OSI model and PDUs

pduTwo concepts from my Network+ study: the OSI model and PDU.

OSI stands for Open Systems Interconnect and is used as a conceptual model to ease data transmission and communication. Read more about it here and here.

PDU stands for Protocol Data Unit and describes the unit that is being used on a particular layer on the OSI model. This and this have some good explanations.

Both concepts I was already familiar with a long time, but it’s good to come across both once more and read them in a new context. I like repeating stuff on occasion.

In this fast post I want to quickly explain both. The more detailed explanation you can get from the links above if needed.

OSI Model

The OSI model has 7 layers which you can memorize with the phrase All People Seem To Need Data Processing. This is from top to bottom:

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data Link

1. Physical

PDU

On these layers all different Protocol Data Units are operating, and that was the key point of this post anyway. These Protocol Data Units are, from top to bottom:

7. APDU – Application PDU – or just ‘data’

6. PPDU – Presentation PDU – or just ‘data’

5. SPDU – Session PDU – or just ‘data’

4. Segment or Datagram

3. Packet – IP Packet

2. Frame

1. Bits

I’ve learned the OSI model about 20 years ago already but it is nice to recap, learn a bit more and put certain things into place (again). I’m all pro repetition really. Although I’ve been working with networks on almost a daily basis, the OSI model is not something that goes through your head on that same frequency. So rereading it is good and I really enjoy it as well.

UPDATE: Small addendum.

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