When users attach to the same shared segment, all of them
share the bandwidth of the segment. Every additional user attached to the shared
medium means there is less average bandwidth available for each user. If
the sharing becomes too great, user application performance will begin
to suffer. The network administrator will begin to suffer as well
because users will begin complaining and asking for more bandwidth.
VLANs, which are usually created with LAN switch equipment, can offer
more bandwidth to users than is inherent in a shared network.
Remember that each interface on a switch behaves like
a port on a legacy bridge. Bridges filter traffic that does not need to
go to segments other than the source. If a frame needs to cross the
bridge, the bridge forwards the frame to the correct interface and to no
others. If the bridge or switch does not know where the destination
resides, it floods the frame to all ports in the broadcast domain (VLAN)
except the "source port."
In a switched environment, a
station will usually see only traffic destined specifically for it. The
switch will filter most of the other background traffic in the network.
This allows the workstation to have full, dedicated bandwidth for
sending or receiving interesting traffic. Unlike a shared-hub system
where only one station can transmit at a time, the switched network in
the Figure allows many concurrent transmissions within a broadcast
domain without directly affecting other stations inside or outside of
the broadcast domain. Station pairs A/B, C/D, and E/F can all
communicate without affecting the other station pairs.