Computers are able to talk to each other because they are networked together, we all know that. But what does that really mean?
Wether we know it or not, we all kind of know how that works already! We have all interacted with quite a complex network and monitored how it works. Being able to track our package through the mail system gives us a very clear look into how a network functions.
We all have a basic understanding of how physical letters are sent; but to recap and draw comparison later:
We package the letter in an envelop, we notate who the recipient is, we even mark a return address in case something goes wrong. Finally, we add a stamp (or two) knowing that is what is needed for it to be passed through the system.
The letter then sits in a mailbox waiting to be picked up by a mail person, at which point it is brought back to the local post office.
Once accepted at the post office it is sent out to a mail sorting facility which engages in appropriately directing it towards its destination.
The letter then makes it way to the mail sorting facility nearest your destination. Where it is routed to the local post office of the destination.
Once the local post office receives it, the letter is given back to a mail person who then delivers it to the recipient.
A digital network is kept in order by seven layers, called the Open Systems Interconnection Model, most often referred to as the OSI Model.
Layer 7: Application
This layer is where we interact, aka this is where we would write the letter! We use a human understandable interface to compose information that we can then pass on to the next layer.
Layer 6: Presentation
This layer takes our data and handles formatting, encryption, compression and other transformations. So for our example, we compress and format; put it in an envelope and write the address in a standardized format with a stamp!
Layer 5: Session
This is where we check our formatting, and make sure we can figure out who the data is intended for. If we cannot an error is sent back. Just like in our post office, if there is no stamp or the recipient address is unclear it gets returned to sender.
Layer 4: Transport
In terms of Data there are different protocols that are used for maintaining a session as data is transmitted. Depending on the type of data and its destination we can pick which protocol we want to use. In our physical mail example we can think of this in terms of ground versus air transport depending on what makes sense for the package and its volume.
Layer 3: Network
This layer's job is to locate the destination of the data. Making sure the best route is taken in order to get the data from its originating location to the destination. By looking at the sending IP address and the recipients it can make a plan for which routers to push the data through in order to get it from point A to B. For our example this is the role the Mail Sorting Facilities take, taking into account knowing where the stops are along the way where the mail is scanned in to report to best get the package through quickly and without complication.
Layer 2: Data Link
Here we handle the physical addressing of the data. By taking the IP address and adding the MAC address of the device it should be sent to; ie, the computer of who your email is intended for. This is the equivalent of the destinations local post office handing the letter to the mail person whose route includes the recipients address, or one step further even, the recipient address knowing who the letter is intended for because we put a recipients name in our addressing.
Layer 1: Physical
This is the hardware of the computer, the part that actually takes 1's and 0's and turn them into signals able to be transmitted across the network, how a signal becomes a readable message in the inbox! This layer also does the exact opposite and takes signals and converts it back into binary. This layer is the actual mail person or mailbox, be it the final one to deliver or the first person to pick it up. They take your letter and begin or finish its process into the whole mail system.
The OSI Model in its complexity handles a myriad of things with each layer having its own category of tasks to achieve. This is meant to be a brief overview to get a basic understanding of how digital networks interact.
Check back in later to read more in depth about each layer and its responsibilities, not with the US postal service.