My last YouTube video was rather large. About 4.2 GB to be precise. The upload started out fine and then all of a sudden dropped down to about 1 MBit/s and later to 300 kBit/s. What happened? Network problems? YouTube having problems? I remember the exact same thing happening with older video uploads. So it was something consistent that happens every time I upload a video. After a bit of research, I found that Comcast actually has a Network Management Policy that lines out when and why they throttle bandwidth .
Can you give me some “real world” examples of how much bandwidth consumption would be considered too much? For example, how many movies would I have to download to be affected by this congestion management technique?
Since the technique is dynamic and works in real time, the answer really depends on a number of factors including overall usage, time of day and the number of applications a customer might be running at the same time. First, the local network must be approaching a congested state for our technique to even look for traffic to manage. Assuming that is the case, customers’ accounts must exceed a certain percentage of their upstream or downstream (both currently set at 70%) bandwidth for longer than a certain period of time, currently set at 15 minutes.
Alright, so using just 71% of the Bandwidth I pay for, for more than 15 minutes will get my bandwidth throttled to a ridiculously small amount. Naturally, being pissed off about this, I fired a tweet at Comcast:
Now watch this, Comcast is slick. While normal companies respond to problems directly from their main account, Comcast has Mike Lewis. Mike Lewis is a Comcast guy responding to customer complaints. In reality, Mike Lewis is, of course, more than one person. But giving that account a personal touch is their idea of giving the customer a warm and fuzzy feeling, I guess. By using a secondary account, they of course make sure that the main account doesn’t draw any attention to your problem so that other Comcast customers don’t read about your issue.
But the amazing thing is his statement. He insists that they don’t throttle bandwidth. Which is funny, because we just read in their official policy that they do. Interesting, isn’t it?
I gave him some education on the fact. Turns out he didn’t want to help me with this issue anymore after that. Go figure.
So what’s the workaround to the problem? Luckily, I have a more advanced router with Quality of Service (QoS) profiles. I pay for 15 MBit/s upload speed. 70% of this is 10.5 MBit/s. So, I set the QoS profile up to never allow the outbound traffic to exceed 10 MBit/s. That way, I can never possibly exceed the 70% condition of Comcast’s Network Management Policy. So even though I throttle myself to about 2/3 of the bandwidth I pay for, this is still a whole lot better than 1 MBit/s or even 300 kBit/s.
It’s still sad, though, that Comcast does heavily limit you this way. If you pay for 15 MBit/s upstream, you expect this to be available 24/7. What’s even more sad is that apparently the customer service representatives at Comcast don’t even have a clue about their own policies.
Links and Sources:
 Comcast, Network Management Policy: http://customer.comcast.com/
The concept I will outline in this article is absolutely nothing new. Back in the days, engineers and hobbyists would use analog noise sources in combination with a spectrum analyzer to determine the frequency response of a filter. The concept is very simple: the wide-band noise source is connected to the input of the filter and the output of the filter is connected to the input of the spectrum analyzer. As long as the noise level is somewhat flat over a wide frequency range, one can read the frequency response directly on the spectrum analyzer. So the question is, does this work with a cheap DDS signal generator and a scope with FFT spectrum display, as well?
I used a Rigol DG1022 signal generator as noise source. The generator has a “noise” option. The first thing I wanted to know is how linear the output spectrum of this noise generator is. I captured the spectrum with a Teledyne LeCroy HDO4024. The frequency range shown is 1 MHz to 80 MHz.
Looks like it’s pretty usable ’til about 10 MHz. If you need more bandwidth, you either need a better dig gen or you could build a simple analog noise source using a reverse biased Zener diode and a couple of transistor amplifiers. For very high frequencies and high linearity, there are special noise diodes available, as well. But for HF and HF experiments, a Zener diode and a few 2N2222 as amplifiers are more than enough.
The first filter I tested is a bandpass filter for the 40m amateur radio band. Since the filter was designed for a 50 Ohm impedance, care needs to be taken to set the scope and the signal generator to 50 Ohms, as well. The FFT spectrum view of the oscilloscope was set to “Max Hold.” After a few seconds of noise input, this is what I got:
But how accurate is the result? Let’s compare.
The following image shows a 10.7 MHz ceramic IF filter measured using the noise setup as described before.
As you can see, the filter curve is not very smooth, but the general filter parameters are clearly visible. In order to compare the measurements, I used a classic sweep generator setup. The generator swept between 9.7 MHz and 11.7 MHz over a time span of 10 seconds. I picked 10 seconds so that I could simply use the Max Hold function of the oscilloscope’s spectrum analyzer without having to bother about synchronization. The following picture does not only show the results, but it also shows the exact setting I used to set-up the FFT spectrum analysis.
The comparison shows that this method words very well to use a modern oscilloscope with FFT spectrum analysis to determine the characteristics of a filter’s behavior in the frequency domain.
Here’s the drawing to the more than over-due KF5OBS #6 giveaway.
Quick look at the Jackson Labs Chip Scale Atomic Clock (CSAC) including a very brief jitter analysis using the LeCroy WaveRunner 640 Zi 4 GHz, 40 GS/s oscilloscope.
Ti TMS320VC5505 eZdsp USBtick
Altera Cyclone II FPGA Starter Development Kit
Analog Devices BF592 EZLite kit
To participate, you must subscribe to my YouTube channel, like this video, comment under this video and send an eMail to firstname.lastname@example.org stating your name and that you would like to participate.
If you want to double your chances, just make this giveaway known through a social media site of your choice. Make sure you submit proof (e.g. screenshot or link) to me via eMail.
Naturally, the more effort one puts into prohibiting the handling of certain items, the more interesting they seem. The electric drill is a great example for that. But an old drill bit was the solution to the problem. It’s sharp enough to cut empty paper boxes but not even remotely sharp enough to do some serious damage.
The oscilloscopes, just like pretty much any other test & measurement device, has always been interesting to her. All the colorful buttons and traces. Plus one can actually watch Elmo on it. One of her newest ideas is to use the LabNotebook function of my Teledyne LeCroy HDO4024 oscilloscope as a modern Etch A Sketch®.
LabNoteook is a feature of many upper-level LeCroy oscilloscopes. It allows the user to store waveforms, a screenshot and the current channel / acquisition status. The user can also graph over the screenshot and place text labels. Paired with the oscilloscope’s touchscreen display and a stylus pen, this makes for a perfect pastime activity for a toddler.
After the screen gets too crowded, she usually hits the “delete all” button and starts over. Every once in a while, I actually manage to save some of her artwork. Here’s an example:
Links and Sources:
 LabNotebook, Teledyne LeCroy http://teledynelecroy.com/
 LabNotebook Report, Lena http://jaunty-electronics.com