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a) Network
Signaling Protocol |
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The signaling
protocol (eg SIP, H.323) can be tested for compliance with the appropriate
standard. Protocol analysis tools and interoperability tests are used
to verify compliance. |
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b) Acoustic
Performance |
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The performance of the
acoustic to electrical and electrical to acoustic conversion can be
measured. Tests are performed by the measurement of various electro-acoustic
parameters. |
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c) Response
to Network Impairments |
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The telephone is tested
to determine how well it handles packet loss, network jitter and other
network disturbances. Tests can be performed using mean opinion score
(MOS) prediction algorithms (such as the ITU-T P.862 PESQ) and a network
impairment simulator. |
| Acoustic
Performance Tests |
The Telecommunications
Industry Association (TIA) has established standards for the acoustic
performance of digital telephones. TIA-810-A (currently being revised
to TIA-810-B) specifies requirements for narrowband telephones, and
TIA-920 specifies requirements for wideband telephones.
Some of the acoustic tests that affect user’s perceived quality are
listed below: |
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Frequency
Response Tests |
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These tests measure the
frequency dependence of the acoustic to electrical and electrical
to acoustic conversion. The frequency response is measured for Send,
Receive, and Sidetone (except for handsfree) modes. The measurement
of frequency response is important to ensure that the telephone does
not inappropriately emphasize or attenuate certain frequencies. |
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Loudness Tests |
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A weighted average of
the frequency response is calculated to provide a single number representing
perceived loudness of the telephone. The Send Loudness Rating (SLR),
Receive Loudness Rating (RLR) and Sidetone Masking Rating (STMR) are
calculated. The nominal loudness ratings are specified in the network
loss plan to ensure that loudness is consistent between different
telephones on different connections and that there is adequate head
room to accommodate peak signal levels. |
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Volume Control
Tests |
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The volume control is
tested to determine the range of loudness provided. This is not only
a performance requirement, but also a legal requirement in many countries;
for example in the U.S.A. the Hearing Aid Compatibility Act of 1988
(HAC Act) requires a certain range of volume control and magnetic
output to ensure reasonable access to telephone service by persons
with hearing disabilities. |
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Noise &
Distortion Tests |
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These tests are performed
to measure the amount of noise and distortion generated by the telephone.
Several different stimulus levels and frequencies are typically tested.
Noise and distortion can fail due to a number of causes: poor codec
implementations, bad or improperly installed transducers, faulty electrical
components, poor connections, handset plastics flaws, etc. Good distortion
performance indicates that the VoIP phone can reliably reproduce signals.
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Noise and
Single Frequency Interference Tests |
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This test detects spurious
signals in the noise generated by the telephone that might be perceived
as a tone. This test is performed to verify the generated noise is
minimal and because telephone users find that a tone is more objectionable
than random noise. |
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Weighted Terminal
Coupling Loss Test |
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This test measures how
much of the receive signal that is sent to the telephone is re-transmitted
to the network. Because of delay inherent in VoIP telephony, a re-transmitted
signal will be heard at the far end telephone as an echo. The echo
frequency response is measured and the weighted terminal coupling
loss (TCLw) is calculated as a measure of the echo from the telephone. |
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Latency Tests |
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The time delay of the
telephone is measured. The delays in the VoIP telephone add to the
network delays and can have a significant affect on the call quality.
Generally VoIP telephones should add minimal delay; TIA-810-A requires
that the send latency be less than 35 ms and the receive latency be
less than 65 ms. |
Acoustic
performance tests are typically done with a closed network to isolate
the IP phone from the effects of network variations. The acoustic
tests are also performed using the G.711 codec for narrowband operation
and linear 256 kbits / sec PCM codec for wideband operation. These
tests are not intended to compare or verify the performance of different
voice codecs. To test a codec implementation, test vectors and other
performance criteria are specified in the standards defining the
codec. For comparison of the performance of different codecs or
network conditions, MOS estimation algorithms such as PESQ are often
used.
The acoustic tests should be performed
for handset, handsfree, and headset operation. The standards’ requirements
are different for each mode of operation. |
Test Configurations
There are two approaches to performing acoustic performance
tests on digital telephones. |
The first, preferred, approach to testing is “direct digital”
(see Figure 1) where the test equipment generates and analyzes
digital signals directly without conversion to analog. Test
equipment designed specifically for testing VoIP telephones
is required, but the results obtained are more accurate because
the reference codec is not used. When
using the direct digital method, a telephone call is established
between the telephone under test and the test equipment. This
requires that the test equipment support the protocol used
by the telephone. This approach works well for telephones
using common protocols such as SIP and H.323. For cases where
the telephone uses a proprietary protocol not supported by
the test equipment, it is sometimes possible to bypass call
establishment and start the Real Time Protocol (RTP) used
for audio transport by placing the telephone into a manufacturing
test mode. Most VoIP telephones use RTP for audio transport.
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Figure
1- Direct Digital Test Configuration |
The second, alternative, approach is use to use a “reference
codec” (see Figure 2) to provide an analog interface to the
telephone. This has the advantage of allowings testing to be
performed using general purpose instruments or traditional telephone
test equipment. When using the reference codec approach, it
is important to be aware of the effect of time delays in the
reference codec and telephone. The combination of the reference
codec and telephone can result in more than 100 ms between application
of a test signal to system, and the output signal being available
for measurement. Most traditional telephone test equipment was
not designed with the expectation that this signal path delay
would be present, and the equipment must be configured to account
for it. The reference
codec is intended to have ideal characteristics, but a physical
implementation is never perfect and the resulting measurements
include the effects of both the device under test and the
reference codec. It can be difficult to determine how much
effect the reference codec has on the readings in a given
test setup. Latency tests are often not possible with the
reference codec method because the exact delay of the
reference codec is unknown.
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Figure 2- Reference Codec Test Configuration
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Direct Digital
test equipment is designed for testing digital telephones and
takes into account the signal delay through the telephone. Time
alignment of the measured signal can be used for analysis to
compensate. The internals delays of the test equipment can be
determined to allow measurement of the telephone’s latency.
For either the reference
codec or direct digital methods, an acoustic fixture is required
to interface to the telephone’s speaker and microphone. The
type of fixture used can vary depending on whether handset,
headset, or handsfree operation is being tested.
The recommended acoustic interface
for headset and handset testing is a Head and Torso Simulator
(HATS). The HATS simulates the shape of an average human head
and torso and includes a mechanism to position and hold a
handset. The HATS is equipped with one or two ear simulators
that simulate human ear characteristics and a mouth simulator
to apply stimulus to the telephone’s microphone. For Handsfree
operation, a different test fixture with a free field microphone
instead of an ear simulator is typically used.
Many of the acoustic tests must be performed in an quiet environment
free from acoustic reflections. The telephone and test fixture
are often placed in an anechoic chamber to achieve these conditions.
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Microtronix Systems Ltd., a worldwide telephone test system
provider since 1972, provides VoIP manufacturers and developers
with a test solution to evaluate the acoustic performance of
an IP phone (Desktop, WiFi, WLAN). The unique feature that Microtronix
provides is Direct Digital Generation with the IP Phone. This
allows direct digital communication with the IP phone without
analog conversion. High speed, accurate, and repeatable standards-compliant
results allows you to reduce development time, costs and increase
production.
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The IP test solution can measure Send / Receive
Latency (the amount of time the phone requires to encode and
decode audio). This ensures the phones’ delay does not affect
the quality of the call. Tests such as Send / Receive,
Frequency Response and Loudness Rating, Echo and Weighted Terminal
Coupling Loss, Distortion and Noise are provided by the test
system. Microtronix provides applications for handset, handsfree,
Analog Telephone Adaptors (ATA), headsets and custom applications.
Microtronix IP test solution can measure the latency (the amount
of time the phone requires to encode and decode audio).
Microtronix supports protocols
such as Session Initiated Protocol (SIP) IETF RFC 3261, H.323
(ITU-T H.323 version 4), Session Initiated Protocol (SIP)
IETF RFC 3261, Wideband and directly over RTP (Real-time Transfer
Protocol). The system architecture is also designed to implement
custom protocols. A pre-programmed VoIP test suite- for specifications
such as TIA/EIA-810-A and TIA-920 standard or custom test
suites are also available.
Microtronix test equipment is being used globally in countries
such as China, Thailand, United Kingdom, Malaysia, and the
United States. To date, Microtronix has test equipment in
over 40 countries worldwide. Many contract manufacturers from
small to large use Microtronix test equipment to ensure the
products they supply to their principal companies meet the
quality and volume goals.
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