Teleconference Bridge Speech Quality Testing (Are you Getting What You Are Paying For?)
Cell phone use has lowered expectations of voice quality in telecommunications except for conference
calls. Clients will complain about voice quality during a conference call, especially when the conference
call is recorded. Is your conference service provider giving your clients the best possible voice quality, or
are they cutting corners to save costs?
In the teleconference industry there has been a rapid transition from the older TDM (Time Division
Multiplexing) and the newer VoIP (Voice over Internet Protocol) technology. Although technically you
can get better than toll-grade speech quality using high bandwidth VoIP technology, but you can also get
a lot worse quality. The bottom line is there is no real correlation between what you are paying for
teleconference services and the speech quality you are getting. Voice quality and delay vary widely
between the eight teleconference bridges we tested. I hired an outside consulting firm to test eight
different teleconference bridges to compare quality and the results are summarized below.
Teleconference Bridge Technology Overview
The workhorse teleconference bridges of the industry such as the Compunetics and Spectel(Avaya)
started as TDM bridges using very powerful Digital Signal Processors (DSP) to mix the calls together.
These older TDM bridges have adapted to receiving VoIP traffic by keeping the DSPs and replacing the
Time Division Multiplexing (TDM) T1-PRI ports with an Ethernet card. The next generation of
teleconference bridges, the ThinkEngine, WydeVoice , Avaya and others were designed from scratch to
be VoIP bridges. For backwards compatibility some bridges such as the ThinkEngine 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.
The Public Switched Telephone Network (PSTN)
The backbone of the majority of PSTN traffic carried by the Inter Exchange Carriers has migrated to VoIP
but has retained the same 64 Kbps channels as the older TDM circuits to minimize any speech quality
issues. The same 64 Kbps channel of TDM takes over 80 Kbps of bandwidth with the extra overhead
required by VoIP. The real advantage of VoIP versus TDM for the IXC is because the nature of human
speech, the majority of the time there is no information being sent. So during the time a conversation is
silent many other conversations are being carried over the same circuit that with TDM was limited to
only one conversation. The IXC networks are designed to have a minimum Quality of Service (QOS) with
minimum delays and congestion. In many facilities the incoming PSTN is on TDM circuits that are
connected to a soft-switch that converts the incoming TDM to VoIP but only travels a few feet to a local
VoIP bridge. There is minimal delay or speech quality degradation introduced at the local facility during
this conversion. There are also no variable delays caused by running VoIP traffic over the Internet as
long as the traffic remains inside the facility.
What Causes VoIP Quality Issues
If any portion of the route the VoIP traffic takes is using the Internet, then all bets are off as they say in
Las Vegas. If you cannot control packet delay, you really have a difficult time controlling speech quality.
Another way speech quality is degraded is changing the VoIP protocol to use a lower sampling rate.
Going from a 64 Kbps protocol such as G.711 to the 8 Kbps protocol such as G.729 changes the Ethernet
Bandwidth per call from 87.2 Kbps to 31.2 Kbps. You can definitely hear the difference between these
two protocols. The TDM bridges using DSPs are also doing a lot less processing than what is going on in
a VoIP bridge. TDM bridges are doing all processing on a synchronized basis so delays are always fixed.
A variable delay in a VoIP bridge will effect speech quality.
The testing results are shown in Table 1. The acronym PESQ LQ stands for Perception Evaluation Speech
Quality/Listening Quality. In reality the lower scoring teleconference bridges sound a lot worse than the
numbers represent. Notice Bridge #3, it is the same exact type of bridge as bridge #2 and #4 but it has
the worse sound quality. The best sounding bridge was #1 which is using the oldest technology and the
older TDM telephone connection.
RTD (ms) Min Max Ave
#1 TDM Bridge, TDM Connection
Pass 1 316.8 3.6 4.07 3.91
Pass 2 311.7 3.59 4.1 3.85
#2 VoIP Bridge, TDM Connection
Pass 1 559.7 3.8 4.09 3.96
Pass 2 569.3 3.51 4.14 3.94
#3 VoIP Bridge, VoIP Connection
Pass 1 116.7 2.76 3.55 3.08
Pass 2 -- 2.61 3.58 3.03
#4 VoIP Bridge, VoIP Connection
Pass 1 456.7 3.32 4.08 3.82
Pass 2 454.1 3.26 4.12 3.87
#5 VoIP Bridge, VoIP Connection
Pass 1 477.4 3.32 4.14 3.92
Pass 2 447.6 3.78 4.09 3.95
#6 VoIP Bridge, TDM Connection
Pass 1 660 3.63 4.1 3.93
Pass 2 648.5 3.48 4.08 3.85
#7 TDM Bridge, TDM Connection
Pass 1 522.1 3.64 4.17 3.96
Pass 2 -- 3.52 4.2 3.93
#8 TDM Bridge, VoIP Connection
Pass 1 559.1 3.71 4.13 3.94
Pass 2 552.7 3.7 4.04 3.89
You really do not have to understand the results from the table above, the best quality teleconferences
are using TDM bridges with TDM circuits. The best way to determine if your provider is offering high
quality teleconferencing is to first ask questions about what type of bridge and network connections
they are using. Then make multiple test conference calls using an analog telephone lines and speak into
one telephone while listening to the other line. All the VoIP bridges had a delay that could easily be
heard. Most of the TDM bridges had a delay so short it was hard to hear. Speech quality is subjective so
test as many services as possible.
Is This Important?
Not long ago I spoke to the principal of a teleconferencing company that recently sold their base to one
of the largest teleconference service providers. So I asked him what he was going to do next. He told
me he was going to start another teleconference company and wait for his old clients to find him. I
asked him why would his old clients go through the efforts of changing toll-free numbers, access codes
and all the other work required to move to a new service provider. He said his old clients were used to
the high level of speech quality from his old TDM bridges. The purchaser of his base to save money was
moving his clients to a software based VoIP bridge. He knew that his clients would recognize the lower
quality of speech immediately and leave. Teleconferencing is a commodity but quality expectations of
the clients can put speech quality ahead of cost as a decision basis.
Herb Levitin is the founder of WholesaleConference, a Santa Barbara, CA hardware based service
provider of fully branded e-commerce sites for toll-free, flat-rate toll teleconferencing and web
conferencing. WholesaleConference specializes in providing advanced web-based user features for