Interference in a Central Office Environment
the real challenges on a wireless project occur after it is deployed
in the field—beyond the controls of the lab environment.
courtesy of CISCO SYSTEMS (Richfield, OH)
occur after wireless equipment is already deployed on customer premises.
In these cases, the manufacturer must not only solve the interference
issue but must also address the customer's concerns. This can present
a tough challenge because it is often not the wireless device acting
up, but rather the customer's equipment that is not properly immune
from the nearby transmitters, specifically portable transmitters.
This same situation
transplanted into a central office environment could be a disaster.
Shutting down a phone network is a real concern.
The issue is
whether wireless devices would disrupt the equipment in a central
office environment. Most equipment installed in a central office
environment is required to be tested for network equipment building
system (NEBS) compliance. This article discusses an on-site evaluation
of wireless devices that was conducted to assess their effect in
the intended environment.
discovered some network interference issues when they deployed 802.11b
wireless in their central offices. Before deployment, they hadn't
performed due diligence or EMI studies. The carriers' senior management
required that the engineers address the interference issues before
they could continue with the wireless rollout. We worked with their
engineering staffs to troubleshoot the situation. We then developed
a generic way for the field technicians to predict and test for
There are no
actual in situ test procedures for this type of testing except ANSI
C63.18, which addresses the medical environment. Therefore, in order
to solve the issue, a repeatable test procedure had to be developed
for use in testing the equipment in the central office environment.
to assessing the equipment's ability to operate normally, testing
was also conducted with the covers removed and cabinet doors opened.
Both of these
conditions generated some very interesting results. From these observations,
new testing methods were developed for testing and mitigating interference.
With the requirements
for NEBS compliance, EMI issues in a central office environment
should be of less concern than in other environments. However, that
was not the case in this evaluation. With the cabinets open, the
devices were no longer technically NEBS compliant. They had never
been evaluated for that mode of operation. NEBS testing is all done
at 3 m. In this test, the customer wanted to insert the antenna
into the cabinet to see whether a problem existed.
To better understand
the radio-frequency (RF) interference issue, it is important to
know what FCC means by harmful interference. As defined in 47 CFR
Part 2 and Part 15 Sections 2.1 and 15.3(m), harmful interference
is an emission that endangers the functioning of a radionavigation
service or seriously degrades, obstructs, or repeatedly interrupts
a radiocommunications service.
A Part 15 device
that is being interfered with, either by another Part 15 device
or by any licensed radio service, cannot claim protection. Even
though Part 15 says the device must accept interference, in the
United States and Canada, there are no mandatory regulations requiring
the testing for compliance to demonstrate this for Part 15 devices.
The requirements to comply with specific susceptibility standards
such as Telcordia Generic Requirement (GR-1089-CORE) are industry
driven rather than regulatory driven.
in a central office generally operates without error when a transmitter
is several meters away. This is because most central office equipment
is evaluated for susceptibility from transmitters operating in the
far field. This testing is done as part of GR-1089-CORE, which tests
devices for normal operation when exposed to E-field strengths of
8 or 10 V/m.
office equipment will most likely not be affected by the wireless
access point equipment, it could very well be affected by a portable
transmitter operating in close proximity to the device. In this
instance, the portable transmitter would be the mobile handheld
device interfacing with the access point equipment. This effect
is known as near-field coupling, the most common source of interference
problems. Near field is defined as a spherical area with a radius
that is one to two times the radio wavelength, written as
the radio frequency, the shorter the wavelength and the smaller
the near-field effect. An illustration of the difference in near-field
range between two operating frequencies is shown in Figure 1.
1. An illustration of the difference in near-field range between
the two operating frequencies, 5 GHz and 2.4 GHz.
was developed for in situ ad hoc testing in a central office environment.
GR-1089 does not require tests for the effects from near-field devices;
its requirements address only far-field measurements, and the standard
does not use complex waveforms such as the orthogonal frequency
division multiplexing (OFDM) modulation scheme. We observed that
the OC48 modules did not have a problem with an 80% modulated AM
signal when measured at 6 in. at 5 V/m; however, they would lock
up when at an OFDM signal greater than 3.5V/m at any distance.
GR-1089 is ineffective when the covers of the Telco equipment are
removed during service. This was the biggest hindrance to solving
the issue. When the cabinets were opened, the network would go down
if the transmitters were within several inches. The carrier found
this unacceptable. In addition, two specific pieces of older gear
that had been tested to an earlier version of GR-1089 had issues
with 2.4 GHz.
needed an on-site test method to verify that interference will either not occur
(GR-1089-CORE does not guarantee that no interference will occur)
or a test that would at least determine which areas to restrict
from the operation of portable wireless devices. This required us
to develop a test in a real-life situation that would enable a field
technician to verify there were no problems.
We had to address
the interference caused by systems with cabinet doors open with
part of the rack operating normally and part of it not operating
normally (these scenarios are according to the Cisco best practice
GR-1089-CORE emissions, if the Cisco radio meets the GR-1089 emissions
standard in the 2.4 GHz or 5 GHz U-NII bands, it is called "broken."
The fundamental power of a Cisco transmitter exceeds GR-1089-CORE
emissions limits by their very nature. A Cisco wireless device can
operate at up to 4 W in parts of the bands.
Cisco has previously
written an ad hoc test procedure for its medical guide for use in
testing its WLAN in the medical environment. That procedure is based
on the more generic ANSI C63.18 medical test standard. This Cisco
document was used as the basis for developing the test procedure
for this evaluation. The goal was to develop a procedure that customers
could use in the field to determine whether any problem existed.
It was designed to be a fairly quick procedure without being overly
For this test,
the radio mode of operation was to perform a file transfer using
an access point. This simulated a typical real-world scenario.
For this particular
evaluation, the procedure used a total of 16 access points around
the equipment under test (EUT). The number of access points can
vary. The measurement started with the radio at about 1 in. from
the EUT at the first test point, and the final measurement was done
at 1 m. A sliding scale was used at 1 in., 3 in.,
6 in., 1 ft, 1/2 m, etc. Other distances can be used. Testing was
done at one test angle first, and then upon completion of all test
distances, the test was repeated again at the next test point. It
is critical to observe and record any anomalies that occur during
It was determined
that a 2.4 GHz WLAN was more disruptive to some of the systems than
a 5 GHz U-NII WLAN.
There were several reasons for this result.
of the central office equipment had internal clocks that were running
at or close to 2.4 GHz, which made them more susceptible to interference
when the boxes were opened up. Second, the 2.4 GHz radios generally
operated at higher power than the 5 GHz U-NII radios that were evaluated. Part of this
was due to the threshold of pain for these systems, which we determined
was when the field strength exceeded 3.5 V/m. Hence, the 2.4 GHz
systems were more disruptive when in close proximity (less than
8 in. ) than the 5 GHz U-NII systems were at 3 in. It is important
to note that we were not simply reinventing the wheel.
We had to provide
not only a prediction model, but also a test methodology. As requested
by the customer's senior management, both the prediction model and
the test method had to be easily understood. Therefore, the approach
was simple and very effective. Until this point, no one had documented
these procedures in a best practice guide for deploying wireless
LAN in a central office environment.
In each case, all wireless projects are going forward, and
calls reporting interference have stopped. That was the goal.
It was determined
in this particular case that deploying a 5 GHz U-NII
radio rather than a 2.4 GHz radio reduced the risk of RF
interference. RF interference not only correlated to proximity,
but also correlated to the level of transmitter RF power and frequency.
The higher the RF power, the higher the probability of RF interference
at the same distance. Therefore, reducing the RF power did help
to mitigate RF interference.
was determined that a minimum separation distance was required between
a wireless handheld device and the central office equipment being
tested. To determine a uniform separation distance, it was necessary
to determine the precise field-strength level that caused interference
to the EUT.
(with 8 dBm TX
dBm TX power
I. Testing and simulation data, 5 GHz.
the testing and simulation data and with respect to the anomalies
observed on some legacy equipment, it was determined that the system
operated without error when the E-field was equal to or less than
3.5 V/m for the 5 GHz radio transmitter. The field strength of the
5 GHz radio emissions at 5 in. with a power output of 8 dBm into
a 2.1-dBi dipole antenna was found to be 3.25 V/m (see Table I and
Figure 2). Issues arose when the 5 GHz device was either operated
at a higher power or at a closer proximity than 5 in. (where the
E-field would exceed 3.5 V/m).
2. Near fields from 5300 MHz dipole. Far-field behavior begins
after 3–4 in.
The use of
wireless devices in a central office environment is challenging.
It is even more so if the requirement is that no interference can
occur when equipment is open for service or testing.
industry needs to develop a standardized test procedure for testing
wireless devices within central office environments. The development
of an industry test standard for near-field measurements is currently
being undertaken by the industry. As part of this effort, the procedure
used for this testing has been forwarded to the relevant standards
central office operators need to consider developing operational
procedures or best practices for wireless use, specifically for
handheld radio devices. These practices should include either exclusion
zones or minimum separation distances between wireless devices and
central office equipment.
and near-field-emission study data were provided by Steve Saliga
and Fred Anderson of Cisco Systems Wireless Business Unit.
Case, NCE, NCT, is senior reg-ulatory engineer, corporate compliance
EMC standards and operations, for Cisco Systems Inc. (Richfield,
OH). He can be reached at firstname.lastname@example.org.