OSI Understanding the Model of All Models

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To understand the OSI-Open Systems Interconnection model one needs to understand that models and standards are used in all industries to create the means to work together called interoperability or in this case, communicate together whether for voice, video, data or any other media. The OSI model touches IT, VoIP/SIP, Security, jacks, cables and every other aspect of our lives, so understanding it in the information technology world is as important as breathing.

Starting with the Physical Layer 1, shown here are the tracks that railroads use to support the trains and boxcars. In the telecom world the tracks are DSL-digital subscriber lines, business lines and trunks, T-1-Transmission Layer One circuits and optical fiber circuits that support streams of digital bits.

Moving up to Layer 2, the Data link layer is where trains, boxcars or in the telecom world, packets are formed. There are literally hundreds of types of data packets such as Ethernet, Frame Relay, ATM, and SCADA as there are types of railroad boxcars. Each data packet format was created for a unique function. In this analogy, we want to show that the train or box car carries the data from one destination to another.

Before we move ahead, let's review two key concepts - routing versus switching
In switching all packets are "flooded" to all locations until they find their destination device. Originally known as TLS-Transparent LAN-local Area Network Service and now often called VPLS-Virtual Private LAN Service, here are some reasons why you should consider this service for your customers: First, it is easy-to-implement with plug-and-play installation. Next, LAN Switches are cheap. Third, VPLS provides very flexible BOD-bandwidth on demand options. Fourth, it is a great solution for same-city customers with many locations. Here are some reasons against: First, it doesn't grow or scale well to tens of sites and hundreds of users Second, faster bandwidth doesn't solve flooding of packets due to large applications.

In routing, only the packets that are destined for the other location are sent. That is, packets are routed based on the destination IP address.
Here are some reasons for routing: First, it scales or grows essentially infinitely for customers with domestic and global locations. Second, routing supports all kinds of IT systems new or legacy
Third routing provides higher security. Reasons against routing: First, there is a high cost with routing and routing will always be more expensive than switching. Second, routing is complex to manage. Third, routing requires very knowledgeable and technical staff.

Now back to the OSI Model, using train metaphor, one key but subtle point is that the packet carries the data without knowledge of the location of its ultimate destination. The knowledge about what routes to take comes from Layer 3 or the Network Layer where routing occurs. Simply, a router supports different protocols or train cars, different routes and different speeds or bandwidth. Importantly, the Network Layer 3 provides routing functions that determine how, when and if a data packet will be sent at all. Again, without going into all the details about firewalls, firewalls operate primarily at Layer 3 controlling incoming and outgoing IP-Internet Protocol packet transmission.

Transport Layer 4 is used to provide reliable confirmation of the packet. Much like when you sign for a FedEx or UPS package, you are confirming receipt of the package. This type of confirmation or protocol is the function of TCP-Transmission Control Protocol which you will see in detail in a few minutes. However, like normal first class mail, UDP or User Datagram Protocol guides the IP-Internet Protocol packet to its destination without confirming receipt.

At Layer 5, the Session Layer additional functions provide for maintaining the connection, in simple terms, logging in and logging off functions.

At Layer 6, the Presentation layer, data is being organized and processed for "presentation" to the user.

At Layer 7, the Applications layer, the user interface and data input such as typing your email or submitting figures in a spreadsheet or database is controlled. As you can see each layer has unique and distinct functions. And for the record, error checking is done at every layer. That is, quality control checks are needed to make sure that the cables are plugged in corrected at Physical Layer 1 all the way up to the Applications Layer 7 where you see and touch the data.

In summary, the OSI model is more than an architecture or framework for IT and communications but a way of life in organizing systems and services efficiently and effectively.

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