Interference Considerations; Distance Limitations - Cisco 7000 Series Hardware Installation And Maintenance Manual

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 electromagnetic interference, especially as caused by lightning or radio transmitters, can
destroy the signal drivers and receivers in the router, and can even create an electrical hazard by
conducting power surges through lines and into equipment. (Review the safety warnings in the
section "Working with Electricity" earlier in this chapter.)
Note
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. When exceeding the recommended distances,
use a high-quality twisted-pair cable with one ground conductor for each data signal when
applicable.
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 (EMP)
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.
Most data centers cannot resolve the infrequent but potentially catastrophic problems just described
without pulse meters and other special equipment. These problems can cost a great deal of time to
identify and resolve, so take precautions by providing a properly grounded and shielded
environment, with special attention to issues of electrical surge suppression.
The ground screw on the left side of the interface processor end of the chassis provides a connection
point for connecting a ground wire or antistatic device to chassis ground. (See Figure 1-5.)

Distance Limitations

The length 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 these 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. The following distance limits are provided as
guidelines for planning your network connections before installation.
Ethernet Connections
The maximum distances for Ethernet network segments and connections depend on the type of
transmission cable used: 0.4-inch diameter coaxial (10Base5), 0.25-inch diameter coaxial
(10Base2), or unshielded twisted-pair (10BaseT). The term 10BaseN is an abbreviation for 10 Mbps
transmission, Baseband medium, and N is the maximum cable length in hundreds of meters.
To predict and remedy strong electromagnetic interference, you may need to consult experts
Site Requirements
Preparing for Installation 2-5