Information Technology Reference
In-Depth Information
backup UPS. When power is cut off, an inverter generates AC voltage from power
stored in batteries. The generated power is switched to the connected equipment in
a relatively short time period so that the equipment sees only a small break in the
voltage. Another type of supply, the true uninterruptible power supply (true UPS),
constantly generates power to the connected equipment.
When there is normal line power available, the true UPS uses the AC line as its
source of power. If there is a break in AC power, the UPS switches almost instanta-
neously to the battery source, with no appreciable interruption in voltage to the con-
nected equipment. As you might expect, the true UPS is considerably more
expensive than the backup UPS.
What type of equipment should be provided with emergency power? You
should provide for equipment needed during an emergency, or equipment that can
be harmed or disrupted by a power outage. Equipment needed in an emergency
might include the telephone system, the alarm system, the video security system, or
any network servers and hubs needed in an emergency. Some servers and other com-
puter hardware are disrupted in a power outage and often must be manually
restarted or “cleaned up” before normal use can resume. Emergency power, while
temporary, may eliminate the restart process completely in a brief power outage.
Some UPS systems alert servers to begin the shutdown process automatically,
if it appears that the outage may be sustained. It also may be necessary to provide
emergency power to network hubs to assist in the shutdown process. Unless user
workstations are somehow provided with emergency power, it may be of little use
to provide such power to the hubs and other networking equipment.
LAN backbones and cabling backbones unfortunately share a common name.
During most of this topic, we have been referring to backbone cabling rather than
LAN backbones. However, in this section, we will actually be talking about the type
of logical backbones that are used in LAN systems.
LAN backbones are often used to tie sections of the networking system
together, so that workstations anywhere on the network can use resources, such as
servers, that are not exactly on the same portion of the network. The workstations
in a work area are connected to a hub-type device in the wiring closet. Hubs may
be interconnected in a variety of ways, but often they are connected to a higher level
of the network, called the backbone.
In a classic repeater backbone system, as shown in Fig. 14.3a, several stand-
alone 10/100BaseT hubs are connected together via a 10Base2 coax segment.
Alternatively, these days, they would be connected via 1000BaseT or fiber optics at
