Information Technology Reference
In-Depth Information
devices are not 100BaseT compatible. This is a real advantage if you are gradually
upgrading to 100 Mbps.
You can install dual 10/100 Mbps NIC cards in all your new workstations at
relatively little extra cost. As a matter of fact, at this time, it may be rather difficult
to locate 10 Mbps-only cards. When you change your hubs to 100BaseTX, the dual-
speed cards will automatically upshift to 10 times the old speed, and the link will
run at 100 Mbps.
A 100BaseTX network has some rather severe distance restrictions in a shared
hub-only environment. Because of the timing constraints of CSMA/CD, the total
“radius” of the network is one-tenth the size of a 100 Mbps Ethernet network.
Hubs may be linked, but only on a very short backbone of 5 m.
The hub ports (which function as repeaters) can be linked to stations by 100
m of cable, which fits into the TIA-568-C limit of 90 m + 10 m nicely. This com-
putes to a maximum station-to-station distance of 205 m (100 + 100 + 5). A fiber
link from a hub to a bridge, router, or switch can be up to 225 m, and a nonhub
fiber link can be 450 m with CSMA/CD enabled and 2 km, disabled. These limits
encourage the use of Fast Ethernet switches. Switches, strictly speaking, are not
repeaters, but rather Layer 2 bridges, and absolve you from the tight timing limits
of hubs and repeaters.
An interesting problem exists if the environment has both 10 and 100 Mbps
stations. Any simple hub must be only one speed, either 10 or 100, because a 100
Mbps station obviously could not transmit to a 10 Mbps station, without some con-
version. The hub (or switch) must buffer the high-speed data packets and restrain
the higher-speed port to allow time to send the packets out at the slower data rate.
100BaseT4.
Originally, for networks built to accommodate 10BaseT wire speeds,
the move to 100 Mbps was a tough one. Many twisted-pair networks were installed
with Category 3 wiring standards that do not accommodate the 100BaseTX line
speed. For those installations that have installed 4-pair Category 3 cable, a standard
called 100BaseT4 allows the operation of a 100 Mbps CSMA/CD connection over
Category 3 cable, by using all four pairs, rather than only two.
For older installations, the implications of this are enormous if you have a large
installed base of 10 Mbps (or 16 Mbps) users. You can selectively increase your
LAN connections to 100 Mbps without pulling new Category 5 cable. This can save
a lot of money if you have a large number of terminals.
As with normal 10/100Base T, many of the suppliers of 100BaseT4 worksta-
tion adapters offer dual-speed operation at 10 and 100 Mbps. The adapter automat-
ically detects which speed is in operation and makes its connection at that speed.
The main drawback to 100BaseT4 is its low acceptance in the market. The choices
