Throughput vs data rate
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Throughput vs data rate

• Frequency bandwidth does play a part in the eventual throughput of the data, but many other factors also determine throughput.
• In addition to frequency bandwidth, data encoding, modulation, medium contention, encryption, and many other factors also play a large part in data throughput.
• Data encoding and modulation determine data rates, which are sometimes also referred to as data bandwidth. Simply look at the 5 GHz channels and OFDM as an example. OFDM 802.11a radios can transmit at 6, 9, 12, 18, 24, 36, 48, or 54 Mbps, yet the frequency bandwidth for all the U-NII band channels is the same for all of these speeds.

What changes between all of these speeds (data rates) is the modulation and coding technique.

• The proper term for the changes in speed due to modulation and coding is data rates; however, they are also often referred to as data bandwidth.

Actual throughput that an 802.11 wireless network provides is based on the following:

• A medium access method known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/ CA) attempts to ensure that only one radio device can be transmitting on the medium at any given time.
• Because of the half-duplex nature of the medium and the overhead generated by CSMA/ CA, the actual aggregate throughput is
  • typically 50 percent or less of the data rates for 802.11a/ b/ g legacy transmissions
  • typically 60-70 percent of the data rates for 802.11n/ ac transmissions
• In addition to the throughput being affected by the half-duplex nature of 802.11 communications, the throughput is affected differently based on the frequency used.
• HT and OFDM technologies are used in both the 5 GHz and 2.4 GHz bands.
  Because of the higher level of RF noise that is typical in the 2.4 GHz ISM band, throughput of 2.4 GHz devices will typically be less than the 5 GHz devices.
• It is also very important to understand that the 802.11 RF medium is a shared medium, meaning that in any discussion of throughput, it should be thought of as aggregate throughput.For example, if a data rate is 54 Mbps, because of CSMA/ CA, the aggregate throughput might be about 20 Mbps. If five client stations were all downloading the same file from an FTP server at the same time, the perceived throughput for each client station would be about 4 Mbps under ideal circumstances.
• When 802.11n and 802.11ac radios are used, the aggregate throughput is typically 65 percent of the advertised data rate.
• The medium contention overhead created by CSMA/ CA is typically about 35 percent of the bandwidth.
• Medium contention overhead is 50 percent or more when using legacy 802.11a/ b/ g radios.
• RTS/ CTS can also affect throughput by adding communication overhead.
• In some environments, fragmentation and RTS/ CTS can actually increase throughput if the initial throughput was low because of communication problems.
• Variables at almost all layers of the OSI model can affect the throughput of 802.11 communications.
• It is important to understand the different causes, their effects, and what, if anything, can be done to minimize their effect on overall data throughput.

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.