Hitachi US7070447-001 manual Chapter Troubleshooting at the MAC Layer Ethernet

Chapter 9

Troubleshooting at the MAC Layer

Ethernet

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.

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