My First Design part 2 – Overall design

I want to thank everyone for their feedback on the first post in this series! Next I’m going to share the “Overall Design” section of my first predictive design project. This section describes some of the specifications that the predictive design is based on, like the AP transmit power and placements.

This section is a little verbose – the client does not have a lot of expertise in RF so I wanted to give them some context to read; but in the future I’m hoping to clean this up. Recommendations or examples are welcome!

A couple of notes:

  • While this section is technical, I did try to keep it understandable for non-experts. We had an in-person meeting where we went over the document and they asked questions. At this time, they are primarily concerned with telling the architects where the Ethernet pulls should be located (with service loops to allow for repositioning).
  • You will see RRM referenced. Yes, the client (and I) are planning on allowing RRM to make channel and power level decisions – but I will be tuning RRM from the defaults.  I would love to hear recommendations on good ways to do this to work with this design. I have a few years of experience with Cisco’s implementation, but could certainly use a refresher. I’m planning to decide on the RRM settings during the validation phase.
  • I’m curious to hear feedback on the AP power settings – did I make a good choice?
  • Also curious to hear feedback on the choice to use 40MHz channels. There are non-VoIP clients which have less stringent RF needs, but who can make use of the bonded channels. I think the indirect benefit to the VoIP handset in addition to the other clients makes this a good choice… am I right?

I realize that some of these items rely on details regarding the building. Here’s a sneak peek of the placements and map of the 2nd floor:

2nd Floor - Access Points

Aperture science will occupy all 3 floors of this side of the building, which is a mix of office space and research/lab areas. I’ll go into more detail on some of the materials and spaces in upcoming posts.

Credit to Steve McKim of Great White Wifi for his post on AP to client power matching which I referenced in describing the power settings for the design.

I’m really looking forward to hearing the Wi-Fi community’s opinion on some of these points!

2.4.  Overall Design

Design Power levels:

Device 1702I AP 7925G Handset
Max Transmit Power 22 dBm 16 dBm
Antenna Gain 4 dBi 3.11 dBi
Design Minimum Transmit Power 13 dBm 13 dBm
Design Maximum Transmit Power 16 dBm 16 dBm

In keeping with Aperture’s current WLAN hardware, the WLAN is modeled using only the Cisco 1702I model AP, which has an internal omni-directional antenna.

The design assumes that the APs are mounted at a ceiling height of 8 feet, with standard alignment. For the 1702I, the standard alignment is horizontally mounted, with the Cisco logo facing the floor.

APs are all modeled using a reduced power output in both the 5GHz, and the 2.4GHz band, where enabled, of 20 mW (or 13 dBm), which, when the antenna gain is taken into account, amounts to a total EIRP (Effective Isotropic Radiated Power) of 17 dBm. The 7925G-EX handset is capable of a maximum power output of 40 mW, which is 16 dBm. This means that the maximum output power of the handset is higher than the modeled power of the AP. Note that the handset will likely operate at a lower average power level (i.e. 13 dBm) when possible to conserve battery power.

A higher power on the AP compared to the client device can cause issues where the client may “hear” the AP at a strong signal level (e.g. “Full Bars”), but the AP may not be able to hear the client at an adequate level. However, antenna gain (in contrast to output power) improves the ability to both hear and to be heard. Therefore, an increase in antenna gain is preferred over and increase of output power when there is any imbalance between EIRP of the client devices and the APs.

With this design, an example can be made showing the signal levels from the perspective of the AP and the handset; in this case, at 50 meters apart. Power gains and losses are simple additions/subtractions:

Link Budget – Cisco 1702I AP at 13 dBm and Cisco 7925G Client at max power

Downlink Uplink
AP sending to Client Client Sending to AP
Transmit Power 13 dBm 16 dBm
Transmit Antenna Gain 4 dBi 3.1 dBi
Effective Power (EIRP) 17 dBm 19.1 dBm
Free Space Loss -80 dBm -80 dBm
Receive Antenna Gain 3.1 dBi 4 dBi
Received Signal Level Client hears -59.9 dBm AP hears -56.9 dBm

Here the client hears the AP at -59.9 dBm, and the AP hears the client at -56.9 dBm. This means the AP is hearing the client better than the opposite. This also means that the AP could increase its own power slightly (matching the client) if necessary.

At one power level higher, the client hears the AP at the same level as the AP hears the client – a good, equal link:

Link Budget – Cisco 1702I AP at 16 dBm and Cisco 7925G Client at max power

AP sending to Client Client Sending to AP
Transmit Power 16 dBm 16 dBm
Transmit Antenna Gain 4 dBi 3.1 dBi
Effective Power (EIRP) 20 dBm 19.1 dBm
Free Space Loss -80 dBm -80 dBm
Receive Antenna Gain 3.1 dBi 4 dBi
Received Signal Level -56.9 dBm -56.9 dBm

Therefore, the design allows some room for transmit power to be adjusted by Cisco’s Radio Resource Management (RRM) to close perceived coverage holes where clients are at the edge of a service area.

Finally, if the AP is operating at 13 dBm and the handset reduces power to 13 dBm to save battery life:

Link Budget – Cisco 1702I AP and Cisco 7925G Client both at 13 dBm

AP sending to Client Client Sending to AP
Transmit Power 13 dBm 13 dBm
Transmit Antenna Gain 4 dBi 3.1 dBi
Effective Power (EIRP) 17 dBm 16.1 dBm
Free Space Loss -80 dBm -80 dBm
Receive Antenna Gain 3.1 dBi 4 dBi
Received Signal Level -59.9 dBm -59.9 dBm

In all 3 examples, the Received Signal Levels are quite strong, and the handset RSL is well above the -67 dBm requirement.

 

The 5 GHz band is the recommended band for serving VoIP due to the higher number of available channels and significantly lower likelihood of interference from non-Wi-Fi sources such as microwaves and cordless phones. As such, the design is for the requirements to be met only in the 5 GHz band. This design uses 40 MHz channels, which bonds two adjacent channels together, for more than twice the available bandwidth than a single channel.

While the 7925G handset is not able to use 40 MHz channels, most data devices can, and these devices require less airtime to send data when more bandwidth is available. Since they require less airtime, more is left available for the devices that can only use a single 20 MHz channel, so there is an indirect benefit for the handset.

Once the requirements were met using the 5 GHz band, the 2.4 GHz band was designed without attention to meeting the requirements. Instead, coverage levels were maximized as best as possible with the minimum amount of co-channel interference. The intent is for Aperture to push all capable client devices to the 5 GHz WLAN, and to only use the 2.4 GHz band with clients that are unable to use 5 GHz until they can be replaced.

Again, let me know what you think in the comments or on twitter @bmroute.

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