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Collisions

Posted on 2004-10-17
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Ok, is it true collisions are impossible to come by in Full duplex ethernet? If its true, why? (my Cisco press book said its impossible)

Is it also true that collisions cannot occur in half duplex as well because only one host transmits at a time?

When DO collisions occur and why?
thanks
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Question by:dissolved
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by:NicBrey
NicBrey earned 400 total points
ID: 12335899
Theoreticaly collisions cannot occur in full duplex mode. But with damaged cabling and hardware problems on ports they do. Also, bad packets can casue it.
Collisions will occur when you use a hub. All the ports on a hub is in one collision domain. So only one host can transmit at any givin time. When 2 or more hosts transmit, you get a collision, because a hub broadcasts a packet received on one port out of all the other ports.
Every port in a swtich is in it's own collision domain. The switch builds a forwarding table, and only transmits a packet received on one port out on the port where it is destined to go. If the switch does not know a destination port for a packet, it will broadcast, learn the destination and adds it to the table.
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by:The--Captain
The--Captain earned 600 total points
ID: 12336318
Googling for:

full duplex hub

yields this nugget:
http://www.cormantech.com/news.asp?SpecificNewsID=77

Cheers,
-Jon

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Felix2000 earned 800 total points
ID: 12336834
A Collision happens when 2 hosts try to send onto the network at the same time.
Collision cannot happen on Full duplex links because Transmit and Receive are on difference phyical wires.  If frames are bad or have errors this is not considered a collision this results in Framing Error, CRC Errors.  

Half Duplex those same physical wires are now shared between host and switch or hosts on a hub. Before sending they check the line see if it's free and send away. If 2 hosts both check the line at the same time and it's clear and then send this is where collisions occur.

Think of your wire as a highway, in Full everybody has their own lane and can't hurt anyone else.  In half duplex the highway is shared and now packets can collide with each other on that same link.

Hope it helps
-= Felix =-
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by:dissolved
ID: 12338534
Thanks guys! So CDMA collision detection is NOT used on full duplex I'm assuming?. Since collisions arent possible ?
Thanks!
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by:forrestd
forrestd earned 200 total points
ID: 12341670
That's correct CSMA/CD comed from the days of shared ethernet segments (& later hubs & switched runnign at half duplex).  In a full duplex environment no collision detection takes place i.e. a NIC can transmit & receive at the same time (i.e. FULL Duplex) as opposed to only being able to do one or the other.

This comes from the days of Ethernet when Ethernet was a single shared cable (collision domain) sometimes bridged with other cables.
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