Answer:
A network-layer packet is a data gram. A router forwards a packet based on the packet's IP (layer 3) address. A link-layer switch forwards a packet based on the packet's MAC (layer 2) address.
Explanation:
When the end system receives the unstructured bit stream from the physical wire, each layer removes the header information applicable to it until the application receives the data.
1. An application, such as an email program, creates data that will be sent by an end user, such as an email message. The Application layer places a header (encapsulation) field that contains information such as screen size and fonts, and passes the data to the Presentation layer.
2. The Presentation layer places layer 6 header information. For example, the text in the message might be converted to ASCII. The Presentation layer will then pass the new data to the Session layer (layer 5).
3. The Session layer follows the same process by adding layer 5 header information, such as information that the Session layer will manage the data flow, and passes this data to the Transport layer (layer 4).
4. The Transport layer places layer 4 information, such as an acknowledgement that the segment was received in the header, and passes it to the Network layer (layer 3).
5. The Network layer places layer 3 header information, such as the source and destination address so the Network layer can determine the best 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 synchronisation. Bit synchronisation 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.
