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New Testing Approaches for Mobile Phone Quality
Understanding voice quality issues

Unlike wireline service, mobile phones rely on often precarious over-the-air radio transmissions to carry time-sensitive voice traffic.

It's been more than five years since Verizon first aired its popular "Do you hear me now?" marketing campaign, yet a majority of consumers still wonder why they don't get the same level of call quality portrayed in the commercials.

Many tend to blame the poor sound quality on the phone itself. But, it takes a lot more than a high-quality handset to make a high-quality phone call. Most phone manufacturers don't focus their testing on how handsets perform over particular networks, and some wireless carriers don't do comprehensive testing on how specific handsets' perform in their networks. Therein lies the problem.

Unlike wireline service, mobile phones rely on often precarious over-the-air radio transmissions to carry time-sensitive voice traffic. Add to this the transforming of voice calls into IP-routed traffic; microphones and speakers disconnected from the device via Bluetooth; high-bandwidth competition from e-mail, data, and video downloads - plus multiple hand-offs between network elements, like routers, switches, and gateways - and there's a Pandora's box of traps in the transaction where voice quality can degrade.

At the same time, new converged services are being added to the mix. Wireless carriers are creating 4-in-1 deals that put your mobile phone, home phone, broadband, and digital cable services in one package. T-Mobile offers its @Home service on top of its UMA-based wireless service. AT&T offers its full-service U-Verse quad-play service. With its over-the-top VoiceWing broadband phone service, along with wireless service, FiOS and DSL, Verizon Wireless can create packages for any user. New, similar converged services will continue to tax the bandwidth that network operators currently offer, and the introduction of WiMAX as DSL replacement adds another wrinkle to the mix.

Most wireless carriers continue to follow Verizon's model in promoting quality based on the percent of dropped calls, network reliability, and the speed of their wireless cloud. But what really concerns them is providing the highest quality of experience (QoE) to a customer, no matter how that customer is accessing the network. While most operators have systems in place for measuring things like a handset's RF signal strength, it's not enough information to identify a host of other potentially damaging quality issues.

Much work is already being done behind the scenes to migrate infrastructure to higher bandwidth and higher speed connections to the base stations. Carriers are working hard to create and enforce service level agreements (SLAs) between network providers and network peers in order to measure the customer experience across the network. The goal is to proactively identify performance degradations before they impact users, satisfy contractual SLAs, and cost-effectively troubleshoot user-impacting problems when they do occur.

To this end, network operators who take quality seriously are rolling out converged service assurance solutions for voice that address the user's QoE two ways: passive (live) monitoring of traffic and active (on-demand) testing.

Passive monitoring assesses voice traffic across the network to discover quality trends before the problems become widespread. Active testing injects small amounts of synthetic voice traffic between network segments to isolate disruptions before they affect the user's perceived voice quality.

In both testing approaches, the mobile handset is key to understanding voice quality issues. One passive monitoring approach is to deploy software agents on handsets to measure quality using measurement standards like a Mean Opinion Score (MOS), R-factor, echo, and jitter. But, agent technology requires handset manufacturers to install the operator's software on every device they ship. This may be good for the operator, but it can be a pain for the manufacturer and thus a contract stumbling block.

A simpler passive alternative is for the handset manufacturer to adopt standard protocols, such as Internet Engineering Task Force (IETF) RFC 3611, RTP Control Protocol Extended Reports (RTCP-XR), which provides a way to measure and manage voice performance by reporting detailed RTP and VoIP metrics.

Operators can combine RTCP-XR with the "Johnston Draft," also known as the SIP Service Quality Reporting Event, a draft standard specification from the IETF. With these standards, operators can define a method for retrieving voice quality information at the end of every user's call. The typical approach is to have a SIP message send quality metrics at the conclusion of a call to a service assurance system to correlate information across multiple points in the network and generate targeted reports on voice quality. While few operators are using SIP all the way to handsets today, they definitely will as they migrate toward IP Multimedia Subsystem- (IMS-) based networks.

Adding another wrinkle to the wireless environment mix is the emerging picocell/femtocell market. Femtocells are wireless communication devices that cover a small area, such as the home, through which mobile phone users can access the operator's network wirelessly through their home broadband connection rather than over-the-air to the cell tower. It's similar to a WiFi access point, but for mobile handsets, and operators such as Vodafone, Sprint, Verizon Wireless, and T-Mobile have already announced and are trialing offerings.

Femtocells will become another point in the network where operators will need to know about quality. Using the same passive agent or protocol-based approaches will enable operators to extend quality testing to another point in the network where their customers may experience voice quality issues.

The other approach is active (on-demand) testing. Here, standards are key enabling technologies. With active testing, an operator triggers a transaction to the user's handset or - femtocell completely undetectable by the user - to measure quality and troubleshoot issues. Standards used by the operator and handset manufacturer include RTCP-XR and the IETF drafts for SIP/RTP Media Loopback and the Two-Way Active Measurement (TWAMP) protocol.

Typically, the operator deploys a service assurance appliance in the middle of its network. This appliance initiates a call to the customer's device, with protocols notifying the network device to act as a reflector with the payload looping back to the appliance so that it can be measured. The network device can insert timestamps and measure quality as well so that one-way characteristics of the testing are available to the network operations personnel.

The active testing approach enables operators to test devices across the network 24/7, and also allows them to detect degradations in the network - not just outages - so that it's easier to identify potential problems before customers detect them and either call for assistance or return their equipment to the store.

Active testing combined with passive monitoring provides carriers with critical visibility to service performance across their networks, resulting in reduced truck rolls and tighter SLA enforcement, and gives consumers higher quality calls every time they dial. With better service assurance coverage, someday soon the wireless carriers' various claims of having the "highest quality network" will sound more like fact than hype to the average user.

About Charlie Baker
Charlie Baker is director, product management at Chelmsford, Mass.-based EXFO Service Assurance (formerly Brix Networks). He holds a BS in management from the United States Air Force Academy, and an MBA from Boston University.

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