CCNA Cisco Certified Network Associate Study Guide - FTP Server

18 Chapter 1 � Internetworking
Switching and VLANs and are covered in Chapters 2 and 6, respectively.
� Routers can provide Quality of Service (QoS) for specific types of network
traffic.
The Data Link Layer
FIGURE 1.9 Data Link layer
The Data Link layer ensures that messages are delivered to the proper device
and translates messages from the Network layer into bits for the Physical
layer to transmit. It formats the message into data frames and adds a customized
header containing the hardware destination and source address.
This added information forms a sort of capsule that surrounds the original
message in much the same way that engines, navigational devices, and other
tools were attached to the lunar modules of the Apollo project. These various
pieces of equipment were useful only during certain stages of space flight and
were stripped off the module and discarded when their designated stage was
complete. Data traveling through networks is similar. Figure 1.9 shows the
Data Link layer with the Ethernet and IEEE specifications. Notice in the figure
that the IEEE 802.2 standard is used in conjunction with the other IEEE
standards, adding functionality to the existing IEEE standards.
You need to understand that routers, which work at the Network layer,
do not care about where a host is located but only where networks are
located. They also keep track of the best way to get to a remote network. The
Data Link layer is responsible for uniquely identifying each device on a local
network.