Chapter 1014: The Ignored Broadcast Effect
"Mr. Chang, I think you shouldn't be too anxious..."
Wu Wei, who was a little overwhelmed, spoke to comfort him:
"The transmission rate and 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 firmly:
"The transmission rate of the wireless system is indeed not as good as that of the wired system, but our test has controlled the transmission method. Both the experimental group and the control group use wireless transmission, so there should be no difference in the rate increase."
"As for stability..."
He paused for a moment, raised his head and looked at Wu Wei with a serious look:
"If we analyze it from this perspective, the improvement brought by network coding technology should be more obvious for transmission methods with less stability."
"In short, the improvement is less than 5 times... There must be something wrong..."
"..."
Wu Wei really wanted to express some objection to the description of "only five times", but seeing that Chang Haonan looked serious and didn't seem to be joking at all, he swallowed the words back.
For a moment, the progress of the test came to a halt.
West in June.
Sunny day.
It's afternoon again.
Even with air conditioning, the car is still stuffy.
Everyone’s attention was focused on the test just now so we didn’t feel it, but now it makes people feel a little upset.
Even Chang Haonan could not avoid being affected.
Anyway, I couldn't figure out where the problem was right away.
It’s mainly about wireless communications, there are too many influencing factors.
Just like in aircraft design, humans only apply it based on engineering experience, and there are actually many fundamental parts 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 thoughts were fruitless, Chang Haonan had no choice but to use his unique skills in electronic product repair -
reboot.
Maybe it was just an accident.
His instructions were quickly transmitted to each test vehicle via radio.
The technicians are already familiar with this kind of thing.
So, after some trouble, the new control group and experimental group data were soon measured.
The results are somewhat fluctuating compared to before.
But not much has changed.
That's 1.6 and 7.4 kilobytes per second respectively.
Still a result of less than five times.
If you think about it carefully, it’s 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 throbbing.
He just used the system during the process of algorithm development, which was already completed.
We are currently in the engineering testing phase, so the system naturally does not include after-sales service.
Moreover, he had no idea what the problem was, and he did not meet the basic requirements for system startup...
"Boss Chang, our next step..."
Wu Wei was mainly engaged in ground single-hop communications, so he thought that this result was already quite good and there was no need to be hard on himself.
However, he certainly couldn't say this directly, so he just asked Chang Haonan hesitantly what to do next.
The latter looked at the time and knew that the testing progress could not be stuck here.
The results were the same for both repeats, indicating that the problem is reproducible.
It is not caused by unexpected interference factors.
On the other hand, since there is a 5-fold improvement, it means that in principle, it is feasible to apply the combination of MRD and network coding to multi-hop communications.
As for why it is only five times, we can study it slowly later when we go back.
"Let's move the cars and continue the 10-kilometer test..."
As he gave the order, he wrote down the question and several possible answers he had currently speculated on in a notebook beside him.
Wu Wei, who was standing nearby, also breathed a sigh of relief when he heard the instruction and quickly directed the other three cars to the next test location.
Chang Haonan sat nearby, continuing to be lost in thought.
Next, there is a long wait...
After an unknown amount of time had passed, he suddenly heard Wu Wei's voice again:
"Car No. 5, adjust the antenna angle, deflect 30°' to the right..."
Obviously, at this time, each communication vehicle has arrived at the scheduled second round position, preparing for the final debugging before testing.
"Antenna angle..."
For a moment, a light bulb went off in Chang Haonan's mind, as if he had grasped something.
Logically speaking, due to the relatively poor directivity of wireless communications, such an antenna adjustment of just a few degrees is unlikely to have any significant impact on the communication effect.
The reason for the adjustment is just to control variables and ensure the rigor of the test.
To some extent, it is an engineer's professional instinct.
But now thinking about it carefully, it gave Chang Haonan a new idea.
"Mr. Chang?"
Wu Wei suddenly heard Chang Haonan repeat the word "antenna angle" just now, and thought that there was something wrong with his operating steps, so he quickly looked at the latter with an inquiring look.
"It's okay, continue testing!"
Chang Haonan immediately regained his spirits.
His idea just needs to be verified by the results of several subsequent tests.
……
Since this was the third operation, the testing team was familiar with the entire process.
The efficiency is much faster than the previous two times.
After an interval of 10 kilometers, the speed of the control group remained the same.
1.5 Kbytes/sec.
However, the situation in the experimental group changed dramatically.
10.3 Kbytes/sec.
When the result came out, Wu Wei's eyes widened and he stood there in a daze.
"I c..."
The quintessence of Chinese culture was almost at his fingertips, but he still forcibly took it back:
"Seven...seven times?"
Compared with the test results at a distance of five kilometers, this time the changes seem to be small, but it clearly shows one thing -
Chang Haonan was right just now.
Five times is definitely not the limit.
However, the more confusing thing is that the effect becomes better when the distance is farther...
You should know that the main distance for direct communication of the divisional communications vehicle developed by the 5th Institute is between 15 and kilometers.
Therefore, he is most concerned about the results within this range.
"Boss Chang, do you need to repeat it again?"
It took Wu Wei a long time to recover from his shock.
Chang Haonan raised his wrist to check his watch, and then calculated the time in his mind:
"Repeat it, quickly, and then immediately move on to the 15km 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 vehicles were 30 kilometers apart, the throughput data measured by vehicle No. 17.3 had reached kilobytes per second.
In terms of absolute speed, it is of course still not fast.
It can even be said to be very slow.
However, compared with the control group, the improvement 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. 15 itself is also counted as one relay) is only about % lower than that of one round of relays.
The relay loss has been almost minimized!
"call--"
Chang Haonan finally breathed a sigh of relief after his guess was completely confirmed.
For most scientific research work, the most torturous part is finding the problem from the confusing clues.
Once this step is completed, the subsequent part of solving the problem will be relatively easy.
At least on a psychological level.
After a short pause, Chang Haonan issued new instructions over the radio:
"Attention all cars, today's test ends here, prepare to withdraw..."
Naturally, there would be someone specifically responsible for these finishing tasks, and there was 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 question that had just occurred to him:
"Mr. Chang, why is it that when the communication distance is extended, the transmission effect of using network coding technology is better?"
"Because of the broadcast effect..."
Chang Haonan pushed the door and left the car, then found a position upwind, ready to enjoy the refreshing evening breeze after the sun set.
Although it smells a bit like 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, the temperature will drop immediately as long as the sun is out of sight.
"Broadcast effect?"
Wu Wei thought about it for a moment, but couldn't find the 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, all information in the physical layer of a wireless network is transmitted in the wireless medium via electromagnetic waves, so that multiple receiving nodes can simultaneously monitor the electromagnetic wave signal transmitted by a sending node. Conversely, 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 the current hop in the same service flow. Most wireless communication systems only allow one source to transmit information at the same time, or suppress the superimposed signals from other sources as interference, which leads to a significant decrease in the actual throughput of wireless multi-hop networks compared to wired networks..."
"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 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 network throughput of the control group is the transmission delay caused by repeated packet transmission, and self-interference has little effect on it."
"After applying network coding technology, the proportion of transmission delay is replaced by processing delay, so the broadcast effect becomes more obvious..."
