Facing Wi-Fi reality in the Middle East
Vendor theoretical throughputs are more hazardous than helpful in today’s Wi-Fi decision making process, says Arnaud Le Hung, Ruckus EMEA marketing director
Vendor theoretical throughputs are more hazardous than helpful in today’s Wi-Fi decision making process, says Arnaud Le Hung, Ruckus EMEA marketing director.
In the age-old battle of one-upmanship, vendors continue to mislead potential buyers with theoretical speeds that look good on boxes and data sheets, but are never actually experienced by the expectant customers.
Despite what vendors (including us) spew, the speeds you achieve on your Wi-Fi network in the real world will be lower, even with emerging dual-band 802.11n APs that can now support multiple spatial streams in each band, 2.4 and 5GHz.
Assuming the use of wider, 40MHz channels in each band, the use of the narrower 400ns guard interval (GI), and the ideal channel quality required to support 64-QAM, a theoretical throughput of 300 Mbps per radio is possible.
A guard interval is a small delay that ensures that distinct OFDM symbols don’t interfere with one another. Their purpose is to introduce immunity to propagation delays, echoes and reflections, to which digital data is normally very sensitive. And QAM, or quadrature amplitude modulation, is an analog and digital modulation scheme that conveys data by changing some aspect of a carrier signal, or wave, (usually a sinusoid) in response to a data signal.
The IEEE standard defines a 400ns GI option, but increased bit error rates in practice generally disqualify 400ns guard intervals from utility in the real world of live networks. Operation with an 800ns GI is much more common. The more common 800ns GI would effectively lower peak data rate to 270 Mbps.
With these wider channels and smaller gaps between bits, 600Mbps throughput for a dual-band, two stream 802.11n AP is theoretically achievable. But you’ll never get it. Such theoretical throughput numbers, while enticing, are based on a number of big assumptions.
Reality in the Not-So-Perfect World of Wi-Fi
In most Wi-Fi networks today, the 2.4GHz band is forced to do the majority of the heavy lifting. This is because so few clients — especially existing smart mobile devices that are the source of a rapidly growing majority of traffic — support 5GHz. While this is changing fast, it’s still the reality for IT manager trying to build fast and efficient Wi-Fi networks.
Each channel within a Wi-Fi network typically consumes 20MHz of spectrum or bandwidth. To achieve higher data rates, one technique used by 802.11n is bonding or combining these channels into larger 40MHz lanes.
Since 40MHz operation in 2.4 GHz is rare, 20MHz channels are to be expected. With a much more practical 800ns guard interval and 20MHz channel, IT managers are now looking at about 130 Mbps of throughput per radio.
Inefficiencies and errors that arise in any real, commercial chipset implementation and radio hardware help to bring the peak down into the neighborhood of 110 Mbps depending on the quality of the radio implementation.
UDP traffic can come close to the 110 Mbps, but most of the traffic in today’s mobile internet world is TCP. This delivers a performance decrement on the order of 15 to 20%, yielding a new peak throughput of about 90 Mbps.
So far we’ve been talking about the raw symbol rate in the over-the-air Wi-Fi protocol. A symbol is either a pulse or a ‘tone’ representing an integer number of bits. A theoretical definition of a symbol is a waveform. But these symbol rates exclude the effects of Wi-Fi protocol overhead as bits are disaggregated from and reassembled into layer 2 frames and layer 3 packets on both ends of the link - lowering the actual throughput experienced by users.