Hitachi US7070447-001 User Manual

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Ethernet is a LAN based technology with data transmission rates of 10 or 100 
Mbps. CSMA/CD (Carrier Sense Multiple Access with Collision  Detection) 
protocol dictates how Ethernet frames traverse the network. In CSMA/CD, an 
Ethernet station checks for the presence of a signal on the cable.  If there is no 
signal, the station begins transmission.  However, more than one station can sense 
an idle channel and simultaneously begin to transmit data resulting in a collision.  
When this occurs, all stations involved detect the collision and immediately stop 
transmitting.  After waiting a random time interval and ensuring that the line is 
free, each station will retransmit.  If another collision occurs, the station must wait 
another random time interval before attempting to retransmit.  After 16 tries, the 
station reports that the network is unavailable and discards the frame.

Based on the CSMA/CD methodology, collisions are inherent to Ethernet and 
considered ‘normal’.  Retransmit delays due to collisions usually go unnoticed by 
the users, however, on larger networks with hundreds of users, excessive 
collisions may cause slow response times.  Segmenting the network into smaller 
collision domains should significantly improve network performance.