2-5
Cisco 7204 Installation and Configuration Guide
OL-5101-02
Chapter 2 Preparing for Installation
Site Requirements
Interference Considerations
When wires are run for any significant distance in an electromagnetic field, interference can occur
between the field and the signals on the wires. This fact has two implications for the construction of plant
wiring:
•
Bad wiring practice can result in radio interference emanating from the plant wiring.
•
Strong EMI, especially when it is caused by lightning or radio transmitters, can destroy the signal
drivers and receivers in the Cisco 7204, and can even create an electrical hazard by conducting
power surges through lines and into equipment. (Review the safety warnings in the section
“Safety
with Electricity” section on page 2-3
”.)
Note
To predict and remedy strong EMI, you may also need to consult experts in radio frequency
interference (RFI).
If you use twisted-pair cable in your plant wiring with a good distribution of grounding conductors, the
plant wiring is unlikely to emit radio interference. If you exceed the recommended distances, use a
high-quality twisted-pair cable with one ground conductor for each data signal when applicable.
Warning
A readily accessible two-poled disconnect device must be incorporated in the fixed wiring.
If wires exceed recommended distances, or if wires pass between buildings, give special consideration
to the effect of a lightning strike in your vicinity. The electromagnetic pulse caused by lightning or other
high-energy phenomena can easily couple enough energy into unshielded conductors to destroy
electronic devices. If you have had problems of this sort in the past, you may want to consult experts in
electrical surge suppression and shielding.
Distance Limitations and Interface Specifications
The size of your networks and the distances between connections depend on the type of signal, the signal
speed, and the transmission media (the type of cabling used to transmit the signals). For example,
standard coaxial cable has a greater channel capacity than twisted-pair cabling. The distance and rate
limits in the following descriptions are the IEEE recommended maximum speeds and distances for
signaling; however, you can usually get good results at speeds and distances far greater than these. For
example, the recommended maximum rate for V.35 is 2 Mbps, but it is commonly used at 4 Mbps
without any problems. If you understand the electrical problems that might arise and can compensate for
them, you should get good results with rates and distances greater than those shown here; however, do
so at your own risk.
Note
We recommend that you do not exceed specified transmission rate and distance limits.
When preparing your site for network connections to the Cisco 7204, you must consider a number of
factors related to each type of interface:
•
The type of cabling required for each type (fiber, thick or thin coaxial, shielded twisted-pair, or
unshielded twisted-pair cabling)
•
Distance limitations for each signal type
•
The specific cables you need to connect each interface
