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Chapter 2
The Open Systems Interconnection Specifications
Introduction to Encapsulation

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When a host transmits data across a network to another device, the data goes through encapsulation: It's wrapped with protocol information at each layer of the OSI model. Each layer communicates only with its peer layer on the receiving device.

To communicate and exchange information, each layer uses Protocol Data Units (PDUs). These hold the control information attached to the data at each layer of the model. They're usually attached to the header in front of the data field but can also be in the trailer, or end, of it.

At a transmitting device, the data-encapsulation method works like this:

1. User information is converted to data for transmission on the network.

2. Data is converted to segments, and a reliable connection is set up between the transmitting and receiving hosts.

3. Segments are converted to packets or datagrams, and a logical address is placed in the header so each packet can be routed through an internetwork. A packet carries a segment of data.

4. Packets or datagrams are converted to frames for transmission on the local network. Hardware (Ethernet) addresses are used to uniquely identify hosts on a local network segment. Frames carry packets.

5. Frames are converted to bits, and a digital encoding and clocking scheme is used.

Figure 2.11 shows how user data is encapsulated at a transmitting host.


Figure 2.11 Data encapsulation


After you learn more foundational material about networking in the next few chapters, I'll come back to the encapsulation method and discuss it in a lot more detail in Chapter 6.

CompTIA Network+ Study Guide

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