Summary：We frequently encounter specifications citing theoretical transmission rates—such as 300 Mbps, 150 Mbps, or 100 Mbps—for devices like wireless bridges and COFDM equipment. While we all recognize these as units for measuring network data transmission speed, few people have likely stopped to consider how these theoretical rates are actually calculated.
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We frequently encounter terms like 300Mbps, 150Mbps, and 100Mbps when dealing with wireless bridges and COFDM equipment. While we recognize these as units measuring network data transmission speed, few people have likely stopped to consider how these theoretical rates are actually calculated.
We should adopt the spirit of Newton and ask "why?"—just as one might wonder why an apple falls or why these specific theoretical values exist.
To understand why an apple falls, one needs a grasp of physics; similarly, calculating these theoretical transmission rates requires knowledge of the relevant communication technologies.
Take WCDMA (Wideband Code Division Multiple Access), a type of 3G cellular network, as an example. HSDPA (High-Speed Downlink Packet Access) is a mobile communication protocol with a theoretical rate of 14.4Mbps. This figure is derived as follows: the HSDPA service protocol allows up to 15 codewords with a Spreading Factor (SF) of 16 to be used for HS-DSCH channel transmission. The number of symbols transmittable on each SF=16 code channel is calculated by dividing the chip rate by 16. Given that the WCDMA chip rate is 3.84 Mchips/s, each code channel can transmit 240 symbols. With 16QAM modulation, each symbol carries 4 bits; after coding and rate matching, the effective coding rate is 1. Thus, the HSDPA rate is calculated as: 15 (code channels with SF=16) × 4 (bits per symbol with 16QAM) × 240 (symbols per SF=16 code channel) × 1 (coding rate) = 14.4Mbps.
Next, let’s examine the origin of the transmission rate for China Unicom’s "21M" network.
By applying 64QAM modulation to the existing HSDPA framework, the number of bits per symbol increases to 6—a 1.5-fold increase over the 4 bits per symbol provided by 16QAM. Consequently, the calculation is 14.4Mbps × 1.5 = 21.6Mbps, resulting in the "21M" transmission rate. Later, China Unicom rolled out a 42 Mbps network—doubling the previous speed—by employing dual-carrier modulation technology. This technique allows a mobile phone to transmit data across two carriers simultaneously; essentially, it adds an extra carrier channel. The math is simple—21 × 2 = 42—so everyone gets the idea, right?
To fully grasp exactly how these transmission rates are calculated, however, one would need to understand concepts like chip rate, code channels, and modulation schemes, as well as why 64QAM offers only 1.5 times the data capacity of 16QAM. Explaining all of that in detail would practically require writing a book; those interested can easily look up the information online.
Of course, these are merely theoretical transmission rates. In real-world scenarios, both equipment limitations and environmental factors significantly reduce actual speeds; for instance, a device advertised with a 300 Mbps rate is doing quite well if it actually delivers over 200 Mbps.
When purchasing wireless surveillance or transmission equipment, you shouldn't focus solely on theoretical specifications; the supplier's service quality matters, too. After all, this is high-tech gear. While tech-savvy users might enjoy tinkering with it themselves, those who consider themselves novices are better off choosing a reliable manufacturer to ensure access to high-quality products and proper technical support.
So, here is my advice: don't get hung up on surface-level figures. When buying equipment, if the product meets your actual usage requirements, the so-called theoretical data doesn't really matter.
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