Information Technology Reference
In-Depth Information
Problems to watch for in thinnet installations are intermittent connectors and
Ts, connectors that may have pulled loose from the cable, and easily disconnected
BNC-Ts. The T may be disconnected from the workstation without disrupting the
network, as long as both cables to the next adjacent workstations remain connected
to the T. In large installations, ground loop currents between areas of separate elec-
trical service may cause network errors and even safety problems.
Ethernet Grounding
Proper grounding of coaxial LAN cables is very important, both for safety and oper-
ational reasons. Both types of Ethernet coax use unbalanced “ground-referenced”
signaling. That is, the LAN signal is applied to the center conductor and is refer-
enced to a signal ground, connected to the shield. Most people assume that a ground
is a ground. However, it is intended that this signal ground be independent of the
“earth” ground (sometimes called
chassis
or
safety ground
). Unfortunately, the inde-
pendent signal ground of the network and workstations can sometimes get intercon-
nected with safety grounds. If the signal ground on a cable and the chassis ground
are not at the same electrical potential, significant signal distortion and even poten-
tially lethal voltage differences can result. Grounding is illustrated in Fig. B.5.
The Ethernet specification calls for the trunk cable to be grounded at each
wiring closet or thinnet cable to be connected to earth ground at one end (not at
both ends). In addition, you should ensure that the building electrical system is
properly bonded and grounded together, preferably at the one point where the
power entry to the building is located. All electrical distribution rooms in the build-
ing should be grounded back to the same entry point, as should all power trans-
formers. This is clearly an area for an experienced electrician, but as the network
manager or contractor, you should know how to assess a proper building ground
system. EIA/TIA-607 gives extensive information on providing proper building
grounding and bonding.
Coax connectors have been used for LAN connections for a very long time. The two
most common LAN coax connectors are the BNC and the Type N connectors. The
Type N was used on the original “thick Ethernet” or thicknet cable that first intro-
duced Ethernet to the world.
The connector is designed for the large cable diameter of coax cables such as
that used for thicknet. The Type N is shown in Fig. B.6. Type N connectors are
expensive and bulky, but they are commonly available and are very reliable. The
