Open System Interconnection MEBS 6000 2010 Utilities services ISO 1984: ISO Standard 7498 Information Processing System Open System Interconnection Basic Reference Model OSI is itself not a standard, but offers a framework to identify and separate the different conceptual parts of the communication process. 7 functional layers are defined in OSI Each layer communicates directly only with the layers above and below it, requesting services from the layer below and providing services to the layer above. K.F. Chan (Mr.) Page 1 of 5 July 2010
OSI Layer Function Application This layer is the window to user program and transfers information from program to program. FTP, SMTP are protocols on this layer. Presentation Handles semantics. This layer is responsible for data presentation, encryption and compression. Session Establishes, maintains and coordinates communication. It establishes and brings down a connection, and handles communications in which an ongoing dialogue occurs. This enhances the transport layer by allowing users on different machines to establish full SESSIONS between them Transport (#) It is responsible for end-to-end reliability and ensures accurate delivery of data. Error recovery is the duty of this layer. TCP and UDP protocols belong to this layer Network Determines transport routes and handles the transfer of messages. Message segmentation and reassembly are duties of the network layer. IP protocol belongs to this layer. Routers are devices on this layer. Since operators of the subnet may expect remuneration, there is often some accounting function built into the network layer Data Link (*) This layer is concerned with orderly access to the media, thus manages codes, message framing and hardware addresses. Ethernet is both a data link and physical layer technology. Physical This layer is concerned with the raw bits on the physical medium, and manages hardware connections. This layer is the only real connection between 2 communicating nodes. RS232C protocol is a physical layer technology. (*) In IEEE network standard, data link layer is further divided into 2 sub-layers Logical Link Control (LLC), (IEEE 802.2) and Medium Access Control (MAC) (IEEE 802.3/802.4/802.5) (#) Layer 4 (transport) is the actual interface between the machine software and the external network. The highest 3 layers are implemented in the application software of a target computer. The lower 3 layers are network or communication layers. Layer 4, the transport layer, is the link between the application and the communication oriented layers K.F. Chan (Mr.) Page 2 of 5 July 2010
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TCP/IP model compared with OSI model (Adopted from DEAN, T. Network+ Guide to Network) Although the OSI model can be used to explain TCP/IP, TCP/IP actually predates the OSI model. In fact, TCP/IP, which is based on a 4-layer US Department of Defense model, is very often compared with the OSI model. The IP protocol provides connectionless mode of data transfer. The data are transmitted as independent units called data grams. The IP provides services such as data exchange, addresses, and routing independent of the media and network topology. TCP is a connection-oriented protocol which provides end-to-end transmission services and data integrity support such as sequencing, flow control, reliability data checks, etc. K.F. Chan (Mr.) Page 4 of 5 July 2010
TCP/IP and OSI [Adopted from FREEMAN, Roger, Fundamentals of Communications] Another layer 4 protocol, User Datagram Protocol, is also popular in the cyber world. UDP is a connectionless protocol providing routing information with each message, allowing name assignment to each protocol address, and providing for remote network management. The popular building management systems communication protocol BACnet is based on a connectionless communication model, and does not have a transport layer in its protocol stack. K.F. Chan (Mr.) Page 5 of 5 July 2010