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Communications in Seattle in our Virtual Museum Partners
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described in this exhibit, see the Science of Phones
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OSP Display Cases
“Outside Plant” refers to telephone equipment that was located outside of the telephone office or outside the customer’s home or office. This includes wire, cable, telephone poles, and terminals. All of these components are used to carry the telephone signal from your telephone to the telephone office. On display here are examples of splicing equipment and specialized tools. Some of these are still used today to ensure a good connection between your telephone and the telephone office.
The cable splicing display
Splicing telephone cable required great skill. The cable splicer removed a small amount of insulation from each wire and then carefully twisted the two ends together. An insulating sleeve was then slipped over the splice. This was repeated for each wire in the cable. Afterwards, the splicer installed wrapped muslin cloth around the splice bundle and then sealed the spliced area with lead to make a water-tight seal. It’s very important to keep telephone cable dry. Any moisture inside the cable causes electrical leakage, which can add noise to the telephone signal or even cause the signal to fail completely. Look at the large cable at the bottom of this display. This was the largest cable used in the Bell System. It has 4200 pairs of wires inside—that’s 8400 individual pieces of copper wire. We’ve removed the cover from the splicing case so you can see just how many wires are in this cable. In this type of copper telephone cable, each pair of wires was dedicated to just one customer. When you aren’t using the telephone, your pair of wires was not used. So at any given time, most of the wires inside a large telephone cable were not in service. To increase the capacity of the telephone network without adding more of these large cables, Bell System researchers in the 1970s developed a new type of cable—fiber optic cable.
Copper vs. Fiber
You’ve probably heard about fiber optic cable. Today it is mostly used for long-distance telephone communication and sometimes from one local telephone office to another. Fiber optic cable is made from very fine strands of specialized glass. To send your voice over a fiber optic cable, the telephone office converts your voice to a very fast stream of digital pulses, which are transmitted as bursts of light through the glass fiber. These pulses are converted back to an analog signal at the other end of the cable so that the party you called can hear your voice normally.
Fiber optic transmission is so efficient because each of these tiny glass fibers can carry many voices at the same time. This is called time division multiplexing. It simply means that your voice is sent over the fiber for a tiny fraction of a second, and then someone else’s voice for a tiny fraction of a second after yours, and then yet another voice is transmitted after that. The switching equipment at both ends of the glass fiber keeps track of each time increment and reassembles your voice without interference from the other parties sharing the same glass fiber.
A two-way telephone conversation requires two fiber optic strands. One strand is used to send your voice, and the other to carry the voice of your called party back to you. Just two strands of fiber optic cable can carry as much telephone traffic as six of those enormous 4200-pair cables. This enormous increase in telephone capacity has made possible new uses for the telephone network, including access to the internet and other computer-related activities.
Open wire, transpositions, bird wire, aerial cable, and B-Box
This connector box was removed from service in a neighborhood that had many apartment buildings. Since residents moved very frequently, telephone pairs were connected and reconnected many different times—the result is an out-of-control connecting box!
Manhole and underground ducts
Much of the Outside Plant infrastructure is out of sight. Cables were placed in a network of underground tubes or ducts or directly buried. In this display we have simulated a part of the underground duct system with a variety of ducts and an underground vault known as a “manhole.”
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