untangling a web: the internet gets a new look

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components, Dravid expects his systemto be useful in biological research.Unlike other scanning-probe micro-scopes, the acoustically enhanced systemdoesn’t require the tip to come in con-tact with the surface. Therefore, it can beused on “soft” structures, such as cells,says Dravid. —A. CUNNINGHAM

Untangling a Web The Internet gets a new look

The Internet may be everywhere andnowhere, but that’s not stopping informa-tion engineers from mapping it. An atlasthat accurately shows the physical path ofinformation from one computer to anothercould protect the Internet from massivefailures after, say, an earthquake or a ter-rorist attack. The latest finding by Internetcartographers offers some good news:Routing computers—the hardware thatdirects bits of information around theworld—are linked in a way that makes theInternet less susceptible to a centralizedattack than past studies had indicated.

In an upcoming issue of the Proceedingsof the National Academy of Sciences, JohnDoyle of the California Institute of Tech-nology in Pasadena and his colleagues offera new mathematical model of the Internet.The team used a class of models known asHOT, for highly optimized tolerance.

This approach differs from the so-calledscalefree models that many Internet mod-elers have favored (SN: 1/29/05, p. 68).Extrapolating from small local networks,the scalefree model indicates that a fewwell-connected master routers direct Inter-net traffic to numerous, less essentialrouters in the network’s periphery. Themodel predicts that over time, the busy cen-tral routers acquire ever more links in arich-get-richer effect. From a security per-spective, a targeted attack on a centralrouter could halt virtually all data flow insuch an Internet.

Doyle and his team present a HOT modelin which the Internet has no vulnerable cen-tralized hubs and any highly connectedrouters lie at the periphery. If one of thesewell-connected, outlying routers were takenout, Internet traffic would simply divert toanother well-connected router.

That approach incorporates current tech-nological and economic constraints on theInternet. For instance, routers are limitedin the amount of information that they candirect at any given instant. Additionally,Internet-service providers that own majorrouters can allocate only limited resourcesto their hardware. By optimizing these fac-tors in their model, Doyle and his cowork-ers aim to present a picture thatmatches today’s Internet.

The team has tested itsmodel with the knownmap of Internet2—anacademic subnet-work within thelarger Internet. Theresearchers reportthat their proposedmodel correspondswell to the structureof Internet2, whichDoyle says is a “good rep-resentation” of the struc-ture of the entire Internet.

Identifying the strengthsand weaknesses of differentInternet architectures isimportant, says Fan Chungof the University of Califor-nia, San Diego. She notesthat HOT models can alsobe used to analyze other complex net-works, such as the immune system andprotein interactions.

Although the researchers’ model revealsthat the Internet ought to efficiently adapteven if the busiest routers were removed,Doyle notes that the system is still vul-nerable to hijacking via malicious softwarethat leaves the hardware intact. Even inthat case, however, a good map could helpInternet custodians track harmful soft-ware and ultimately protect the Internetagainst it. —K. GREENE

Fertility and Pollution Dirty air, ozone linkedto sperm troubles

Men might improve their fertility by reduc-ing how much pollution they breathe in.The dirtier the air, the lower a man’s spermcount and the more sperm with fragmentedDNA he produces, two new studies suggest.

However, neither report directly links thedecline in sperm quality to fertility problems.

“The decrease is not enormous,” com-ments environmental chemist BrianMcCarry of McMaster University in Hamil-ton, Ontario, who was not involved in eitherstudy. “There’s no evidence that it has animpact on fertility.”

In one study, ozone appeared to be a cul-prit behind diminished sperm counts, sug-gesting that it’s a “sperm toxicant,” sayRebecca Z. Sokol of the University ofSouthern California in Los Angeles andher colleagues. They had looked for a cor-relation between the quality of semen from48 local sperm donors and air-quality datafor the zip code in which each donor lived.

The donors were healthy men whohad given 10 or more donations

to a sperm bank over atleast a year.

Sperm counts werelower when ozone con-centrations where themen lived had beenhigh during the pre-vious 90 days, Sokoland her team report in

an upcoming Environ-mental Health Perspec-

tives. Sperm take nearlythat long to develop. The

researchers took intoaccount the effectsthat temperatureand season have onmen’s sperm counts.Airborne particulatematter, nitrogenoxides, and carbon

monoxide weren’t associated with reducedsperm concentrations, the team says.

In the second study, Jiri Rubes and twoof his colleagues at the Veterinary ResearchInstitute in Brno, Czech Republic, workedwith U.S. scientists. They examined up toseven semen samples from each of 36 menliving in a polluted region of the CzechRepublic.

Each September for 3 consecutive years,the researchers collected a sample from mostof the men. The team took as many as fourmore samples from each man during the twowinters of the study. Wintertime pollutantconcentrations in the region can be doubleto quadruple those measured in September.

In most winter-air samples, a cubic metercontained 60 to 80 micrograms each of par-ticulate matter, nitrogen oxides, and sulfurdioxide and about 150 nanograms of poly-cyclic aromatic hydrocarbons, exceedingcommon regulatory limits. Semen sampleshad more fragmented DNA at those timesthan they did in September, the team reportsin the October Human Reproduction.

“This is certainly an important finding,”says Ashok Agarwal of the Cleveland Clinic.DNA damage to sperm has been linked tolow pregnancy rates, although the damagefound in the Czech study may not have beenenough to impair fertility, he says.

Despite the heavy pollution, theresearchers found no differences in spermcounts or several other measures of spermquality. But, McCarry notes, “they didn’tmeasure the ozone.” —B. HARDER

SCIENCENEWSThis Week

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NET WORKS The Internet doesn’thave centralized master routers,according to a new analysis. Itsdistributed structure, as depictedhere, protects it against a widelydebilitating failure.

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