One of the mistakes that I 've made during the time when I was still learning OSI and its seven layers is the process of encapsulation. I have always thought that the process begins from Layer 1, the bottom layer. It was in fact the other way around. It begins on Layer 7, and works its way down to the bottom before it's de-encapsulated on its way up again on the receiving end. Here are some few illustrations behind the concept.
Protocol data Units (PDU) on every Layer of the OSI Model where every layer adds its own corresponding header.
Data Encapsulation in the Lower Level protocols where data will be encapsulated into TCP segment, IP datagram or packet, Ethernet Frame before being transformed into bytes and bits as it is sent through the physical medium.
