[SATLUG] Network+ question collision domains

Bruce Dubbs bruce.dubbs at gmail.com
Sun Jan 10 20:27:12 CST 2010


Todd W. Bucy wrote:
> 
>>
>> I don't know about your setup, but if it were HW, a collision domain 
>> is where frames being sent out can collide.  Bridges do a Carrier 
>> Sense with Multiple Acesss and Collision Detection CSMA/CD.  Hubs do 
>> not and the collision detection has to be at the NIC.   That is fairly 
>> old technology with Thick-net or Thin-net.  10*-BaseT is generally 
>> hooked up to a bridge or a router that inherently prevents collisions.
>>
>> A bridge receives a frame and can hold it until the destination is free.

> If a bridge holds the data until the line is free wouldn't that make it 
> CSMA/CA rather then CSMA/CD?

No.  There is only 2 devices on the physical line -- the bridge and the 
nic.  Both know what is going on and no collisions occur.  You need a 
third device to cause a collision and there are none present.

CSMA/CA lowers the probability of a collision by listening to a line for 
traffic and waiting a random amount of time to start transmitting, but 
doesn't prevent them completely.

   -- Bruce


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