OCS-Office Communications Server Exposed – Microsoft Media Stack

April 20, 2008
Among the many ways OCS improves on voice quality is via:
1 - QoS-Quality Controller
OCS provides for "dynamic adaptation" to real-time network conditions with progressive bit rate adjustments and insures that no voice sessions are dropped during this process. However this does not prevent content loss or jitter.   According to various sources, even a 1% loss can significantly degrade the user experience with G.711, which is the standard for toll quality for voice CODECs.   That is, different CODECs (e.g. G.711/729) may be used at different locations within the same company and more likely between different companies. MOS-Mean Opinion Scores reduces rapidly with each time a voice conversation is processed by a CODEC. Whether you call it Transcoding or Tandem Encoding is a network element to be standardized when ever possible.
2 - Voice Activity Detection/Silence Suppression
Voice clipping, chopping or dipping occurs as the result of the VAD-Voice Activity Detector. VADs are used for silence suppression in packet voice systems, due to the need not to use bandwidth to send packets when there is no voice; think of this as not sending silence.   If below the signal threshold, then the media is clip/cut as it is perceived as noise not voice. This is called "front end speech clipping." "Dipping" is clipping at the end of the voice segment.  VAD also reduces or suppresses echo suppression in echo cancellers. 
VADs are also used to reduce room or background noise in IP phones, speakerphones and microphones. VAD systems determine the difference between human voice, tones, unvoice, "white noise" (similar in concept to white light (composed of equal amounts of all visible light frequencies) - a sound composed of an equal mix of all audible frequencies) and comfort noise (noise generated to let the user know the call has not been disconnected) and other sources. There are three or more types of noise or non-noise detectors: known energy level, adaptive energy level and spectral energy based on compression. 
 
3 - Audio Optimization
Advanced voice engineering is used to enhance noise suppression, AGC-Automatic gain control, automatic echo cancellation and other techniques.  Echo is caused by three principle factors: talker echo, listener echo and loss of interaction (human and cultural influences). Talker Echo disturbs the speaker who hears an attenuated and delayed echo of his/her voice. This is caused by a reflection on the distant end. EL-Echo Loss is defined at the ratio of the power (voltage) of the arriving voice signal to the power of the reflected echo signal expressed in dB (deciBels). If there is no echo, the loss is infinity. Listener Echo also influences the speaker who hears the signal from the other party followed by an attenuated echo of the signal. Listener Echo is caused both by a reflection close to the speaker and a reflection on the distant end. Loss of interaction is frustration by either party from the echo on the line.
 
If you want to know more, this information is part of OCS-101 and SIP Essentials 2.0c courses available onsite and online. Th online version is $299 for SIP 2.0c and $499 for OCS-101 Office Communications Server per person (volume and site license discounts available).   For more information go to http://www.techtionary.com or please call Tom Cross at 303-594-1694 or cross@gocross.com. 
Discounts are also available to members of the SIP Forum.


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