Global Mobile Stats - Google Our Mobile Planet

In a project called Our Mobile Planet, Google's been collecting mobile stats from 27 countries. Dan Swinhoe from IDG Connect Global has picked this data apart and written a very nice post titled The App Revolution: How this Varies By Market. Here's a few interesting tidbits from Swinhoe's excellent piece:

• Japan is the most ‘appy', but Germany is amongst the most keen to pay.
• According to 148apps, the Apple store has 719,452 apps available, and to buy them all would set you back a hefty $1,307,715.69.
• Angry Birds Star Wars is currently dominating the App store charts.
• By the end of the year, over 45 billion apps will have been downloaded - around 15 billion of those from Google, but you can expect Android to take the majority share in 2013 due to the sheer number of devices being sold using the search engine's OS. 
• Microsoft's own appstore is yet to make significant inroads in any market but, depending on the success of its Surface tablet this could well change after Christmas.
• Custom-app building continues to grow, today's estimates putting the average cost of development at around $30-40,000. 
• According to a report by Appaccelerator, Apple has become the chosen platform for enterprise app development, with 53.2% of developers picking iOS for corporate app development.
• In all the charts, no matter what system or country, games feature heavily in both free and paid for.
• While things such as social media and certain business software are now fully-apped, other areas are still a while off. For example media outlets are still struggling to cope with apps (web is still a struggle for many), while the largest programs - CAD/CAM and other large engineering/graphics programs simply are too big and complex for apps and mobile devices. At least for now.

Be sure to check out Dan's full post linked here and also take a look at Google's Our Mobile Project - all pretty interesting stuff.

Bridged Taps - More On The Local Loop

A bridged tap is an unterminated wire pair that sits in parallel to the main wire pair. Ideally, the local loop is a continuous wire point-to-point connection. At one time, the local loops were all setup this way but, with the growth of neighborhoods, new unused wire pairs got added. Typically, extra pairs are included though not initially used when cable is run down a street. When a new house is built, or a line is added, a phone company technician taps into one of the unused pairs. The technician typically does not cut the pair, the wires are just “tapped,” leaving the unterminated ends running down the street. This way, if the line is no longer needed, a technician can come out, remove the tap and still use the pair for another customer farther down the street. This leaves a bridged tap with the tap point being where the technician spliced into the wire pair on the street. 

Bridged Tap Example

Bridged taps can create an impairment to the transmission system. A signal on the loop moves down the un-terminated cable and will reflect back to the main pair affecting the main signal. A bridged tap will typically not be noticed at voice transmission frequencies because the wavelength of voice frequencies is always greater than the line length. All that is experienced is a slight increase in attenuation due to added capacitive load which is usually so small it is not detected by the human ear. 

However, when it comes to Digital Subscriber Line (DSL) technologies, bridged taps can cause major data communications problems and frequently require cleaning up by telecom technicians. I'll discuss how DSL technologies work in a future post.

Loading Coils - More On The Local Loop

Early in the development of the telephone system infrastructure designers realized our everyday speech lies in between 125 Hz and 8 KHz with most voice centered between 400 and 600 Hz. With more studies designers realized that humans can recognize and interpret voice if they stayed within this frequency range. Voice frequencies below 200 Hz and above 2 KHz play very little role in voice recognition.

Frequency Range Diagram 

Since the early 1900’s the infrastructure has been tuned to match these frequency requirements using devices called loading coils.

Both George Campbell at AT&T and Michael Pupin at Columbia University were working in 1899 on wire pair mutual capacitance problem. Both realized that, by adding a lump series inductance called a loading coil, resonance could be used to cancel the effects of shunt capacitive reactance and increase signal strength over long local loops. Michael Pupin ended up getting the patent and by late 1899 loading coils were being installed in the field on copper wire pairs longer than 3 miles.

Western Electric 25 Pair Loading Coil Cable Case (circa 1977)
[and.... circa 1977 is the cable case, not me!]

Loading coils are a simple lump series inductance that produce an effect called loading. Loading increases the series inductance of the loop and effectively makes the loop a low pass filter, increasing the impedance of the line which drops signal attenuation. A typical 26 gauge local loop pair is loaded with a 26H88 loading coil. The letter H designates a coil that is added every 6000 feet, 26 represents 26 gauge wire and 88 indicates the inductance of the coil is 88 mH. This loading makes the loop perform as a low pass filter and cuts the frequency off sharply at around 3.4KHz. Loading coils  work great for the low bandwidth requirements of voice but causes problems when you want to transmit data at higher bandwidths over these same wires.

Loaded and Unloaded Loss

 

At voice frequencies, the cutoff frequency (f_C) for a transmission line can be approximated as follows:

     where:    L = Loading Coil Inductance

                       D = Distance in miles btwn loading coils

                       C = capacitance per mile

Example 1

Calculate the cutoff frequency and sketch the frequency response curve for a local 3 mile loop using 26H88 loading coils spaced every 6000 feet. 

Solution:

                                                               L = 88mH 
                                                  D = 6000 feet ≈ 1 mile 
                                                  C = .083 μF/mile                                              

Notice using this formula total loop distance is not used in the calculation – only distance between coils is required.

In addition to H (6000 ft) load coil spacing, there are also B (3000 ft) spacing and D (4500 ft) spacing loading coils. By changing coil spacing along with coil inductance the loop cutoff frequency can be adjusted or tuned to the proper value. Let's look at another example.

Example 2

22mH loading coils are spaced every 3000 ft on a local loop. Calculate the cutoff frequency.

Solution:

                                                                    L = 22mH
                                                      D = 3000 feet ≈ .5 miles
                                                      C = .083 μF/mile

Notice in this example by reducing the distance between coils and decreasing the individual coil inductance values, we can increase the cutoff frequency. There are three commonly used loading coils in the United States and the coil specifications are listed below:

Loading coils have been used over the last 100 years and are an excellent way to tune a local loop to voice frequencies between 300 and 3300 Hz. As carriers move to provide high bandwidth data services such as ADSL on the same local loop being used for voice the low pass filter characteristics of the loaded local loop provide significant bandwidth limitations. We can see frequencies above 4000 Hz on loaded loops are blocked. For this reason loading coils are being removed from the local loop.

Transmission Lines and the Local Loop

I know this post gets a little mathematical. Try and think of the math in simple terms - in the examples below we're dealing with some basic division:

Answer = Numerator / Denominator 

That's numerator (top number) divided by denominator (bottom number) in the equation.

If the numerator is large compared to the denominator then the answer is going to be relatively large (think big number divided by small number gives big number answer and remember...... everything is relative :) ). And vice versa - if the numerator is small compared to the denominator then the answer is going to be small ((think small number divided by big number gives small number answer).

This should help to understand the examples below.

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In my last post I wrote about the local loop - that pair of copper telephone wires most of us still have coming into out homes.These wires have been used for voice in some places for close to 100 years and now, using DSL technologies, to deliver voice and data. AT&T UVerse is even using the local loop to deliver triple play services - voice, video and data. In this post, let's take a little close look transmission lines.

The local telephone loop (also referred to as the subscriber loop) is the dedicated copper wire twisted pair connecting a telephone company Central Office (CO) in a locality to a customer home or business. The loop resistance is critical in the local loop and phone companies have had to “tune” the loop to transmit high-quality voice. Typically, companies have used 19 gauge (1.25 decibels [dB] attenuation per mile) to 26 gauge (3 dB attenuation per mile) copper wire for the local loop. The average customer local loop is about 2 miles and attenuation on this loop is ideally kept below 8 dB.

We can look at a typical transmission line model and use it to represent a subscriber loop:

Transmission Line Model

We can see that the inductance (L), resistances (R for series resistance and S for shunt resistance), and capacitance (C) are distributed throughout the model. We can also show that these values cause signal loss and distortion.  A local loop copper wire pair effectively forms a capacitance since you have two conductors (copper wire) separated by an insulator (wire insulation). Shunt or mutual capacitive reactance is independent of wire gauge and local loop wire pairs designed for voice have a capacitance value of about .083 μF/mile.

In addition to local loop cable, copper cables designed for higher frequencies like those used for T carrier systems are designed to provide a capacitance of .066 μF/mile.

Two Wires Separated by Insulation Forming a Capacitance

Capacitive reactance is basically the resistance of a capacitance and it changes with frequency.   The formula for capacitive reactance is:

The units for capacitive reactance are Ohms (Ω). Looking at the formula you can see as frequency increases the denominator gets larger so the capacitive reactance drops. On long local loops (3 miles and greater) shunt capacitance values increase to the point where significant signal leakage occurs at frequencies greater than 1000 Hz. If you look at the formula, you realize the higher the frequency the greater the leakage loss. Let’s look at some examples:



Example A

A local loop is 1 mile long. Calculate the capacitive reactance for the loop at 2KHz.

Solution:
Using          f = 2 KHz     




Example B

This same local loop is extended to 3 miles. Calculate the new capacitive reactance for the loop at 2KHz

Solution:
Using          f = 2 KHz  




In the example you can see that, by increasing the length of the loop by two miles, shunt capacitance drops by a factor close to 10.

In addition to length, higher frequencies also cause shunt capacitance reactance to increase.


Example C

Let’s increase the frequency in Example B to 3KHz and calculate the capacitive reactance of the local loop.

Solution:

Using          f = 3 KHz    




Example D

Let’s now decrease the frequency to 1KHz and calculate the capacitive reactance of the local loop.

Solution:

Using          f = 1 KHz



Now consider a voice conversation on the Example C local loop. We know the frequency range of the local loop is approximately 300 Hz to 3300 Hz. We know the human voice can produce frequencies of both 3KHz and 1KHz and the average ear can hear these frequencies. At 1 KHz we have a shunt capacitive reactance of 639Ω and  at 3 KHz we have a shunt capacitive reactance of 213Ω. You can see more signal is lost due to capacitive shunting at the higher frequencies than at the lower frequencies. When it comes to voice - the listener will notice these differences – the lower frequencies in a voice conversation will appear louder than the higher frequencies in a conversation.

Over 100 years ago telephone companies figured out they could "load" a transmission line with inductors (loading coils) to reduce the effects of capacitive reactance. I'll discuss loading coils in a future post.