Chapter Summary

Building an Ethernet LAN using a hub significantly reduces the cabling effort required, while allowing all devices that are attached to the hub to communicate with all other devices. However, the hub does allow collisions to occur.

The CSMA/CD algorithm defines how to reduce collisions and how to react when collisions occur. A NIC first listens and then sends only when the LAN is silent. If a collision does occur, all NICs that were sending frames wait for a random time and then try to send their frames again.

Ethernet standards define the details of an Ethernet header and trailer that encapsulates the data before being sent over the network. The IEEE defines an 802.3 header, as well as an 802.2 header, with a single trailer defined by the 802.3 committee.

The header defines a source and destination Ethernet address, each 6 bytes in length. When a computer receives an Ethernet frame, it must first look at the destination address of the frame to see if it matches the Ethernet address of its own NIC. If the addresses match, the frame was sent to that device. The receiver can look at the source Ethernet address, which is typically most useful when replying to whatever is inside the data. The receiver must also check the FCS field to ensure that the frame did not experience errors during transmission.