Analysis of the relationship between MHz and Mbps

Mbps is the unit of data traffic and MHz is the unit of frequency.

Hz (Hertz) refers to the BandWidth of the cable. It represents the number of oscillations of the electrical signal in the line per unit time. For example, the frequency bandwidth of Category 5 twisted pair is 100 MHz. This value is fixed, that is to say there is a fixed medium. mHzj is megahertz (MegaHertz, MHz) is one of the units of fluctuation frequency. The bps generally refers to the data rate (also known as the port rate, DataRate), which measures the number of binary bits transmitted on the line per unit of time. Mbps is an abbreviation of Millionbitssecond, and 1 Mbps represents 1,048,576 bits per second (1 Mb=1024 Kb=1024*1024 bits), ie, 1,048,576/8=131,072 bytes=128 KB=0.125 MB per second. Mbps is the rate unit, megabits/second

Concept analysis

With the popularization of the Internet, the application of integrated cabling has become increasingly widespread, and the transmission level has become higher and higher. From Category 3 to Category 4 to Category 5, up to now, there are 6 types of cabling products on the market. The main parameter that defines the level of these descriptors is the transmission bandwidth (MHZ).

At the same time, there are endless web applications. Transmission media from 10Base5 (thick cable), 10Base2 (thin cable), 10BaseT (twisted pair), 10BaseFL (fiber) to 100BaseTX (STP/UTP), 100BaseT4 (4/5 UTP), 100BaseFX (fiber), up to the present Gigabit fast network has emerged. The main parameter used to describe these applications is rate (Mbps).

In fact, Shennong formula has already summarized the relationship between bandwidth B and rate C:

C=B*Log(1+SNR)

Where B is the channel bandwidth, so-called bandwidth refers to the frequency range that can transmit signals with appropriate fidelity, its unit is like Hz, it is the channel itself and it has nothing to do with the contained signal. SNR is the signal-to-noise ratio, which is determined by the system's receiving equipment and the electromagnetic environment in which the transmission system is located. The rate C is a calculation result, which is determined by B and SNR, and its unit is bps, conceptually expressed as binary bits transmitted per second.

It can be seen that for a given channel, the bandwidth B is also given, and a different transmission rate C can be obtained by changing the SNR. There is a one-to-many relationship between MHz and Mbps, that is, the same bandwidth can transmit different bitstream rates. At the same time, Mbps is application-dependent; MHz is independent of the application.

Technical discussion

If you want to give a metaphor for an image, then the car's speed and engine speed are just right. When given the rotational speed, the speed of the car can be calculated if the gear is known. In this analogy, gears act as a bridge. In fact, gears are used in cars and engines just like coding systems for speed and bandwidth.

Encoding is used by computers for information transmission. By coding information, many technical problems, such as synchronization, bandwidth limitations, etc., can be solved. Coding is crucial for the reliable transmission of information.

There are currently two basic coding series. The first is to add one synchronization bit per N bits to make synchronization possible (eg Manchester encoding when N=1, 4B5B encoding when N=4), but this requires a larger bandwidth than the original . And the more synchronization bits, the greater the bandwidth needs. In order to reduce the bandwidth, it is possible to use an encoding system that adds a synchronization bit (ie, 7B8B encoding) every 7 bits, but then, when a longer bit stream of the same type is transmitted, the synchronization becomes Very difficult.

Another encoding series is to reduce the bandwidth by increasing the number of levels, the more the number of levels, the less bandwidth is needed. However, when a long series of 0-encoded continuous signals are transmitted, synchronization becomes almost impossible. For example, when we use five levels, we need four comparators, and each comparator should have its proper tolerance range. This means that when we choose the total number of levels, we should also take the signal-to-noise ratio (SNR) into account in order to be able to identify these different levels.

Manchester, NRZ1, and MLT-3 encoding are currently the three major encoding systems used. Their transmission factors are 1, 0.5, and 0.25, respectively. These conversion factors can be defined as the ratio of MHz pairs. Table 1 lists the summary characteristics of several encoding systems in terms of synchronization and bandwidth.

From this point of view, any kind of coding system has its technical limitations. In addition, there are some parameters such as DC components also put some restrictions on the encoding, in practical applications, the current major encoding systems are used in order to make a trade-off between bandwidth and synchronization, or biased, for example, a pair Applications with higher synchronization requirements may choose the Manchester encoding system or other encoding methods that can generate timing. For another example, a 100Mbps application using MLT-3 encoding requires 25MHz of bandwidth; when using 4B5B encoding together, the system needs to add an additional 25Mbps overhead, and the entire system needs 31.25MHz of bandwidth. The advantage is that the system changes in synchronization. It's easier. In addition, it is worth mentioning that 100 Fast Ethernet uses the 5B6B encoding system (IEEE802.13), which is a typical example of bandwidth and synchronization tradeoffs. Table 2 lists some of the current applications and the encoding system used.

in conclusion

As a user, the most interesting is the communication rate. The rate is to describe the communication from the application level. In order to increase the communication rate, there are two ways to consider: One is to improve the transmission performance of the cable system, which determines the bandwidth; the other is to select the appropriate coding system, which determines the conversion factor.

The wiring manufacturer has already developed a Category 5 cable system capable of supporting more than 100 MHz. And they continue to invest in research and development of higher performance cable systems. International organizations such as EIAAA/TIA and ISO/IEC have developed standards that define the level of LAN components through bandwidth.

Although the bandwidth is physically limited, a higher communication rate can be obtained by a suitable encoding system. In particular, it should be pointed out that the coding system is application-dependent, which means that a new application with the same bitstream rate but using a different coding method may not necessarily be supported by the original system, so when designing, if only Consider wiring components that support existing application systems and describe the bitstream rate in MHz. This will result in serious and erroneous decisions. From this perspective, any open system should be independent of the application. And only by using MHz to describe the communication rate, we can make full choices from the current and future broad application fields. For the performance grading problem of the integrated cabling system, we can only measure the bandwidth but not the rate.

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