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Chapter 1
Internetworking
Routers can provide quality of service (QoS) for specific types of
network traffic.
Switching and VLANs and are covered in Chapters 2 and 6, respectively.
The Data Link Layer
The Data Link layer provides the physical transmission of the data and han-
dles error notification, network topology, and flow control. This means the
Data Link layer will ensure that messages are delivered to the proper device
on a LAN using hardware addresses, and translates messages from the Net-
work layer into bits for the Physical layer to transmit.
The Data Link layer formats the message into pieces, each called a data
frame, and adds a customized header containing the hardware destination
and source address. This added information forms a sort of capsule that sur-
rounds 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 1.10 shows the Data Link layer with the Ethernet and IEEE spec-
ifications. When you check it out, notice that the IEEE 802.2 standard is
used in conjunction with and adds functionality to the other IEEE standards.
F I G U R E 1 . 1 0
Data Link layer
It's important for you to understand that routers, which work at the Net-
work layer, don't care at all about where a particular host is located. They're
only concerned about where networks are located, and the best way to reach
them--including remote ones. Routers are totally obsessive when it comes to
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