George Dai's Blog: What I learned this week

Friday, April 24, 2009

How to transfer purchased app to another PC

How to transfer purchased app to another PC


Assumption

1.

You have two PCs PC-A, PC-B, and two iPhone-A/B or iPod Touch-A/B. A and B is in different locations. You can’t connect iPhone-B with PC-A. But iPhone-A can be brought to PC-B.
2.

App-X was purchased through PC-A and installed on iPhone-A.
3.

Now you want App-X available on PC-B, through which iPhone-B can install App-X as well.


Let’s go, steps.

1.

Make sure the latest iTunes version is installed at PC-B
2.

Authorize PC-B with the same iTunes account through which you purchased App-X at PC-A.


Option-1

3.Copy the ipa file from PC-A and put the same folder on PC-B

4. Double-Click the subjected file, you will find the app in the application folder of iTunes


5.

Connect iPhone-B with PC-B, and synchronize applications.



Option-2

3. Connect iPhone-A (App-X installed) with PC-B through USB cable

4.

Transfer the App-X from iPhone-A to PC-B

5.

After couple minutes (depends on the app size) when the transferring is done, you will see the app in your application folder.


6.

Connect iPhone-B with PC-B, and synchronize applications.
7.

It’s done!

Monday, April 13, 2009

MTBF calculation for redundant system

Lambda = failures per million hours

Serial Failures:

Let's do the serial failure case first. In this case, the failure of
any one component will cause failure of the system. This may sometimes
be illustrated as:
A -> B -> C -> D
So if you have four components, A, B, C, and D each with MTBF of
20,000, 10,000, 15,000, and 30,000 hours respectively, using this
method, the MTBF of the system is calculated as:
Lambda A = 1,000,000 / 20,000 = 50.0
Lambda B = 1,000,000 / 10,000 = 100.0
Lambda C = 1,000,000 / 15,000 = 66.67
Lambda D = 1,000,000 / 30,000 = 33.33
Lambda (composite system) = 50+100+66.67+33.33 = 250
MTBF (composite system) = 1,000,000 / 250 = 4,000 hours

Redundant Components:

If you have two components in parallel (e.g., dual power supplies)
where a failure of both components is required to fail the system, the
failure rate of the system is MUCH less than either component.

Assume A and B both have MTBF of 100 hours or Lambda = 10,000. The
failure rate F for A and B would then be 0.01 for each. For
comparison, the serial solution has Lambda = 20,000 failures per
million or MTBF = 50 hours.

For the redundant case, the probability (F) that both items are failed
at the same time is:
F = FA * FB
F = 0.01 * 0.01
F = 0.0001
Solving for lambda gets
Lambda = 100
or
MTBF = 10,000 hours
So there is a substantial improvement in reliability when using
redundant components. Note that if you have serial components before /
after the redundant components, you still need to handle those in
series with the redundant components.

A composite system:

If you have both serial / parallel components, break up the system
into pieces and do the lambda calculations as serial or parallel. I
usually end up with several serial items to add at the end and then
compute the overall system MTBF value.

Sunday, April 12, 2009

One on One

One on one,
face to face,
more and more