The Dreaded Hallway Design

Well I have seen my fair share of Hallway designs, and I can tell you I am not a fan. I find the reason they have been used is legacy designs, and companies trying to keep costs down. I have in some situations also found that people don’t like APs placed above their desks as they fear they will be irradiated by them (but surprisingly they carry their phone in their pocket next to their vital organs with no second thought)

Lets consider the negatives around hallway designs:

  • No one working in the hallway – normally
  • Low SNR
  • Channel overlap
  • Negative RRM effect
  • Coverage holes – in the rooms off the hallways
  • To fix coverage holes normally transmit power is set at the highest manually
  • Poor roaming decisions
  • Not designed for capacity

With all these negatives it can be easy to forget to design the wireless keeping in mind that the hallway needs coverage. Some of the reasons to keep the hallway in mind when designing a building are:

  • Plan on the number of APs that will be placed in hallway
    • Important to help facilitate roaming
    • Signal propagates far in the hallway
    • These areas are transit areas that people utilise to traverse between rooms
  • Lower transmit power or use thresholds
    • Number of APs that can hear each other will have a negative effect
  • Consider RRM in the design
    • Transmit power levels
    • Set thresholds – don’t allow the AP to go too high or too low
  • Use building obstacles to help attenuate the signal
  • Place APs where the users are
    • Are there areas in the hallways where people gather for example outside a meeting room as these will need to be considered.
    • Higher SNR
    • Better data rates
    • Users are in the rooms
  • Use a proper channel plan
  • Plan for roaming
    • Wi-Fi calling now available, users will walk out of their workspace

I recently have had lots of wireless issues logged for a particular building that had a hallway design installed prior to my starting with the company.

Investigation found that the users were not receiving adequate signal levels where their desks were located, and in some cases for them to use the wireless they had to go out into the hallways, so not ideal. The walls in this building had a 8-12 dBm loss through them

Unfortunately I never managed to get a full site survey completed prior to the new RTLS design being installed (due to access issues and timings) – only some spot checks with an Aircheck showing RSSI of -75 dBm or worst at users desks.

Below is what the existing design was and the predictive survey from this design:

Building 11 Hallway design
Original Design
Building 11 Hallway heatmap
Original Design – Predictive Survey

For the new design it was decided – due to business requirements – to be a RTLS design, deployed along with Cisco’s Hyperlocation modules to potentially give greater location accuracy. Now I am not sure if the Hyperlocation modules will give any advantage in this building when this was deployed as the Hyperlocation Deployment Guide states:

A general rule of thumb is to have 3 or 4 access points that are within line of sight of the device at a distance of less than 70 feet

I have an upcoming post showing the comparison of the location accuracy of both RTLS and Hyperlocation.

The requirements of the design were:

  • RSSI of minimum of -67 to all areas – with the exceptions of the stair wells, and toilets.
  • Ability to see 3 APs at minimum of at -75 dBm or better as per Ciscos best location based services best practices

For mobile devices to be tracked properly, a minimum of three access points (with four or more preferred for better accuracy and precision) should be detecting and reporting the received signal strength (RSSI) of any client station, asset tag, or rogue device being tracked. It is preferred that this detected signal strength level be -75dBm or better.

  • SNR of 20 or better.
  • Account for capacity – Large teaching spaces holding up to 100 students at a time.
  • Keep AP numbers to a the minimum required to meet the customer requirement of keeping costs down.
  • Location Accuracy of <3m required for associated Clients, <10m for probing clients.
  • Toilets, Stairwells and Lifts did not reauire coverage, although spill coverage would be accepted

For the design I used Ekahau Site Survey utilising Cisco 3702i APs. I might have considered the 3800 series but currently they don’t support Hyperlocation although I have been told these are on the way.

Survey was done with TX power set to 25mW. An assumption was made about the wall’s thickness of 10 dBm and that they were all uniform as on site pre-deployment survey could not be arranged. This dB drop was calculated from couple of spot checks – although turned out that might not have been 100% uniform.

Building 11 RTLS design
RTLS Design
Building 11 RTLS Predictive heatmap
RTLS Design – Predictive Survey (5GHz)

Following the design being installed a Post implementation survey (leaving RRM and TPC  to take care of Channel Assignment and Power Levels along with default RF Groups) was conducted the results were:

Building 11 RTLS Post Install Survey
RTLS – Post Installation Survey (RSSI – 5GHz)
Building 11 RTLS Post Install Survey SNR
RTLS – Post Installation Survey (SNR)

End Results:

Following the site survey and spending hours in the building conducting location accuracy testing, I was able to speak to many of the users, all who advised that the wireless service was heaps better and they were no longer having coverage issues.

 

 

 

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