IEEE 802.6 Standrad Network Essential

Logic Link Control (LLC) is an IEEE 802.2LAN protocol that specifies the implementation of the data link layer on the LLC sublayer.  IEEE 802.2 LLC is used in IEEE 802.3 and IEEE 802.5 logic ring LANs to perform the following functions.  Following are the functions of the LLC Protocol

(i) Messaging data link communication,

( ii ) Link Addressing , 

( iii ) Determining Service Access Points ( SAPs ) , 

( iv ) Sequencing .

LLC is derived from High Level Data Link Control (HDLC) and uses a specialized subclass of HDLC.  The LLC defines the following three types of operations for data communication -

(i) Connectionless - Connectionless operation is basically seeding but there is no guarantee of receiving.  

(ii) Connection Oriented - Connection oriented operation for the LLC layer provides the following four services -

(a) Establishment of connection, 

(b) Authentication and receipt of acceptance,

(c) Recovery of errors by resending incorrect data requested.  

(d) Sliding window which is a method of increasing the rate of data transfer, 

(iii) Acceptance with connectionless service

Protocol Structure - The following are the different fields of the LLC header -

8	16	24 or32 bits	Variable
DSPA	SSAP	Control	Data or LLC Information

( i ) Destination Service Access Point ( DSAP ) - DSAP is a 7 - bit address in which the 8 ° bit is a specific address ( 0 ) or a group broadcast address ( 1 ).  DSAP is not a station or device address.

( ii ) Source Service Access Point ( SSAP ) - SSAP is also a 7 - bit address but in this case the 8th bit is used to indicate whether this message is a command ( 0 ) or a response ( I ).  Like DSAP, SSAP is a control point and not a station address.

( iii ) Control - The control field is either 8-bit or 16-bit large.  16-bit fields are used to exchange sequence numbers while 8-bits are used for random information. 

1.   IEEE 802 Modals