Chapter 1014: The Ignored Broadcast Effect

"Mr. Chang, I think you shouldn't be too anxious..."

Wu Wei, who couldn't hold himself any longer, opened his mouth to comfort him:

"The transmission rate and transmission stability of wireless systems cannot be compared with wired systems. The results are slightly worse... This should be normal, right?"

"wrong!"

However, Chang Haonan's response was to shake his head decisively:

"The transmission rate of the wireless system is indeed not as good as the wired system, but our test has controlled the transmission method. The experimental group and the control group are both wireless transmission, so there should be no difference in the magnification rate."

"As for stability..."

He paused for a moment, raised his head and looked at Wu Wei with serious eyes:

"If we analyze it from this perspective, then for transmission methods with less stability, the improvement brought by network coding technology should be more obvious."

"In short, there is only less than a 5-fold improvement... There must be something wrong..."

"..."

Wu Wei wanted to express some objections to this description of "only five times", but seeing that Chang Haonan's face was serious and did not look like he was joking at all, he swallowed the words that came to his lips.

For a moment, the progress of the test stopped.

West in June.

Sunny day.

It's afternoon again.

Even with air conditioning, the cabin is still stuffy.

Everyone's attention was focused on the test just now, so they didn't feel it, but this meeting made people a little upset.

Even Chang Haonan was inevitably affected.

In short, I couldn't immediately figure out where the problem was.

The main thing is wireless communication, which has too many influencing factors.

Just like aircraft design, humans only apply it based on engineering experience. In fact, there are still many principle parts that are not fully understood.

So sometimes we really can only consider the possibility that the machine soul is displeased...

"Reset the device and try again."

Since his thinking was fruitless, Chang Haonan had no choice but to use his electronic product repair skills——

Restart.

The lack of protection was just an accident.

His instructions were quickly transmitted to each test vehicle via radio.

Technicians are already familiar with this kind of thing.

So, after a lot of fuss, the new control group and experimental group data were quickly measured.

The results were a little fluctuating compared to just now.

But not much has changed.

These are 1.6 and 7.4 kilobytes/second respectively.

Still less than five times the result.

After careful calculation, it's actually a bit worse than the first time...

Looking at the numbers on the screen in front of him, Chang Haonan felt the veins on his forehead throbbing.

He only used the system in the process of algorithm development, which was long over.

It is already in the engineering testing stage, and the system naturally does not include after-sales service.

And he has no clue as to where the current problem lies, nor does he meet the basic requirements for system startup...

"Mr. Chang, our next step..."

Wu Wei mainly engages in ground single-hop communications, so in his heart he actually feels that this result is quite good, and there is absolutely no need to make things difficult for him.

But he certainly couldn't say this directly, so he just asked Chang Haonan what he should do next with a little hesitation.

The latter looked at the time and knew that the test progress could not be stuck here forever.

The results are the same twice, indicating that the problem is reproducible.

It is not caused by accidental interference.

On the other hand, since it can be improved by 5 times, it means that in principle, it is feasible to apply the combination of MRD and network coding to multi-hop communication.

As for why it is only five times, we can study it slowly after we go back.

"Let each car move its position and continue the test with a distance of 10 kilometers..."

As he ordered, he jotted down the question and several possible possibilities in the notebook next to him.

Wu Wei next to him was relieved when he heard this instruction, and quickly directed the other three vehicles to enter the next test position.

Chang Haonan, on the other hand, sat beside him and continued to be lost in thought.

Next, there is a long wait...

After an unknown amount of time passed, he suddenly heard Wu Wei's voice next to him again:

"Car No. 1, adjust the antenna angle and deflect it 5°30' to the right..."

Obviously, at this time, each communication vehicle has arrived at the scheduled second round position and is ready for final debugging before testing.

"Antenna angle..."

For a moment, an idea flashed in Chang Haonan's mind, as if he had grasped something important.

Logically speaking, due to the relatively poor directivity of wireless communications, it is almost impossible for such a few adjustments of the antenna to have a huge impact on the communication effect.

The reason for the adjustment is just to control variables and ensure the rigor of the test.

To a certain extent, it is the professional instinct of engineers.

But now that I think about it carefully, it provides Chang Haonan with a new idea.

"Mr. Chang?"

Wu Wei suddenly heard Chang Haonan repeat the words "antenna angle" just now. He thought there was something wrong with his operation steps, and quickly looked at the latter with questioning eyes.

"It's okay, keep testing!"

Chang Haonan immediately cheered up again.

His idea needs to be verified by the results of several subsequent tests.

…

Because it was already the third operation, the testing team was familiar with the entire process.

The efficiency is much faster than the previous two times.

At an interval of 10 kilometers, the speed of the control group is still the same as before.

1.5 kilobytes/second.

However, the situation of the experimental group has changed dramatically-

10.3 kilobytes/second.

When this result came out, Wu Wei's eyes widened and he was stunned.

"I f--..."

The quintessence of the country was almost on the tip of his tongue, but he still forcibly took it back:

"Seven... seven times?"

Compared with the test results at a distance of five kilometers, this time it seems that there is not much change, but it clearly shows one thing-

Chang Haonan was right just now.

Five times, it is indeed not the limit.

However, the more confusing point is that the effect becomes better after the distance is farther...

You know, the main distance for direct communication of the divisional communication vehicle developed by the 54th Institute is between 5-15 kilometers.

So he is most concerned about the results within this range.

"Mr. Chang, do you need to repeat it again?"

After a long time, Wu Wei recovered from his shock.

Chang Haonan raised his wrist to check his watch, and then calculated the time in his mind:

"Repeat it, move quickly, and then immediately switch to the 15-kilometer test..."

...

The following test results, from a qualitative perspective, were completely consistent with Chang Haonan's expectations-

As the communication distance increased, the network throughput of the control group began to decline significantly.

But the experimental group has been rising against the trend.

When the four cars were 30 kilometers apart, the throughput data measured by car No. 4 had reached 17.3 kilobytes/second.

In terms of absolute speed, of course it is still not fast.

It can even be said to be very slow.

However, compared with the increase in the control group, it is basically close to the level when Chang Haonan tested it in the wired network before.

19.7 times.

Moreover, after adopting the new technology, the network throughput measured after three rounds of relays (car No. 1 itself is also counted as one relay) is only about 15% lower than that of one round of relays.

The relay loss has almost been minimized!

"Huh——"

Chang Haonan, who had completely confirmed his conjecture, finally breathed a sigh of relief.

For most scientific research work, the most torturous part is to find the problem from a mess of clues.

Once this step is completed, the part of solving the problem later is relatively easy.

At least psychologically.

After a short pause, Chang Haonan issued new instructions on the radio:

"Attention, all cars, today's test ends here first, prepare to pack up..."

These finishing tasks are naturally handled by special people, and there is no need for him or Wu Wei to handle them personally.

Therefore, the latter took advantage of this free time to ask Chang Haonan the doubts he had just had:

"Mr. Chang, why is the transmission effect of using network coding technology better after the communication distance is extended?"

"Because of the broadcast effect..."

Chang Haonan said as he pushed the door to leave the car, and then found a position upwind, ready to blow the refreshing evening breeze after the sun sets.

Although it smells a bit of sand, it is much better than the stuffy operating cabin.

This is the good thing about inland areas in mid- and high-latitudes.

No matter how hot it is at noon, as long as the sun is out of sight, the temperature will drop immediately.

"Broadcast effect?"

Wu Wei thought for a moment, but did not find this keyword in his brain.

"Ah... yes, I just came up with this term, it's normal that you haven't heard of it..."

Chang Haonan nodded and replied:

"That is to say, all information in the physical layer of the wireless network is transmitted in the wireless medium through electromagnetic waves, so that multiple receiving nodes can simultaneously monitor the electromagnetic wave signal transmitted by a sending node. Conversely, the information sent by multiple sending nodes can also be received by a receiving end at the same time."

"This causes the packet data of adjacent hops to cause "self-interference" to the data of this hop in the same service flow, and most wireless communication systems only allow one source to transmit information at the same time, or suppress the superimposed signals sent by other sources as interference, which also leads to wireless multi-hop networks. The actual throughput of the control group is significantly lower than that of the wired network..."

"When the distance between the vehicles increases, the self-interference caused by the broadcast effect decreases, so the throughput is actually improved."

But this explanation did not completely eliminate Wu Wei's confusion:

"But... the control group should also have the same effect, right?"

"Of course."

Chang Haonan put his hands behind his back and acted like a big boss:

"But the main factor affecting the control group's network throughput is the transmission delay caused by repeated packet transmission, and the self-interference has little effect on it."

"After applying network coding technology, the proportion of transmission delay gives way to processing delay, so the broadcast effect is more obvious..."