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CS 635 Lecture 3, Part 1 Logical Link Control


Unacknowledged connectionless service

Datagram style service, send PDUs without acknowledgment. This service provides two primitives:
  1. DL-UNIDATA.request(sa,da,data,priority) - used to pass a block of data down to LLC for transmission
  2. DL-UNIDATA.indication(sa,da,data,priority) - used to pass a block of data up to the destination use from LLC upon reception

source address and destination address specify the local and remote LLC users. Each address is a combination of LLC SAP and the MAC   address. MAC layer is responsible for implementing a priority mechanism. Token bus and token ring are capable of enforcing priority, CSMA/CD system is not.

Connection mode service
Enables two communicating LLC users to set up a logical connection for data transfer. Provides for establishing and terminating a connection, flow control, sequencing and error recovery. This service supports individual addressing, no broadcast or multicast. This service provides the following primitives:
Connection establishment
  1. DL-CONNECT.request(sa,da,priority) - used to requests a connection
  2. DL-CONNECT.indication(sa,da,priority) - used to indication connection request to the use
  3. DL-CONNECT.response(sa,da,priority) - used to refuse the connection
  4. DL-CONNECT.confirm(sa,da,priority) - used to indicate refusal of connection

If connection is established, the priority is assigned to the connection as a whole. In addition, the priority is passed down to the MAC entity for each data unit transferred over the logical connection.

Data transfer

There are no response and confirm primitives because LLC guarantees to deliver all data in the proper order, with no losses.

Connection termination
  1. DL-DISCONNECT.request(sa,da) - used to terminate the connection
  2. DL-DISCONNECT.indication(sa,da,reason) - used to inform the user of connection termination
  3. DL-RESET.request(sa,da) - resynchronize in case of data loss due to terminate connection
  4. DL-RESET.indication(sa,da,reason) -
  5. DL-RESET.response(sa,da,reason)
  6. DL-RESET.confirm
Flow control
  1. DL-CONNECTION-FLOWCONTROL.request(sa,da,amount) - used to regulate flow control across the LSAP. Local flow control mechanism. Specifies the amount of data that may be passed across the LSAP.
  2. DL-CONNECTION-FLOWCONTROL.indication(sa,da,amount)

Acknowledged Connectionless service

A user can send a unit of data and receive an aknowledgment that the data was delivered, without the necessity of setting up a connection.
  1. DL-DATA-ACK.request(sa,da,data,priority,service-class)
  2. DL-DATA-ACK.indication(sa,da,data,priority,service-class)
  3. DL-DATA-ACK-STATUS(sa,da,priority,service-class,status)

The DL-DATA-ACK service is a guaranteed delivery service, in which data are sent from the an originating LLC user, and acknowledged.

  1. DL-REPLY.request(sa,da,data,priority,service-class)
  2. DL-REPLY.indication(sa,da,data,priority,service-class)
  3. DL-REPLY-STATUS(sa,da,data,priority,service-class,status) - provides ack to the sending user; the status parameter indicates whether or not the data unit was received successfully by the peer LLC entity.

The DL-REPLY service is a poll with a guaranteed response. It enables a user to request a previously prepared data unit from another user or to exchange data units with another user. The service-class parameter specifies whether or not an aknowledgement capability in the MAC layer is to be used for the data unit transmission.  Only 802.4 supports this capability.

  1. DL-REPLY-UPDATE.request(sa,data)
  2. DL-REPLY-UPDATE-STATUS.indication(sa,status)

The DL-REPLY-UPDATE service allow a user to pass data to LLC to be held and sent out at a later time when requested to do so ( by a DL-REPLY primitive) by some other station.

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