Technology Roadmap for Intelligent Buildings
Appendix B: OSI Seven Layer Communication Model

Frequent references have been made to the use of communications as a utility within an Intelligent Building. Little emphasis has been given to the mechanism by which the infrastructure is supported with the diverse systems and technologies implicit in an Intelligent Building. The OSI (Open System Interconnection) model is a standard for communications that defines networking in terms of seven layers. This standard was developed and is supported by the International Standards Organization, a United Nations agency located in Geneva, Switzerland.

Control in each case is passed from one layer to the next. This model allows layers to be tuned and modified without prejudicing the other layers. This theory does not work with all modern protocol implementations because not everybody adheres to all of the standards. In some cases, as noted below, some of the layers (5 and 6) are effectively bypassed. The concepts, however, are crucial to any form of network and communications system. The traditional seven layers are:

1. Physical Layer: this defines the hardware for sending and receiving data, including cables, cards and electrical interfaces.
2. Data Link: this handles errors in the physical layer, places bits into frames, packetizes the data and validates some error correcting protocols for example, checksums. The Media Access Control (MAC) and the Logical Link Control (LLC) are sub layers of the data link layer.
3. Network Layer: this provides switching and routing technologies, including virtual circuits for data from node to node. Routing and forwarding, including addressing, internet working and packet sequences are included. Protocols such as X.25 and IP are part of this layer's responsibilities.
4. Transport Layer: this layer provides transparent transfer of data between end systems and does not deal with any lost messages, except for the provision of a reliable service. It may reduce the size of messages from higher level layers in order to ensure effective transportation. If packets arrive out of order (diverse routing) they must be resequenced. The most common internet protocol, Transport Common Protocol (TCP) mates with layer 3, IP protocols.
5. Session Layer: manages connections between applications and sets up, coordinates and terminates conversations. It is widely ignored in practical terms.
6. Presentation Layer: this provides independence from differences in data representations or encryptions by translating from application to network format and vice versa. It transforms data into a form that the application layer can accept.
7. Application Layer: this is a collection of miscellaneous protocols for high level applications, including electronic mail, file transfer, remote terminal access and terms such as SMTP, FTP, Telnet, HTTP are widely evident in this layer.

While the OSI model serves as a valuable concept for outlining network and communications systems, it should be noted that this model has not been adopted by all manufacturers.

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