How do I estimate Layer 7 throughput from Layer 1 raw data rate?

Aruba Employee
Aruba Employee

Product and Software: This article applies to all Aruba controllers and ArubaOS versions.

The 802.11 physical layer operates at half-duplex because only one station can transmit on a wireless channel at the same time. 802.11 data rates such as 54 Mb/s or 300 Mb/s refer to one-way, "raw" physical layer bandwidth. No consideration is given for latency from an application perspective and no consideration is given for transmission "overhead". In addition, such data rates represent single client to single access point (AP) speeds under best-case conditions. Actual client bandwidth will be lower under real-world conditions and client counts.

By contrast, application performance is heavily dependent on round-trip, full-duplex network performance. Even applications that have asymmetric traffic profiles, such as video cameras, must use delivery guarantee mechanisms that require acknowledgments, windowing, and sequencing at upper layers of the protocol stack. Latency in all layers and in any network element can significantly reduce performance.

As a result, application developers typically express their network requirements in terms of throughput. Throughput is defined as the effective data transfer rate at the application layer, and can be measured as an average or a peak.

Therefore, we need a mechanism to convert between the bandwidth values that will be used for WLAN design and the throughput values that must be guaranteed to applications. The simplest and most conservative technique is the following formula:

802.11a/b/g Throughput = Bandwidth * 0.40

802.11n Throughput = Bandwidth * 0.50

In other words, 54 Mb/s of half-duplex bandwidth provides 22 Mb/s of full-duplex throughput. This conversion is well-known among wireless designers, and it is suitable for most purposes.

For more information, see the "RF Design" chapter in the Retail Wireless Networks Validated Reference Design document.

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‎07-05-2014 04:24 AM
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