Our internal network has recently been rebuilt using equipment generously donated by 3Com Corporation. We now have a high-speed ATM fiber backbone network. Whoa! What does all that mean? Let's take each term separately.

A quick word about ATM. First, in our case this does not stand for "Automated Teller Machine." It stands for "Asynchronous Transfer Mode." This is just the "language" or "protocol" that is used to send data across the network. It's crucial that all ends of the network agree on HOW the data is sent (the format, timing, etc.) before anything can happen at all. ATM is a very efficient method that allows many different types of data (computer, voice, video) to efficiently share the same wire or, in our case, optical fiber.

We need to distribute data to hundreds of computers within the Exploratorium. Most of these machines are on people's desks. Some of them are used in our store and admissions desks. A few are in exhibits, and several are used to supply Internet services like our e-mail and the Web page you're reading right now. That means there's a lot of data flowing around the Exploratorium. Each individual computer may not receive or produce much data, but collectively quite a few bits are flying back and forth. If you build a city and want to supply all the houses with water, you install a few very large pipes, or water mains. Each house taps into the water main with a smaller pipe. The water main carries the collective water for the city. Likewise, we have a "data main" called a "backbone" that carries the bulk of the data at a high speed. Each computer taps into this backbone with a smaller data "pipe."

Most data flows through copper wires. If you want really high speed (many bits per second), another good way to go is with optical cable. With this method, the electrical pulses are turned into flashes of light that are fed into the end of a "wire" made of glass. This wire is called a "fiber optic." The fiber optic is designed to allow the flashes of light to travel great distances without (much) absorption. When the flashes of light reach their destination at the far end, they are turned back into electrical signals. The fiber is our big "data pipe" (even though it's actual diameter is about the same as a human hair!). We run this fiber between several distribution points within the museum. At each of these distribution points, the optical signal is converted back to an electrical signal and we use standard copper wire to connect to the individual computers. The map (above ) shows the fiber and distribution points (called "distribution frames") within the Exploratorium. The MDF is the "Main Distribution Frame" where all the data converges, and the IDFs are "Intermediate Distribution Frames" where data goes out to the various machines.

The distribution frames are where everyone connects. How do we connect hundreds of computers to the same optical fiber? We use two devices, one called a "hub" and another called a "switch."

A hub is a simple device that will allow many people to attach their computers together. The hub makes a connection to all of the machines simultaneously. For those old enough to remember, this is like having a telephone "party line," where everybody in the building had to share the same telephone line. If someone in apartment 3 was using the phone and another person in apartment 7 picked up their phone, they would hear apartment 3's conversation and would have to wait until 3 was finished before placing a call. This is efficient usage of a single wire, but not very convenient for everyone. The hub acts the same way. Only one device can talk at a time and the others wait. If two start talking at the same time, they detect this and each computer waits a random amount of time and tries again. If your data traffic is very high, many data collisions take place and everything slows down. This is where the switches come into play.

The switch looks almost identical to a hub, but it works very differently. The switch works more like our current phone system. If a computer on one of its ports needs to send data to a computer on another port, the switch makes a connection between these two computers only. None of the other computers plugged into the other ports on the switch "hear" anything at all. The computers on the other ports can also make their own connections because the switch can allow many "conversations" to take place simultaneously. The computers don't have to share a common data path -- each gets its own connection on demand. This is just like dialing a friend on the telephone. You get a private wire between both of you. (Actually, the computer at the phone company that does this for you is called a "switch.")

As you might guess, switches are much more expensive than hubs (by a factor of about 10!). It would not be economical to connect every computer to its own port on a switch. The compromise is to connect several hubs to a switch, each hub with its own port. This way several machines must "share" a hub, but each hub gets an individual line or port on the switch. If a machine on one port of a hub talks to another machine on the same hub, the rest of the network never sees that traffic. It is therefore wise to intelligently plan which machines plug into which hubs.

The switches at the IDFs are hooked together by fiber-optic cable at the MDF with a special switch capable of handling multiple fibers all transmitting at very high data rates. All the fibers converge here and the data is switched appropriately.

 
Let's wire it all together see what it looks like:

Note that the different IDFs have different numbers of hubs. This is because there are differing numbers of computers located near each IDF. If more computers are installed, we simply add more hubs. Also note that there is additional equipment at the MDF. This room also houses all of the Exploratorium's servers (both in-house and Internet servers). It makes sense to hook these higher usage machines to their own port on the switches. We may put higher speed connections (100 Mbps or even direct fiber) directly on these machines. This additional equipment will allow us to do so.

The Exploratorium's networking components have been generously donated by 3Com Corporation.