Part three of the series showed you the terrifying effect of a disastrous increase in the noise floor using a real-life example from a production network’s access point’s perspective. This time I want to show a similar event but from the other side of the link: at the customer premises.
Interference cropping up from various consumer-grade products is a commonly occurring problem for WISP and fixed wireless operators, not to mention their interference-oblivious customers. The list of electronics that can interfere with wireless broadband signals is long indeed.
Here are just a few popular, and more often than not, guilty suspects to look for:
- Cordless phones
- Wireless speakers
- Microwave ovens
- Garage door openers (remotes)
- Baby monitors
- WiFi routers (Netgear, DLink, Linksys, etc)
- Smart meters (installed by electrical utility, ie. hydro)
- Bluetooth devices
- Etc., etc., etc…
In this particular example, the customer called in, quite irate, complaining that his Internet ‘kept breaking’ twice a day for a period of an hour or so. He also added that it rarely worked at night. Apparently this had been going on since the installation of the service two months previous (but was just calling into support now).
So what was happening here? Well, time to put the assessment and troubleshooting process used in part three to work.
GENERAL ASSESSMENT
Scale: Single customer with intermittent service interruptions, integrated radio/antenna with 42º beamwidth install at 909.0 MHz pointing to omni sector at < 1 km.
Internal factors: No known changes or additions to the WISP customer’s wireless network.
External factors: No known tower (or other) construction projects in customer’s coverage area.
Timing: Customers claims stability and/or performance is affected 3x daily; twice during the day, once at night for various periods of time.
TROUBLESHOOTING PROCESS
|
Step |
Test |
Result |
|
1 |
Baseline review |
Normal receive signal strength from AP -70 dBm, signal-to-noise ratio ~25-30 dBm, throughput 2-2.5 Mbps |
|
2 |
Configuration changes |
No changes to hardware or software since installation (~2 mths) |
|
3 |
Customer feedback |
Instability/performance issues at specific times of day |
|
4 |
Radio configuration |
Configuration correct |
|
5 |
Radio operation and monitoring |
Daily automated and manual spectrum analysis results reviewed, no indication of noise or interference found; however, MRTG performance (Kbps) and signal-to-noise ratio data coincident with customer’s reports on timing |
|
6 |
Hardware inspection and component swapping |
Site visit scheduled to coincide with one of the time periods that customer reports outage. Spectrum analysis reveals channel interference (see scan below) |
|
7 |
Parallel system testing |
Unnecessary (see conclusion and resolution below)
|
There’s that noise floor again, walloping the legitimate signal coming from the tower. So much so, the scan can’t even pinpoint the tower’s transmitter (CCU – pink square).
CONCLUSION AND SOLUTION
Unfortunately this scenario happens all too often in the fixed wireless broadband world. If you’re lucky, and gently ask the customer the correct questions, you can locate the culprit. In this case, the customer eventually revealed that they were using an older baby monitor. Further prodding led to the admittance that the monitor was only powered on when their 6-month old took his 2 naps during the day and overnight. And wouldn’t you know it, they were in the middle of nap #1 on the day we arrived on site.
The customer complied in the request to power off the baby monitor so I could take another scan.
And voila…
…a nice, clean signature of the expected 11 MHz channel bandwidth from the tower, complete with the center frequency plotted at a level consistent with the original baseline. A quick speed test between both radios also resulted in throughput consistent with the baseline. Finally, I had the customer confirm resolution of the problem by reconnecting the service to his computer to test. In the end it was all gummy bears and rainbows.
The next article will cover interference from neighboring systems (read competitors). Be sure to review the first article in this noise and interference series that speaks to cooperation with your wireless brethren.
Author Profile: Tim Preston is a Senior Network and Systems Analyst with experience dating back to 1998. He started on the front lines of technical support for a large northern Ontario Internet service provider while earning his diploma in Computer Programming and Network Analysis. After being hired by a major wireless broadband radio manufacturer, Tim moved to Toronto in 2001. In 2009 Tim started Haven IT Consulting. Examples of work he has done for his clients include providing management and troubleshooting services to wireless ISPs, interconnecting retail outlets for an equipment supplier, and providing technical auditing, network design, operations advice, and technical support for various local businesses and network solutions providers.
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