This article is from the August 15, 2001, issue of Red Herring magazine.

From the outside, the Internet appears simple: just power up your computer, launch your Web browser, and you're able to search and retrieve data from servers all around the world. But under the surface, it's a complex system.

Where there's intricacy, it's necessary to clarify the pieces. And with the Internet there are plenty of moving parts. For starters, the Internet is conveniently organized into an "edge" and a "core." The edge contains thousands of local "points of presence" (POPs), which provide last-mile network connections for individual homes or businesses. The core consists of hundreds of routers, huge switches that receive the data packets from one POP. Each router determines the best way to send the data packets to their final destination, and then dispatches the data. The core's routers and the edge's POPs are interconnected by a global web of fiber-optic cables.

To better visualize POPs, think of them as local post offices, which send and receive mail packages using "low bandwidth," pedestrian letter carriers. Next, think of the core routers as regional mail sorting centers, which are linked to the local post offices by "high bandwidth" trucks, and to each other by fast vehicles like trains and airplanes.

Just as an efficient post office requires efficient mail sorting centers, the Internet's performance and reliability hinges on those huge core routers, which sell for $500,000 or more from companies like Cisco Systems (Nasdaq: CSCO), Juniper Networks (Nasdaq: JNPR), Lucent Technologies (NYSE: LU), Nortel Networks (NYSE: NT), and Alcatel (NYSE: ALA) (see "Architecturally impaired").

ESPRIT DE CORE An Internet core router looks quite boring. It's about the size of a dorm-room refrigerator, typically fitting inside a 6-foot rack. The front of a core router has up to 18 replaceable cards called blades, which provide the physical interfaces between the router and the outside world. Most of these blades contain several high-speed fiber-optic links that run at speeds ranging from 51 Mbps to 2.5 Gbps. Engineers at the router manufacturers are working on faster interfaces of 10 Gbps and 40 Gbps. Those interfaces would be connected to other core routers as well as to smaller edge routers at local POPs.

A router's job looks easy: when a packet of data arrives at one interface, the router must quickly ascertain where that packet must go, determine the best route for sending it there, and then send it out through the appropriate interface -- without delay. What's so difficult about that?

Determining where the packet needs to go requires knowledge of nearly every IP address (the unique identifying code of a specific computer or local area network), and its local POP. The second job is figuring out the best path from the core router to the POP's edge router. This is a process that might entail a dozen or more intermediate "hops." Finally, shooting the packet out through the appropriate interface might require sending the data from one blade to another through a high-speed backplane, a common data pipe that links all the router's blades and interfaces together. It also requires that routers convert data from one network protocol format to another to fit the particular interface.

In order to perform their function, Internet routers must constantly communicate with one another and the devices to which they're connected. That helps each router maintain its routing table, a type of Internet map, as well as "weather reports" that show which links are running fast or slow. Based on this data, the router can compare different data paths for each packet, quickly assessing the optimal path.

Sound straightforward? Keep in mind that a modern core router might forward hundreds of millions of data packets every second -- 24 hours a day, seven days a week -- without delaying a packet.

ROUTE OF THE PROBLEM One of the major hurdles to building bigger routers is constructing an efficient backplane. Currently, the industry is deploying terabit backplanes, which can shuttle 1 trillion bits -- enough data to fill 200 CD-ROMs -- per second.

A second challenge is designing faster individual network interfaces. Equipment makers are beginning trials of the 10-Gbps routers, and are experimenting with 40 Gbps in their laboratories. The problems are twofold: first, manipulate the extremely complex beams of light traversing the fiber-optic cables, and then slice and dice the millions of packets coming out of each interface every second.

To help improve overall performance in these routers, engineers use a new protocol called multiprotocol label switching (MPLS). Typically, each core router examines every packet it receives, recomputes the best path, and then sends the packet along to the next router in the sequence. With MPLS, the packet's originating edge router (which isn't as busy) takes the time to determine the complete path for the packet. That edge router wraps the entire path's information, stored in a label, around the packet's data, and sends it to the first core router. The first core router examines the label to determine the name of the next router and ships the packet to the next stop; the second router examines the label for the next router in the sequence, and so on until the packet reaches its destination. By eliminating the need for path computation at each step along the way, each core router can process the packet more quickly.

In some cases, service providers can predefine specific MPLS paths, called label switched paths, which are configured for high quality-of-service access. This reserves specific types of traffic routes much like an express lane on a freeway.

Behind your favorite e-commerce site, downloaded MP3 songs, and "You've got mail," the Internet's core routers keep all the traffic humming along. As the Internet continues to handle more and more traffic, these routers are increasingly busy, but thanks to advances in fast backplanes, high-bandwidth optical pipes, and new routing protocols, they have room to grow.

Alan Zeichick is principal technology analyst with Camden Associates and is editor in chief of BZ Media's SD Times. Write to alan@bzmedia.com.

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