Shulou(Shulou.com)11/24 Report--

What do we often mean by gigabit broadband and 100G optical networks?

That's right! Is the bit rate (Bit Rate)! Smart, you've got it right! Similar to the bit rate are baud rate and spectrum bandwidth.

But how much do you know about the relationship between bit rate, baud rate and spectrum bandwidth?

Don't worry, the following document will decrypt it for you.

What is the bit rate and baud rate of PART.01? The gigabit 1000Mbit / s mentioned in the broadband network generally describes the maximum number of 0 and 1 bits (bit) that our home network port can receive per second, that is, it can receive 1000x106 bits per second. Obviously, the higher the bit rate, the more bits are transmitted per second. A large number of bits means that more information is obtained per unit time, and the faster the network speed is naturally.

In general, the bit rate is the number of bits transmitted per second, also known as the transmission rate. The basic unit of bit rate is bit / s or bps, which is called bit per second. If a large number of bits are transmitted per second, the bit rate units can be converted to Kbit / s, Mbit / s, Gbit / s. Here K, M and G represent 1000, 1000000 and 1000000000, respectively.

What is very easily confused with the bit rate is the baud rate.

The baud rate is defined as the number of symbols (symbols) transmitted per second, also known as the bit rate, in baud (Baud, B, or symbol / s). In a communication system, a signal unit carrying data information is called a symbol, also known as a symbol (symbol).

In fact, Baud already means speed, but don't translate the baud rate into Baud Rate, just use Baud to express it. Rate is superfluous. However, Chinese has become accustomed to baud rate, so it still uses the term "baud rate" to mean "Porter".

Let's give an example of life to deepen our understanding of these two concepts.

The communication system is a bit like our public transportation system. We can compare the symbols in the communication system to public transport vehicles, such as buses, subways, taxis. If the number of bits transmitted by the communication system is compared to the number of people who travel, then the bit rate is the flow speed of the travel population, and the baud rate is the departure rate.

What is the relationship between PART.02 bit rate and baud rate? To discuss the relationship between bit rate and baud rate, it is necessary to understand the relationship between symbols and bits. Just like buses, subways and taxis mentioned in the example just now can take different numbers of passengers, different symbols can also be represented by different bits. The number of bits required by the symbol is determined by the number of states supported by the symbol.

The following is a table of the relationship between symbols and bits in 2, 4, 8 states:

Total number of bits required for symbol type symbol state total number of bits required for symbols in two states: 0,01,010,011,100,101,110, 11183 in two states: 0,001,010,011,100,101,110, 11183 in two states, the number of bits required for this symbol is as follows:

Drawing shows that the transmission relationship between symbols and bits is as follows:

It is easy to deduce from the figure that when the baud rate Rs and the total number of symbol states N are known, the corresponding bitrate Rb relationship is as follows:

If the bit rate Rb and the total number of symbol states N are known, the corresponding baud rate Rs relationship is as follows:

The above bit rate and baud rate formulas only consider the information payload and do not consider other factors such as signal modulation and error correction coding.

What happens to the relationship between bit rate and baud rate when considering symbol multiplexing?

For example, if we use polarization multiplexing (PDM,Polarization Division Multiplexing) to modulate the symbols to be transmitted, we can achieve simultaneous two-way transmission, which doubles the bit rate of the signal.

Polarization means that using the polarization dimension of light, two independent data information are transmitted simultaneously through two mutually orthogonal polarization states of light in the same wavelength channel, which is equal to the realization of dual-channel transmission, so the bit rate of the signal can be doubled.

For a more detailed description of the signal modulation, please refer to the article "the strongest Advanced Science Popularization on Optical Communication" in the Wechat public account "fresh Jujube classroom". )

Obviously, with the addition of multiplexing technology, the relationship between bit rate Rb and baud rate Rs becomes:

What happens to the relationship between bit rate and baud rate when considering information error correction coding?

With the addition of factors such as information error correction coding, the actual bit rate will also increase according to the ratio of error correction coding overhead. For example, in the 200G optical network system, the 16QAM modulation method is adopted, and the coding error correction method adopts SD-FEC with 20% coding overhead and baud rate of 32GB.

The actual bit rate is 258.48Gbit / s

For a more detailed description of the error correction code, please refer to "do you know how hard FEC is trying to convey New year's wishes clearly?" . )

What is the relationship between PART.03 bit rate, baud rate, and spectrum bandwidth? The spectrum bandwidth is actually the difference between the highest frequency and the lowest frequency of the communication signal. The higher the baud rate of the signal, the greater the spectrum bandwidth of the communication channel when the signal is transmitted in the communication channel. For example, in a transportation system, the larger the model is, the wider the road will be.

Limited by the processing speed of the hardware chip, in order to improve the bit rate, the bit rate can be increased by increasing the baud rate and the number of bits of a single symbol. In addition, the communication system also requires the device chip baud rate ≥ signal baud rate, usually the chip baud rate is 45GB, 69GB, 96GB, 128GB. If the baud rate of the designed signal is greater than the baud rate of the device chip, it means that the signal can not be realized, because no chip can support the generation of this signal.

At the same time, according to Shannon's theorem and experience, the required spectrum width of the signal should be more than 1.2 times of the baud rate of the signal in order to ensure that the signal can be transmitted with high quality. Without considering other factors that affect the transmission, we can roughly calculate the relationship between the baud rate and the required spectral bandwidth according to Shannon's theorem and experience.

For example, if 27% FEC and 8QAM modulation techniques are used in a 200G optical network, and the signal bit rate is about 219Gbit / s, the relationship between signal baud rate and frequency is as follows:

Considering the margin, it is recommended to use the spectrum bandwidth of 62.5GHz rather than the spectrum bandwidth of 50GHz to transmit signals.

PART.04 concluded that today, Mr. Archive has learned knowledge with you:

Bit rate, baud rate and spectrum bandwidth are important indicators in communication systems.

The higher the baud rate and the higher the bit rate, the greater the channel spectrum bandwidth.

This article comes from the official account of Wechat: ZTE documents (ID:ztedoc)

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