Designing Wi-Fi for High Density

In technical interviews, I often ask (and am often asked):
How would you design a Wi-Fi network to support a large room with 1000 devices?

The question is purposely vague to identify how someone thinks through a problem that doesn’t have a single answer, and to observe how thoroughly they respond. Below I’ll take my own stab at a response.

Step 1: Requirements Gathering

Starting off by talking about antenna types or software tuning is the wrong first step, every time. As much information as is provided in the question, it’s never enough. Wi-Fi is a fickle beast, and collecting requirements is certainly the most important step. I would start by asking qualifying questions such as:

  • What types of devices will be associating?
  • What types of applications are we expecting to support, and/or how much bandwidth is needed per client?
  • What is the construction and layout of the room?
  • What are the restrictions on AP location, such as cabling, mounting, or aesthetic requirements, etc.?

Step 2: Hardware

The correct hardware choice is usually determined by the answers to the questions in the previous section. Some environments, such as stadiums, allow for access points to be mounted under seats, where integrated omnidirectional antennas are adequate. In other areas, such as conference centers where chairs and tables may be moved, access points need to be mounting on walls or high ceilings.

Above 25ft, omnidirectional antennas lose a lot of their performance, as most of the attenuation is into space where there are no clients. In these cases, downtilt omni-directional antennas can provide a similar horizontal range, but better propagated toward the floor. In cases where limiting the propagation is desired, semi-directional or directional antennas will limit the horizontal propagation while also improving the vertical reach.

Step 3: Software Tuning

While every environment is different and requires unique exact configurations, a high density environment almost certainly requires a high density of APs, and with that there are a certain set of options that are best practice for almost all such deployments

Data Rates

In a well designed Wi-Fi environment, it’s a best practice to increase the minimum data rate above the default. 12-18Mbps is a common setting, as it prevents 802.11b devices from joining the BSSID and bringing other clients down, and it reduces the airtime required for management frames, leaving more space for meaningful traffic. It can also reduce effective cell sizes by not supporting clients that have too weak an RSSI to transmit at the increased minimum rate. However, caution is needed as setting the minimum data rate too high can lead to high amounts of corruption

Channel Planning

More APs means more chance for co-channel contention, which negatively impacts all clients on that channel. Where possible, enabling the use of 5GHz UNII-2 extended channels allows for more non-overlapping channels, as long as clients support them. On the 2.4GHz spectrum, with only 3 non-overlapping channels available in the US, disabling the 2.4GHz radio on select APs will reduce the number of APs in an area fighting for the same frequency.

In addition to enabling more 5GHz channels, it’s important to reduce the channel width to allow for more channels to be used concurrently. A high density environment configured for 80MHz-wide channels may only have six non-overlapping channels available, while the same environment configured for 20MHz-wide channels will have 25 non-overlapping channels. There is a tradeoff in throughput by reducing channel width, but that’s usually less important than having more channels available.

With a reduced number of 2.4GHz radios compared to 5GHz, band steering can also be effective at encouraging dual-band clients to connect on the 5GHz channels where there is less congestion.

Power Levels

With high client density, access points are generally placed to cover a chosen number of client devices. Because those clients are in a smaller area than lower density deployments, the AP doesn’t need to cover as large a physical area. Lowering the transmit (TX) power of the APs will reduce the cell size, and thus reduce the amount of co-channel contention.

Advanced Options

Some vendor-specific options, such as Cisco’s RX-SOP, can also impact client connectivity and roaming. While RX-SOP is marketed as helping to “reduce cell size ensuring clients are connected using the highest possible data rate”, this is not what it’s designed for, and improperly configuring these options can negatively impact connectivity. RX-SOP is used to lower the possible contention between APs on the same and adjacent channels by reducing the APs “sensitivity” to packets in determining transmit opportunity. When tuned correctly, it can increase the overall available airtime available.

Most vendors offer some type of Radio Resources Management (RRM) capabilities to automatically tune the settings above, to provide features such as: coverage hole detection and correction, dynamic channel assignment, dynamic transmit power control, and client balancing. However, many RRM solutions don’t do a great job of tuning for high-density environments out of the box, and almost always need tweaking and tuning.

As with any Wi-Fi deployment, there is no “one-size fits all” answer. Site surveys, both pre- and post-installation, are vital in ensuring success.