Layers of the OSI ISO 7-Layer Model

When software engineers think about communications they visualize this type of architecture

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You will notice at the very bottom, something called TCP/IP (more on this later)

Sofwareware engineers are thinking JSON, XML, or a binary formatted payload being transmitted between two application/service endpoints. The architecture to support this type of transmission must go beyond these types of payloads because both XML and JSON are current phenomena

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Application Layer (7)

Applications that create and consume data, such as browsers, Zoom, Email, and proprietary applications

At this level, data is structured as messages

Presentation Layer (6)

How is the data for the specific applications structured on the wire? For example, when dealing with web applications using HTTP/S data is structured according to the HTTP protocol format (text file but data within that text file has to be structured in a specific way). Another example is when organizations transmit JSON data between endpoints they have to agree on the structure of this data (text file but data has to be structured in a specific way). CORBA uses the GIOP protocol at this level.

Outgoing data from the application is extracted and formatted ready for transmission. Incoming data to the application is taken off the wire and presented in a meaningful way to the application.

At this level, data is structured as messages

Session Layer (5)

Establish a connection, maintain sessions, authentication, and ensure security. 

This layer allows two processes to establish, use and terminate a connection.

This layer allows a process to add checkpoints which are considered synchronization points in the flow of the data. These synchronization points help to identify possible errors in the transmission of the data, if there are it can be re-synchronized properly, it also ensures that the ends of the messages are not cut prematurely and data loss is avoided.

The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

At this level, data is structured as messages

Transportation Layer (4)

It is responsible for the end-to-end delivery of the complete message from one application to another application. It also provides the means by which messages are acknowledged to have been successfully transmitted or not. If data is not successfully transmitted because of an error, those segments that are in error will be retransmitted.

It is positioned in the 7-Layers so that it may provide services to the application layer(s) and takes services from the network layer.

The segment header contains the source and destination service point address (port number)

For connection oriented communications

For connectionless communications

At this level, data is referred to as Segments.

Network Layer (5)

Has the responsibility for routing the data between networks. Data is transmitted as packets. Each segment (Layer 4) is enveloped with more structure (a packet) so that the segments can be routed from one network to another. Sender and Receiver IP addresses are added to the packet header. The network layer will look for the shortest to get from its source to its destination through a series of routing tables located at well-designed points in the network as a whole.

The network layer uses the Envelope/Letter pattern

At this level, data is referred to as Packets

Data Link Layer (6)

Has the responsibility of ensuring the error free transmission between Nodes on the network over the physical infrastucture. At this level the MAC address of the Nodes is used for idientifying the Nodes.

In order to successfully transmit data, the packets are further broken down into frames. Frame sizes are determined by the sender's NIC.

Sender’s and Receiver’s MAC address placed in the packet headers.

The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.

At this level, data is referred to as Frames

Physical Layer (1)

Responsible for the actual physical connection between the Nodes. The physical layer has no framing, packets, or segments, its currency of exchange is bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s, group these 1s and 0s into bytes and send them to the Data Link layer, which will reconstitute the frame.