OSI/RM - Open Systems Interconnection/Reference Model
The OSI/RM sometimes just called, the 'OSI' provides standards to be used and referenced by vendors and developers so the network and its various systems all speak the same language. This language comes in the form of protocols, and provided systems are all using the same protocols, they can all be interoperable.
Open Systems Interconnection Reference Model is a standard model and is used by networking engineers and protocol developers to design network systems that conform to this model. It is comprised of seven different layers that each actions certain protocols and processes the data packet ready to be sent to the next layer. This OSI is a conceptual model, you do not actually see it working but you can observe the result. Data enters the model at layer 7 and exits at layer 1 to be sent across the network to its destination address. As the data is processed and packaged, it is often referred to as a packet.
Going down the model the seven layers are:
Layer 7 Application
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Data
Layer 1 Physical
Layer 7 Application
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Data
Layer 1 Physical
The OSI describes the process of packet creation known as encapsulation and also describes how a packet is de-capsulated after delivery. When data travels down through the model it is encapsulated, this process is not that dissimilar to wrapping up a present for someone and using seven layers of wrapping paper. Each time a new wrapper is added, another item of the delivery address is added. This process is a very basic analogy for Encapsulation.
In terms of encapsulation a common mnemonic to remember it by:
All Application Layer
People Presentation Layer
Seem Session Layer
To Transport Layer
Need Network Layer
Data Data Link Layer
Processing Physical Layer
All Application Layer
People Presentation Layer
Seem Session Layer
To Transport Layer
Need Network Layer
Data Data Link Layer
Processing Physical Layer
When the transmitted data arrives at the destination it is processed in reverse and goes up through the model beginning at layer 1 and finishing at layer 7. This process is known as demultiplexing. During de-capsulation at the other end, these additional wrappers are peeled away one by one until the original data is presented. It's like a game of (pass the pacel) until the original gift is exposed, this process briefy decsribes De-capsulation.
In terms of de-capsulation a common mnemonic to remember it by:
Please Physical Layer
Do Data link Layer
Not Network Layer
Throw Transport Layer
Sausage Session Layer
Pizza Presentation Layer
Away Application Layer
Do Data link Layer
Not Network Layer
Throw Transport Layer
Sausage Session Layer
Pizza Presentation Layer
Away Application Layer
When data enters the top three layers of the model, it remains relatively unchanged and is essentially still data, it is called a PDU or protocol datagram unit at this point. When it reaches layer 4 it is known as a Segment, at layer 3 it becomes a Packet and at leyer 2 it becomes a Frame. The bottom layer converts the frame into Bits or (one’s and zeros) for transport across the network medium.
Below describes the model and its functions.
The seven layers of the OSI model
ASCII – American Standard Code for Information Interchange
EBCDIC – Extended Binary Coded Decimal Interchange Code
EBCDIC – Extended Binary Coded Decimal Interchange Code
The data encapsulation process adds headers - (like new wrappers!) to the data as it travels down through the OSI model, it is also de-encapsulated at the receiving end, and the additions are stripped away in reverse order to finally reveal the original data. At each layer a new header is added providing the packet with information relevant to that layer, or ‘peer layer’ at the receiving end. Thedata also changes state as is it is passed down.
Application, Presentation and Session Layers: PDU
The data at these layers receives its first headers and is known as a PDU or Protocol Data Unit.
Transport Layer: Segment
When the data reaches the transport layer it is encapsulated with source and destination port numbers that indentify the application the data is expected to use. It may be port 125 (SMTP) or 69(TFP), the data is also segmented into smaller chunks and becomes a segment at this layer.
Network Layer: Packet
When a segment is passed down from the transport layer it becomes a packet and is further encapsulated with source and destination IP addresses.
Data Link Layer: Frame
The packet is passed down to the data link layer where it is encapsulated with source and destination MAC addresses, a CRC or cyclical redundancy check is also added at this point, this uses a metric (counts the number of one’s) that matches the data at the receiving end to ensure the successful delivery of data. The CRC is added as a trailer to the end of the frame. If the CRC does not match the entire packet is discarded. At this point the packet becomes a frame.
Physical Layer: Bits
Frames are passed down to the physical layer where they are converted into bits ready for transmission. Simple one's and zero's
As data is processed first down through the model a packet is built, eventually it contains several headers and the original data resides in the middle known as the payload and the end of the packet has a trailer which may contain information that validates the packet. The trailer often contains a CRC and if the CRC is not matched at the destination end, the entire packet will be discarded and a request for a new one is sent.
Right shows the basic construction of a packet: |
Basic Packet Construction
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Several protocols are mapped to specific layers of the OSI, some overlap and others pertain just to one specific layer, below tables some protocols mapped to layers. Many application protocols also map to the presentation and session layers.