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Cisco VG202 and Cisco VG204 Voice Gateways Hardware Installation Guide
OL-15959-01
Chapter 2 Planning Your Installation
Interference Considerations
FXS Analog Voice Port Maximum Distance
The maximum distance is established by a total allowable loop resistance, including the phone or
terminal equipment, of 600 ohm.
Interference Considerations
When you run cables for any significant distance in an electromagnetic field, interference can occur
between the electromagnetic field and the signals on the cables. This has two implications for the
installation of terminal plant cabling:
• Unshielded plant cabling can emit radio interference.
• Strong electromagnetic interference (EMI), especially that caused by lightning or radio transmitters,
can destroy the EIA/TIA-232 drivers and receivers in the Cisco VG202 and Cisco VG204 voice
gateways.
If you use twisted-pair cables with a good distribution of grounding conductors in your plant cabling,
then emitted radio interference is unlikely.
If your cables exceed the recommended distances, or if your cables pass between buildings, you must
give special consideration to the effect of lightning strikes or ground loops. If your site has these
characteristics, consult experts in lightning suppression and shielding. The electromagnetic pulse caused
by lightning or other high-energy phenomena can easily couple enough energy into unshielded
conductors to destroy electronic devices.
Most data centers cannot resolve the infrequent but potentially catastrophic problems of EMI without
using pulse meters and other special equipment. Take precautions to avoid these problems by providing
a properly grounded and shielded environment and by installing electrical surge suppression.
If you remove any module, you must either install a module in its place or install a cover plate over the
opening. To prevent electromagnetic interference, all module openings must be either occupied or
covered.
For advice on the prevention of electromagnetic interference, consult experts in radio-frequency
interference (RFI).
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