wi-fi system maintenance - 4liberty, inc. system... · 5 martin w greenwood, vp network design,...

1 Martin W Greenwood, VP Network Design, 4Liberty Inc, [email protected]

www.4liberty.biz, Direct Telephone (858) 610 4832

White Paper – Wi-Fi System Maintenance

Martin W Greenwood VP Network Design, 4Liberty Inc September 23, 2013

One of the regular requests that we receive is for a “health check” of a Wi-Fi network. This turns out to be more demanding than it might at first appear and this paper highlights the steps in the process needed to perform a full maintenance review.

Pitfalls of the “Break Fix” Model

Provision of “public access” Wi-Fi is considered to be an important value-add for many types of enterprise. A typical approach to IT infrastructure maintenance is “break-fix”, but in the case of a public Wi-Fi network, it is not immediately obvious when something is “broken” and the perception often is that “we need an extra one or two access points”. This is often the wording that we see on the work order, and by the time the issues have become thus perceptible, there may be quite an accumulation of faults and issues.

Objectives of the Typical Maintenance Visit

The motto is “Assume Nothing!” The questions we ask ourselves are:

(a) Was the original design capable of giving good coverage? (b) Was the network built to the design, in the first place? (c) Is all the equipment present and working? (d) Do the physical switch port allocations correspond to the “as builts”?



(e) Are additional access points needed, and are there any that are redundant and should be removed?

(f) Does the network need to adapt to additional load or interference that has arisen since the original design was completed?

Step 1 – Decide on the Design Rules and Target Area

Right up front, it is important to understand what you are trying to achieve in objective terms. For a public access network we typically would specify -75 dBm signal strength and +20 dB signal-to-noise-and-interference ratio, though this does also depend a bit on the type of access point.

Secondly, we discuss with the customer what area we are trying to cover. Are bathrooms included? Stairwells? Management offices? We also like to have a percentage coverage figure in mind too – usually 97% is adequate for public access, though on a new build we would normally design to 99.5% or 99%. To specify 100% coverage is asking for trouble!

Step 2 – Create A Heat Map

This is the simple and obvious first step. We would normally use Ekahau Site Survey, but Air Magnet is equally effective. Recording on both the 2.4 GHz and 5 GHz bands, we would walk the entire area at least twice, and if possible use a continuous PING test too to identify any access points that are putting out a Beacon but lack Internet connection.



Section of a typical heat map

Step 3 – Locate the Access Points

The next step can be a little tricky. The idea is to identify the access points physically and locate them onto the heat map. This involves careful inspection of the heat map and estimation of the most likely spot for the access point, and then walking the site to see if you can physically locate – and photograph – it. When working with both bands, it is necessary to inspect the MAC addresses with care inside the mapping software to make sure that the 2.4 GHz and 5 GHz radios are combined into the same physical access point.

At the same time, the heat map and walk-around should reveal any defunct radios from previous installations that are still transmitting. This is a surprisingly regular occurrence, causing confusion to visitors and clogging the airwaves unnecessarily.

Physical location of the access points is an important step in creating an accurate “as built”. Backed up by photographs, this creates a high degree of confidence that the actual build-out has been established. At this stage it is also obvious if there is an access point that is transmitting on one band but not the other and any gaps in coverage may readily be identified.



This is also a good opportunity to inspect the access points for physical damage (such as missing or broken external antennae) or poor installation. Examples of poor installation would be having the access point directly behind some obstruction that blocks the radio signals, or it may be that something has been built in front of the access point in the time since the original installation.

Step 4 – Missing and Broken Access Points

Upon finding a gap in the coverage, it is often the case that an access point has failed. Sometimes, finding it can be as simple as walking to the most likely candidate location and taking a look. At other times it is necessary to refer to the original design documentation (if available), or looking at the port allocation sheet to see if all the access point switch connections are accounted for.

Access Point with Damaged Antenna

Step 5 – Identity of Access Points

It is important, in producing a complete as-built, to relate the name of the access point as it appears on the port allocation sheet, to the physical AP location. This is not always as simple as it sounds. The usual way is to relate the BSSID (RF MAC address) of the access point to its LAN-side MAC address as it appears on the switch. Some manufacturers – Cisco-Meraki notably – also allow interrogation of the AP and this gives the required information (http://my.meraki.com when associated with a particular AP).



Meraki over-the-air diagnostics

Step 6 – Surveying Coverage Gaps

Having identified coverage gaps, the survey team wants to be sure that putting in a new or replacement AP will be effective in plugging the hole. This is best done at the time of the survey, by temporarily placing a live access point and re-surveying the relevant area. It is also possible, in Ekahau, to persuade the software to simulate placement of an access point and superimpose the simulated coverage on the real coverage map from the physical survey.

Step 7 - Physically Check Switching Plant

Each IDF closet should be inspected to see that the actual AP connections match what is shown on the port allocation sheet. It is surprising how often these two things diverge.

Step 8 – Measure Interference – Interference Survey

We normally make a second sweep of the property specifically to identify interference. Use of a spectrum analyzer is essential, because it picks up both Wi-Fi and non-Wi-Fi interference, and does so across the full spectrum. Sometimes you can find the exact source of interference (wireless CCTV being a particular culprit) and oftentimes you cannot, but it gives a good overall picture of the radio environment within which the network must operate and identifies any particular trouble spots.



Waterfall Diagram showing interference

Step 9 – Measure Congestion

Congestion is slightly different from interference and we measure it in a different way. We normally use a radio packet “sniffer” with appropriate software (such as Metageek’s PCAP) and go to a high traffic area where we measure the occupancy of each channel. It is important to do this at a busy time, and often this will reveal, for example, that the 2.4 GHz channels are severely congested. When we see occupancy on a channel of over 50% and a re-try rate of 75% we can see right way that the public access is simply not going to give good performance on that channel, whatever we do with the radio coverage. This is useful information for the site owner, at least, and takes the pressure off the IT department to attempt the impossible!



Packet Analysis showing heavy airtime congestion

Step 10 – Summarize Findings

When there’s a lot going on with a large and complex network, this can be the most difficult step in the process. It usually takes us more than a day’s intensive desk-based effort, in addition to the time in the field, to produce a high quality report. In addition to the as-built information, we will produce a pictogram that shows the exact equipment locations, plus the locations where various types of maintenance work are required:

- APs to remove - APs to replace - New APs needed

wi-fi system maintenance - 4liberty, inc. system... · 5 martin w greenwood, vp network design,...

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