Throughput with Wi-Fi is not the same thing as your Internet connection speed. Throughput measures how quickly you can transfer data between two devices on the local wireless network. The maximum theoretical throughput on your network depends on the standards supported by your equipment. For several technical reasons, the actual speeds you achieve will be lower than the theoretical maximum.
Older Wi-Fi Standards
Two main Wi-Fi standards were published before 2009. The 802.11b standard has a maximum theoretical throughput of 11 megabits per second. The 802.11g standard has a maximum theoretical throughput of 54 megabits per second. The 802.11a standard also has a maximum theoretical throughput of 11 megabits per second; note that it operates on the 5-GHz frequency rather than the 2.4-GHz frequency of the other two.
Newer Wi-Fi Standards
There's more variation in the maximum theoretical throughput of the more recent Wi-Fi standards. The 802.11n standard can support a maximum theoretical throughput of 600 megabits per second, but only if your router is configured in a particular way to transmit data across multiple channels at once. Some routers will only be able to support theoretical maximums of 150, 300 or 450 depending on their configuration. The 802.11ac standard has a theoretical maximum speed of 1300 megabits per second. Note that 802.11n can operate on both 2.4-GHz and 5-GHz frequencies, while 802.11ac is 5GHz only.
Even in perfect conditions, you can't actually achieve the maximum theoretical throughput as a real transfer speed. One reason for this is that the throughput figure covers all traffic both back and forth, meaning that at full speed, you can't have device A sending data to device B and device B simultaneously confirming to device A that it has received the data. Another reason is that data is sent in packets, which contain information (a bit like a packing note and checklist inside a parcel); this header information counts towards throughput and thus reduces the speed at which the data you actually want to transfer is carried.
Throughput figures assume perfect conditions. In reality, speed is affected by the distance over which the data is travelling and whether or not you have any physical barriers such as walls or ceilings, particularly if they contain metal. Signals can also be slowed down by other wireless information in the area such as signals from baby monitors, cordless telephones and microwaves; this interference is much less of a problem on the 5-GHz frequency.
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