Voice Communications

Voice communication is the transmission and reception of audio and other signals that can be represented by the frequency band used for voice signal transmission. Telephone systems transfer voice signals in a variety of forms through by wire, radio, light, and other electronic or electromagnetic systems. These forms include analog and digital voice signals. Options for voice communications include different voice quality of service levels and voice privacy options.

Voice Quality
Voice quality is a measurement of the level of audio quality, often expressed in mean opinion score (MOS). The MOS is number that is determined by a panel of listeners who subjectively rate the quality of audio on various samples. The rating level varies from 1 (bad) to 5 (excellent). Good quality telephone service (called toll quality) has a MOS level of 4.0.

The first telephone systems used analog signals to represent the voice. To overcome the cumulative noise limitations of analog signal transmission, digital transmission systems were created. These digital transmission signals represented voice signals by discrete levels that can be recreated eliminating the noise. As a result, in the 1960’s, many modern telephone systems began to offer digital voice communications.

The first digital voice services converted (digitized) the analog voice signal to a 64 kbps digital signal. This 64 kbps digital channel called a DS0) provided “toll quality” voice with a MOS score of 4.0 or above.

Generally, there is a tradeoff between system efficiency (bandwidth used) and the level of voice quality. To gain system efficiency (to add more customers per interconnection line), some telephone systems compress the voice using speech-coding (data compression) technology. The first compressed voice service uses adaptive pulse coded modulation (ADPCM) that further compresses the 64 kbps DS0 to 32 kbps ADPCM.

Other voice compressed voice service have been developed that can use low bit-rate standard or proprietary speech compression algorithms. These can further compresses the 64 kbps DS0 to below 16 kbps or even 8 kbps.

Voice Privacy
Voice privacy is a process that is used to prevent the unauthorized listening of communications by other people. Voice privacy involves coding or encrypting of the voice signal with a key so only authorized users with the correct key and decryption program can listen to the communication information.

Digital systems are inherently more secure than analog systems because they can easily use an encrypted mode of operation. This encrypted mode of operation “scrambles” voice data before it is sent to other users in the network. The encryption uses a key (mask value) that is calculated from some form of secret data. When the voice data is received, it must be decrypted using the same mask value that was used to encrypt it. Although an interceptor may be capable of receiving the data signals, they cannot learn the true data value unless the secret number that was added to it is also known.

While the telephone system can offer an encryption mode that encrypts the signaling between the end-user’s phone line and the telephone network, it is more common for the end-user to maintain their own voice encryption system. This does help to prevent unauthorized access to the telephone system. This also allows the end-user to have many different voice encryption algorithms. The voice encryption algorithms are typically stored on the end-user’s telephone devices.