Logical Link Control
| layer | originally | expanded |
|---|---|---|
| 7th | application | application |
| 6th | presentation | presentation |
| 5 | meeting | meeting |
| 4th | transport | transport |
| 3 | Mediation | Mediation |
| 2 | Fuse | Logical Link Control |
| Media Access Control | ||
| 1 | Data transfer | Data transfer |
Logical Link Control (LLC) is the name for a network protocol of telecommunications , which by the Institute of Electrical and Electronics Engineers has been standardized as IEEE 802.2. The aim of the protocol is the transparency of different methods used at MAC level for media allocation. It offers an unconfirmed, connectionless service (LLC 1), a connection-oriented service (LLC 2), and a confirmed connectionless service (LLC 3). LLC is assigned to Layer 2 of the OSI model . The sublayer on which the protocol works is also referred to as LLC. This is why LLC is sometimes also called layer 2b and thus forms the security layer (2) together with the MAC layer (2a). It distributes incoming data by forwarding it to the corresponding instance protocols of OSI layer 3 . Data that OSI layer 3 sends for transmission is passed on from LLC to the MAC layer.
structure
The LLC protocol adds two 8-bit identifiers called DSAP ( Destination Service Access Point : entry address of the recipient) and SSAP ( Source Service Access Point : entry address of the recipient) to a given data packet from a higher-level layer (usually the OSI layer 3 "network layer") Sender). There is also an 8 or 16 bit field ( control ) with control information for auxiliary functions such as data flow control.
| DSAP (1 byte) |
| SSAP (1 byte) |
| Control (1–2 bytes) |
The LLC protocol is based on the bit-oriented HDLC protocol. LLC only uses Asynchronous Balanced Mode (ABM), so any station can be the primary station.
LLC frame types
|
U-Frames unnumbered frames (LLC Type-1) |
SABME set asynchronous balanced mode extended |
Command |
|
DM disconnected mode |
Response | |
|
DISC disconnect |
Command | |
|
UA unnumbered acknowledge |
Response | |
|
FRMR frame reject |
Response | |
|
TEST test |
Command / Response | |
|
UI unnumbered information |
Command | |
|
XID exchange identifier |
Command | |
|
S-frames supervisory frames (LLC Type-2) |
RR receive ready |
Command / Response |
|
RNR receive not ready |
||
|
REJ reject |
||
|
SREJ selective reject |
||
|
I-Frame information frame (LLC Type-3) |
I information |
Command |
LLC classes
Individual stations can offer more than one type of service. Classes of services are combinations of available types of services.
| Class I. | Class II | Class III | Class IV | |
|---|---|---|---|---|
| LLC Type 1 Unconfirmed Unlinked Service |
X | X | X | X |
| LLC Type 2 connection-oriented service |
X | X | ||
| LLC Type 3 Confirmed Service Without Connection |
X | X |