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Touch Tones and VoIP
Saturday, May 10, 2008
A standard analog telephone line is capable of transmitting all kinds of things, ranging from the human voice to music to data--think fax machines and analog models. The Bell System's focus on audio quality early on in the development of the phone system made this possible.
In the 1950s, AT&T was using specific tones--multifrequency tones--on telephone lines in order to route calls between switching facilities. This practice of in-band signaling was used well into the 1980s, but has since been replaced with much more secure out-of-band signaling.
The Bell System created a derivative system for use by telephone customers called dual-tone multifrequency, or DTMF. They began marketing this service under the trade name Touch-Tone in 1963. This allowed people to push buttons to dial their phone instead of using a rotary dial.
While the touch tone phone could be used to dial a telephone number, it could also be used to signal the remote end of the call in some way-the reason they created the * and # keys. They also created explicit A, B, C, and D keys, but they were never widely used.
While not entirely common back in the 1960s, using your touch tone telephone to navigate a telephone tree is a fairly common occurrence today. Therefore, if your using a VoIP telephone service, touch tone service had better work.
On VoIP, you can signal touch tones one of two-ways: in-band or out-of-band. In-band signaling only works with G.711 and even so, it's generally not recommended. The preferred method is therefore out-of-band signaling for touch tones, and is the only method supported with codecs like G.729.
There are two standards for DTMF signaling out-of-band in VoIP: RFC 2833, which is the common method, and SIP INFO, which can also signal this information. In both cases, they transmit when a particular touch tone has been activated and when the touch tone has been deactivated.
The challenge with both methods is correctly indicating when a particular button has been pressed and, more importantly, when it has been released. Both RFC 2833 and SIP INFO take this into account, but use different methods. RFC 2833 is the most common method and seems to be a bit more reliable than SIP INFO, based on my experience.
G.729 versus G.711
Friday, May 09, 2008
In the last article, we discussed the G.711 codec, which is often used to give you "toll quality" calling. There are situations where you do not have the bandwidth for G.711 calling. Perhaps you have a restricted upstream speeds on your Internet connection or you need more bandwidth for other applications.
Another common choice for encoding your voice into data is the G.729 codec. Unlike G.711, it is able to transmit voice very efficiently--at about 32 kilobits per second versus 87 kilobits per second for G.711. Unlike G.711, however, the human voice is synthesized by something called a vocoder.
The vocoder uses both a tone generator, a white noise generator, and a filter that shapes the sound as the throat, mouth, tongue, lips, and nasal cavities do. By itself, the vocoder produces intelligible speech, but it sounds like a robot is speaking.
Since that's clearly unacceptable, G.729 also uses samples of the actual human speech to set the vocoder settings properly. It also compares the actual voice from the synthetic voice to come up with a "code." The code along with the vocoder settings are what's sent to the remote end. The remote end takes the code and vocoder settings and plays the sound.
The result of all this work is voice quality that sounds similar to G.711 but at almost a third of the bandwidth requirements. It takes a lot of processing cycles to do all this, too, which is why some voice over IP gear limit the number of streams it is capable of processing G.729 for.
There are some challenges with G.729. While it works great for voice communication, it doesn't do very well for things like faxing, data, and transmitting touch tones. This is because G.729 is designed specifically to translate voice, not data.
So if G.729 doesn't support touch tones, how is it you can do touch tones on a VoIP line? We'll explain that next time.
How Does A Phone Work?
Thursday, May 08, 2008
Here's an interesting video from the 1960s that explains how a telephone works:
The video goes a bit longer, but for the purposes of this discussion, it covers the basics. Sound is modulated in via the mouthpiece and turned into electrical impulses, which are then transmitted out the wire to the central office, where it is then sent to the other party, who hears it faithfully reproduced on the other end through the earpiece.
That's the way it works with a traditional telephone. In the voice over IP world, it's even more complicated than that. Sure, there's something, be it a telephone handset or a headset on your microphone that modulates your voice into electrical impulses. While VoIP does go out a wire, the wire isn't analog like a traditional telephone line, it's digital, or at least the part it uses is.
Specifically, the voice must be translated into something that can be sent and received on an IP network. The voice has to be turned into ones and zeros--relatively quickly, I might add--to get to the other end quick enough that the conversation seems natural. Too much delay in that process and the call becomes difficult to have.
Sampling your voice is fairly straightforward. Transmitting it efficiently while maintaining the quality of your voice is a completely different matter. This is done through codecs, which compress your voice into data and decompress it at the other end.
G.711 is the codec most often used to create "toll-quality" voice. If you want support dial-up modems, faxes, or other non-voice uses of a voice over IP line, then G.711 is what you must use. Assuming there is no packet loss and the jitter remains relatively constant, G.711 provides the closest thing to a call over a conventional landline.
The problem with G.711 is the amount of bandwidth you need. It encodes 8-bit samples at 8kHz , which gives you 64 kilobits per second of transmission. When you add the extra overhead of communicating over IP, it takes roughly 87 kilobits per second. (Incidently, that's more bandwidth than a traditional telephone line requires).
There are other codecs as well, which we will get into next time.
How Sound Synchronizes Up With Pictures
Wednesday, May 07, 2008
In this short film from 1929, a couple of animated characters explain the process of synchronizing sound with pictures--something that back in those days was still relatively uncommon in films.
What I find interesting about this process is that they described microphones in terms of the telephone, i.e. it's it just like a telephone. It's an apt description, given the film was made by Western Electric, the manufacturing arm of the Bell System (AT&T and associated companies) until it was ordered to be split up in 1984.
Nearly 80 years later, of course, getting sound and video synced up on motion pictures is old hat. But where it has been a challenge in recent history is in another promised innovation from Ma Bell--video calling. Even into the early 1990s, AT&T was putting out ads saying video calling was going to be a reality--soon.
Now there are a number of solutions for video calling, and they all have to solve the audio/video sync problem. Unlike with recorded video and audio, where it is generally trivial to fix any synchronization issues, you don't have the luxury of time in real-time video calling.
In my experience with a number of different video calling solutions, when push comes to shove, video is put aside in favor of maintaining audio quality. The theory is that you can still have a conversation with audio, even if the video isn't up to snuff. Also, video takes substantially more processing power and bandwidth than audio.
The codecs used to compress audio and video take this into account, though it is a challenge to get back into sync when you get out of sync. With many video calling solutions, it can take several seconds to get back into sync. One company in particular has patented technology they employ to keep audio and video synchronized, even in changing bandwidth conditions.
The World Of Communications, Then And Now
Tuesday, May 06, 2008
I've been spending some time looking through some old movies to try and uncover some interesting tidbits about the history of communications. In this particular short from 1947, a telephone line repairman explains to a couple of young boys about communications. Interestingly, the movie spends a lot more time talking about ways other than the telephone.
How did people communicate back in 1947? They communicated by the telephone, of course, but they also communicated by handwritten letters, radio, the newspaper, and television. I'm surprised they left movies out as a way to communicate. After all, that's what this short was--a movie.
Today, communications isn't all that different. We still send letters to people, we still listen to the radio, read the newspaper, and watch TV and movies. At least conceptually speaking, anyway. All of these activities have now taken on different forms.
Let's take a look at letters. I can't remember the last time I hand wrote a letter to someone. I bet you can't, either. However, I send electronic letters (i.e. email) constantly. Instead of having to wait for days for a response, we can get a response in a matter of minutes, sometimes.
The radio? It's still there and operates much the same way. However, we can also listen to our own radio station, as it were. Portable Audio Players like the Apple iPod can contain your own collection of music and audio programs, commonly called podcasts or netcasts. You can be your own disc jockey, as it were.
The newspaper and TV is still there, also. But the newspapers are rapidly moving online. Television is doing the same thing, with the major networks making their most popular shows available over the Internet. Even the evening news can be seen online.
What about telephony? That's moved online as well. voip.com and a number of other services deliver phone calls over the Internet, much like the other forms of communication are starting to do.
Some things don't change. While it's true the delivery method for all these communication methods is changing, the fact is that people are still using essentially the same methods to communicate then as they are now.
Click To Call
Monday, May 05, 2008
Click To Call is a feature that many services are starting to offer. Instead of a customer picking up a phone to dial you, the customer clicks on a link. The customer is taken to a page where they enter their telephone number, and possibly other information about the transaction. When they click submit, their telephone rings and they are connected. It is similar in principle to the Make A Call page on voip.com.
One reason why Click to Call is desirable: it's actually cheaper than an 800 number. An 800 number is a free call for the people that are calling, but it's by no means free to the business or person paying for the 800 number. Paying for what ends up being two calls and bridging them is actually cheaper than what a typical 800 number charges per-minute.
Second is that with Click to Call, you have the opportunity to collect more data before the call is even made. Instead of just having customers blindly dialing into your 800 number and have to navigate the phone tree in order to get to the right person, you can collect a lot information up-front to route the call to the right person and shorten the amount of time it takes to handle that call.
A third reasons you might consider Click to Call, particularly as an individual, is that you don't want to expose your real telephone number, yet provide a way for people to talk to you in realtime for free. Click to call hides your real telephone number. In some cases, it can also hide the person's telephone number who is calling, making the system "double blind" in a sense.
Of course, this technology has some challenges. It won't work if the person initiating the click-to-call doesn't have a direct dial telephone number, such as in some office environments or in hotels. Even so, Click to Call is a nice feature to be able to provide.
How Operators Dialed Long Distance Calls
Sunday, May 04, 2008
Lately, I've been delving into the history of how telephone calls used to be made. It's fascinating to see where things started--manual operators connecting cords--and watch the progression to where it is today--customers direct dialing their own numbers almost 100% of the time with no human intervention.
Of course, several decades ago, dialing anything outside your local area required human intervention. It might explain why long distance calling was so expensive. To get a sense for what the operators had to do, check out this training movie, voice by the late Jane Barbe, whose voice you might recognize if you had a land line in the United States:
Why was a human operator needed? Mainly because the entire Bell System was not at the same technological level. You can see this in the movie.
In larger cities, they switched over to the automated dialing and switching systems quickly. In some of the more rural parts of the Bell System, they still used manual operators and cord boards. The operator would have to figure out the right way to connect the call, depending on the location.
Another thing to notice about the movie was how they focused on the right way for the operator to dial. Oh no, you couldn't use your finger to dial, you had to use this pencil-like thing with a metal ball at the end and hold it perpendicular to the face of the dial, which was slanted. Not only did your form had to be right, you had to dial quickly and efficiently to save time on the equipment and circuits. Very important, back in those days.
Give Your Mobile Phone That Old Time Feel
Saturday, May 03, 2008
Chances are, you've got a mobile phone. Do you have a landline too? An increasing number of people are ditching their land lines for mobile phones. They could also be ditching their landline for VoIP service also.
If you miss the days where you had to hold a receiver up to your head or you're afraid the various reports about cell phone radiation being bad for you are true, then you might want to have a look at this thing called the P*PHONE from Hulger.
What does it do? It plugs into your headset jack on your mobile phone. It also includes adapters for a number of different headset plugs, so it should plug into just about anything--including your computer. Not without an extra purchase, of course (e.g. the Hulger USB*ADAPTOR). Once you've done that, it is possible for this headset to be plugged into a computer and used with any VoIP client, such as the one that voip.com makes available.
The next question, of course, is where are you going to put the handset when you're done? Hulger has an answer for that, too: the P*BASE. It apparently includes the USB*ADAPTOR and gives you a place to hang up your old school headset.
What's missing here? The old ma-bell ringer. Surprised Hulger hasn't created a solution for that.
How Rotary Phones Affected Area Codes
Friday, May 02, 2008
Image via WikipediaDid you ever wonder why a place like New York City got an area code like 212 and a place like Hawaii got an area code of 808? It boils down to one simple fact: the rotary phone and how fast you could dial it.
If you're under the age of 30, chances are you may not have used an old rotary phone. I grew up at a time where touch tone telephones were starting to become popular, but many of the older members of my family had an old rotary telephone.
If you want to see how to use an old rotary telephone, here's a nice introduction from YouTube from the 1920s which explains how to do it.
Prior to the introduction of direct dial service, telephone calls were made with the help of an operator. Larger cities were first to switch to this direct dial service with smaller cities coming along as time went on.
Notice something with the dial telephone. The larger the number, the longer it took for the dial to return back to it's original position. Larger numbers took longer to dial than shorter numbers.
Something else about area codes in the United States: the original rules for area codes were as follows: a digit 2-9 followed by either a zero or a one and then a digit 2-9. These rules have been long since abandoned, given the proliferation of telephone numbers and the advent of automated switching equipment.
What does this mean? The largest US city, New York, got the area code 212. It was the shortest to dial. The next shortest area codes to dial are 213 (Los Angeles) and 312 (Chicago), two other large, major cities.
Of course, these days, with touch tone phones, it doesn't take any longer to dial someone in Los Angeles than it does in North Carolina, which has the area code 919--the longest to dial on a rotary phone under the old rules. The nice thing is, it costs a heck of a lot less than it did back then, too!
VoIP Over The VPN
Thursday, May 01, 2008
Chances are, if you work for a large corporation, and you do any kind of work from home, you have to have some kind of access to the office remotely. This typically happens through the help of something called a VPN, or virtual private network. Through encryption and strong authentication, and some software magic, your remote PC becomes part of the corporate network.
First of all, there's usually some piece of client software that must be loaded on your system. The exact software will vary depending on the vendor that provides the VPN solution. Despite the fact that most VPNs these days employ either IPSec or SSL--both well known, well-understood standards, the client software is specific to each vendor's implementation.
One challenge that VPNs must deal with is the fact that most clients are behind an address translation device of some sort. The clients are assigned an address by their local router that are not only not reachable from the greater Internet, but may be the same or similar to an IP address on the corporate network.
To resolve this issue, VPN clients create the equivalent of a new networking adapter. This adapter, like your Ethernet or WiFi adapter, are assigned an IP address through the VPN. This IP address is unique within the corporate network and makes each client connected appear as if they are somewhere on the corporate network.
It should, therefore, be possible to use a voice over IP client over a VPN. It's true, it does work, but there are some challenges to make this work acceptably. The main problem is that you're now adding the overhead of encryption. A VPN, as part of the encryption process, must also add a certain amount of data to each packet to ensure the confidentiality and integrity of the data.
Essentially, a VPN adds more overhead into a process where anything over 300 millisecond round-trip times might be a noticeable difference in the quality of the audio. More data that has to be processed more to get where it needs to go will for sure slow things down, but under the right conditions, it can be just like you're in the office.