Wireless LAN Capacity Planning
By Laure Garcia On 12/18/2024
Wi-Fi has become a prevalent method for network access, often supporting mission critical devices and applications. The ubiquity of smartphones has acclimated users to always being connected. Heightened expectations for Wi-Fi performance have made it akin to tap water, only noticed when it doesn't work well. Capacity planning is the key to delivering network performance that meets users' needs.
The unbound nature of wireless networking that makes it so convenient also makes it challenging to design. There are numerous factors that must be considered because WLAN capacity does not exist in a vacuum. These include user-oriented factors such as density, devices, and applications. Other factors are environment-driven: attenuators, interferers, and constraints.
Defining Requirement Areas
WLAN designs may use a coverage-based approach to provide basic connectivity for areas with a low user density or a capacity-based approach to deliver a defined level of performance for areas with high user density. WLAN designs commonly require a mixture of these two approaches as requirements can differ between areas. Capacity cannot be delivered without adequate coverage. However, achieving optimal WLAN capacity also necessitates additional factors beyond just coverage.
The first step is to identify the physical areas that require WLAN coverage. Good quality floor plans with proper scale and zone labels are preferred for this step. Coverage areas can then further be divided based on usage requirements. Identify the number of users, user-to-device ratio, types of devices and their capabilities. Wi-Fi is a shared medium, so determine the least capable, most important device because it will dictate the minimum performance of the WLAN in each area. Information about the client device mix may be obtained from IT personnel, the resident WLAN controller, or other network management/analytics platforms.
The second step is to identify the applications and services in use for each requirement area. It is important to remember that not all services require constant communication over the network. VoIP and video conferencing are latency sensitive and are constant bit rate services, while services like web browsing and email are not. Also note that while most client devices may be associated with the WLAN, only a portion will be actively using the network concurrently. If historical WLAN usage data is available (such as from a WLAN analytics platform), it may be possible to identify "peak busy" times and investigate association count, number of active clients, active services or applications, and throughput achieved during those times.
Considering the Radio Frequency (RF) Environment
Beyond clients and services, physical limitations or other constraints may influence the WLAN design. Signal attenuators such as building materials or large groups of people affect AP density. Interferers, Wi-Fi or not, need to be factored into channel planning to mitigate impact on WLAN performance. Spectrum availability may also be limited by the regulatory domain and client device channel support (check drivers/firmware). Mounting restrictions, related to aesthetics or other factors, may require thoughtful AP model selection to achieve the WLAN's goals.
Capacity Planning Variables
Information gathering is critical to successful WLAN design and deployment. The activities discussed so far are pre-design efforts that replace assumptions with known data to improve the WLAN design. Capacity planning puts this information together to estimate the minimum AP count needed to handle data demand. The WLAN is a system, and its performance is based on the aggregated throughput of simultaneous transmission by multiple stations.
It is recommended to avoid including 2.4 GHz in capacity planning. With the large number of 2.4 GHz devices, including Bluetooth and microwave ovens, it should be considered for legacy devices or utility usage only. With Wi-Fi 6E and the advent of Wi-Fi 7, the 6 GHz frequency band adoption process is underway across the globe. This new frequency band will be a boon for WLAN capacity as it delivers a large amount of clean spectrum. However, client device support of 6 GHz must be confirmed or forecast before using it in capacity planning.
Capacity planning can be distilled into this simple idea: (Number of active users x Service throughput demand x Future growth factor) / AP Throughput Capacity = Estimated AP count
Unfortunately, this simple idea hides several important considerations. To start, a strong signal-to-noise ratio (SNR) is required to use a fast modulation and coding scheme (MCS) for better throughput. However, that is not the only determining factor. The MCS depends on common Wi-Fi standard, channel width, spatial streams, and guard interval. The latest feature enhancements can only be used if both the transmitter and receiver share those capabilities. Planning with the least capable, most important device will estimate the worst-case performance. When choosing a MCS as a performance target, make sure the appropriate SNR (usually 25-30 dB or higher for the faster MCSs) can be achieved. This MCS/SNR target will guide coverage planning in high-capacity areas.
Operating on wider channel widths will unlock higher data rates but there are several additional considerations. The first is that for every doubling of the channel width, the noise floor increases by 3 dB. A 40 MHz channel will add 3 dB while an 80 MHz channel will add 6 dB to the noise floor. It's important to consider this during coverage planning to maintain the desired SNR. The second consideration is spectrum availability, including channels and transmit power levels allowed within the regulatory domain. Channel planning to avoid overlapping Basic Service Sets (OBSSs) is critical as they will share airtime, degrading performance as contention increases and airtime efficiency decreases. Simultaneous transmissions increase airtime efficiency so high-density environments benefit from 20 MHz channels in most cases, especially where channel reuse can be entirely avoided.
Don't overlook the network backbone. The purpose of a WLAN is to allow wireless clients access to a wired network. The wired infrastructure must also support the capacity the WLAN can deliver. It is not uncommon for the wireless network to be blamed for poor performance when an upstream LAN issue is the cause.
Capacity planning ensures reliability and efficient network performance. However, WLAN designs are not just based solely on capacity planning. AP count estimates will vary between throughput capacity, association limits, or coverage requirements. Always choose the largest AP count estimate to meet all WLAN requirements.
Maximizing the WLAN requires understanding its clients, services, airtime, and the backhaul network. Requirement areas and design approach are defined by user density, device types, services, and throughput demand. The frequency bands used, channel widths, channel reuse, signal strength, and SNR create WLAN capacity potential. Capacity planning is the process of putting these pieces together to solve the WLAN performance puzzle.
Tagged with: cwnp, wireless, lan, capacity, planning, wifi, wireless networking, wlan design, wifi design
Blog Disclaimer: The opinions expressed within these blog posts are solely the author’s and do not reflect the opinions and beliefs of the Certitrek, CWNP or its affiliates.