My first design

I’m wrapping up my first predictive design – 3 floors of a building that is being heavily renovated for a client. The renos are in progress so much of the construction does not yet match the drawings; and the client wanted to know where to have the electricians run cabling for APs.

Channeling Keith Parsons – the most important part of this design is the validation stage; which hasn’t happened yet; but WILL be happening once renovations are near completion. When that happens, I’ll find out how far off the predictive design was, and make adjustments to make sure I’ve met the requirements.

I’d like to share the process I went through for two reasons:

  1. So my experienced colleagues on twitter can offer advice that I can use to improve
  2. So my novice colleagues on twitter can see the process and the expert advice

I realize that I will make mistakes, but I will learn from them, and hopefully give some of the other up and coming WLAN experts something to glean. I’m definitely uncomfortable about posting my first design for the community to see, but I’m confident that a lot of good can come from it. With that in mind, please be gentle!

We’ll call this client Aperture Science. They are a research facility.

Step #1 ? REQUIREMENTS (see Keith, I’m listening!). Today’s post will be about the specification of the requirements. Here is a snippet of that section from my design report.

NOTE: The design is for the 5 GHz band. 2.4 is just a “bonus” and I’ll show that in an upcoming post.

3. Design Requirements

The most important step in proper WLAN design is determining the requirement metrics to be met. These requirements can then be used to derive a design, measure deviations in production environments, and validate that requirements are met.
Aperture Science has indicated that the portal building WLAN will service a number of client devices:

  1. Laptops
  2. Microsoft Surfaces
  3. Wireless VoIP handsets – Cisco 7925G-EX model

These devices will transmit a mixture of traffic types:

  1. General web data traffic
  2. Internal corporate file sharing and web traffic
  3. VoIP calls
  4. Real-time videoconferencing

Based on these needs, the predictive design is based on the most stringent client need; in this case, the VoIP call traffic is the most sensitive to delay and loss. Minimizing delay and loss is accomplished by:

  • Maintaining strong WLAN signal strength (RSSI)
  • Maintaining high Signal-to-noise (SNR) ratios
  • Minimizing co-channel interference
  • Ensuring the handset can always hear two or more suitable APs to seamlessly roam

Maintaining strong signal strength and high SNR; as well as minimizing co-channel interference allow the client device to accomplish several things:

  • Spend less time waiting to transmit
  • Use higher data rates when transmitting
  • Receive data (including VoIP call packet data) faster, with fewer retries.

Cisco has a published set of recommended requirements when designing for VoIP over WLAN; therefore, these are the requirements that were used for the predictive design.

The requirements for this design are set as follows:


Signal Strength – sometimes called coverage – is the most basic requirement for a wireless network. As a general guideline, low signal strength means unreliable connections, and low data throughput. Signal Strength details in this report are shown in two ways:

  1. Whether the strongest AP in each location meets the requirement
  2. Whether the two strongest APs in each location meet the requirement

The second item is notable because the Cisco recommendation for VoIP is for the handset to be able to hear a minimum of two APs at the recommended signal strength in each location. This is slightly different than the Number of Access Points requirement, which has a slightly lower requirement of -75 dBm.

Signal-To-Noise Ratio indicates how much the signal strength is stronger than the noise (co-channel interference). Signal must be stronger than noise (SNR greater than zero) for data transfer to be possible. If the signal is only barely stronger than noise, you may encounter occasional connection drop-offs.

Data Rate is the raw bit rate in a single direction. It is not indicative or closely related to overall bandwidth or throughput, unlike wired Ethernet mediums.

Number of Access Points indicates the number of access points audible at each location, at minimum signal strength levels. This is important to facilitate roaming from one AP to another without loss of packets.

Channel overlap indicates the number of access points audible at each location in a single channel. The total capacity of a channel is fixed, regardless of how many APs there are. Adding APs within range of other APs on the same channel does not increase capacity; in fact, it creates co-channel interference (CCI). It is, therefore, necessary to reduce overlap, and CCI as much as possible.

I’d like to stop there for now, and take some time to get some comments. Bear in mind that I will be sharing additional details over the coming weeks!

Let me know what you think in the comments or on twitter @bmroute. Perhaps a hashtag is in order… how about #1stwifidesign ?


5 thoughts on “My first design

  1. I’ve been browsing online more than 3 hours today, yet I never found any interesting article
    like yours. It is pretty worth enough for me. In my opinion, if all web owners and bloggers made good content as you did, the net will be much more useful than ever before.


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