This past April, I wrote a Top Story titled: It's Channel Bit Rate, as Advertised. Since then I have continued to read product reviews in which writers complain that the actual data rate of a device is way below the advertised value. It’s amazing how this is so misunderstood, especially by writers who are eager to present a professional opinion about a product. More needs to be said about this until reviewers finally get it.

The advertised data rate is the maximum bit rate (bits per second) that a technology can produce through the physical-layer medium. The physical (PHY) layer in a transmit chain starts at the last digital assembly point before the data is encoded as an analog signal and sent out over the physical medium. The physical medium is ‘air’ for Wi-Fi and wire for powerline communications. Sometimes, we refer to the ‘PHY rate’ as the ‘channel rate’ because it is the rate of bit transfer across the channel.

Digital communications devices are designed in accordance with some global or proprietary standard, which by design dictates the maximum PHY rate. It is reasonable that the manufacturer will state on the package and in advertising materials the maximum PHY rate and even state what standard is being used, i.e. IEEE 802.11g.

The PHY, or channel, datastream contains digital bits of many kinds – not all are actual payload, meaning, not all of the bits are those that make up a text file or a picture that is being sent - in fact, most of the time, the payload bits are very few compared to the total, which includes a lot of ‘overhead’ bits. That is the reason that the measured throughput rate is very low compared to the advertised PHY rate. Consider these categories of overhead bits that must be included in the PHY datastream (depending on protocol):

- forward error control and preamble;
- redundant information (error detection and correction);
- addressing/routing;
- acknowledgment packets;
- retransmissions of failed packet transmissions; and
- control information.

The above categories of overhead bits serve to package, route and protect payload data. This overhead is essential.

To press this further, consider the overhead involved in sending a letter. The payload is written on the paper. The paper is overhead. The paper is placed in an envelope that is addressed. The envelope is also overhead. The letter goes into sorting bins – the bins go into routing containers – the containers go into trucks and airplanes – all types of necessary overhead. At some point in the ‘payload’s’ journey, the process is reversed until the letter is opened and the paper is held in the hands of the recipient with nearly all overhead stripped away. This analogy can be developed much further, but you probably get the point.

So, when software on your PC tells you that the throughput rate is a certain amount, an amount much less than advertised, understand that the bit-rate measurement you are seeing is taken at a level well below the PHY, or channel, layer where there is little overhead (less bits per second). Understand that no-one is being cheated or lied to.

Data such as e-mail and file transfers (text, pictures) are handled using an intermediate-layer protocol known as transmission control protocol (TCP). When TCP is used, each packet of data that is sent must be acknowledged by the receiver and a retransmission of this data takes place if an error is detected. As a result of this back and forth communication, the throughput is slowed and the accuracy of the data is preserved. So, when you are measuring the rate of a file transfer using TCP, the throughput rate will be much lower than the advertised PHY rate.

Another intermediate-layer protocol that is used for streaming media, such as music and video, is known as the user datagram protocol (UDP). When you stream music and video, the bits are sent without asking for any acknowledgement from the receiver. If an error occurs, you will hear an interruption to the music or a disturbance in the video picture, but there is no error correction or retransmission. Thus, the throughput rate for UDP traffic is higher than TCP traffic, but still significantly less than the advertised PHY rate because there is still a need for many overhead bits.

There are other protocols that operate at other levels in the data ‘stack’. These, too, affect the throughput rate because they add to the overhead and determine the timing of data in relation to other data and users wanting to use the channel. It all adds up.

In addition to the above, channel conditions such as distance/attenuation and noise will affect the measured throughput. Also, measurements at different times and settings present different conditions that influence throughput results.

Finally, it’s perfectly valid to make opinion statements based on the comparative performance of devices under the same operating conditions. However, based on the above discussion, it is not valid to conclude that a device underperforms or that its manufacturer was less than honest just because measured throughput rate is not close to the advertised PHY rate.

I hope this will help you spread some understanding on this subject. If reviewers don’t eventually catch on to this, consumers never will. Let’s educate.

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