16.1 Network Infrastructure Concepts
16.1.4 Shared Ethernet

Shared Ethernet or standard Ethernet is a bus-based network where all workstations can try to send packets simultaneously. Early versions of Ethernet used a single coaxial cable known as a bus. Workstations tapped into the bus to gain access. Ethernet networks today are almost exclusively star-based designs where the bus has been compressed into a small concentrator known as a hub or switch. These star networks typically use Unshielded Twisted Pair (UTP) category 5 cabling that supports speeds up to 100Mbps.

Earlier versions of Ethernet used the CSMA/CD method of access since only one packet can be on the wire at a time. This allowed many workstations to share the wire. CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. Carrier Sense means the workstation (NIC) is sensing or listening to the carrier (the wire or media) to see if there is any traffic. Multiple Access means that more than one computer may be trying to send data simultaneously. Collision Detection means that if there is a collision the computers will detect it and retransmit. Every NIC has a transmit and receive circuit so that it can not only send data, but can receive data, listen for traffic and detect collisions. In a shared Ethernet network the NIC is either transmitting or receiving but cannot do both at the same time. This is referred to as half-duplex communication.

The disadvantage to early Ethernet and CSMA/CD was that with many workstations creating traffic, a lot of collisions and significant slowdowns in network performance occurred. The early versions of shared Ethernet ran at 10Mbps (10 megabit per second), so actual throughput might only be 5 Mbps when collisions were taken into account. If there were 10 workstations connected to the bus or hub, they had to share 10 Mbps of bandwidth resulting in a 10 node collision domain. The Figure shows a shared Ethernet LAN with two nodes colliding. If there were twenty nodes, the size of the collision domain would be 20 nodes and so on.

As network applications began to require more speed, a 100Mbps version of Ethernet was introduced known as Fast Ethernet. This helped to alleviate congestion, but although the speed was 10 times faster, it was still shared Ethernet and there were still collisions to deal with.