High Density Design Guidelines

Vendor Recommendations on High Density Design - 5GHz

    According to CWNP's CWDP certification study guide, wireless engineers agree that the Aruba Networks high density design guides are among the best in the industry. Due to this, recommendations will be pulled from Aruba's design guide and CWNP's recommendations form their various certification teachings. This post will focus mostly on the need for smaller channel widths in high density deployments. Channel bonding is one of the very important considerations when deploying to a high density environment.

Upsides to channel bonding:

    Bonding channels increases throughput, which can improve performance. Thus, the difference between using 20 MHz and 40 MHz channels is throughput. 40 MHz has higher throughput, around double that of 20 MHz and 80MHz has higher throughput than 40MHz thanks to channel bonding. In order to achieve the highest data rates available in 802.11ac, 80MHz(867Mbps) or 160MHz(1.7Gbps) wide channels must be used.

Downsides to channel bonding:

Channel bonding increases the noise floor:

    Each time you bond a channel together, it increases the noise floor. Meaning if you bond two 20MHz channels to make a 40MHz channel, the noise floor increases by 3dB, if you bond two 40MHz channels it increases by 6dB. Note this is Logarithmic, meaning a 3dB increase in the noise floor is actually DOUBLE and a 6dB increase is actually QUADRUPLE the noise floor of a 20MHz channel. Meaning, if you want to capitalize on the higher throughput of 80MHz wide channels, you must also ensure you have a very strong signal above the noise floor to achieve those data rates. Bonded channels are also more susceptible to interference meaning there’s a larger chance for frames to have the need to be resent.

    In reality, most client stations are unable to achieve the highest data rates in 802.11ac on 80MHz channels due to WiFi being a half-duplex medium, channel contention, and limitations of the client wireless antenna. To hit maximum speeds on an 80MHz .11ac channel, your device needs to be 4x4:4 capable. Meaning 4 transmit radios, 4 receive radios, and 4 spatial streams. Many device manufacturers do not include 4x4:4 antennas in their devices because they cause a larger power draw on the device, lowering battery life. Instead, most devices are 1x1:1 or 2x2:2, meaning the theoretical maximum data rate of 80MHz .11ac is not within reach. Please note, a 1 or 2 spatial stream device WILL still notice higher speeds on bonded channels, but not to the maximum theoretical which is usually calculated off of the largest channel width possible.

Aruba's Recommendations:

Aruba highly recommends sticking to 20MHz channels instead of bonding channels, though they do value a use case for 40MHz, they especially recommend staying away from 80MHz channels due to the number of problems they can cause.

Both screenshots pulled from Aruba's Large Campus Deployment Guide: https://www.arubanetworks.com/assets/tg/AVD_Large-Campus-Design-Deploy.pdf

Screenshot pulled from Aruba's VHD Guidelines found at cwnp.link/arubavhd

One of the most important considerations are that they do not recommend channel bonding in high density deployments:

    "For 802.11ac, Aruba is restating its longtime guidance that all VHD WLANs should only use 20-MHz channel widths. We have studied channel width in the lab, and it’s clear that 20-MHz is the optimal channel width for large numbers of clients. Never use bonded channels in VHD areas, and here are four main reasons why." (cwnp.link/arubavhd)

    "Using 80-MHz (VHT80) or 40-MHz (VHT40) channels reduces the number of radio channels by bonding them together. Having fewer channels means that the distance between same-channel APs is also reduced because there are not as many channels to spread around. If you use 40-MHz channels, you have half the number of channels as 20-MHz, therefore each channel must be used twice as often."(cwnp.link/arubavhd)

    "Reduced retries due to lower interference – Wider channels double or quadruple the exposure of each and every transmission to interference from legacy devices transmitting with a smaller bandwidth. As a result, using narrow 20-MHz channels reduces retries, which allow some channels to get through even if others are blocked."(cwnp.link/arubavhd)

    "Higher SINRs – 20-MHz channels experience up to 6 dB more SINR than 80-MHz channels for the same data rate, and up to 3 dB more SINR than 40-MHz channels. This increase is because the noise floor increases by about 3dB with each doubling of channel width. Higher transmit power is therefore required for any given MCS data rate, as compared with a narrower bandwidth. In practice, since many APs in VHD areas already operate at maximum power, the result is to reduce the SINR of the 40-MHz and 80-MHz signals. This extra SINR can make a significant difference in the throughput of a VHD network. In essence, it provides additional margin in the link budget in both directions. This gain can help keep data rates higher when dealing with crowd loss, obstructions and other impairments."(cwnp.link/arubavhd)

    "Higher performance – It is far better to have 25 users each on four different VHT20 channels than 100 users on one VHT80 channel. Four clients can be served at the same time in the same amount of spectrum, instead of having to be served consecutively by a single VHT80 access point. The Aruba VHD lab evaluated this recommendation with 100 smartphones and 100 laptops. Each group of clients was tested with a single VHT80 AP, then split into two groups of 50 and tested with two VHT40 APs. Finally, they were split again into four groups of 25 clients and tested against four VHT20 APs. In all cases, we used the same 80 MHz of spectrum (channels 100 – 112)."(cwnp.link/arubavhd)

Aruba provides a great chart on the throughput drop when putting dozens of clients on an 80MHz channel. Note that wider channels can actually handle less throughput than smaller channel widths when saturated with a lot of clients:

TLDR: Bonding channels does not always increase performance

My recommendations:

    Channel bonding is a double edged sword. It can greatly increase speed and performance, but not in high density deployments. High density deployments have different requirements that typically require you to manage clients efficiently instead of giving them a massive channel to try to reach theoretical throughput. I highly recommend sticking with vendor recommendations and only use 20MHz or 40MHz wide channels depending on your deployments and client density. I believe we will be able to get a lot of use out of 80MHz wide channels in 6GHz, but for 5GHz, there are too few benefits for the amount of risks. I am one for following vendor recommendations, since they are who design the solutions, and especially since an expert vendor agnostic wireless company recommends their design guide.

Source:

Aruba Networks high density design guide