Introduction
While studying for CompTIA Network+, I couldn't totally grasp the PDU names in the OSI model. I could memorize them, but I never understood why the names are what they are.
One day, while ordering some items on Amazon, it hit me — the ordering process maps perfectly to the OSI model. So I decided to write it down.
What is OSI model
The OSI model is a framework that divides network communication into 7 layers. Separating responsibilities by layer makes it easier to identify which layer is causing a problem.
| Layer | Name |
|---|---|
| 7 | Application |
| 6 | Presentation |
| 5 | Session |
| 4 | Transport |
| 3 | Network |
| 2 | Data Link |
| 1 | Physical |
Note: This mnemonic goes from Layer 1 to 7 (bottom to top).
Physical → Data Link → Network → Transport → Session → Presentation → Applicaion
What is PDU
PDU (Protocol Data Unit) is the name given to data at each layer. Each time data passes through a layer, it gets a new name.
| Layer | PDU name |
|---|---|
| 7–5 | Data |
| 4 | Segment |
| 3 | Packet |
| 2 | Frame |
| 1 | Bit |
We can memorize them:
" Do Some People Fear Birthdays?"
Data → Segment → Packet → Frame → Bit
What is Encapsulation and Decapsulation
-Encapsulation: The process of adding headers to data as it moves down the layers (sender side).
-Decapsulation: The process of removing headers from data as it moves up the layers (receiver side).
We can memorize them:
-En = put in an envelope → Top to Bottom (7-1), adding header when sending.
-De = take out of the envelope → Bottom to Top (1-7), removing header when receiving.
Understand the concept with Amazon ordering
This section is the main part of this article.
When I think about Amazon ordering process - from ordering items to delivering them, I could understand them easily.
Layer 7-5 (Data) - Items themselves
Data represents the items we ordered in Amazon.
T-shirts, gadgets, or other items stays in Amazon warehouse.
It is not wrapped yet.
Layer 4 (Segment) - Dividing and Numbering them
It can be difficult to deliver them, if the order consists of multiple items.
Therefore, we need to divide into small portion and numbering them 1,2,3…
Without numbers, it would be impossible to identify which piece is missing if they arrive out of order.
Layer 3 (Packet) - Putting in boxes and Labeling them
In this process, divided items are put into boxes and labeling each of them.
Labels show the destination address about receiver (IP address).
The multiple boxes come to do have same address labels.
The labels are useful to determine routes to deliver.
Layer 2 (Frame) - Adding relay labels about next delivering center to Boxes
The destination address (IP address) always stays the same, but the package doesn't travel directly to the final destination -it goes through multiple delivery centers along the way.
The actual delivering process is going through many delivering center in each locations until completing delivering, and relay labels, shows destination center, are replaced each time.
This label is the Frame (MAC Address).
-Destination address label (IP Address) = Final destination = stays same
-Relay label (MAC Address) = Next center destination = replaced whenever the items go through delivering center
Layer 1 (Bit) - Transporting
Actually, those boxes are transported physically by track.
Items go through cables or wireless as electrical or light signal (0,1) .
Ultimately, any data are exchanged to 0 and 1.
Conclusion
[Sending side: Encapsulation]
Items (Data)
↓ Dividing and numbering
Segments
↓ Labeling with destination address (IP Address)
Packets
↓ Labeling with next delivery center (MAC Address)
Frames
↓ Physical transport
Bits → Network
[Receiving side: Decapsulation]
Bits ← Network
↓ Removing relay labels at each delivery center
Frames → Packets → Segments
↓ Reassembling and unpacking
Items (Data)