Hardware Reference
In-Depth Information
UTP Ethernet networks were originally wired using hubs. When a specific computer sends a packet
of data to another specific computer through a hub, the hub doesn't know which port the destination
computer is connected to, so it broadcasts the packet to all of the ports and computers connected to it,
creating a large amount of unnecessary traffic because ports and systems receive network data even if
it is not intended for them.
Switches, as shown in
Figure 17.17
,
are similar to hubs in both form factor and function, but they are
very different in actual operation. As with hubs, they connect computers on an Ethernet network to
each other. However, instead of broadcasting data to all of the ports and computers on the network as
hubs do, switches use a feature called address storing, which checks the destination for each data
packet and sends it directly to the port/computer for which it's intended. Thus, a switch can be
compared to a telephone exchange, making direct connections between the originator of a call and the
receiver.
Figure 17.17. The back of a typical eight-port, 10/100/1000 Ethernet switch.
Because switches establish a direct connection between the originating and receiving PC, they also
provide the full bandwidth of the network to each port. Hubs, by contrast, must subdivide the
network's bandwidth by the number of active connections on the network, meaning that bandwidth
rises and falls depending on network activity.
For example, assume you have a four-station network workgroup using 10/100 NICs and a Fast
Ethernet hub. The total bandwidth of the network is 100Mbps. However, if two stations are active,
the effective bandwidth available to each station drops to 50Mbps (100Mbps divided by 2). If all
four stations are active, the effective bandwidth drops to just 25Mbps (100Mbps divided by 4)! Add
more active users, and the effective bandwidth continues to drop.
If you replace the hub with a switch, the effective bandwidth for each station remains at the full
100Mbps because the switch doesn't broadcast data to all stations.
Most switches also support
full-duplex
(simultaneous transmit and receive), enabling the actual
bandwidth to be double the nominal 100Mbps rating, or 200Mbps.
Table 17.8
summarizes the
differences between the two devices.
Table 17.8. Ethernet Hub and Switch Comparison
