Does heavy volume smooth Net traffic?

By Kimberly Patch, Technology Research News

It's difficult to figure out the best way to distribute traffic, whether you're talking about where to build a road to alleviate rush-hour congestion or what type of Internet connection is best. It is complicated enough that no one really knows precisely how data packets flow through the deepest innards of the Internet.

Researchers from Lucent Technologies' Bell Laboratories have shown that network traffic gets smoother when communications lines become fairly full. These counterintuitive results mean that bigger is not necessarily always better when it comes to network links.

An email message or Web page request is broken up into many packets of data before traveling through a network like the Internet. The packets are then reassembled when they arrive at the server hosting the e-mail address or Web page.

An Internet connection with a light to medium amount of packet traffic is bursty, meaning packets of data tend to travel in bunches, said William Cleveland, a mathematician and statisticians at Bell Labs. Bursty traffic on a road means clusters of cars interspersed with periods of little or no traffic.

When a 45-megabits-per-second line has as many as 50 or 60 connections per second, however, the traffic starts to smooth out, said Cleveland. "The burstiness disappears and [packets] are more randomly distributed," he said.

The researchers noticed a trend toward smoothness when they were measuring connections on the Internet for other research. They came up with a mathematical theory that explained why this might happen, then tested the theory by measuring traffic at six large Internet links.

"The theory tells you that the arrival times for packets should head to a Poisson process," said Cleveland. A Poisson process is essentially random, said Cleveland. "The [packet] inter-arrival times are independent of one another," he said.

The finding is significant because smoother traffic means fewer dropped packets. "It's better in the sense that you can use more of the link's capacity. [A link] that's bursty will drop more packets than one that has this Poisson character to it," said Cleveland. In addition, bursty traffic requires larger buffer sizes in equipment like routers that queue up data packets as they travel around the Internet.

When a packet of data travels around the Internet, it hops from server to server to get to its ultimate destination, very much like a car that has to turn onto different roads during a trip. Bursty car traffic means a greater chance of a longer wait at an intersection. When a data packet is delayed, however, it must be stored on a router's buffer, and if the buffer is full, packets get dropped, meaning they must be sent again from the original server.

Smoother traffic means fewer dropped packets and lower buffer requirements. "If you can even out the load, the queuing is less, and if the queuing is less the buffers can be smaller," said Cleveland. "Overall, the characteristics of the traffic are very important for network design because... you've got to know the nature of the traffic to know how you should design the devices to accommodate it," he said.

The study is important work, but is not comprehensive enough to be conclusive, said Ahmed Helmy, an assistant professor of computer networks at the University of Southern California. "These kinds of studies are hard due to the dynamic nature of the Internet traffic. We need [more] samples to get anywhere near the big picture," he said.

Several previous studies of the Internet have shown bursty characteristics even at high traffic levels, said Helmy. A comprehensive study of Internet traffic would require a larger number of samples that use more protocols over a long period of time, he said.

The Bell Labs studies looked at half a dozen links, but "there are potentially hundreds of thousands of links in the Internet, with varying loads and characteristics," Helmy said. A more representative sample of links would be, for example, the 100 most important links that connect the biggest ISPs, like AOL and MSN to the biggest backbones, like Sprint, MCI and AT&T, said Helmy. The trouble is, "it is difficult for researchers to get such information on links," he said.

Network software has several layers. The media layer determines how electrical or optical signals carry data. The routing layer controls how packets of data get from point A point to point B. The transport layer puts information into small packets for sending and reassembles the information on the receiving end. The application layer handles commands specific to programs, like fetching a Web page or playing a music file.

Several versions of these layers of software are used on the Internet. Different portions of the Internet use Ethernet, Asynchronous Transfer Mode (ATM), or Synchronous Optical Network (SONET) at the media layer. The application layer includes many different protocols like Hypertext Transfer Protocol (HTTP) for accessing web pages, File Transfer Protocol (FTP) for direct file transfers, and streaming audio for music files. "The paper only looked at HTTP at the application layer, and four out of six links traced were ATM networks," he said. ATM in particular may sway the analysis because it transports data in fixed packet sizes, he said.

The most difficult part of getting a comprehensive picture of Internet traffic is the time dimension, Helmy said. Long-term studies need to be carried out continuously in order to arrive at meaningful conclusions, he said. "For example, two years ago most of the traffic was... Web traffic. However, recent killer applications, namely Napster [have spawned] huge amounts of music files... and those files have very different characteristics. In the next years, perhaps another killer app such as videoconferencing or short messaging between wireless devices may alter the current characteristics," he said.

Although difficult to achieve, a better understanding of Internet traffic has the potential to increase network efficiency in many ways. In addition to dictating the amount of buffer memory to include in hardware like routers and switches that route traffic, the way traffic flows influences the design of software protocols that control Internet congestion and queuing, Helmy said. "You can draw parallels with building a highway. The designer needs to know [the] average and maximum weights of cars [the road needs] to withstand, and that leads to traffic analysis," he said.

The researchers plan to use their increased knowledge of traffic characteristics to better design traffic engineering equipment, said Cleveland. Lucent will also use the information to better size links for its ISP customers, he said. "We want to size networks so they don't have to get more than what they need," he said.

The work could find its way into commercial products in one to two years, according to Bell Labs spokesperson Patrick Reagan.

Cleveland's research colleagues were Jin Cao, Dong Lin and Don X. Sun of Bell Labs. The research was funded by Lucent Technologies.

Timeline:   1-2 years
Funding:   Corporate
TRN Categories:   Internet
Story Type:   News
Related Elements:   Bell Labs technical report, "Internet Traffic Tends to Poisson and Independent as the Load Increases."


August 15, 2001

Page One

Atom lasers made easy

Molecule makes mini memory

Does heavy volume smooth Net traffic?

Mind game smooths streaming audio

Quantum effect for chipmaking confirmed


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