Learning by Doing: CISCO Certified Network ... - SCN Research

Layer 4: The Transport layer
The official definition of the transport layer is that is "Responsible for reliable network
communication between end nodes and provides transport mechanisms for the
establishment, maintenance, and termination of virtual circuits, transport fault detection
and recovery and information flow control."
This is the layer where information is readied for transmission. For example, if we were
to make a large packaging machine about 60 feet long and 20 feet wide that we wish to
ship we would have to "break it down" into smaller chunks before sending it. These
chunks would be numbered 1 of x, 2 of x, 3 of x, etc. In this manner we could assure that
all packages were sent and received. All of the chunks would be placed into the semitrucks
for transport. Could they be delivered now? Nope, lets move on to the next layer.
Visual Representation: Semi-truck
Layer 3: The Network layer
The official definition of this layer is that it "provides connectivity and path selection
between two end systems where routing occurs. Segments are encapsulated into packets
here."
Now before we can start sending out our shipment we need to give it a destination and
the directions on how to get from here to there. This layer is also in charge of logical
addressing.
Visual Representation: map
Layer 2: The Data Link layer
The official definition of this layer is that it is "concerned with physical addressing,
network topology, and media access. Packets are encapsulated into frames here.
The data link layer is in charge of physical addressing and a little bit of error checking
called "cyclic redundancy checking." CRC calculates the total size of the packets,
divides the total size by a unique prime number (a number divisible only by itself and
one) and attaches it to the packet. This is also the layer where the NIC card functions.
Visual Representation: MAC, CRC, LLC.
Layer 1: The Physical layer
The official definition of this layer is that it "describes the various types of networking
media. Frames are converted into bits here. Defines the electrical and functional
specifications for activating and maintaining the link between end systems.
The physical layer is, simply put, the media or cabling.
Visual Representation: cables
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