Dialtone Generation

In my last telephone set post I described what happens when a telephone handset is picked up. Here's a diagram showing that process.

Dialtone Generation

Dialtone is a signal formed by the simultaneous transmission of a 330 Hz tone and a 440 Hz tone. After the first number is dialed by the caller the dial-tone generator is shut off. 

Quick Thoughts on Google Acquiring Motorola

Here's some quick thoughts on today's Google Motorola announcement:

• It's been done before - Microsoft cut a similar deal earlier this year with hardware manufacturer Nokia.
• Google is looking to further follow Apple's vertical integration strategy - basically creating device hardware and operating systems.
• Motorola has been struggling. The company is a goldmine of patents and technology. Merge Google's software,marketing and innovation with Motorola's hardware engineering and some really good things should start happening. Good for consumers.
• There's been a lot of legal/patent back and forth with Android and this has slowed development. The Motorola deal should smooth some of the legal issues out. Good for consumers.
• Google's Larry Page said today that Google will keep Motorola and Google as separate businesses and will continue to keep the Android operating system as an open platform for other hardware suppliers. This is an interesting strategy - will a hybrid closed/open strategy work? Even with questions - I still believe this is good for consumers.
• Other Android device hardware manufacturers including Samsung, Huawei and HTC are going to have to adjust to this. This may not be good for consumers in the short term.
• I've got a Motorola broadband cable modem in my house. Google has done some experimenting with Google TV. Combine the two and we could see some interesting and innovative content coming into our homes soon. This could be really good for consumers.

The acquisition requires both U.S. and European Union approvals and should be completed around the end of this year.

The Basic Telephone Set Fundamental Functions

With my recent posts on the Public Switched Telephone Network (PSTN) I've been getting some email questions and suggested posts. I've received a few questions on telephones (what I would call  end user devices) so I thought I'd take a few posts to describe how a basic telephone works.

The basic telephone set connected to the telephone network we are all very comfortable with using, has 4 basic functions:

1. To provide a signal to the telephone company that a call is to be made (off-hook) or a call is complete (on-hook). 
2. To provide the telephone company with the number the caller wishes to call.
3. To provide a way for the telephone company to indicate that a call is coming in or ringing.
4. To convert voice frequencies to electrical signals that can be transmitted at the transmitter and convert those electrical signals back to voice frequencies at the receiver.

The Federal Communications Commission (FCC) has set standards for the above features and all manufacturers selling telephones in this country must match these standards or the phone will not work properly. In addition many modern telephones also come with features like speed dial, redial, memory, caller ID, voice mail, etc. These are all additional features that are not necessary to make or receive calls.

Let's look at Telephone Set Function 1: To provide a signal to the telephone company that a call is to be made (off-hook) or a call is complete (on-hook).

The switchhook gets its name from the old telephones that had a hook on the side. On modern phones the switchhook is a button that is depressed when the handset is put on the cradle of the telephone. 

According to telephone company specifications individual telephone set DC resistance should be 200 Ω but in reality most telephones range between 150 and 1000 Ω of DC resistance. When a user picks up a connected telephone handset to make a call the switchhooks in the figure below (S1 and S2) close (off-hook condition) and the local loop circuit is complete. 

When a handset is picked up, a DC current ranging between 20 and 120 mA flows on the pair of wires connecting the telephone to the CO. This current flow causes a relay coil to magnetize and it's contacts close.

In the CO current flows through a relay coil attached to the local loop wire pair. The coil energizes, it’s contacts close and the CO switch knows a phone is off hook somewhere. A line feeder in the CO switch looks for the off-hook signal, finds it and sets up a connection. In the CO switch a dial-tone generator is connected to the line so the caller knows they can dial a number. 

I'll cover dial-tone generation (and why cell phones don't use dial-tone) in my next post.

Q&A: Wireless Data Caps And Other Wireless Stuff

I started working on this post on Friday with the intention of posting today. It seems even more appropriate today with the Verizon strike starting over the weekend......

I've had the chance to speak at four or five conferences over the past few months or so on emerging broadband technologies and services. A good chunk of one of these presentations always covers wireless technologies with a focus on 4G (WiMax and LTE) upgrades. One thing I can count on are questions from the audience about wireless data caps. With Verizon Wireless halting their all you can eat plan on July 6th, these questions have become even more frequent. Here's some of those questions and my answers:

Question: Why are the wireless providers doing this?

Answers:

• Expensive to maintain landline divisions of the big providers (AT&T and Verizon), still comprise approximately half of their business. In comparison, wireless is less expensive to install and maintain.
• Average wireless subscriber voice plan revenue has dropped from $50 per month in 2005 to approximately $33 today. Wireless voice plans have become commoditized.
• Smartphones with expensive data plans have allowed the wireless providers to replace the loss in voice revenue.
• These same smartphones have put incredible demand on wireless bandwidth, wireless spectrum and fiber backhaul connecting cell towers. 4G technologies operate at approximately 10 times the rate of 3G and will continue to challenge the providers when it comes to capacity needs.
• A recent Business Week article refers to cellular upgrades by the major providers in the U.S. as "Money Pits".  In 2010 the telcos cost of capital exceeded their return on invested capital - which may mean they're throwing good money after bad.

Question: My students, my own kids, grandkids, etc are constantly texting. Hasn't text messaging provided some nice revenue for the wireless providers?

Answers: 

• comScore reports 69.6 percent of U.S. mobile subscribers used text messaging on their mobile devices in 2010.
• The same Business Week piece referenced above estimates text messaging represents 16% of Verizon Wireless revenue and contributes as much as 40% to total profit. AT&T numbers are likely similar.
• Text messaging was designed for voice phones - basically phones that do not have alphabet keyboards. You don't see too many of those around anymore. Smartphones have more capabilities and options.
• The problem is, with a smartphone you don't need a text plan from a wireless provider to send text messages. Smartphone users can send and receive unlimited text messages using free apps like Skype, Google Voice, TextNow and GroupMe using the smartphone's data connectivity. Using many of these apps users can also text from there computers at work, laptops, iPads etc as long as they have an Internet connection. 
• Apple will be releasing a new messaging feature called iMessage and the  next version of Microsoft Windows Phone will integrate texting with Facebook chat. Both of these services will also bypass the wireless texting technology.
• Text message revenues are going to drop rapidly as users move to apps that run on smartphones.
• Capping data is one way wireless providers will try to recover some of this revenue. There is a bit of a flaw in this strategy though - the average text message (unless pictures are attached) uses very small amounts of data bandwidth. 

Question: 4G services are fast and a monthly 5GB cap is not going to last long. Are you concerned?

Answers:

• Right now I'm a little concerned. For example, I'm on an unlimited data plan now from AT&T and I'll keep that as long as I can. Even though I never come close to 5GB per month, if I were to end up switching, I'd go with a provider that does not cap bandwidth. I would not lock into a long term capped data plan with any provider. 
• Right now Sprint is still offering unlimited data plans - the big two (AT&T and Verizon) are not. If you are concerned about caps I would take a close look at Sprint.
• In places where I do use my phone for data I would estimate over 50% of the time I'm in a location where I have secure WiFi access. As long as I've got the WiFi radio on I'm not using wireless (3G/4G) bandwidth.
• I live in a small town that only has a couple of gas stations. The price per gallon is always high. The next town over has a number of gas stations in a relatively small area and price per gallon competition is fierce. When one drops their price the others follow almost instantly. Competition is good. Right now AT&T and Verizon have agreed to implement caps. If one of them cracks or another alternative provider comes along (Google, Microsoft or who knows who?) with unlimited service at a competitive price they'll all have to drop their caps fast to prevent customer churn.

Over the long term, I'm not really worried about data caps. 

Verizon Strike - Could Be A Long One

I'll front this post by saying my Dad worked for AT&T as a technician for about 30 years, retiring right around divestiture. I was only 11 but remember the long strike (over 100 days) in 1968 like it was yesterday. I've also been teaching telecommunications classes to NYNEX, Bell Atlantic and now Verizon technicians since 1995 as part of the Verizon NextStep program, I've had hundreds of Verizon technicians in my classes and have always been impressed.

Verizon is actually two separate companies when it comes to unionization. There is the landline side - what most of us would call the traditional telephone company. 45,000 Verizon landline employees are unionized and represented by the Communications Workers of America (CWA) and the International Brotherhood of Electrical Workers (IBEW). These employees are on the east coast, ranging from D.C. north to Massachusetts. Verizon Wireless is non-union and has coverage across the United States.

It's no secret the landline business is in decline and the company is currently focused on building out the wireless business. Here's the sticky points as listed by William Huber, president of IBEW Local 827 in New Jersey:

• Verizon wants to tie pay increases to performance review.
• Verizon wants the union workers to contribute to health-plan premiums.
• Verizon wants to freeze pensions at the end of this year.
• Verizon wants to eliminate the sickness and death program
• Verizon wants to cut the sickness disability benefits from 52 to 26 weeks.

I've always been impressed with the work ethic of the technicians, starting with my Dad who went about 19 years without taking a sick day. I see the same level of work and family commitment I saw in my Dad in today's Verizon technicians.

I do think this strike could rival my Dad's long 1968 strike in length. Hope I'm wrong.

Update 8/8/11 at 6:39 AM

Here's a piece of an interesting email I got from one of the technicians I had in class a while ago:

"We were told for the last year that Ivan was stepping down as CEO after the contract was done. When the news came out a couple weeks ago that he was stepping down a week before the contract expired, we should have known that was not good. We always received decent contracts from Ivan. From what I hear the company would not back down on any of their demands, even late Saturday, I also heard the union had agreed to pay some towards healthcare."

Ivan Seidenberg worked his way up in the company the old school way, starting as a cable splicer (he was in the union) out of high school. Lowell McAdam, the new President and Chief Executive Officer of Verizon, came up a different path.

More on CODECs: Quantization + Sampling Rate = A PCM Wave

In my last post I started writing about about CODECs and how they are used in a communications system to convert analog signals to digital signals on one end and, on the other end, convert a digital signal back to an analog signal. I discussed sampling rates and the Nyquist Sampling Theorem. It turns out there's more to generating the Pulse Code Modulated (PCM) wave shown in that last post.

Quantization is used along with the sampling rate to generate a PCM wave. The instantaneous voltage value of an analog signal is quantized into 2⁸ (256) discrete signal levels. With each sample the signal is instantaneously measured and adjusted to match one of the 256 discrete voltage levels.

PCM Wave Generation (note: not to scale)

Since quantization adjusts voltage levels to match one of 256 discrete voltage levels it is easy to see that there will be some signal distortion. This distortion is known as quantizing noise and is greater for low amplitude signals than high amplitude signals. A technique called companding is used to compress and divide the lower amplitude signals into more voltage levels and provide more signal detail at the lower voltage amplitudes. In North America and Japan, this is done using an algorithm called the μ-law. Other countries in the world use the A-law algorithm so conversion is required when calls are made between countries using different algorithms.

Once a piece of an analog signal has been quantized and companded it is then given an 8 bit binary code. This process is referred to as encoding.

After a single analog signal has been encoded it is multiplexed, or combined, with 24 other encoded 8 bit signals. This generates a 192 bit (8×24) sequence for the 24 signals. A process called framing then adds one framing bit to create a 193 bit frame. The framing bits are used to keep the receiving device in synchronization with the frames it is receiving and follow a 12 frame pattern that is repeated with each 12 frames. This sampling rate has determined the Digital Signal (DS) Level System I'll cover here in a future post.

Analog to Digital (and Digital to Analog) with CODECs

In this post I continue to discuss the (rapidly disappearing) Public Switched Telephone Network (PSTN).
CODECs are used to convert analog signals to digital signals on one end and, on the other end, convert a digital signal back to an analog signal.

 CODEC Conversions

CODECs use a method called Pulse Code Modulation (PCM) to convert the analog signals to digital bit streams. PCM uses a technique called sampling to obtain instantaneous voltage values at specific times in the analog signal cycle. This sample generates a Pulse Amplitude Modulated (PAM) signal.

PAM Signal Generation

The diagram above shows an analog signal multiplied with a digital pulse train instantaneous point by instantaneous point with the result being a PAM wave representation of the analog waveform. The digital pulse train determines the sampling rate and it is easy to see if the analog signal is not sampled enough, the analog signal will not be properly represented by the PAM signal. 

In 1924 while working for AT&T Henry Nyquist studied this sampling technique and developed the Nyquist Sampling Theorem. This theorem states that an analog signal can be uniquely reconstructed, without error, from samples taken at equal time intervals if the sampling rate is equal to, or greater than, twice the highest frequency component in the analog signal or:

Sampling Rate = 2(BW)

Example

The Public Switched Telephone Network (PSTN) has a bandwidth of approximately 3300 Hz. Using the Nyquist Sampling Theorem calculate the minimal sampling rate of the PSTN.

Solution: 

Sampling Rate = 2(BW) = 2(3300 Hz) = 6600 Samples per Second

This means, for minimal analog to digital conversion, an analog voice telephone line must be sampled minimally 6600 samples per second. Sampling at a rate of less than 6600 samples per second will not reproduce the signal properly.  Sampling rates of greater than 6600 samples per second will produce more detail. Designers of the voice network used Nyquist’s Sampling Theorem to determine the proper sampling rate. They knew they could not sample under the 6600 samples per second rate and also knew going over the 6600 samples per second rate would produce higher quality. A PCM sampling rate of 8000 samples per second was selected.

In my next post I'll discuss Quantization, which is used along with the sampling rate to generate a PCM wave.