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Testing VoIP infrastructure
Measuring the quality of a VoIP signal is easier said than done, says Rick Pearson, marketing manager of Agilent's computer and networking solutions group (Palo Alto, CA). Quality of service (QoS) is not inherent to packet-based networks, which were originally best effort. When Ethernet (over which many VoIP calls now run) began operating as a shared service, it followed a scheme called carrier-sent multiple access with collision detect. "You listened on the network, and if nobody was transmitting, you transmitted," notes Pearson. "If packets collided, then you stopped transmitting, waited until there was nobody else transmitting, and then you transmitted again."The combination of IP and Ethernet has made the technology more reliable. "It adds switching technology, so you won't have to contend with other packets, and optical technology, so you can get longer lengths of high-quality transmission," says Pearson. "Tags are added to the front of the Ethernet packet, and those tags allow people to prioritize. Now we can say, 'This information is more important than the following packet'." But that still leaves carriers with the same problem: all those packets or streams of packets are still contending for the same bandwidth. According to Pearson, carriers need to go beyond bandwidth tagging to more advanced schemes for providing guaranteed QoS. "How can you actually simulate the ability to always have high-quality streams of packets that you can put voice or video services over, while at the same time having the flexibility to statistically multiplex together the extra data associated with printers or file backups, things like that," he offers. "That's really the state-of-the-art."
At SuperComm last year, Agilent debuted its N2X device, which is a combination of several existing platforms, including the company's router tester, SAN tester, and OmniBER XM. The OmniBER XM is aimed at simulation or emulation of edge access network capabilities. "What's been done in the past for IP test has been to send a packet, count a packet, and look at packet loss and packet delay. Now, if you start to say, 'I'm going to have 50,000 people in a metro area on VoIP service, can I support 60,000?', how do you correlate that with packet delay and packet loss?" reasons Pearson. "We make sure that the infrastructure can handle the level of thousands of users." The N2X stresses the VoIP devices by sending thousands of streams of traffic, each of which can represent a connection between two sites.
"It's all about stress testing," agrees Ixia's Buerger. "How many calls can you make and at what quality? How many video streams can you support and at what quality?" Ixia's IX Chariot, acquired about 18 months ago from NetIQ, is the industry's most popular IP test software, claims Buerger. It fits into the company's existing chassis and enables the user to emulate hundreds of protocols across thousands of network endpoints, he says.
Though no-one has yet deployed IPv6 commercially, Ixia's test equipment also helps carriers make the transition from IPv4 networks to the more sophisticated IPv6. Dual-stack testing ensures that routers and other network equipment can handle IPv4 and IPv6 triple-play traffic with or without QoS policies.
Testing VoIP services
After the network has been deployed, carriers face challenges on the VoIP services testing side as well, contends Bill Dentinger, senior director of Spirent Communications' service assurance group (Rockville, MD). "How do they take this completely new technology and be as effective as they tend to be in the analogue world with a completely different set of tools?"
According to Dentinger, carriers are asking for three basic operations. First, they need to run an IP QoS test, which is roughly the equivalent of the line integrity test in the analogue world. A POTS technician tests for shorts, opens and grounds, but an IP technician tests for packet loss, delay, jitter, packets out of order and connectivity. "When you talk to the service providers [about VoIP testing], they ask, 'How can I prove I have connectivity?' Basically, they are talking about a line integrity test; is the connectivity there and is it of sufficient quality to actually run the service being offered," he explains.
Like their POTS counterparts, IP technicians also must be able to passively monitor the customer experience, says Dentinger, "and that means analysing the VoIP signaling and media streams for congestion, voice quality and QoS issues."
Finally, the carrier or technician needs to be able to experience the service in the same way the customer does. "In the IP world, the test set must be able to drop into the line and basically make a call with VoIP or answer a call," says Dentinger, "then measure the signalling performance, media stream and voice quality."
From an architecture level, IP service creates another set of challenges for both the carrier and test equipment vendor. The analogue POTS service looks the same throughout the US; it was either digital loop carrier or traditional copper, says Dentinger. "One of the challenges for the service provider rolling out VoIP service is that the architecture over which it can ride can vary greatly," he observes. "It could be that you're dropping off a T1 or T3 to an enterprise, and you're running VoIP over that. It could be that you're dropping off a Gigabit Ethernet connection or even a 10/100 to a small to medium-size business and you want to run VoIP over that. It could be that you have DSL running to a small or medium business, and you want to run VoIP over that. FTTP is another architecture approach."
The challenge for the service provider is not only finding a test set that provides the requisite functionality, but also finding a variety of test sets that can fit into these different architectures. While reliability, scalability and performance testing are all critical, at the end of the day the carriers just want to make sure their networks are futureproof. "In the future, everything but your toaster will have an IP address," quips Buerger.
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