What is OSI model

OSI model:-
       The Open Systems Interconnection (OSI) model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar communication functions are grouped into logical layers. 
         The layers are
                    Layer 7: Application Layer 
                    Layer 6: Presentation Layer 
                    Layer 5: Session Layer 
                    Layer 4: Transport Layer 
                    Layer 3: Network Layer 
                    Layer 2: Data Link Layer 
                    Layer 1: Physical Layer

What is the OSI Model
         The OSI model is a framework for network communications. The OSI model is the fundamental model  for developing proper network communications. Consisting of 7 layers the OSI model frames communication from the Physical Media all the way to the Application (envision a web browser). Message traffic moves up and down the OSI model depending on the purpose of the message.
1. Physical Layer- This is the media you use to communicate by use of RS-485/Ethernet ect..
2. Data Link Layer- This is the subnet communication or for example the communication of MS/TP on a field bus.
3. Network Layer- This is typically were your supervisory devices lie and is where IP messages exist and route outside of the local subnet.
4. Transport Layer- This is where the data verification exist for communication. Typically when you are transferring code or you are logging into a system the verification of the transport is made at the Transport layer.
5. Session Layer- This is where the conversation is initiated. Often times two supervisory devices will create a session to discuss. As you learn more about TCP and UDP you will learn that UDP is a connection-less protocol essentially it just sends out data. Our control systems usually require verification of commands as such our systems rely on TCP sessions.
6.Presentation Layer- This is where the data is taken and formulated into view-able information. Often times this is when the raw BACnet/IP data shifts into a view-able data format whether that be ASCII or UNICODE. Additionally the presentation layer can translate data into XML which is utilized by the majority of the web based building automation systems.
7.Application Layer- Here programs run to visually present and physically interact with the data from the previous layers. Typically building automation systems will use applications built around the HTTP or SNMP protocols. Additionally applications can create FTP connections between one another to transfer data this is often seen in high level peer to peer supervisory communication.



 OSI Layer Model for concentrators
Hubs/Repeaters are found in the Physical Layer.
Switches /Bridges/Wireless Access Point are found in the Data Link Layer.
Routers are found in the Network Layer.
Gateway are found in All 7 of the OSI Layers.
Brouter(Bridge Router) are found in both the Data Link and Network Layer.


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