For much of the past week, two large stretches of the Internet turned invisible to each other. Unlike the electrical outages plaguing California, which no one wants, this intentional blackout suited the purposes of one side in the collapse of talks between two major Internet service providers, Cable & Wireless and the financially strapped PSINet. A critical link between the two networks was cut, blocking some companies from seeing their own Web sites, and stalling e-mail between thousands of sources. Although a connection between Cable & Wireless and PSINet was re-established Tuesday night, the squabble illustrates just how fragile the Internet's series of connected, largely unregulated private networks can be. The Net has built its strength in part on this decentralized, unregulated environment, but the ISPs' fight underscores that very little can prevent future blackouts like this from happening. Some who closely watch the Net's daily health say such blackouts could even become more frequent, as cash-hungry telecommunications companies stop benefiting from deals with peers and rivals to swap expensive data traffic. "I think this is the beginning of a cascade failure," said Lloyd Taylor, vice president of operations at Keynote Systems, a company that monitors connection speeds across the Web. "I think you're going to see a lot more of this over the next few months." The blockage that stopped traffic flowing between two of the top ten networks in the United States for more than four days stemmed from a relationship called "peering," in which two networks agree to swap traffic back and forth without charge. In this case, Cable & Wireless has stopped peering with PSINet, saying the struggling company no longer had enough traffic to make the relationship worthwhile. Because of the complicated set of histories and relationships that make up the Internet, this had far-reaching ramifications. Cutting off this link prevented either network from seeing the other, though both could use different routes to get anywhere else on the Net. The problem might best be illustrated by comparing the Internet to a road map. In this case, the blockage would be similar to Los Angeles blocking all roads to San Diego. Anybody coming from any other destination could get to either city. But from the perspective of visitors from one of those two cities, the other city might as well have been wiped off the face of the map, so completely that even alternate routes won't work. From the individual customer's perspective -- the hapless traveler from Los Angeles to San Diego, to continue the example -- the outage was inexplicable. Robb Brynn, vice president of operations at Cfwebmasters.com, a small North Carolina ISP that buys its Net connectivity from Cable & Wireless, says he had no idea what was happening until he started getting complaints from his customers early this week. At least one of his corporate Web hosting accounts gets regular ISP service from PSINet, Brynn said. That meant that this company was unable to see its own Web page. Many more of his customers were simply unable to surf to Web sites that were hosted by PSINet, which still maintains a large hosting business despite its financial troubles. E-mail that had to go between the two networks backed up. "We only found out about it after calls came in for a couple of days," he said. Cable & Wireless gave him no advance notice and has said that it is up to PSINet to fix the problem. This kind of blackout would never happen in the old telephone world. Those networks are still tightly regulated, ensuring that calls will get through. But data networks are not regulated the same way, a fact that many insiders contend has helped spur the Net's fast growth. The unregulated status has left the Internet's "network of networks" to determine its own rules. As financial pressures loom over the companies that run these networks, they are finding less wiggle room available to choose the health of the Net over their own financial realities. In Cable & Wireless' case, this has meant tightening its policies on which companies it is willing to trade traffic with for free and which it asks to pay. PSINet has long been on the free side because the two networks were of roughly equal size. But as PSINet began losing customers in the wake of its financial difficulties, far less traffic began to flow to Cable & Wireless' network, and the company said the exchange no longer made financial sense. The two companies have been talking over the issue since February, a Cable & Wireless spokesman said. The shutdown therefore could have come as no surprise to the financially struggling company. Cable & Wireless has been pushing PSI to pay for exchanging traffic in a "transit" agreement that would have averted the blackouts. PSINet could sign a similar agreement with any large network company to achieve the same end, however. "Internet backbone companies invest billions of dollars to build and maintain their networks," said Cable & Wireless spokesman Chad Couser. "It wouldn't make any sense to enter into an agreement where all the financial benefits go to one side." The network connection between the two companies was restored late Tuesday, after PSINet sent a letter of intent indicating it would raise its traffic to levels that met Cable & Wireless' standards. But this is potentially only a temporary fix. A spokesman for Cable & Wireless said that if PSINet is not able to prove it has enough traffic, it will once again be ineligible for the free "peering" arrangement. But if this week's flare-up has been smoothed over, insiders say it served to shine an unusual light on just where the Net's frailties lie, and which, in cases like this, can serve to hold entire unsuspecting regions of the Web hostage. "Without this kind of cooperation that goes on the back end, the Internet can't function," said Jay Adelson, founder and chief technical officer of Equinix, a company that provides private, secure places where companies can link their networks together. "If this cooperation breaks down, parts of the Internet cease to be accessible." Have your say instantly, and see what others have said. Click on the TalkBack button and go to the Telecoms forum. Let the editors know what you think in the Mailroom. And read other letters.