Many new telephone services, such as Click to Call, Virtual Call Centers, Hosted IVR and Voice Broadcast, rely on voice over IP (VOIP) telephone connections to deliver services at the lowest possible cost. While some of these services sound great, others deliver voice quality no better than the walkie talkies your child recently received for Christmas. This blog entry will attempt to present, for a non technical audience, the basics behind VOIP communications that determine the quality of a phone call. Specifically this blog will address VOIP traffic transmitted over the public Internet.
Your voice starts out as an analog wave or vibration. Remember the telephone you built as a child with two cans and a string. The bottom of the first can vibrates, this vibrates the string, which then vibrates the bottom of the second can. In a traditional telephone these same vibrations are used to modulate an electrical signal transmitted over a wire which replaces the string.In VOIP communications your voice is converted from an analog wave into a series of data packets. These data packets, which share the Internet with email, web page transmissions and file transfers, are then transmitted from one computer to another where at the destination the data packets are converted back into a sound wave you can hear.
The process of digitizing sound is well understand and as in the case of DVDs can be very high quality. In fact many DVDs are better quality than traditional analog recordings.Unfortunately, unlike high quality DVDs, which have the capacity to transport a large amount of data, the Internet has limited bandwidth. Therefore when we digitize voice for transmission over the Internet we compress the data significantly more than the music on a typical DVD. This compression reduces the quality of the sound.
There are many standards for the compression of voice. However, in a typical engineering trade off; the more we compress the voice transmission, the more conversations we can transmit over the same Internet connection. The other side of this trade off is that the more we compress the voice the worse it sounds.
The vast majority of telephone calls today, even the calls made on your traditional home phone, are digitized. The traditional telephone companies use a compression standard called PCM or G.711. In fact VOIP telephone services that use G.711, such as DialogTech, sound just as good as traditional telephone calls.
Another common compression standard is G.729. Telephone calls compressed with G.729 are compressed on average four times more than G.711 or traditional telephone calls. This additional compression results in lower overall call quality.The telephone industry has a standard measurement of telephone call voice quality called the Mean Opinion Score or MOS. The generally accepted typical MOS (just Google MOS for more examples) for a number of compression standards follow:
Traditional PSTN phone calls: MOS is close to 4.5
G.711: 4.4 or better
G.729 about 4
G.723 about 3.5
G.726 between 3.5 and 4.3
Unfortunately this is not the whole picture. The quality of the connection of to the Internet used by your VOIP service provider also affects the quality of your telephone call. And finally some VOIP vendors use a Voice Activity Detection algorithm to reduce the bandwidth they need for your calls. VAD attempts to listen for silence between the callers in a telephone conversation and stop transmitting packages when no one is talking. If the VAD algorithm is inappropriately tuned the ends of your words will be clipped and overall voice quality will decline.
At DialogTech we are committed to the delivery of high quality voice services designed for business customers. We exclusively use G.711 for our business services. Since many of our services rely on automated voice recognition we need to set very high voice standards for the recognition to function properly.
Please feel free to post comments to this blog entry about your VOIP experiences and information you may have about the codex or compression standard used by your VOIP vendor.