A Few Additional Telephone System Features

In this post I continue to describe the legacy Public Switched Telephone Network (PSTN), looking at a few other common telephone system features we are all used to having and relying on. These are additional handset signals and PIC. I would also want to include Caller ID here but I've already covered how it works in a previous post.

Some Common Handset Signals

We are all used to hearing these additional common signals coming from our telephone. 

Line Busy Signal - 480 Hz and 630 Hz tones on for .5 seconds and off for .5 seconds, then repeats. 

Block Signal - 480 Hz and 620 Hz tones on for .2 seconds and off for .3 seconds, then repeats. This signal is often referred to as fast busy.

Off-Hook - 1400 Hz, 2060 Hz, 2450 Hz and 2600 Hz tones on for .1 seconds and off for .1 seconds, then repeats with a duration of 40 seconds. This signal is designed to be heard from across a room and is very loud.

Preferred Interexchange Carrier (PIC)

Since the 1976 MCI ruling AT&T has been required to open the long distance market to other long distance providers. Prior to this, all long distance traffic in the United States was handled by AT&T and users would just dial a “1” to connect to an AT&T long distance trunk. As other long distance carriers entered the market, AT&T had a big advantage. Customers were already used to dialing a “1” for long distance and placing a long distance call to anywhere in the United States involved dialing a minimum number of numbers. – only 11. This included “1”, the area code, and the 7 digit number. Customers that wanted to used other long distance carriers had to dial 25 numbers to make a long distance call. These calls required an 800 number be called initially (11 numbers), a 4 number personal identification number (PIN), the area code, and the 7 digit number.

In 1987 a method called Feature Group D was implemented to automatically pass calls to the customers preferred long distance carrier using something called a Preferred Interexchange Carrier (PIC) number.  Customers are required to select a preferred carrier and the preferred carrier information is added to the local switch database the customer is connected to.

Feature Group D also allows a customer to bypass the preferred PIC by dialing a 101XXXX number and use another long distance carrier. These 101XXXX are commonly referred to as dial-around service numbers.

RIP Steve Jobs

“.. almost everything – all external expectations, all pride, all fear of embarrassment or failure - these things just fall away in the face of death, leaving only what is truly important. Remembering that you are going to die is the best way I know to avoid the trap of thinking you have something to lose. You are already naked. There is no reason not to follow your heart.”

Steve Jobs

1955 - 2011
Rest In Peace

Telephone Set Function 4. To convert voice frequencies to electrical signals that can be transmitted

In my last few legacy Public Switched Telephone Network (PSTN) posts, I covered pulse or rotary dial service, dual tone multi frequency (DTMF) dialing service and what makes a telephone ring. In this post let's look at microphones and speakers.

A telephone converts voice frequencies to electrical signals and electrical signals back to voice frequencies using basic microphone transmitter and speaker theory and application. 

Transmitters

A telephone transmitter is built into the handset of the phone and is responsible for converting sound waves into electrical signals that can be transmitted.

Telephone Carbon Granule Transmitter

Carbon granule transmitters are still common in wired home phones. Sound travels in waves that are actually variations in air pressure. Some of these waves enter the mouthpiece and cause a diaphragm in the transmitter microphone to vibrate back and forth. These vibrations put either more or less pressure on carbon granules in the base of the microphone. If more pressure is applied, the granules pack more tightly and conduct electricity more efficiently. Inversely, in between the waves the granules unpack and do not conduct as well. Voltage is applied across the electrical contacts and the varying amounts of resistance caused by the carbon granules in the microphone cause varying amounts of current to flow. This current variation is an electrical representation of the sound waves (voice=analog signal) entering the microphone. 

In addition to carbon granule transmitters many modern telephones use dynamic transmitters that function by moving a coil of wire inside a magnetic field to produce an electrical current in response to soundwaves or electret transmitters, also known as condenser microphones, which use a capacitor for a transducer and generally contain an amplifier circuit. 

Receivers

The telephone handset receiver is just a simple speaker. It performs the opposite function of the transmitter in that it takes the incoming electrical signal and converts it to sound waves that can be heard by the listener. 

Simple Speaker Diagram

The incoming electrical signal flows through a magnetic coil in the speaker. The magnetic field surrounding the coil changes in conjunction with the changing current flowing through the coil. This changing magnetic field causes a cone in the speaker to vibrate. These vibrations create air pressure waves forming sound.

In my next legacy PSTN post I'll describe how some additional telephone features work.

Early Fall Walk Around the STCC Technology Park

I took a quick walk at lunch time today around the STCC Technology Park. The Tech Park is part of Springfield Technical Community College and located directly across the street from the main campus. Our NSF funded National Center for Information and Communications Technologies is located in the Scibelli Enterprise Center building in the Tech Park. Lots of cool technology going on in the Tech Park - things like photovoltaics, electric cars and of course Information Technology and Communications! Here's some of the pics I took today.

gordonfsnyder's photostream on Flickr.


A great place to locate a business and work!

Telephone Set Function 3 - To provide a way for the telephone company to indicate that a call is coming in or ringing

In my last two legacy Public Switched Telephone Network (PSTN) posts I covered pulse or rotary dial service along with dual tone multi frequency (DTMF) dialing service. In this post let's look at what makes a telephone ring.

When the user begins dialing the phone each sequenced number is stored in the central office computerized switch at the Local Exchange Carrier (LEC) Central Office (CO) and analyzed. The first three digits determine if the call is local or long distance. If the call is local, the switch determines if it can complete the call itself or the call needs to be forwarded to another local LEC CO that handles that telephone number. If the call is long distance, the call needs to be forwarded to the customer's long distance carrier.

Once the call destination is determined a switch on the receiving end sends a repeating 90 Vrms 20 Hz ringing signal (on for 2 seconds with a 4 second pause) called a ring or alerting signal to the receiving phone causing it to ring.

Ring or Alerting Signal

Notice the ringing signal has an inaudible frequency of 20 Hz - this is why different phones can have different ring styles.

At the same time, a ring back signal that is a mix of 440Hz and 480Hz is sent back to the caller. This signal is on for 2 seconds and off for 4 seconds and indicates that the phone being dialed is ringing. When the receiver picks up the handset the telephone goes off-hook.  The switch hook on the receiving phone closes, current flows and the CO switches turn off the ringing signals.

Facebook Will Need Its Own Operating System

Gabby came home today for a little while and she had her Chromebook with her. If you haven't seen one yet - it's a pretty cool little notebook running Google's Chrome operating system. It boots up in about three seconds and has a battery that lasts over 8 hours. Everything is stored in the cloud so everything is accessible with a web connection. Pretty nice for a first generation device.

It got me thinking - Facebook does not have a mobile operating system (or any operating system for that matter). Apple does, Microsoft does and Google now has more than one. Poking around on the web I found an interesting report from ABI Research titled Mobile Social Networking. Here's some quotes from that report:

The number of people accessing social networks from mobile phones will exceed 550 million in 2011, and that figure will more than triple to over 1.7 billion by the end of 2016. 

For Facebook, the growing importance of mobile is both an opportunity and a serious strategic challenge. On one hand, mobile allows the world’s leading social network to engage with millions of new consumers, but on the other hand its ability to make money from mobile users remains untested. 

Senior analyst Aapo Markkanen is also quoted in the report saying, "A huge problem for Facebook is that while on the web it is a platform, on mobile it’s just another application. To strengthen its hand in the short term we expect Facebook to aggressively take advantage of HTML5, but in the longer term it should absolutely become a mobile operating system of its own."

Facebook is lagging. Google+ is tied in very tightly with Android and Chrome already. Twitter is going to be built into Apple's iOS 5. Practice director Dan Shey is also quoted saying, “The interesting aspect in Apple’s and Twitter’s partnership is how it can provide iPhone users with a verifiable social identity for websites and apps. That gives developers a lot of scope to innovate in areas such as authentication, personalization and advertising. It’s a hint of things to come.”

Rural Broadband - Holland, Massachusetts

The Springfield (MA) Sunday Republican runs a Q&A piece every week titled Just Ask. this past Sunday a woman who lives in Holland, MA posted a pretty interesting question. Before we get to the question though, let's take a quick look at Holland (source Wikipedia). According to the United States Census Bureau: 

• The town has a total area of 13.1 square miles (33.9 km²), of which, 12.4 square miles (32.1 km²) of it is land and 0.7 square miles (1.8 km²) of it (5.34%) is water. 
• As of the census of 2000, there were 2,407 people, 898 households, and 668 families residing in the town. The population density was 194.2 people per square mile (75.0/km²). There were 1,317 housing units at an average density of 106.3 per square mile (41.0/km²). 
• The median income for a household in the town was $52,073, and the median income for a family was $57,024.
• Holland has it's own elementary school but is considering merging its elementary school with the town of Wales. Holland students attend Tantasqua Regional Junior High School (grades 7-8) and Tantasqua Regional High School in Sturbridge.

A typical rural New England town that can be compared to likely hundreds of other rural communities across the U.S. Now for the newspaper reader question.

Question: Is it true that Verizon Communications does not have fiberoptics in my home area, Holland, and that there are no plans to install same? What I have now is a dial-up modem with Verizon, and it is very slow. It takes 10 full minutes for me to access my checking account online, after four screen changes. I understand the need for security, but this is ridiculous. I’m a teacher and need to cover a lot of ground on the Internet in a single day. At such snail speeds, I’m limited to very few online tasks like collecting emails. Over the past three years I have made numerous calls to Verizon service to ask if I could sign-up for high-speed Internet service, and the answer was always “no.” 
In this year of speed-of-light communications, do I have any other options? 
– Kathleen McGrory, Holland 

And the answer from reporter Jim Kinney:

Answer: While it is true Verizon does not offer DSL service for people in your area of Holland, there are other options available. Cox Communications, Holland’s cable provider, does offer provide high-speed Internet in this area. Since you sent us this inquiry, we understand you signed up with Cox’s Broadband service and can now quickly access your checkbook online. Welcome to the 21st century, Kathleen. 
Verizon spokesman Philip G. Santoro said, “There just aren’t enough customers there (for Verizon) to justify the expense.” He suggested people like you contact their local cable operator.  He also pointed out that there is a third option, and that is Verizon Wireless’ 3G coverage, which would provide high-speed Internet through cell phone coverage.

Many small rural towns in the U.S. do not have any option except dial-up. It is upsetting to read the Verizon spokesman's honest answer regarding high speed landline based service in Holland. Nothing against Verizon - it's the frustrating reality of situations across our country. From a business perspective it does not make sense for a traditional telephone company to offer high-speed data service in a town like Holland. With current data caps in place from Verizon Wireless and other providers I don't see 3G (or upcoming 4G) services as a competitive alternative. 

Holland residents are fortunate they do have a cable option. Many similar communities in our country are stuck with dial-up as their only option.


9/14/11 at 7:28 PM

This comment came in from retired Cable Executive Steven Solomon via Google+.

Gordon, I would add this comment to your post. What is often invisible to the public is the real demographics of a community like Holland. I know about this first hand. I helped negotiate the cable TV franchise with the town on the part of the predecessor provider to Cox, Continental Cablevision.

As of 1994, of the 2,400 or so residents of the town, only about 600 homeowners lived in their homes all year round. The rest kept their residences as vacation or second homes. This put the full-time residents at a great disadvantage in getting broadband by landline of any sort.

Putting in broadband plant at about $20G a mile is not a reasonable business proposition for the private sector if the company is relying only on the margin earned from Internet (which now competes with phone and cable TV). Thus, I don't see any alternative to the private sector stepping in with the 21st century equivalent of rural electrification.