OSI Reference Model


The ISO (International Organization for Standardization) developed a standard defining a model for a general purpose data communications architecture.

Each of the seven layers of this model is implemented in the LonTalk protocol. Each has a purpose to make the technology robust and provide room to grow the network. The most important benefit of this approach is that each layer performs services for the next higher layer so that details are hidden to the higher layer. Changes can be made in a layer without changing any of the other layers.

Layer 1: Network Physical Layer - Electrical Interconnect

This layer addresses specifics of wiring and connections. The specification of the 78 kbps twisted pair media with 2000 meter range, 64 nodes per network segment, and network isolation characteristics is an example of one physical layer type of media. LONWORKS technology provides many different communications media options including 1.25 Mbps twisted pair, power line, fibre optic, and RF transceivers. This provides you with a wide range of choices for communicating your data.

Layer 2: Data Link Layer - Media Access and Framing

This layer defines the rules of access to the physical layer. For example, this corresponds to the dial tone on the telephone network.

Services provided by this layer include:

  • Error Detection (CRC)
  • Flexible allocation of bandwidth
  • Priority access mechanisms
  • Graceful behaviour under overload (p-persistent CSMA)
  • Message collision avoidance
  • Optional collision resolution, collision detection

Layer 3: Network Layer - Destination Addressing

This layer specifies the destination of a message on the network. This corresponds to the area and long distance codes on the telephone network. Services provided by this layer include:

  • Node address information
  • Routing of messages to segment and control network bandwidth usage
  • Services such as determining which nodes on the network receive various messages.
  • The ability to provide routers to segment the traffic and communicate between different physical media.

Layer 4: Transport Layer - End to End Reliability

This layer establishes the type of services required for the node messages depending on the level of reliability required by the application.

The services provided are:

  • Broadcast addressing
  • Unicast addressing
  • Multicast addressing
  • Repeated service
  • Acknowledged service
  • Unacknowledged service
  • Duplicate packet detection
  • Authentication

The level of service required by the application is established when each node is installed on the network. This is all handled by a network management installation tool, and the node’s design.

Layer 5: Session Layer - Remote Actions

This layer provides the communications to request action from another node.

Examples of the services include:

  • Acknowledgment of received message
  • Application to application communication
  • Retry if the correct response is not received from the remote node
  • Request to a destination group and receive individual responses from the group
  • Request - response message authentication

Layer 6: Presentation Layer - Data Interpretation

This layer provides translation of the network data for the application.

Examples of services provided in this layer include:

  • Input, output, and configuration variables for the node
  • Standard data representations for physical quantities
  • Network variable description

The standard data representations are important to assure interoperability between products from different manufacturers.

Layer 7: Application Layer- Application Compatibility

This layer includes services to simplify development of application programs to interface to specific sensors, actuators, and external microprocessors.

The services provided in this layer include the following:

  • Memory storage for application program
  • Built-in real-time operating system
  • Device drivers for the I/O hardware on the Neuron Chip
  • Standard Network Variable Types (SNVTs)


We hope this has been a useful introduction to Echelon Corporation’s LONWORKS® technology, and not too technical.

Davmark Group make things work. We do the hard work, not you.

Davmark Group provides advanced solutions using LONWORKS.

Extensive portions of this article were quoted verbatim from Motorola document BR1108/D, LONWORKS Product Line Brief and EB161/D, LonTalk Protocol. Davmark Group wishes to express their grateful acknowledgment to both Echelon and Motorola Semiconductor for these reference documents.


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