The maths behind 4 stream 2.4GHz data rates explained
As any wireless professional will tell you, data rates or signalling rates are the theoretical speeds that a wireless access point can provide under perfect lab conditions, using all available spatial streams at the highest possible modulation rate. The truth of it, is that real-time throughput speeds are always a mere percentage of these data rates, usually less than 50% due to wireless being a half-duplex medium. However, calculating data rates and using these for advertising purposes is common practice among wireless manufacturers and resellers alike.
So how can you calculate an Access Points data rate?
Quite typically, this can be achieved by following a set pattern based on that Access Points specifications. For example, an 802.11ac 2x2:2 wireless access point of any manufacturer, will commonly report its data rate within the 2.4GHz spectrum as 300Mbps, 867Mbps in the 5Ghz spectrum and aggregate speed of 1267Mbps of 1.2Gbps (300Mbps + 867Mbps). Again, this is assuming that the AP is performing in perfect lab conditions and utilising all of its available spatial streams at the highest possible modulation.
|2.4GHz 802.11 Wireless Networking Standards|
|802.11 Protocol||Advertised Speed (Mbps)||Real World Throughput Speed (Mbps)|
|802.11g (11b compatibility on)||54||= 14.5|
|802.11g MiMO||108||= 45|
So what happens if you have more or less spatial streams/radios available?
Well for 5Ghz this is a quite a different beast, which is discussed further in our blog on 5GHz speeds. For 2.4GHz however, this tends to be a lot more straightforward. Conventionally it's been quite safe to assume that with each radio/spatial stream pairing an AP has, then the data rate is 150Mbps x the number of pairs, i.e. each spatial stream is worth 150Mbps. For example:
|AP Specifications||Advertised Data Rates|
|1 Stream||150 Mbps|
Why is it then, that when it comes to the industry’s latest 4x4:4 APs such as the Ruckus R710, Ubiquiti’s AC HD and Meraki’s MR52, that 150Mbps x 4 = 800Mbps and not 600Mbps!?!
Well these Access Points are able to advertise such data rates, due to the use of higher channel bonding and denser modulation techniques. Whilst channel bonding is ill-advised in the 2.4GHz band in a real deployment scenario, 802.11n technologies using MIMO-OFDM can make use of 40MHz wide channels, as opposed to the commonly used 20MHz wide channels. 40MHz wide channels are essentially impossible to use in practice, due to their occupation of an already narrow/saturated spectrum.
The other side to this otherwise unexplained improvement, is the use of 256QAM (Quadrature Amplitude Modulation) or TurboQAM as Broadcom call it. Typically, most clients will only support 64QAM, which provides you with the 300Mbps, 450Mbps, 600Mbps data rates for 2, 3 & 4 streams respectively. However, for those (few and far between) clients which support 40MHz wide channels, 4 spatial streams and 256QAM, the theoretical data rates improved by almost 30% to 800Mbps!
Ultimately, 800Mbps on the 2.4GHz band is not an achievable throughput for even the most up to date AP/client device combination, and 5GHz should be used where possible. However, it’s important for us to understand how the progression of AP and client technologies, as well as the savvy marketing of manufacturers, will influence both our perceived and real time network performances!
If this product is of interest, or you would like a free consultation and a quotation for both a capex option and our new opex based, Managed service with flexible payment and duration options please do not hesitate to give our wireless bridging team a call. please do get in touch. You can reach us on 01202 61240