Would you believe that digital telephony is nearly 50 years old? Would you believe that digital telephony is nearly 50 years old?

It seems like VoIP telephone systems are a relatively new technology without the hundred years of experience that built the solid, dependable public switched telephone system. Would you be shocked to learn that digital telephony has been part of telephone technology for nearly 50 years? Would you also be surprised to learn that most phone calls you make today are at least partially digital? Digital voice is not only a proven technology, it offers quality advantages over old fashioned analog telephone.

The Story Behind the Digital Telephone Revolution

When Alexander Graham Bell yelled for Mr. Watson, his invention was pure analog technology. A vibrating microphone converted speech pressure to electricity which then vibrated an earpiece in the next room. That principle was used in every phone and phone line for decades. So, why change it?

For one thing, analog technology can consume a lot of wires... one pair for each conversation. Analog multiplexing electronics can shift frequencies to stack many conversations on a pair of wires, but it's complicated and gets out of alignment. Analog signals also pick up noise every time they are amplified. Anyone who remembers the poor quality of long distance and international calls thirty years ago knows that they sounded nothing like the "next door" quality you experience today.

By the 1950's, digital technology had advanced to a point where phone companies began to use it to transport calls between phone company offices. The T1 digital line carries up to 24 conversations and a T3 digital line carries up to 672 simultaneous phone calls. That's quite a savings in wire. Moreover, as long as the digital signals are automatically regenerated when they get weak, no noise is added to the signal. What comes out is exactly the same as what went in.

Today nearly every long distance call and many local and regional calls have been converted to and from the digital domain without your being aware. What you do notice is the quiet background and clear voice quality even on international calls. The person you are talking to in the next state may just as easily be down the street. That's the value of digital transmission.

How It Works

Your regular phone is connected to the telephone office or your company's PBX system by a pair of copper wires. At that point, an analog to digital converter called a CODEC (coder / decoder) converts it to a digital signal that is a set of numbers rather than varying voltages and currents. Another device called a multiplexer combines your call with others into a single bit stream that is loaded onto a digital trunk line. At the other end, the process is reversed and the digital voice is converted back to analog and sent to the receiving party's phone. Unless you are aware of the inner workings, the conversion processes are transparent and you can't really tell if the phone call was sent by analog or digital means.

How Does This Relate to VoIP?

In a VoIP telephone system, the phone call is converted to digital much earlier in the process. If you have an IP telephone, the circuitry to convert between analog and digital in built right into the phone. Only the handset itself is analog. The line that connects the telephone to the phone network is actually a digital network line. Everything between phones happens in the digital domain.

The other difference is one of technique. In the T1 digital system, each phone call is assigned to one of 23 or 24 channels that the line can handle. If no one is speaking, the channel is still tied up. If only a few calls are taking place, the other channels are idling empty but you still have to pay for them.

In VoIP, the digital voice is sent out as a series of packets that share a common network line with other VoIP phones and computers. The phone generates packets only when a conversation takes place. Otherwise, packets from other sources are free to use the bandwidth available in the network. It's a more efficient system and gives you the ability to converge voice and data into a single network.

What About Quality?

The legacy digital phone system called TDM for Time Division Multiplexing was designed to mimic the characteristics of the analog phone standards. Quality is assured.

VoIP phone systems can sound just a good as the best analog or other digital phone. TDM phones use a conversion process that generates 64K bits per second. VoIP also has this standard available. However, you may wish to opt for a slightly reduced quality standard that generates 8K bits per second. This standard lets 8 phone conversations use the same bandwidth as 1 conversation of the highest voice quality.

There are also lower quality coding standards that offer even higher bandwidth efficiency, but at noticeably poorer voice quality. If your only experience with VoIP phone calls has been with earlier PC based phones on dial-up Internet, you might be inclined to think of VoIP as a poor quality medium. However, with high quality phones on a properly designed enterprise network, VoIP can sound as good and maybe even better than some analog systems.

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