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Chapter 1

Internetworking

network segment--which is the only way to get the information to a router
or host.

It's the Data Link layer that's responsible for taking packets from the Net-

work layer and placing them on the network medium (cable or wireless). The
Data Link layer encapsulates each packet in a frame, and the frame's header
carries the hardware address of the source and destination hosts. If the des-
tination device is on a remote network, then the frame is sent to a router to
be routed through an internetwork. Once it gets to the destination network,
a new frame is used to get the packet to the destination host.

To put this frame on the network, it must first be put into a digital signal.

Since a frame is really a logical group of 1s and 0s, the Physical layer is
responsible for encapsulating these digits into a digital signal, which is read
by devices on the same local network. The receiving devices will synchronize
on the digital signal and extract the ones and zeros from the digital signal. At
this point the devices build the frames, run a cyclic redundancy check (CRC),
and then check their answer against the answer in the frame's FCS field. If it
matches, the packet is pulled from the frame, and the frame is discarded.
This process is called de-encapsulation. The packet is handed to the Network
layer, where the address is checked. If the address matches, the segment is
pulled from the packet, and the packet is discarded. The segment is processed
at the Transport layer, which rebuilds the data stream and acknowledges to
the transmitting station that it received each piece. It then happily hands the
data stream to the upper-layer application.

At a transmitting device, the data encapsulation method works like this:

User information is converted to data for transmission on the
network.

Data is converted to segments and a reliable connection is set up
between the transmitting and receiving hosts.

Segments are converted to packets or datagrams, and a logical address
is placed in the header so each packet can be routed through an inter-
network.

Packets or datagrams are converted to frames for transmission on the
local network. Hardware (Ethernet) addresses are used to uniquely
identify hosts on a local network segment.

Frames are converted to bits, and a digital encoding and clocking
scheme is used.

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