Like you, I have read many product reviews lately in which the author complains that the actual data rate of a particular device is much lower than that which was advertised. The implication is that somehow he or she and consumers are being misled and cheated because of the wide disparity between the advertised and actual data rates. Well, let’s take a moment to examine bit-rate terminology to see if we can’t come to an understanding that allows us to accept the disparity and to realize that companies are not trying to scam us with exaggerated claims.

The first thing to understand is that there is a natural difference between the maximum ‘channel bit rate’ and the actual ‘data throughput bit rate’ of a network device or system.

The maximum ‘channel rate,’ sometimes referred to as the ‘PHY rate’ (bit rate over the PHYsical medium), is the maximum theoretical digital bit rate that can be produced through the network medium by the utilized technology. This rate is realized only under the best of conditions. As an example, Wi-Fi based on IEEE 802.11g has a maximum channel rate of approximately 54 Mbps under the best of conditions (short range, no interference).

The actual throughput bit rate for the data (payload) that is being passed through the system is much less. Why? Because many of the bits contained in the channel rate are used for ‘overhead,’ meaning, link management, quality of service, error detection and correction and more. Obviously, these bits are necessary. Without the overhead bits, data bits (the payload) could not be passed successfully.

In our 802.11g example, the actual data throughput rate, under the best of conditions, is somewhere around 25 Mbps, approximately half of the maximum channel rate. The channel rate is still in the neighborhood of 54 Mbps, but the overhead bits are not counted, or shown, by most network analysis software--they are invisible. The point is that no one is being cheated or intentionally mislead--the overhead bits are present ensuring that the payload gets through.

What is more, both the actual channel rate and the data throughput rate are affected by the connection’s signal to noise ratio. A strong signal with little or no opposing noise will yield channel and throughput rates near maximum. However, in the case of Wi-Fi, increasing distance between network devices (nodes) decreases signal strength, and the signal-to-noise ratio, and deteriorates both the channel rate and the data throughput rate. Therefore, when you are testing the throughput rate between Wi-Fi network nodes, the distance between them will impact the actual rate value displayed. That means an 802.11g network will exhibit throughput bit rates of less, usually much less, than 25 Mbps.

This applies to advanced wired network technologies as well. When a company advertises a HomePlug AV standards-based powerline network device as having a PHY rate of 200 Mbps, the 200 Mbps is the maximum theoretical channel bit rate under the best of operating conditions. The actual data throughput bit rate is in the neighborhood of 100 Mbps, under the best of conditions, which in this case means no AC line noise and very little line attenuation between nodes.

You might argue that you can’t actually use 200 Mbps, so ‘they’ shouldn’t advertise it. But what we must understand is that under the best of conditions, you are using 200 Mbps--your payload data bits plus the necessary overhead bits.

So, why don’t manufacturers just advertise the maximum data throughput rate and forget about the channel rate? It’s because a manufacturer’s device (product) is tied to a particular standard. It’s actually more honest, and safer, for the manufacturer to state the standard-based channel rate than to make a claim about their throughput rate, which varies widely with network channel conditions.

So ease up on the manufacturers and standards organizations a little. Naturally, they are going to state the maximum theoretical channel rate (PHY rate) for a communications standard, IC or network device because they want you to know how fast it can shove bits over the medium as compared to other similar or different technologies. It’s not hype--it’s not marketing--it’s simply a fact that you need to know. Remember, under the best of conditions, you are getting the channel bit rate they state when both payload and overhead are considered.

See the Examples Table in the attached document.

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