Cisco CCNA Certification: An Illustrated Guide To Ethernet CSMA/CD

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Telepresence and Holography / Holograms - November 2008

Editor’s viewpoint on their differences, likenesses and future...

We all just recently viewed an historic CNN holographic interview on an historic Election Night. CNN stated this type of imaging has never been done before on National TV and I believe that. I haven’t seen anything like it. The live interview caught me by surprise and sure had the “wow” factor attached to it didn’t it? Here’s a link to the actual interview.

CNN Hologram Video

The question now has to be asked. Is it Telepresence?

I’m no expert on the subject but I’m going to throw my hat into the ring, go out on a limb and say yes, I believe it is. Although I know many video conferencing purists are rolling their eyes and grabbing their guns right about now with my statement.

While I do know that Telepresence still hasn’t been universally defined (Houston, we have a problem), here’s how I see it…. Broken down to its simplest form - “tele” (through electronic means) and “presence” (present). This is how I’m justifying my opinion by lumping holography and Telepresence together. Just as Telepresence is surely an offshoot of video conferencing, holographic imaging (like we viewed on CNN) surely has to be considered an offshoot of Telepresence. The interviewer was “telepresent” from Chicago and beamed into the CNN Election Headquarters. She was most assuredly - “telepresent”. There was negligible latency, the imaging was fantastic and both interviewers were rehearsed on what they would say. I’ve just recently read that CNN actually “downgraded” the experience to make it “movie” quality and not as good as it could have been. Maybe they thought us mere mortals would be shocked if they showed their complete hand. I have a feeling it was so big to them, they didn’t want it to get upstaged by the historic election. I’m willing to bet we’ll see the “true” power of this technology in the upcoming weeks and months when they can put the pedal to the medal and show it off without having it sidetracked.

What an amazing time for this technology (all differences and opinions aside). Telepresence Doctors, nurses, teachers, schools and healthcare (healthpresence) will benefit from the recent gains in this state-of-the-art industry whether holograms, Telepresence or holography. Airlines also have to see the threat to a portion of their business travel. Hell, who wants go through security and fly to Miami for a two-hour conference when you can look them in the eye via Telepresence?

Beam me up Scotty! Simply amazing…!

Visit the free Telepresence Forum for more information and discussion

Your Editor and Administrator - L II
"Ride on the Next Plane of Existence" TM

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.

To your success,

Chris Bryant
CCIE #12933

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage. The Bryant Advantage offers dozens of FREE CCNA and CCNP tutorials and articles. The Bryant Advantage sells the world's most comprehensive CCNA Study Guide, and my exclusive Binary Math and Subnetting Mastery book. My $299 CCNA Online Fast Track is the world's fastest-growing CCNA course.

Visit http://www.thebryantadvantage.com today!

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To learn more about Telepresence, the revolutionary new style of immersive video conferencing; visit these websites:

Telepresence - "Ride on the Next Plane of Existence" - Telepresence
Telepresence Web Portal - Telepresence Internet web portal
Telepresence Report - 24/7 breaking Telepresence related news and information
Via Telepresence - Video conferencing via Telepresence
Telepresence Forum - Free user discussion forum for everything Telepresence related
Telepresence Today - Telepresence information and editor qualified headline news
Telepresence Resource Directory - Telepresence web resource directory
Telepresence Tube - Telepresence videos, pictures and more