Information Technology Reference
In-Depth Information
Transceivers are mounted directly to the thick cable and secured in place. A
stepped hole is drilled into the cable with a special drill and guide. When the trans-
ceiver mounts to the cable, a probe makes pressure contact with the cable's center
conductor and the ground of the transceiver is placed in contact with the shield of
the cable. (As mentioned, this connection method is sometimes called a
vampire
tap
.) Thicknet is most often placed into ceilings or walls near the workstations.
Prewire methods may place the cable into wall outlets with an integral trans-
ceiver tap. Power for the transceiver is provided from the workstation when the
transceiver cable is connected to the wall outlet.
The cable may also be placed above the ceiling, suspended from structural
members or lying directly on the ceiling grid. The transceivers are tapped into the
cable as needed and the transceiver cables are run to the workstations by dropping
directly or running through a wall opening.
The backbone trunk cable may be run a maximum of 500 m (about 1640 ft)
without using repeaters or bridges, with each of these runs being called a
segment
.
Segments may be joined with a repeater or a bridge. A repeater simply receives,
reconstitutes, and retransmits the Ethernet signal (packet) to the next segment. A
bridge or router, on the other hand, reads the packet and determines if the packet's
destination is in another segment. If so, the bridge or router recreates the packet
and retransmits it on the next segment. Ethernet 802.3 has a maximum limit of
two repeaters between segments (four repeaters, if two of the segments are
repeater-to-repeater links). Ethernet version 2 defines things a little differently to
determine the repeater limit. Bridges and routers may be used to extend the net-
work beyond this repeater limit or to off-premises locations via wide area net-
working (WAN).
The transceiver cable is limited to a length of 50 m (164 ft). While this distance
might mean that it would be possible to run all the way from a wiring closet to a
workstation, the usual procedure is to run the cable to a nearby section of the trunk
cable and install a transceiver. Each transceiver must be separated from others by a
minimum of 2.5 m (about 8 ft, 2.5 in). This minimum distance makes it impractical
to place many transceiver taps in a single location. In instances where several AUI
cables must be run, a multiport transceiver with several AUI interfaces may be used.
The 10Base5 standard allows a maximum of 100 transceiver nodes per segment,
including repeaters.
Thicknet installations are most often found in large buildings and are fre-
quently older network installations. The problems that may occur are often due to
corrosion of the connectors, barrels, or even transceiver probes. Additional prob-
lems may occur because of improper grounding. More information on grounding is
given at the end of this section.
