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CCNA Training Course: OSI Model Part 3

Posted: October 21st, 2016

Physical Flow of data

When the data arrives at its intended destination it will travel up the 7 layers to pop out onto the
Users application as a for example an email or webpage.


When the user on the left hand machine press the send button on their email program the email is sent down the stack starting at layer 7 and through the layers. There is no way of avoiding passing through a layer. As the data moves through each layer the functions of the layers will determine exactly what happens to the data.

Logical flow of data
The data will move physically down through the layers until the data is placed onto the physical wire as 1’s and 0’s, even though the data moves down the stack for example i.e. layer 4 passes the data to the next layer down to layer 3 the actual communications is logical from peer layer to peer layer.


In the diagram above the arrows indicate the logical communication between layers. Layer 3 for instance is only interested in its corresponding peer layer’s information, and layer 4 is only interested in its corresponding peer layers information.

Encapsulation of data
User data moves down the stack from 7 to 1 on the left hand stack, from Layer 4 down to layer 2 the data is packaged or encapsulated in the lower layer data.


1. Layer 4 is encapsulated into Layer 3.
2. Layer 3 data is encapsulated into layer 2
3. Layer 2 data is transmitted onto the wire
4. Data arrives at the destination
5. Receiving layer 1 pulls the 1’s and 0’s off of the wire to reconstruct the data and passes it up to layer 2
6. Layer 2 removes it’s peer layer data and passes the remaining data to layer 3
7. Layer 3 removes it’s peer layer data and passes the remaining data to layer 4

The data enters the stack from the top and makes its way from layer 7 down to layer 1. From Layers 7 to 5 the data is encoded and or encrypted, from layer 4 down to layer 2 new fields are added to the data. These fields are for addressing, sequencing, length, type of data, error correction, error detection, flow control to name a few (We will cover all of these terms).


Once the data now wrapped up nicely inside all the other information a bit like when you send a parcel in the post you would wrap the parcel and apply a to and from label and maybe a sticker which says “fragile” or “This way up”

On the receiving end, the data is pulled off of the wire and reassembled, the parcel of data is then passed up the stack. Each layer will read and remove and discard its corresponding peer layer data.


When layer 2 receives the data parcel from layer 1, it will check the information contained inside the layer 2 header for example at this layer it will check layer 2 hardware addresses, if it is happy with the info it will remove the layer 2 header and tail then pass the rest up to the next layer above.

Each layer will do the same until the data is passed up to layer 7 then presented to the user as an email or webpage etc.

Naming the data parcels
The data parcel otherwise referred to as the PDU (Protocol Data Unit) which is produced at each layer has a specific name and it is important to learn the names so you can become a more efficient fault finding network work engineer.


1. The PDU at layer 4 is referred to as a “Segment”
2. The PDU at layer 3 is referred to as a “Packet”
3. The PDU at layer 2 is referred to as a “Frame”
4. The PDU at layer 1 is referred to as a “Bits”