknowledgement with connectionless service.
The Type 1 connectionless service of LLC specifies a static-frame format and allows network protocols to run on it. Network protocols that fully implement a transport layer will generally use Type 1 service.
The Type 2 connection-oriented service of LLC provides reliable data transfer. It is used in LAN environment that do not invoke network and transport layer protocols.
Protocol Structure
Logic Link Control Layer (LLC) Header:
| 8 bit | 16 bit | 24 or 32 bit | Variable |
| DSAP | SSAP | Control | LLC information |
DSAP - The destination service access point structure is as follows:
| 1 bit | 8 bit |
| I/G | Address bits |
I/G: Individual/group address may be: 0 Individual DSAP; 1 Group DSAP.
SSAP - The source service access point structure is as follows:
| 1 bit | 8 bit |
| C/R | Address bits |
C/R: Command/response: 0 Command; 1 Response.
Control - The structure of the control field is as follows:
| 1 bit | 8 bit | 9 bit | 16 bit | ||||
| Information | 0 | N(S) | P/F | N(R) | |||
| Supervisory | 1 | 0 | SS | XXXX | P/F | N(R) | |
| Unnumbered | 1 | 1 | MM | P/F | MMM | ||
| N(S) | Transmitter send sequence number. |
| N(R) | Transmitter receive sequence number. |
| P/F | Poll/final bit. Command LLC PDU transmission/response LLC PDU transmission. |
| S - Supervisory function bits: | |
| 00 - RR (receive ready). | |
| 01 REJ (reject). | |
| 10 - RNR (receive not ready). | |
| X | Reserved and set to zero. |
| M | Modifier function bits. |
| LLC | information - LLC data or higher layer protocols. |
Related protocols: IEEE 802.3, 802.5, HDLC
Sponsor Source: LLC is defined by IEEE (http://www.ieee.org) in the 802.2 specifications.
Reference:
http://standards.ieee.org/getieee802/download/802.2-1998.pdf: IEEE 802.2 specification.
https://secure.linuxports.com/howto/intro_to_networking/c5048.htm: IEEE 802.2: Logic Link Control Layer