A very common question we wireless professionals get asked is. How many clients per AP can we have on this AP?
To help us determine and better answers this question there are some excellent resource available that I have linked below. Highly recommend becoming familiar with them.
Ekahau Site Survey Pro (any of the latest version) need to pay for software
To determine client capacity, we need to understand what is actually possible, as we have some major limiting factors with our wireless networks “Air-time” and the half duplex nature of the medium.
Capacity calculators like the ones above are used to determine how many AP’s/Radio’s we need based on the amount/types of clients and their throughput requirements.
To get a general understand of how many clients a radio can support I will show some examples using Devin Akins efficiency rules (linked above) “MCS rate x ~ 40% / # of users = per device throughput.
Following examples are based on zero 802.11 and non-802.11 source of interference (perfect world).
802.11n 2×2:2 AP with 20MHz channel with 50/30/10 802.11n 1×1:1 clients at an RSSI -64 that gives them a MCS7 data rate of 72Mbps with a short guard interval at medium/large use.
Data Rate= 72Mbps
40% efficiency= medium/large use
Total throughput capacity= 29Mbps
/50 1×1:1 client devices =570Kbps/client
/30 1×1:1 client devices =650Kbps/client
/10 1×1:1 client devices =2.9Mbps/client
802.11n 2×2:2 AP with 20MHz channel with 50/30/10 802.11n 2×2:2 clients at an RSSI -64 that gives them a MCS7 data rate of 144Mbps with a short guard interval at medium/large use.
Data Rate= 144Mbps
Total throughput capacity= 58Mbps
/50 2×2:2 client devices =1.152Mbps/client
/30 2×2:2 client devices =1.92Mbps/client
/10 2×2:2 client devices =5.76Mbps/client
The following examples show the affect that device capability & number has to overall throughput per client.
Wireless networks 99% of the time contain different chipsets, understanding your device types is a critical component to your wireless network. Mike Albano site has an extensive list of different client chip sets, if the device is not listed, it mentions how to capture the information.
A very common way marketing spin their AP’s is with descriptions like: “Our 802.11n 2×2:2 APs can support 500 clients with a data rate of 300Mbps”. Which is just not possible.
- To theoretical achieve a “data rate” of 300Mbps you need to have 40MHz wide channels, clients are 2×2:2 802.11n with an RSSI -64dBm using a short guard interval.
- The term “data rate” is completely miss leading this is not your actually throughput, wireless has a lot of overheads, basically cut that value in half and there is your starting point.
Now let’s look at some other variables that also affect the capacity of our wireless networks
- Business requirements:
- Different applications have different channel utilisation threshold before services start being affected
- If Voice services are required what are the RF design and configuration requirements,
- RF design
- Vendor specific configuration requirements: i.e. Cisco CAC
- QoS design
- Such as 802.11e, WMM, DSCP & CoS
- Rate limiting
- Can have significant impact to air-time utilisation
- Airtime fairness
- # of SSIDs
- % of associated users or concurrent clients?
- AP’s configuration i.e. 1x 2.4GHz and 1x 5GHz
- AP Chips sets, can they handle all the con-current and client associations
- Sources of RF and non-RF interference
- Wireless design requirements: Mesh with client connect, etc.
- Mesh present a whole number of extra requirements (headaches) when considering overall aggregate throughput per client.
- Channel plan
- Suitable Channel width without increasing CCI & ACI
- Device types, density, & number of devices: A killer especially with legacy devices.
- Client device drivers are up to date.
- Backhaul: extremely important.
While an AP data sheet may say it can support # of clients per radio, we as wireless engineers need to ensure our wireless networks are designed with sufficient AP density to accommodate the numbers/types of clients and their throughput requirements.
Capacity planning is critical step to ensuring the wireless network meets the requirements of the design.