Doubling up

Cable modems add two-way testing challenges for cable operators

A story that has been making the rounds of the cable TV industry demonstrates the perils of cable modem testing and installation.

During a routine residential installation, a technician kept having problems with interference. He tried different tests, then attempted to adjust the settings on the PC browser software, but nothing would get the modem to work properly. Substituting a second modem didn't change things.

Back at the headend, no one could find a physical problem on the cable or the drop. Because it looked like the installation was going to take some time, the technician went out to his truck and shut the engine off. Upon returning indoors, he found the modem working perfectly. Putting two and two together, he realized he had left his truck idling beside the pole where the cable feed split off to the house. The electromagnetism generated by the truck engine created the interference on the line.

There's no way of knowing whether this tale is apocryphal or not. Those who tell it don't know or won't name the cable company for which the technician worked or even where in the U.S. the alleged incident occurred. But it does reveal a weary awareness of the simple things that can affect service provisioning and maintenance.

Discounting the ability of most cable systems to handle a pay-per-view order from the set-top box, the introduction of cable modems requires the CATV network to become a true two-way street. The modems, along with the gradual deployment of hybrid fiber/coax (HFC) systems, have cable companies re-tooling their networks to handle large amounts of upstream traffic from Internet surfers at home.

And as it does in DSL (see story on page 32), testing plays an important role in service rollout and maintenance as TV, Internet and voice services hop onto cable's shoulders.

A two-way street

The gradual revamping of CATV plant from end-to-end coaxial cable to HFC platforms has done much for the rollout of cable modems. New buildouts preclude the need to retrofit aging plant. That's not to say that, where necessary, cable companies aren't modifying older networks. The only question is how it affects the pace of conversion.

Some new companies, such as 21st Century Communications, have the benefit of building fresh. Others such as AT&T Broadband & Internet Services (formerly Tele-Communications Inc.) have deferred to the deep pockets of an acquiring company to finance the HFC conversion of its 10 million customers nationwide. MediaOne Group (which also is in the process of being acquired by AT&T) is doing its share of new buildouts, but its goal is to deploy as many cable modems as possible, even where the networks are all coax, says Tim O'Keefe, director of broadband data services at MediaOne.

Either way, the U.S. cable plant is in the process of rapid modernization, spurred again by demand for broadband access. On the premises equipment side, CableLabs, which is aiming to ensure interoperability between headend equipment and as many cable modems as possible, has been moving forward with its data over cable service interface specifications (DOCSIS). As of last month, 11 companies had cable modems certified as interoperable (see textbox on page 60). With that initiative, CableLabs is hoping to spur a retail market and create more choices for cable companies.

For carriers such as AT&T BIS and MediaOne, DOCSIS serves as a starting point for cable modem selection. Carriers then do more testing in their own labs, followed by a limited rollout to "friendly" customers, usually their own employees. Lastly, there's the service rollout itself.

The Ethernet model

Like DSL, broadband delivery involves assigning different frequency channels within the cable to different services. The 100 or so TV channels take up one group of frequencies; Internet access, with a portion of bandwidth reserved for upstream traffic, takes another; voice telephony, still another.

Unlike DSL, the overall link is much more akin to that of a LAN. For instance, cable modems usually connect to a 10/100BaseT Ethernet card in the customer's PC. If two or more PCs are linked to a single modem, the connection is made through a small Ethernet hub. Back at the headend, data is peeled off the signal at a cable modem termination system and routed to a proxy server (Figure 1). From there, it goes to a back-end server or router linked directly to the Internet backbone or a portal such as Excite@Home.

Because the plant is built for data, testing issues do not concentrate so much on the adequacy of the physical connection, as in DSL's case, but instead focuses on making sure service is functional and reliable. The most difficult part of the process is certifying the plant for two-way transmission.

AT&T BIS, for example, is offering service from an HFC platform, which automatically makes the service more robust, says Luisa Murcia, vice president of technology operations for the BIS Data Services Group at AT&T. The main task is conditioning the lines for two-way transmission, which means upgrading lasers, amplifiers, power supplies and other passive components. Line conditioning for cable telephony is usually done at the same time, she says.

"Construction is a comprehensive process," Murcia says. "There are a lot of factors involved."

"The hardest part is converting one-way to two-way," says MediaOne's O'Keefe. In addition to the infrastructure work, MediaOne had to upgrade its traffic measurement devices so upstream signals can be monitored.

For starters, two-way telecommunications is relatively new to cable companies. In addition, cable modem technology is new itself. Only now is gradual experience creating an effective empirical database of information that would tell a technician "that this amount of data would cause this much loss," says Peter Shaw, marketing manager for the Service Test Division at Agilent Technologies, the test and measurement spinoff from Hewlett-Packard.

AT&T BIS engineers test the two-way performance as the plant is being converted so that any faults are captured early. Once the conversion in a given area is complete, AT&T BIS begins an entire pre-launch process before two-way certification is granted. This involves the participation of the @Home network, the Internet access provider and content aggregator that is working with a large number of cable companies nationwide.

This system readiness test is done from AT&T BIS' network operations centers and tests the readiness of installations at headend termination equipment and its connections with routers, switches, servers and caching systems, Murcia says. The test can involve any number of headends and extend from AT&T BIS' network into @Home's, she adds.

This is only the first phase. After the network is certified, modem deployment can begin.

Recent months have seen the rollout of DOCSIS modems, and MediaOne is the biggest consumer, O'Keefe says.

Likewise, AT&T BIS now uses only DOCSIS-qualified modems, which have been further screened by @Home, Murcia says. After that, the modems are subject to an intense round of lab testing by BIS itself.

"The vendor must pass all the tests or they can't be deployed," Murcia says.

A series of certifications

AT&T's testing does not duplicate either CableLabs' or @Home's work. In fact, each serves its own purpose.

The DOCSIS qualification certifies that the modem will interface with any type of headend equipment. It is fairly confined to physical layer compliance, Murcia says, although the process does incorporate some element management aspects that the cable companies sought. "CableLabs tells us that [the modem] complies, and it's compatible."

The @Home testing looks at a higher layer - how the modem works with the trunk network and what the management aspects are on a broader level, Murcia continues.

After that, AT&T examines "how the box will perform under field conditions," she says. Its lab attempts to replicate the environment and the problems cable modems will find during their lifetime in service. "CableLabs tests component-by-component," Murcia says. "We test on a performance level."

Tests include checking for levels of RF interference, which can come from household appliances such as vacuum cleaners or, as noted, from automobile engines. The carrier then conducts load and stress tests. "We will put as many modems as possible on a [cable modem termination system] and see what happens," Murcia says. AT&T's labs can simulate power failures, cable cuts and variable levels of noise. Test and measurement equipment used include spectrum analyzers, noise generators and call generators. The process, she says, "gives us an idea of how stable the whole platform is."

Thus far, according to Murcia, AT&T BIS has certified cable modems from 3Com, Thomson Consumer Electronics and General Instrument.

MediaOne, with 5 million subscribers in 17 states, has its own labs, and its testing process is similar to AT&T's. "The biggest concern with the DOCSIS modems is how they will run in our operations environment," O'Keefe says. In addition, while CableLabs may certify each manufacturer's modem as DOCSIS-compliant, underlying performance issues vary from vendor to vendor, he says. So like AT&T, MediaOne takes a hard look at each modem before giving a thumbs-up for deployment.

The cable company maintains a facility in Westminster, Colo., where it has replicated an HFC distribution hub, which can simulate a full suite of television, data and telephony services. "We're even running some legacy cable media on that hub," O'Keefe says. Using the captive network, lab technicians throttle data speeds up and down, they test to see if the modems receive filtering commands properly and if they process the dynamic host configuration protocol, which is used for IP address assignment, in a way that is compatible with MediaOne's servers. They make sure signaling traffic in the data channels does not "leak" and cause interference on the frequencies used for TV or telephony.

MediaOne also tests for congestion control. At times of heavy use, network administrators may want to issue a command to all modems to limit accepting downstream transmission to, say, 1.5 Mb/s, O'Keefe says. The company wants to be sure the modems will acknowledge the command. Spectrum analyzers, packet sniffers and load generators are among the test equipment technicians use to evaluate the gear. Most test equipment is off-the-shelf, although MediaOne developed an in-house software tool to make it easier for technicians to generate requests to the modem and register the answer they get back, O'Keefe says.

Thus far, modems from Samsung Information Systems of America and 3Com have made the grade.

Field testing

After lab testing is complete, field tests begin. At AT&T, these last 30 to 45 days, and "alpha testers" tend to be users employed by or associated with BIS. "This is really where we get to see what happens in a normal environment," Murcia says. "Software problems become evident here."

Should a software problem be detected, BIS reports it to the manufacturer and waits for a fix. After that, the trial clock starts again. "We won't launch a city commercially until the technology works," Murcia says.

MediaOne watches that two-way aspect of the modem service very carefully, O'Keefe says. Monitoring equipment tends to be an extension of standard cable signal measurement gear. "You don't monitor [individual] channels with specific gear, but you do run a monitoring system that's up and running," he says.

In a way, because the DOCSIS modems are standardized, they have made testing and deployment a bit easier. At the same time, because they are a new generation of equipment, they bring some new challenges. "As hard as anyone tries, nothing comes out right the first time," O'Keefe says. "When a service is brand new, it takes a lot more testing."

In testing, the services and testing curve slopes downward with time, O'Keefe says. As existing technology matures, a new generation is released, and the process begins all over. For O'Keefe and Murcia, it'swhat keeps the test and measurement process interesting - that and the occasional idling truck.

As of Sept. 30, CableLabs had certified cable modems from 11 manufacturers as compliant with data over cable system interface specifications. Those companies include:

- Arris Interactive

- Askey Computer

- Cisco Systems

- General Instrument

- Philips Electronics

- Samsung Information Systems of America

- Sony

- Thomson Consumer Electronics

- Terayon Communications Systems

- Toshiba America Information Systems

- 3Com

In addition, CableLabs has certified Cisco's cable modem termination system for DOCSIS compliance.