Types of Services provided by LLC

Prerequisite – Services provided by Data Link Layer Logical Link Control (LLC) is one of the uppermost layers of the IEEE 802 Protocol Reference Model. LLC basically provides addressing and control of data links. Optionally, it can provide flow control, acknowledgement, or error recovery. LLC sits in the DLL (Data Link Layer) among the MAC layer and the layer 3 protocols and also forms an essential part of 802.2 specifications. LLC is usually responsible for the transmission of data among the computers or devices on a network. 

Services provided by LLC : LLC basically provides various services to the users using LSAPs (LLC Service Access Points). These services are generally specified in terms of service primitives. LLC standards generally specify and define three different forms of service to the LLC users as given below –

• Unacknowledged Connectionless Service (LLC 1) : Unacknowledged Connectionless Service, as the name suggests, is a service in which data frames are sent or transmitted from destination to source machine without any acknowledgement and without connection established among source and destination machine. In this, Unacknowledged Connection service comprises two words i.e. unacknowledged and connectionless service. The source machine sends or transmits data frames to the destination machine. But in return, the destination machine does not provide any acknowledgment to the source machine, so this service is known as unacknowledged service. Along with this, there is no connection established between the source and destination machine, therefore it is known as connectionless service. So, combined it is known as unacknowledged connectionless service. It is also responsible for providing only two service primitives across the interface among the next higher layer and LLC. This service provides the following functions –
  1. Datagram service with no acknowledgment.
  2. Error control
  3. Flow control
• Connection-Oriented Service (LLC 2) : Connection-Oriented Service, as the name suggests, is a service in which data frames are sent or transmitted from destination to source machine with acknowledgment and connection established among source and destination machine before any transmission of data. In this, Connection-Oriented Service comprises two words i.e. Acknowledged and connection service. The source machine sends or transmits data frames to the destination machines and in return, the destination machine provides an acknowledgment to the source machine, so this service is known as acknowledged service. Along with this, there is a connection established between the source and destination machine before any data transfer, therefore it is known as a connection-oriented service. So, combined it is known as acknowledged connection-oriented service. It does not support any multicast or broadcast addressing. It only supports individual addressing. It also provides a point-to-point link connection among LSAP (Link Service Access Points). It also provides a point-to-point link connection among LSAP (Link Service Access Points). This service provides the following functions –
  1. Flow control
  2. Sequencing of data
  3. Error indication and recovery
• Acknowledged Connectionless Service (LLC 3) : Acknowledged Connectionless Service, as the name suggests, is a service in which data frames are sent or transmitted from destination to source machine with acknowledgement and without connection established among source and destination machine. In this, Acknowledged Connection service comprises two words i.e., Acknowledged and connectionless service. The source machine sends or transmits data frames to the destination machines and in return, the destination machine provides an acknowledgement to the source machine, so this service is known as acknowledged service. Along with this, there is no connection established between the source and destination machine, therefore it is known as connectionless service. So, combined it is known as acknowledged connectionless service. It does not support any multicast or broadcast addressing. It is a rarely used service.