Consonics CyberTek Qualifier Руководство пользователя - Страница 11
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Печатать страницуCATV Pedestal
Downstream Path
Upstream Path
CATV Pedestal
with Local Tap
Typical CATV underground routing to private residence.
The underground distribution system has the cable buried in the ground. The cable only
surfaces into above ground enclosures call pedestals. Opening a pedestal allows access to
the cable system for maintenance. Distribution pedestals contain local taps and an
underground cable is used to supply cable services to the residence. The underground drop
cable usually surfaces just below the ground block.
Cable signals coming to the residence follow the downstream path. Downstream signals are
in the 50 MHz to 1 GHz frequency range. To enable two way communications on a cable
system, the upstream path is used for signals originating within the residence and traveling
to a central location in the cable system. Upstream signals utilize the frequency range of 5 to
50 MHz. The upstream path is used for devices such as cable modems and special services
devices. Examples of special service devices are burglar alarms, fire alarms, and personal
health monitors.
Unfortunately, the reliability of the upstream path is vulnerable to the effects from devices
that radiate electrical energy in the 5 to 50 MHz frequency range. The upstream path uses
digital signals and as such is subject to very unreliable operation if interference signals find
their way into the system. Examples of devices capable of causing interference are: CB
radios, hair dryers, washing machines, food mixers, toasters, vacuum cleaners, and almost
any home appliance that has a motor or a power switch. The interference effect may be for
an instant or continuous while the device is on.
Quality CATV cabling practices utilized within the residence usually reduce or eliminate the
susceptibility of interfering signal ingress.
Ingress problems in the upstream path are compounded by the multitude of residences
connected to the CATV distribution system. Any one residence is capable of adding an
interference signal on the upstream path and inhibiting communications. A multitude of
residences, each only adding only a small amount of interference, can also render the
upstream path useless. Interference signals add on a power basis.
Underground
Drop Cable
Ground Block
7
Qualifier Supplement
Private Residence