WTF is “Exact time 256” : diving into Bluetooth SIG documents

 

WTF is “Exact time 256”    

The Bluetooth Special Interest Group (SIG) has a metric ton of Bluetooth LE device specs in a massively confusing pile. In this walkthrough I’ll show how I figured out the details of a fairly simple LE protocol. In particular, I’ll be creating something that needs to match the “Current Time Service”, services # 0x1805.

TL/DR: the good document is the “Gatt Specification Supplement”.

On the Bluetooth.com site, you want “Specifications” and under specification you’ll need a tab open for both the “Assigned numbers” and the “Specifications” directory.

In “Assigned numbers”, there’s two important documents. The first is “Assigned Numbers Document”. It’s a giant list of all of the GATT services by name and all of the characteristics. The second is “Gatt Specification Supplement”.

In the “Specifications” directory, find the listing for the "Current Time Service” and click the link to get the service page. It's got a bunch of the lest useful programming documents ever. The only document that’s interesting for most developers is the Current Time Service 1.1 PDF file. Click the link to read the long, complicated document.

Page 9 starts to be interesting: the Current Time Service supports three characteristics: the “Current Time” (page 10), “Local Time”, and “reference Time”. On page 10 it’s mentioned that we’ll be reading the Exact Time 256 field. This is the first bit of useful (and critical) information in the spec.

BTW, when they say “Unknown”, it’s not clear to me which “Unknown” value they mean. But in the Assigned Numbers document there are 8 time that “Unknown” is mentioned; all of the numeric values (6 of them) are zero. The others are weird strings for something having to do with telegrams. Or not, it’s a Bluetooth SIG spec, so nothing is clear.

At this point you might wonder what the actual bytes are. The Bluetooth SIG doesn’t care that you’re wondering. You might even try typing “Exact Time 256” into the main page search box on the main page, but you won’t get any hits. However, you can find it in the Assigned numbers where it has number 0x2A0C in the “Characteristics by name” and “Characteristics by UUID” section.

Note that in the “Specifications” it lists “Current Time” as the characteristic; that’s characteristic 0x2A2B.

And then finally, take a look at the GATT Specification Supplement, page 78, section 3.62, “Current Time”. It lists the bytes: there’s an Exact Time 256 and then a U8 “Adjust Reason” which says why the time changed. Even better, there's a decent overview of the Exact Time 256 fields!

And here the path through the forest of all Bluetooth knowledge ends. If only there was some way to know that the "supplement" is in fact the useful document, and not the pile of other docs.

It takes a lot of people to ship a complex product

 I'm a Program Manager (PM) for one of Those Big Companies. I don't actually program except as a hobby, and I don't manage. Where I add value is making sure that what we're building is what our customers need.

"I could build a replacement with just ___ people"

No, you couldn't, if it's a complex project. I was recently reminded of the true history of the automobile: a long, hard slog by an enormous number of people to get each element of a car to actually work. In particular, take a look at Motor-Car Principles on Project Gutenberg. Among the many, many things that had to be painfully figured out

How to stop a car. The book lists a whole series of brake types (none of them disk brakes like in modern cars) 

How an axle works. Car axles are not wagon axels, and you can't use one instead of the other.

Building a transmission. This is something steam engines, for example, simply don't have. A steam engine on a train is directly connected to the wheels with no intervening transmission.

Making a piston. Steam engines, in contrast to car engines, are forgiving: as long as the timing is roughly correct, the steam engine will work. A gas engine has much higher tolerances.

The book goes on with a long, long list of technologies that are unique to cars.

Moral of the story: some projects are pretty simple and can be done with a small team (or even just one person). But once you get to new technology, it's a quagmire that requires an extraordinary amount of work just to know what needs to be done, and from there to co-ordinate the work so that all the parts show up on time.

Fixing "The type or namespace 'Windows' could not be found" error CS0246

"The type or namespace 'Windows' could not be found" solution!

Every time I install a new version of Visual Studio, I run into just a metric hand-full of errors from previously working projects. The projects haven't changed, but suddenly Visual Studio is seemingly incapable of finding the "Windows" namespace.

My Solution: run the Visual Studio Installer". Pick your installed version and click Modify. The modification you probably want is to under Universal Windows Platform development in the Optional section. Look at the list of available Windows SDK and just install them all (assuming you have enough disk space).

There's no harm (AFAICT) in getting them all, and it will save you heartache and frustration down the line.

This entire process is just a cluster. What's more common than building apps for Windows using the compiler tools from Microsoft that are specifically designed to build Windows apps? Well, the only thing more common then developing windows apps is seeing yet another place where Visual Studio once again makes development more painful.

For me, the #1 reason I get the error is that I don't have enough of the Windows SDKs installed. By default, the Visual Studio installer only installs the "most recent" SDK. You might think that's enough, but it's not.

Each project will have a reference path that looks like this:

    <Reference Include="Windows.Foundation.UniversalApiContract">
      <HintPath>..\..\..\..\..\..\Program Files (x86)\Windows Kits\10\References\10.0.22010.0\Windows.Foundation.UniversalApiContract\14.0.0.0\Windows.Foundation.UniversalApiContract.winmd</HintPath>
    </Reference>

Note how the HintPath specifies exactly one particular SDK. If you have a more recent version, it doesn't count even though the contracts are specifically designed to be upwards compatible.

What's worse, Visual Studio loves to kind of silently update your project files, so you'll silently be updated, potentially breaking the project on a different computer.

Error CS0246 The type or namespace name 'Windows' could not be found (are you missing a using directive or an assembly reference?)

Wi-Fi Performance samples

 Wi-Fi Performance, broadly

Number 2 in a series on Wi-Fi performance. This time, Wi-Fi (and networking in general) is compared in difference places is compared. Sneak preview: Bluetooth PAN is the slowest :-)

I ran a simple speed test and gathered latency information (round-trip UDP times) in a variety of places and network. No surprise, Ethernet is the fastest and Bluetooth is the slowest. Airport Wi-Fi had generally high throughput (it's higher throughput than my house Wi-Fi network, although note that I have fiber with a low bandwidth cap) but with medium latency.

PAN? Bluetooth Personal Area Network? What's that?

Bluetooth PAN (Personal Area Network) is a technology that lets you share an internet connection from one laptop to another using Bluetooth instead of Wi-Fi. I'm not quite sure why people would do this, but it's still supported in Windows. It's got a non-winning combination of low speed and high latency. 

To actually share a network connection using PAN:

1. On the "host" laptop, open the Mobile Hotspot settings, and share your Wi-Fi (or Ethernet) connection with Bluetooth. 

2. On the "using" laptop, you need to get to the PAN settings page. AFAICT, there's only one way to do this: 

• right-click on the Bluetooth icon and click "Join a Personal Area Network"
• in the resulting Vista-era "Devices and Printers", select the "host" computer whose network you want to join. You might need to first pair with the "host" computer.
• click connect using and select direct connection.

Wi-Fi: Is it faster on AC? Is it slower on Battery?

 Wi-Fi Performance: AC versus DC

One of a series of comparisons to learn what makes a difference in Wi-Fi performance.

TL/DR: they aren't difference, at least on my setup.

In the screenshot, I've done several latency, download, and upload tests against one specific server.

Test methodology: With the laptop plugged in, run all the speed tests twice with "AC" as the note. Then remove the docking station (the part that is plugged in) and run the same tests against the same server and mark them "DC".

Data Analysis: 

• Download mean throughput increased from 16.2 Mbps to 18.4 Mbps going from DC to AC. This isn't a large difference.
• Upload mean throughput decreased from 31.3 Mbps to 31.28 Mbps going from DC to AC. This is almost certainly not a real difference.
• Mean Latency decreased from 12 msec to 10.3 msec. My stats program tells me this is probably a real difference. The DC had a little bit less jitter, but not by a real amount.

Conclusion: Wi-Fi is about the same speed on DC as it is on AC.

Adding Pivot Item causes massive crash in UWP XAML

 Surprise exception when adding PivotItem

Quick background: my Simple Wi-Fi Analyzer program is getting a hidden feature which is unlocked by manipulating the UX in a secret way. I won't say what the secret is, but you can always look for "Unlock" in the source code :-)

The Analyzer app uses the nice Pivot control: I define a bunch of tabs for different features, and the user picks the tool they want to use. The new feature is a hidden tab, and my unlock code simply unhides it.

Except it doesn't. You can't actually hide a PivotItem in a Pivot control; it's not supported. Instead you have to create the PivotItem in code and then add it to the pivot.Items collection, it is shows right up!

Except you actually get this Vista-era dialog:

The problem (not even on Stack Overflow) is simple: you can't update a pivot.Items collection while you're in a PivotSelectionChanged callback! The solution is to 

Instead add in code like this:

                    var task2 = this.Dispatcher.RunIdleAsync((arg) => {
                        MethodThatAddsPivotItem();
                    });

You don't have to wait for the task; it will just run when it's appropriate.

Calculating Standard Deviation

 Calculating Standard Deviation 

The "classic" formulas aren't always the "easiest to program" formulas. Let's look at the classical formula to calculate a sample standard deviation:

σ = √ (Σ(xᵢ-x̅)²)/(n-1)

where σ is the standard deviation, x̅ is the mean and n is the sample count. But note that we have to walk through the numbers twice: once to calculate the mean, and then again for the rest of the calculations. And yet, old fashioned scientific calculators with just a handful of registers (memory locations) could do this calculation by just entering in a column of numbers. What gives?

The answer is that although conceptually you are summing the square of the difference between each sample and the mean, you can actually do the calculation differently as long as you can keep track of n, the sum of x, and the sum of x². The resulting calculation is:

σ = √(Σ(xᵢ²) - ((Σxᵢ)² / n))/(n-1)

or, slightly more computery: for each new x:

1. n++

2. sumX += x;

3. sumXSquared += x*x;

4. varianceEstimate = (sumXSquared - ((sumX*sumX) / n) / (n-1)

5. stdDevEstimate = SQRT(varianceEstimate)

And you get a running estimate for the standard deviation. You also get the variance, but that's not often really needed.

Statisticians call these the "estimates" because the statistical theory is that there's a magic, universal reality for the actual population variance and standard deviation for which these samples provide an estimate. They aren't wrong :-)