Which layer in the OSI reference model is the only layer that adds a trailer to the data it transmits?

The term encapsulation is used to describe a process of adding headers and trailers around some data. This process can be explained with the four-layer TCP/IP model, with each step describing the role of the layer.  For example, here is what happens when you send an email using your favourite email program (such as Outlook or Thunderbird):

1. the email is sent from the Application layer to the Transport layer.
2. the Transport layer encapsulates the data and adds its own header with its own information, such as which port will be used and passes the data to the Internet layer
3. the Internet layer encapsulates the received data and adds its own header, usually with information about the source and destination IP addresses. The Internet layer than passes the data to the Network Access layer
4. the Network Access layer is the only layer that adds both a header and a trailer. The data is then sent through a physical network link.

Here is a graphical representation of how each layer add its own information:

delivery path for the packets, and passes this data to the Data Link layer (layer 2).

6. The Data Link layer places layer 2 header and trailer information, such as a Frame Check Sequence (FCS) to ensure that the information is not corrupt, and passes this new data to the Physical layer (layer 1) for transmission across the media.

7. The bit stream is then transmitted as ones and zeros on the Physical layer. It is at this point that the Physical layer ensures bit synchronization. Bit synchronization will ensure the end user data is assembled in the correct order it was sent.

8. Steps 1 through 7 occur in reverse order on the destination device. Device B collects the raw bits from the physical wire and passes them up the

Data Link layer. The Data Link layer removes the headers and trailers and passes the remaining information to the Network layer and so forth until data is received by the Application layer. Eventually, Device B will receive an email notification displaying a message to indicate that a new email message has been received.

Familiarize yourself with the OSI model and each layer's responsibility. You should be able to recognize a function of each layer of the OSI model. The seven layers of the OSI reference model are typically divided into two categories: upper layers (layers 4 through 7) and lower layers (layers 1 through 3).

As you can determine from the example of encapsulation, the OSI model provides a service that allows information to flow smoothly from one layer to another. Eventually, the information will be presented to the end device in a readable format. Now that we've reviewed the OSI model, the next section takes a look at how packets are sent across a network using a routing algorithm.

Continue reading here: Routing Vs Routed Protocols

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