Chapter 1014 The Ignored Broadcasting Effect
“Mr. Chang, I think you should not be too anxious...”
Wu Wei, who was a little overwhelmed, comforted:
< br>“The transmission rate and transmission stability of the wireless system cannot be compared with that of wired systems. The results are slightly worse...it should be normal, right?”
“No!”
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 variables for the transmission method. The experimental group and the control group are both wireless There should be no difference in transmission, in terms of magnification."
"As for stability..."
He paused for a moment, raised his head and looked at Wu Wei with serious eyes:
“If analyzed from this perspective, then for less stable transmission methods, 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 really wants to describe this "only five-fold" Expressed some objections, but seeing Chang Haonan's serious face, it didn't look like he was joking at all, so he swallowed the words that came to his lips.
For a while, the progress of the test stopped.
Western in June.
Sunny day.
It’s afternoon again.
Even if the air conditioner is installed, it is still hot and stuffy inside the carriage.
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.
Similar to aircraft design, humans only apply it based on engineering experience. In fact, there are still many principles that are not fully understood.
So sometimes we can only consider the possibility that the machine soul is unhappy...
"Reset the device and try again."
Since his thinking was fruitless, Chang Haonan had no choice but to use his electronic product repair trick -
restart.
The lack of guarantee was just an accident.
His instructions were quickly conveyed 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.
1.6 and 7.4 kilobytes/second respectively.
Still less than five times the result.
After careful calculation, it was actually a little worse than the first time...
Looking at the numbers on the screen in front of him, Chang Haonan felt the veins on his forehead jumping.
He only used the system in the process of algorithm development, and this process ended long ago.
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 no need to have to make things difficult for himself.
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, you can study it slowly after you go back.
"Let each car move its position and continue the test at a distance of 10 kilometers..."
While giving the order, he jotted down the problem and several possible possibilities he had guessed so far. on the notebook next to it.
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.
The next step is a long wait...
After an unknown amount of time passed, he suddenly heard Wu Wei's voice next to him again:
"No. 1 Car, adjust the antenna angle, 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, a light flashed in Chang Haonan's mind, as if he had grasped something important.
Logically speaking, due to the relatively poor directivity of wireless communications, this kind of mere antenna adjustment is almost unlikely to have a huge impact on the communication effect.
The reason for the adjustment is just to control the 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 has provided Chang Haonan with a new idea.
"Mr. Chang?"
Wu Wei suddenly heard Chang Haonan repeat the four words "antenna angle". He thought there was something wrong with his operation steps, so he quickly used the query Look at the latter. "It's okay, continue testing!"
Chang Haonan immediately cheered up again.
His idea needs to be verified by the results of several subsequent tests.
……
Because this is the third operation, the testing team is 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 was still the same.
1.5 kilobytes/second.
However, the situation of the experimental group has changed dramatically -
10.3 kilobytes/second.
When the result came out, Wu Wei's eyes widened and he froze on the spot.
"I c..."
The quintessence of Chinese culture almost reached his lips, but he still forcefully took it back:
"Seven...seven times?"
Compared to five The test results at kilometers apart seem to have not changed much this time, but it clearly shows one thing -
Chang Haonan was right just now.
Five times is indeed not the limit.
However, what is more confusing is that the effect becomes better when the distance is farther...
You know, the division communication vehicle developed by the 54th Institute, The main distance for direct communication is between 5-15 kilometers.
So he is and is most concerned about results within this range.
"Mr. Chang, do you need to repeat it again?"
After a while, Wu Wei recovered from the shock.
Chang Haonan raised his wrist and looked at his watch, and then calculated the time in his mind:
"Repeat it again, move quickly, and then immediately switch to the 15-kilometer test..."< br>
……
The next test results, from a qualitative perspective, are completely consistent with Chang Haonan’s expectations——
As the communication distance increases, the network throughput of the control group begins to decline significantly.
But the experimental group has been rising against the trend.
When the four vehicles were 30 kilometers apart, the measured throughput data of vehicle 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 control group, the improvement factor is basically close to the level of Chang Haonan's previous test on the wired network.
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 been almost minimized!
“Hu——”
Chang Haonan finally breathed a sigh of relief after his suspicion was completely confirmed.
For most scientific research work, the most grueling part is figuring out the problem from a jumble of clues.
Once this step is completed, the subsequent part of solving the problem is relatively easy.
At least on a psychological level.
After a slight pause, Chang Haonan issued new instructions on the radio:
"Attention, all cars, today's test is here first, prepare to close the team..."
Of course, there is a dedicated person in charge of these finishing tasks, and there is no need for him or Wu Wei to handle them personally.
Therefore, the latter took advantage of this free moment to ask Chang Haonan the doubts he had just had:
"Mr. Chang, why do you use the Internet after the communication distance is lengthened? Is the transmission effect of encoding technology better?”
“Because of the broadcast effect...”
Chang Haonan said as he pushed the door open and left the carriage, then found a position upwind, ready to blow in the refreshing evening breeze after the sun set.
Although it smells a little like sand, it is much better than the stuffy and hot operating cabin.
This is the good thing about inland areas at 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 could 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:
< br>"That is to say, all the 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 one sending node. On the contrary, the information sent by multiple sending nodes It 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 business flow, which is the case in most wireless communication systems. Only one source is allowed to transmit information at the same time, or the superimposed signals from other sources are treated as interference and suppressed, which leads to a significant reduction in the actual throughput of wireless multi-hop networks compared to wired networks..."
"As the distance between several vehicles increases, the self-interference caused by the broadcast effect weakens, so the throughput increases."
But this explanation has not been completely eliminated. Wu Wei was puzzled:
"But... the control group should have the same effect?"
"Of course."
Chang Haonan put it Put your hands behind your back, like a boss:
“But the main factor affecting the network throughput in the control group is the transmission delay caused by repeated packet sending, and self-interference has little impact on it.”
“After applying network coding technology, the transmission delay accounts for The ratio gives way to processing delay, so the broadcast effect becomes more obvious..."
(End of this chapter)