Page 41 - M. Abrahim Thesis

Computer Network 2021

Step 1 Verifies that the MAC destination address matches this station’s address or is an
Ethernet broadcast. If neither of these situations is true, the frame is discarded.

Step 2 If the data is in error, it can be discarded, and the data link layer might ask for the
data to be retransmitted. If the data is not in error, the data link layer reads and interprets the
control information in the data link header.

Step 3 The data link layer strips the data link header and trailer and then passes the
remaining data up to the network layer based on the control information in the data link header.
This process is called de-encapsulation. Each subsequent layer performs a similar deenca
psulation process. Think of the de-encapsulation process as the process of reading the address
on a letter to see if it is for you and then removing the letter from the envelope if the letter is
addressed to you.

2.9 Networking Devices

Equipment that connects directly to a network segment is called a device. These devices
are broken into two classifications:

• End user devices: Include computers, printers, scanners, and other devices that
provide services directly to the user.
• Network devices: Include all devices that connect the end-user devices to allow them
to communicate.

2.9.1 End-user devices that provide users with a connection to the network are
also called hosts.

These devices allow users to share, create, and obtain information. Host devices can exist
without a network, but without a network, host capabilities are greatly reduced. Host devices
are physically connected to the network media using a network interface card (NIC). They use
this connection to perform the tasks of sending e-mails, printing reports, scanning pictures, or
accessing databases. A NIC is a printed circuit board that fits into the expansion slot of a bus on
a computer motherboard, or it can be a peripheral device. It is also called a network adapter.
Laptop or notebook computer NICs are usually the size of a PCMCIA card.
Each NIC carries a unique code called a MAC address. MAC addresses are covered more
in a moment. As the name implies, the NIC controls host access to the medium. There are no
standardized symbols for end-user devices in the networking industry. They bear a resemblance
to the real device to allow for quick recognition.

2.9.1.1 Network Interface Cards

Network interface cards (NICs) are considered Layer 2 devices because each NIC
throughout the world carries a unique code, called a Media Access Control (MAC) address. This
address controls data communication for the host on the LAN. The NIC controls the access of
the host to the medium. Figure 2-15 shows a NIC.

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