It's not easy to satisfy a voracious appetite for bandwidth. But that's still the goal for designers of optical networks. Using cutting-edge technologies, such as dense and ultra-dense wavelength-division multiplexing, they're adding more and more capacity to optical fiber. At the same time, designers are increasing the data rates through that fiber. If the old saying is true, and "you can't build what you can't test," this should be a recipe for disaster. But apparently it's not.
The past few weeks and months have seen a flurry of test gear aimed at helping designers create the latest and greatest in optical-communications systems. The hottest areas of activity are communications analyzers, bit-error-rate testers, optical spectrum analyzers, and other types of equipment for testing state-of-the-art optical components, boards, and systems operating at 10 Gbits/s and beyond.
Some of the younger test-equipment companies are building a number of devices, including load modules, bit-error-rate testers, and protocol analyzers, to test fiber networks. Ixia Communications, for example, recently introduced its newest load module at the ComNet show.
"We use optics for testing packet over SONET and for testing Gigabit Ethernet over optical-fiber connectors," says Paul Mallinder of Ixia. "Our equipment uses the different physical-layer media, whether they be single mode or multimode. On top of that, we run different framing, either Gigabit Ethernet or packet over SONET."
The backbone of the Internet consists of routers that typically have OC-12, OC-48, and OC-192 interfaces connected to them. Those routers are responsible for switching IP packets. Ixia's packet over SONET (POS) testing is really testing the backbone infrastructure within the Internet. "We're testing to see what the devices under test do to things like latency. So when we transmit packets and receive them, we're measuring the latency that the DUT induces on that flow of traffic. Also, the jitter."
Two metrics are important in the test, according to Mallinder. One is the delay and the other is jitter, which is the variation of delay on a packet-by-packet basis. A third one is packet loss. As the device under test heads toward line speed, OC-48 or OC-12, what happens in terms of packet loss?
The company's number one customer is Cisco Systems, so it's testing the Cisco 12000 products. "We're not testing the fiber itself," says Mallinder. "We're not doing bit-error-rate tests on the fiber. What we're actually doing is testing above the physical layer—the link layer, network layer, and transport layer that's sitting above those fibers."
Operations like error injection are performed by Ixia's products. That operation introduces errors into the protocol that's running over the fiber. Different bit errors can be induced in the frame to see what happens to the device under test. "We do both destructive testing and proactive performance testing," states Mallinder. "For testing purposes, we generate traffic."
Typically, designers will benchmark the switches in a laboratory within a controlled test environment. They'll then deploy them into the Internet. Once the equipment is deployed, they use their equipment in a very controlled way to make sure they don't adversely affect normal Internet operation.
The product announced by the company at ComNet in January is called the IXIA 100 QoS Performance Tester. With this test tool, designers can understand what happens to quality of service over the Internet. It's not blasting the fiber lines to full frame rates. Instead, different streams use very small parts of that bandwidth and check for ways that the Internet can differentiate its services against each stream.
Designers are creating algorithms to prioritize traffic through a switch. After those are designed, they need to be tested. "What our equipment shows is the degree to which they work," says Mallinder. "Do they work as they should? It gives designers the ability to rectify their mistakes before moving to the manufacturing stage of the product life cycle."
The latest product handles OC-48, whereas earlier products targeted OC-12 and OC-3. Aside from the ability to handle the higher speed, it can test router functionality. One of the routing protocols in the backbone of the Internet is called BGP4 (Border Gateway protocol 4). "If you just have OC-48, it's great," states Mallinder. "We can fully blast the line and see what happens. But the whole key is that you need to be able to set up different routes within the Internet and then check, as you send traffic through those routes, as to when you send and when you receive. This verifies that the routes have been established correctly."
Fiber-optic designs do boast other hot areas, too—one being Fibre Channel systems. Companies like Finisar Corp., Ancot Corp., and FuturePlus Systems make Fibre Channel protocol analyzers and other kinds of equipment to test these systems.
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