Specifications

48 Rockwell Automation Publication AG-SG001G-EN-P - April 2015
Chapter 3 Choosing Data Communication Equipment
Licensed Fixed-frequency vs Unlicensed Spread-spectrum
Considerations
Consider the following when choosing either a licensed fixed-frequency or an
unlicensed spread-spectrum system:
• The licensed fixed-frequency option requires a license - this can be an issue
in urban areas where available frequencies are already allocated.
However, in recent years the FCC has subdivided the existing licensed
frequency bands to make more available. Also, many former users of these
licensed frequency bands have switched to newer technologies such as
cellular, freeing up more bands for new potential licensees.
The unlicensed spread-spectrum option does not require a license, but this
does mean that there is the potential for other users to encroach on the
frequencies being used. However, this is rarely an issue due to the lower
power levels and technologies used with spread-spectrum radio.
• Licensed fixed-frequency systems are allowed to transmit at higher power
levels than spread-spectrum systems (5 watts vs 1 watt) and thus have
greater range capability, are less prone to attenuation from atmospheric
and environmental conditions, and have less need for line of sight
configurations.
• Licensed fixed-frequency systems transmit at lower frequencies than
spread-spectrum systems and thus have greater range capability, are less
prone to attenuation from atmospheric and environmental conditions, and
have less need for line of sight configurations.
• Licensed fixed-frequency systems typically support lower bandwidth
throughputs than spread-spectrum systems (due to smaller channel
widths). However, this is often not an issue for industrial applications
using small packet sized serial protocols.
• Licensed fixed-frequency systems can be more susceptible to noise or
interference as they utilize a single frequency.
Spread-spectrum Direct-sequence vs Frequency-hopping
Considerations
• Spread-spectrum direct-sequence systems typically support higher data
throughputs because of the data transmission being spread over a wider
band. They can be susceptible to noise that occurs over the wideband being
used.
• Frequency-hopping systems support a lower data throughput but have
greater noise immunity because the frequency at which the transmission
takes place is constantly changing.