Analog Signals Beyond the Local Loop

Analog transmission works fine for voice transmission on the local loop and the existing copper pairs coming into our homes will exist in some parts of the United States for a number of years. Several companies have been working towards converting the copper twisted pair local loop to fiber. In addition several cable television companies are now offering dial tone to their customers. 

Let’s continue our focus on the telcos and look at what happens to our analog voice transmission when it gets to the telephone company Central Office (CO). 

Local Loop and CO

Our analog voice signal is converted to a digital signal by a device called a CODEC (short for Coder/Decoder) and is then multiplexed, or combined, with other converted analog signals coming from other telephones or analog modems being served by the same CO. Once multiplexed, the calls are then sent out of the CO along their way on a higher bandwidth transmission medium such as fiber or microwave.

I'll write about CODECs in my next post and multiplexing in a future post.

Locking In To An LTE Provider

Amrisa Bhagwandin has an interesting post over at goingLTE.com titled Verizon Reserving Its Phones for Its Own Network?

In the post, Amrisa speculates that Verizon Wireless is designing its phones so they will only run on the Verizon Wireless network. Bhagwandin also speculates AT&T may end up doing the same. Here's some of the technical details:

The Verizon Wireless and AT&T 4G Long Term Evolution (LTE) networks run on different frequency bands:

• AT&T runs in the 704-746 Mega Hertz (MHz) band

• Verizon Wireless runs in the 746-787 MHz band

There is some slight overlap between the two bands but there is not enough overlap for devices to run on each others networks. It's also important to remember the 4G conversion is not going to be like throwing a switch. Tower antennas will be gradually updated from 3G to 4G. This means 4G phones  have both 3G and 4G radios in them - the 4G radio is used when 4G service is available and the 3G radio is used when 4G service is not available. This fallback also causes a problem. In locations where 4G service is not available, Verizon phones will fall back on the Verizon wireless CDMA 3G network and AT&T phones will fall back on the AT&T HSPA/GSM 3G network.

And..... it gets even more complicated - both Verizon Wireless and AT&T both own spectrum through MetroPCS and Bhagwandin thinks we'll see both companies setting up sales through MetroPCS to try and lock customers into their networks . In addition, we may see similar deals being made with Lightsquared and Cricket since both of these companies are developing their own 4G networks.

I'm in wait-and-see mode right now and not going to lock into any new long term wireless contracts until I get a better idea of how it is all going to shake out.

Mike Elgan On Why He's Loving Google+

This is a great Google+ post from tech writer Mike Elgan. Mike is in the middle of a Google+ experiment he's calling his Google+ Diet. He's using Google+ for all of his online communication and has stopped using Facebook, Twitter, Foursquare, e-mail and several other services. Here's  Mike's July 15 Google+ public post summarizing what he's experienced on the Google+ Diet so far:

Here's what I love about Google+ in general and the Google+ Diet in particular:

Instead of saying, "I'm going to write a blog post now," or "I'm going to send an e-mail" or "I think I'll tweet something" you simply say what you have to say, then decide who you're going to say it to.

If you address it to "Public," it's a blog post.

If you address it to "Your Circles" it's a tweet.

If you address it to your "My Customers" Circle it's a business newsletter.

If you address it to a single person, it can be a letter to your mother.

I'd say this is pretty revolutionary.

I haven't personally ditched any of my other social media yet but I do find myself going to Google+ more and using Facebook and Twitter less frequently.

You can follow Mike's Google+ Diet experiment here. My Google+ public stream is linked here.

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.