Capacity planning for High Density WLAN
Last year one of my projects was a smaller music venue in Copenhagen who had a wish for free WLAN for their music lovers and an internal WLAN for corporate business. The venue is a multilevel building with entrance/wardroom in street level and music hall centered on second to third floor with a capacity max of 1550 standing persons. Surrounding the main hall you´ll find lounge areas with bars serving alcohol. Their business objective was PR through guest uploads to social media.
A project like this is one of the more exciting and challenging once since you as a wireless engineer will be tested in areas as capacity calculations RF planning and preferably live configuration tweaking of supporting features and cells. Deploying a HD WLAN does not leverage much room for Best Practice installations because they are to generic. Deploying high density WLANs involve a lot of different steps but for this article I will focus on the capacity calculations.
Starting with the capacity calculations I began looking into the wireless capabilities of the guest devices. For a venue like this we won’t be seeing guests with laptops or tablets but in stead a lot of smart phones to capture the experience. Given the small size of Denmark we don’t see that big a diversity in smartphones amongst people. Its more or less always iPhones and these like most other vendors at that point supported 802.11n which is great for my design and the RF environment.
A clean 802.11n BSS do not implement any protection mechanisms for extra overhead and allows higher data rates. At that time we saw a mixture of iPhone 5/5s and iPhone 4s. iPhone 5/5s supports dual-band 802.11n but iPhone 4s only supports 2.4Ghz 802.11n without short guard. Another down side is the number of spatial stream which are 1x1:1 meaning STAs can´t take advantage of SDM and due to only 1 Tx/Rx antenna benefit from MRC. STBC and TxBF is only able to increase SNR for higher modulation in the mid signal range. The number of antenna chains on 2.4Ghz without short guard support theoretic data rates up 65Mbps on a 20Mhz wide channel. And on 5Ghz up till 150Mbps on a 40Mhz wide channel with a 400 nsec guard interval. To support this the Cisco´s Aironet 2600 series where "chosen" (sometimes hardware is sold by a seller before consulting the technician, different discussion) as an access point with a 2504 WLC.
Now the goal is to minimize the amount of airtime each transmission utilizes and part of this is reducing overhead freeing up highway. And some of the benefits with 802.11n are exactly that. 802.11n uses frame aggregation with block acknowledges instead of every frame being acknowledged. You`ll find two types of aggregation one A-MSDU which can aggregate a total of 7,935 bytes in size and A-MPDU which aggregates up to 65,535 bytes in size. In a pure 802.11n BSS clients don´t require any protection mechanisms (RTS/CTS) to interoperate with 802.11b/g/a devices also resulting in less overhead and more airtime. But actually when sniffing 802.11 traffic from Apple devices you´ll see they use (RTS/CTS) all the time maybe as some sort of link quality. Either way now we have 802.11ac RTS/CTS is used to test the additional channels used before transmission when channel bonding.
1x1:1 device at 2.4Ghz on 20Mhz channel: best data rate 65Mbps ≈ 30Mbps in TCP
1x1:1 device at 5Ghz on 40Mhz channel: best data rate 150Mbps ≈ 70Mbps in TCP
Next I need to have a projection on the application throughput needed. Guests are mostly prone to use their smart phones for social media like Facebook Twitter etc. at arrangements like this. So an expected throughput in the range from 250Kbps to 1Mbps. For this calculation I am going with some assumptions from how you see the guest behavior during a concert will be. For instance we do not expect all 1550 guests will be in the need of 1Mbps transmission at once and even a 1Mbps stream. We will go with roughly 1/4. Therefor the next calculation I assume a number of 390 simultaneous transmitting clients on the a- & g-band which is about half. A total number of 780 clients transmitting a 1 Mpbs stream at once seems a bit stretched I dont remember myself being able or capable of maybe uploading a picture during my visits. But better to be safe and this gives us an idea to guide us on the right path.
1Mbps/30 = 3,33% airtime x 390 clients ≈ 1298%
1Mbps/70 = 1,43% airtime x 390 clients ≈ 558%
Total airtime for 1Mbps transmissions = 1298 + 558 = 1818% / 80% = 23 radios / 2 dual-band = 12 APs
The 80% is around the max performance you can expect from a enterprise grade AP from Cisco, Aruba, Ruckus etc
Next step would be to look into the psysical surroundings of the venue to determine placement of APs with guest flows and behavior during concerts in mind when performing the site survey for coverage and rate efficiency.