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Testing Conference Call Quality
September 28, 2010 - Article
By Herb Levitin

Cell phone use has lowered users’ expectations for voice quality for calls except for conference calls. Clients will complain about voice quality during a conference call, especially when the conference call is recorded. Their complaints can be linked to the migration from older TDM (Time Division Multiplexing) to newer VoIP technology. Technically you can get better than toll-grade speech quality using high-bandwidth VoIP technology, but you also can get worse quality.

To prove the point, I hired an outside consulting firm, GL Communications Inc., to test eight different conferencing bridges to compare quality. The results are summarized in the table, but generally showed that voice quality and delay varied widely among the bridges tested. The connection type also had an impact. The best sounding bridge used the oldest technology and the older TDM telephone connection. (The full report can be downloaded from

The Bridges. The workhorse TDM bridges use digital signal processors (DSPs). They have adapted to receiving VoIP traffic by keeping the DSPs and replacing the TDM T1-PRI ports with an Ethernet card. The next-generation teleconference bridges were designed from scratch to be VoIP bridges. For backwards compatibility some bridges have a DS3-TDM port, but they are doing an immediate conversion from TDM to VoIP and all processing is done with VoIP packets, which introduces an additional delay versus the older DSP-based TDM bridges.

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