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Cisco CCNA Certification: An Illustrated Guide To Ethernet CSMA/CD |
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When you're studying for your CCNA exams, you're going to
study the theory of technologies we basically take for granted
in networking. CSMA/CD is one of those technologies. It works
beautifully and you don't even have to configure it. But to
be an effective network troubleshooter (and to pass the 640-811,
640-801, and 640-821 exams), you have to know Ethernet inside
and out, and that means knowing CSMA/CD.
The first Ethernet standards were 10Base5 and 10Base2. Network
devices such as hubs, repeaters, and switches weren't in the
picture yet. The sole physical components were the Ethernet
cards in the computers and coaxial cable. The cable made up
a bus that all the connected devices would use. (This type
of bus is referred to as a shared bus.)
If only one of the hosts on this network wanted to send data,
there would be no problem. When two hosts would send at the
same time, though, a collision would occur. When the signals
would collide, both would be rendered unusable. A standard
had to be created that would have the hosts follow rules relating
to when they could send data and when they could not. This
standard is Carrier Sense Multiple Access with Collision Detection,
referred to as CSMA/CD.
If two of the three computers on this segment send data at
the same time, a collision occurs.
To avoid this, CSMA/CD forces computers to “listen”
to the Ethernet before sending in order to make sure that
no other host on the wire is sending. When the Ethernet segment
is not busy, the device that wants to send data can do so.
The sender will then continue to listen, to make sure that
sending the data didn’t cause a collision.
If a collision is heard, both of the senders will send a
jam signal over the Ethernet. This jam signal indicates to
all other devices on the Ethernet segment that there has been
a collision, and they should not send data onto the wire.
(A second indication of a collision is the noise created by
the collision itself.)
After sending the jam signal, each of the senders will wait
a random amount of time before beginning the entire process
over. The random time helps to ensure that the two devices
don't transmit simultaneously again.
In a separate tutorial, we'll discuss more Ethernet fundamentals
and values you should know for your CCNA exams and to be a
more effective network troubleshooter.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, visit the website and download
your free copies. You can also get FREE CCNA and CCNP exam
questions every day! Pass the CCNA exam with The Bryant Advantage!
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