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February 05, 2008

Maintaining Quality Voice Service with VoIP Testing Solutions

By Anshu Shrivastava, TMCnet Contributor


Cable MSOs are able to maintain high-quality phone service by using a test approach that reduces return visits and ensures customers’ satisfaction with their new phone service, according to a recent Tektronix (News - Alert) white paper, Maintaining Wireline Grade VoIP QoE in Cable Networks.
 
Cable operators face many challenges when deploying PSTN-quality VoIP, and it is also not easy to monitor and maintain VoIP service quality. However, in spite of all the challenges, MSOs are successfully making the grade by using Tektronix VoIP service quality test strategies that assure their success.
 
According to the white paper, VoIP problems in cable networks usually in one of the five key areas: at the subscriber premises, in the Hybrid Fiber Coax (HCF) or RF access network, in the IP or Fiber backhaul network, in the voice operations center where call features (such switching, voicemail, and caller ID) are hosted, or at the Media Gateway (News - Alert) to the PSTN.
 
In addition to the problems within the operator’s network, customers also experience quality issues originating off-net as calls get routed through VoIP peering and partner carrier networks to far end destinations.
 
The white paper suggests that thorough Tektronix’s Day-of-Install testing can ensure VoIP meets customer expectations when deployed. Also, a complete end-to-end service quality assessment helps to identify and resolve issues before subscribers begin using the service.
 
There are no guarantees that services will remain trouble-free for prolonged periods, and service quality can degrade for a variety of reasons including congestion caused by increasing voice, HDTV, Internet and VoD traffic, and network upgrades.
 
Additionally, service may degrade because of DoS, security or spam attacks, network reliability issues, and other events that may stress networks. The white paper states that response time is a critical factor, which influences customer perception and service retention.
 
The Tektronix white paper notes that, to fully assess and confirm the problem with an end-to-end test that replicates the customer perception, operators should perform remote loop-back testing to the subscriber’s residence.
 
DOCSIS 2.0 (and later)-compliant MTAs provide both analog and IP/ RTP loop-back functionality that allow test systems to measure more than 50 different IP and analog-based speech or service quality measurements directly from the NOC (News - Alert).
 
Tektronix explained in its white paper that some test systems can initiate loop-back tests from the PSTN-side of the media gateway, usually from a T1 interface, to include all the end-to-end elements phone calls normally encounter. Once the network is segmented and the problem is isolated, traditional domain-specific tools are used to complete problem diagnosis and service restoration—including data from return-path monitoring systems, CDRs from softswitches, and results from handheld test tools used by dispatched technicians, according to the white paper.
 
Additionally, the test systems can also perform IP-layer Real-time Transport Protocol (RTP) loop-back tests to cable transponders located throughout the HFC. Loop-back testing is supported by transponders compliant with DOCSIS 2.0 MTA loop-back specifications or the SMRP remote testing protocol.
 
RTP loop-back tests are capable of assessing speech quality—such as MOS, R-factor, packet loss, jitter—DTMF transparency, delay, and network media streaming performance. In addition, RTP loop-back tests can also be used to assess the network’s ability to deliver digital video including HDTV and VoD streams.
 
By integrating a service level test automation platform with the transponder EMS or topology database, the test system can perform loop-back tests from the Cable Modem Termination System (CMTS) or the Network Operations Center (NOC) to all transponders serving a subscriber in the HFC. Tektronix paper suggests that this technique can determine if a problem originates from the HFC, and its exact location.
 
Providers can evaluate the Fiber or IP network by performing Tektronix VoIP service quality tests from the NOC to CMTS hubs using probe-to-probe testing, where test calls are placed from one probe to another, providing complete control over test stream content and quality measurements in both directions.
 
Probe-to-probe testing offers a wider range of measurements than loop-back testing, which include upstream or downstream evaluation of VoIP, PSTN, video, Internet, and fax/modem service quality.
 
Tektronix noted in its white paper that that, in addition to monitoring access or backhaul networks, probe-to-probe testing allows operations to isolate problems with edge routers and Media Gateways.  
 
The HFC networks are usually subject to RF performance issues caused by faulty transponder performance, aging wiring or damaged fibers, failing amplifiers or optical nodes, corroded cable connections and poor optical splices.
 
The paper notes that when RF signal levels shift, a number of subscribers often experience service quality issues or disruptions, some of which may only affect services in the upstream, the return-path direction from subscriber to CMTS.
 
To ensure that PSTN partner-carriers and VoIP-peering networks are providing service levels defined by service level agreements (SLAs), cable operators can use single-ended (ping-style) testing methods that can test off-net, partner-network VoIP performance to off-net destinations worldwide, according to the white paper.
 
Single-ended testing measures voice, signaling, fax, Internet and modem service quality without far-end test equipment. For single-ended testing , a test probe in the NOC places test-calls off-net to public interactive voice response (IVR) systems, fax machines, modems, video conferencing terminals or IP servers stored in a global test destination database.
 
The calls are recorded and analyzed against the original reference file by the test probe using a series of algorithms that provide speech quality (MOS) scores, connectivity stats, fax transmission rates, IP server availability and performance, and other quality metrics.
By testing to different area codes, cities, and countries, partner networks can be tested against SLA parameters.
 
According to the Tektronix white paper, this method is also used by operators to help them select and validate partner carriers before putting them into service.
 
Also, real- time service quality monitoring lets operators trend quality metrics to detect service degradation, usually before subscribers can perceive them. This level of information is used to schedule preventive maintenance, network management and upgrades to ensure VoIP maintains PSTN-quality.
 
Paper notes that service quality monitoring is performed by scheduling automated VoIP service quality tests to critical locations in the Cable VoIP network using a test-automation operational support system (OSS).
 
By performing regular testing of IP/Fiber, media gateway, and voice operations network domains, operators can determine how service quality varies by location, network segment, and time of day.
 
In addition to preventive measures, by setting service quality thresholds for key VoIP quality metrics—such as MOS, echo, latency, jitter, post-dial delay (PDD), and call completion ratio (CCR)—reports can indicate which aspects of VoIP service are degrading, and how quickly.
 
The white paper concluded that operators extend the value of their test system by using a centralized test management platform that automates remote testing for continuous service quality monitoring.
 
Also, by integrating VoIP test results into existing OSS and fault-management systems, operators can use service quality measurements to drive network planning, preventative maintenance, marketing and business-level decisions, which helps them to efficiently provide PSTN-quality VoIP over large-scale cable networks.
 
Anshu Shrivastava is a contributing editor for TMCnet. To read more of Anshu’s articles, please visit her columnist page.



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