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VoIP Test and Measurement by gabyion


									                                ENGINEERING NEWS 02_05

The Guardian II family
        from Trilithic

                         VoIP Test and
                         ...the Key to IP Telephony

                                           FC network engineers with IP telephony service experience know that high-
                                           quality services are founded on a clear plan for system oversight that includes
                                           monitoring, aligning, maintaining, troubleshooting and proper installation. If
                                           any of these factors are neglected, subscribers are likely to have a negative
                                           experience and quickly head for another telephony provider.
                              Because high-speed data (HSD) and IP telephony are both packetized data services, it
                           would be easy to conclude that a network that provides acceptable HSD performance will
                           provide acceptable performance with VoIP. The reality is that high-speed data services are
                           more robust because they don’t require real-time connectivity. To some extent, transient
                           interference, jitter and other impairments can be shrugged off because errors can either be
                           corrected by FEC (Forward Error Correction) or lost packets can be resent, with the delay
                           virtually imperceptible to the customer.

2 ENGINEERING NEWS 02_05                                                               
                                                                                         How to Install and Troubleshoot
                                                                                         VoIP Without Headaches
Figure 1: Time doman vs. frequency domain
                                                                                         For VoIP, Trilithic’s 860 DSPi offers useful installa-
                                                                                         tion and troubleshooting tests. At installation, for
                                                                                         example, they provide a baseline for future compari-
                                                                                         son, which technicians will be able to use to deter-
                                                                                         mine if there is any degradation in service quality.

                                                                                             One test uses Internet control message proto-
                                                                                         col (ICMP), and the other uses real-time protocol
                                                                                         (RTP). Both look for packet loss, latency and jitter
  Time domain snapshot                        Time Domain converted to                   on the upstream and downstream, all of which
                                              Frequency Domain using FFT.
                                                                                         can degrade voice traffic. It takes latency of just
                                                                                         over 300 msec, jitter above 50 msec and 1 percent
   Voice is more finicky. Because telephone for VoIP services. When it comes to
                                                                                         packet loss to adversely affect a conversation.
is a real-time service, interference and monitoring the return band, speed is
interruptions are harder to repair or hide, essential. “It is especially important to        The ICMP test uses any network device,
and even moderate levels of interference have a fast monitoring system because           while the RTP test communicates with a server-
can cause calls to break up or be dropped. fast transients and ‘bursty’ impulse          side application. The RTP test also calculates a
Aggressive FEC correction or resending noise have a more adverse affect on VoIP          mean opinion score (MOS). The MOS is a score
lost packets are not options for mask- services than they do on high speed data
                                                                                         based on the voice quality perceived by a random
ing interference problems because of the services,” Trilithic’s Product Manager
                                                                                         group of people. The highest rating is 5.0, but it
transmission delays they incur. Clearly, Steve Windle explains.
                                                                                         is unachievable since the codec always degrades
VoIP quality maintenance requires a more       That’s why the industry-leading speed
effective strategy.                         of Trilithic’s Guardian II system is espe-   quality by some amount. To achieve toll quality,

   Trilithic, a leading cable TV instrumen- cially popular with cable engineers who      an MOS of 4.0 is needed, and a rating of less than
tation provider for over 30 years, through are rolling out IP telephony services.        3.6 is considered unsatisfactory.
recent engineering investment has devel-       The return path should be monitored           The success of a VoIP endeavor depends
oped products using DSP technology to continuously for both consistent and
                                                                                         on detecting interference before it becomes
meet the changing needs of HFC networks. transient interference. Even though tran-
                                                                                         problematic. A cable operator must meet these
These products enable systems to achieve sient bursts are a major contributor to
                                                                                         challenges by employing a complete maintenance
and maintain the network health required poor audio quality, detecting them is a
to reliably carry VoIP services, by provid- difficult problem for traditional monitor-   program that includes monitoring, alignment and

ing an optimal means for vigilantly moni- ing systems that use frequency scanning        testing, and proper installation. All are offered as
toring the return band, properly aligning analysis technology. Transients can go         part of Trilithic’s complete, integrated solution,
and maintaining the plant, and quickly but unnoticed because the scanning analyzer       Guardian II system. ■
comprehensively testing installations.      must share its time between many nodes
                                            and wide spans of bandwidth. Noise can
Monitoring: Why Speed Is King               occur while the device is watching anoth-
It is essential to monitor the return er node or scanning a different frequency.
band in order to catch problems early          The use of DSP technology enables
and maintain the high reliability needed Trilithic’s Guardian II System to address                                                                                   02_05 ENGINEERING NEWS 3
                                           Figure 2: In a third-party test, the Guardian II system detected and accurately plotted every
                                           15 msec impulse, every time. Note that the display shows both the impulse at 24 MHz and its
                                           harmonic at 48 MHz.

                                         both consistent and transient problems.           Distribution alignment
                                         The hub of this system, the 9581 SST R4,          and ingress control
                                         captures the entire spectrum at once in the       While important, network monitoring is
                                         time domain, and using FFT (Fast Fourier          only part of ensuring quality VoIP service.
                                         Transform) converts the received informa-         The HFC distribution system must be
                                         tion to a frequency domain (spectrum)             carefully aligned and maintained, or some
                                         display. (Figure 1 illustrates a time domain      VoIP paths will suffer from a poor C/I
                                         snapshot and the resulting spectrum               ratio; others may cause laser clipping.
                                         based on the FFT conversion.)                         Trilithic’s Guardian II system is an
                                             The speed of this process enables             end-to-end solution, improving techni-
                                         this system to capture VoIP service               cian productivity through communi-
                                         degrading impulse noise or ingress                cation. Test signals and return band
                                         that traditional scanning analyzer-based          measurement information are exchanged
 The Guardian II system is an integrated systems don’t see. Independent field              between the 9581 SST R4 in the headend
      system of hardware and software tests have proven that the DSP-based                 and a field analyzer, the 860 DSPi. The
    elements that perform all the return Guardian II is over 10 times more likely          two devices work in tandem to display
 path maintenance applications needed to detect interference bursts than any               return band balance and gain for testing
         to verify VoIP service viability. competing system. (See Figure 2.)               and aligning the return distribution plant.
                                             In addition, transients can be “masked”       The instruments work together, so the
                                         by legitimate signals. To detect interfer-        technician can work alone—no need for
                                         ence hidden within the return traffic itself,     a call to the NOC to find out what’s hap-
                                         the 9581 SST R4 offers TraffiControl              pening there.
                                         mode, which processes the captured                    Trilithic’s 860 DSPi offers superi-
                                         data, eliminates legitimate signals from          or alignment and troubleshooting fea-
                                         the displayed spectrum, and shows only            tures including return alignment, sweep
                                         intermodulation products, ingress and             response and spectrum analysis features.
                                         noise bursts. The result is that operators        Using its return sweep mode, the 860
                                         can see transients that burst in at the same      DSPi helps your techs achieve proper lev-
                                         frequencies as legitimate signals.                els at the headend by optimizing reverse

4 ENGINEERING NEWS 02_05                                                                              
                                                                                 By periodically accessing Trilithic’s free upgrade
                                                                                 Web site, the operator can keep his 860 DSPi
                                                                                 ready for new challenges, and always up to the
                                                                                 standard of currently shipping anaylzers.
                                                                                 The VoIP Test function is an example.

amplifier alignment and assists the tech-   with the ingress present at the point to        information about what parameter(s) are
nician in finding and fixing frequency      which the 860 DSPi is currently connect-        out of line as an aid in troubleshooting.
response issues.                            ed. This comparison of local vs. headend           The automated process or macro
   The unique I-Stop probe is particu-      spectrum aids technicians who are               should be configured to perform a battery
larly useful for ingress troubleshooting,   troubleshooting for ingress problems.           of tests to prove the quality of the install.
and helps technicians isolate the source                                                    1. It is important to verify that down-
of ingress. Working with the 9581 SST       Installation                                       stream analog and digital signal levels
in the headend, and using an I-Stop         To ensure reliability, a thorough test must        are within design limits.
Reverse Test Probe with the 860 DSPi        be done at installation. This ensures           2.Downstream carrier-to-noise, MER,
spectrum analyzer feature, a technician     headroom on critical parameters and that           and BER should be tested to verify
can track reverse ingress sources to the    the service is not on the edge of failing.         proper ratios with margin to ensure
nearest tap. Using this simple test, you    Technicians are more likely to perform a           reliability.
can locate ingress sources down to the      quick, simple test under time pressure, so      3.Upstream levels and carrier-to-noise/
tap without removing reverse modules or     the process should be automated as much            ingress ratios should be tested to
diplexers and without disrupting forward    as possible and provide a pass/fail result.        verify proper receive levels and head-
or reverse service.                         The 860 DSPi can perform automated                 room. For trouble-free installations,
   The 860 DSPi also has a comparison       tests, and in addition to storing the data         the 860 DSPi offers an enhanced form
screen allowing the tech to compare         for baseline records, provides a pass/fail         of Trilithic’s RSVP2 Installer’s Reverse
spectra downloaded from the 9581 SST        result, and in the event of failure, provides      Tester. This feature determines the                                                                                         02_05 ENGINEERING NEWS 5
       AT&T LABORATORIES MIDDLETOWN, NJ         Figure 3: Packet Loss vs. MOS

    required upstream transmit power           ally, it tests the entire HFC network from      But if VoIP services are already in
    from the customer premises and veri-       the subscriber to the PSTN handoff point     place, why can’t the installer simply con-
    fies alignment and proper installation     and provides a simple expedient test that    nect the EMTA and make a phone call to
    at eight user-settable frequencies,        quickly verifies operational headroom for    verify the installation? While this would
    from the subscriber to the headend.        optimal technician productivity.             tell the installer that the phone works
    The 860 DSPi checks levels and car-           Cable engineers are using the Trilith-    now, it wouldn’t ensure that the con-
    rier-to-ingress/noise at the eight fre-    ic VoIP test to prequalify their networks    nection is reliable. To ensure continued
    quencies in seconds and provides a         in large part because it doesn’t require     serviceability the installer must verify that
    pass/fail indication for each.             them to have VoIP service equipment in       there is headroom on critical parameters.
 4.Two-way data communication should           the headend. The communication takes         These parameters are tested in the MOS
    be verified by performing modem            place with a VoIP test server, either on     (RTP) test.
    tests including ping and throughput.       the cable system network or at Trilithic        Trilithic recommends performing this
 5.VoIP tests should be performed to           headquarters. The test can be confined       test first to verify the viability of the net-
    ensure that the network is viable for      to the cable network and includes the        work for the subscriber’s VoIP service.
    the service and has operating margin       entire affected network by installing the    The test verifies packet loss, latency
    to ensure reliability.                     Trilithic VoIP test server in the NOC near   and jitter and provides a score based
                                               the PSTN handoff point.                      on these parameters that relates to the
    In addition to the typical HSD instal-        The Trilithic VoIP test sets up a call    quality of the call—MOS (mean opinion
 lation tests, specific VoIP tests ensure IP   using proprietary signaling with the test    score). In addition, it provides detailed
 telephony success. This is where the 860      server. The communication for the test       information on each of the parameters
 DSPi VoIP test shines.                        takes place using the RTP protocol com-      to enable further troubleshooting to
    That’s because it allows you to            mon to all PacketCable VoIP services.        determine the root cause in the event of
 prequalify your HFC network for VoIP          This means that the quality of the part of   failure. (Figure 3 shows how changes in
 without requiring you to have VoIP equip-     the system over which the technician has     packet loss affect MOS.)
 ment installed in the headend. Addition-      control is verified for service readiness.      By performing this test, you remove

 6 ENGINEERING NEWS 02_05                                                                             
The Versatile 860 DSPi provides a full complement of test features for field maintenance and HSD and VoIP service installation.

the network infrastructure as the potential        ogy to the PSTN handoff point. This        in the NOC that would be common to all
problem, leaving only the server system            configuration confines the test to the     subscribers. Testing the ability to make a
or the EMTA. If there is a server system           cable network and makes it as com-         phone call with a specialized test analyzer
failure, it is very likely that it will be         prehensive as possible. Acceptable         doesn’t prove that a call can be made
uncovered first by someone in the NOC.             measurement parameters will have           with the subscriber’s phone.
   This leaves the EMTA as the source of           been programmed in the test setup.
the failure. A small percentage of EMTAs           The test results will show either a        Conclusion:
are DOA failures, but having proved that           “pass” or ‘fail” for both upstream and     Fast, Visible and Versatile
the network is viable by performing the            downstream communication.                  Service quality is critical to IP telephony
VoIP analysis, and knowing that NOC per-        2.Connect the EMTA and make a phone           success, and proper testing and align-
sonnel vigilantly monitor the servers, the         call, within the network and outside       ment are essential for optimal VoIP
installer can fairly confidently point to the      the network. A successful call veri-       service quality. Trilithic’s Guardian II
EMTA as the source of failure if a call can-       fies that the entire system is working     system monitors the return band with
not be made. You can easily correct this           properly and reassures the subscriber      DSP technology, which enables capture
problem by replacing the EMTA. Alterna-            that the service is up.                    of fast transient impulse noise and
tively, a typo made when provisioning the                                                     ingress, as well as visibility of interfer-
EMTA could even be the cause.                      A subscriber phone goes through            ing signals beneath service carriers
   With that point in mind, when VoIP           an MTA registration process, so using         using a unique TraffiControl feature.
serviceability is being added, Trilithic        an MTA built into an analyzer doesn’t         Comprehensive field testing, including
recommends a simple, two-step VoIP              provide much information. The prob-           interaction with the headend monitoring
installation process:                           lems indicated are either a bad phone,        instrumentation, ensures proper align-
1. Connect the VoIP analyzer to the             a cable modem registration problem            ment and helps to quickly troubleshoot
   cable outlet in the home. Access a           (which would be uncovered with the            ingress problems. Finally, a simple,
   VoIP test server in the NOC, as close        Trilithic VoIP or other installation tests)   quick and comprehensive test during
   as possible in the network topol-            or a problem with the VoIP equipment          installation verifies reliable service. ■                                                                                       02_05 ENGINEERING NEWS 7
For more information
contact your local Trilithic representative,

email us at,

or call 1-800-344-2412

from anywhere in North America.

Trilithic, Inc.

9710 Park Davis Drive

Indianapolis, IN 46235


Toll Free: 800-344-2412

Phone: 317-895-3600

Fax: 317-895-3613

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