How to build your own metropolitan fiber-optic network.. part II
Finally, the day came!
Our conduit was ready. The fiber was here. All that remained was putting it in the ground. How do you do that? What all is involved when you see those guys in the orange vests and hard-hats in the middle of the street, crawling in and out of manholes?
A lot, it turns out. The first step is “pulling” a permit with the city. To get a permit, you need construction experience, a performance bond, and a traffic plan. The traffic plan details where work is to be done, how you are going to block the street, and how you will re-route traffic around the blockage. At least in Denver, it seems pretty straightforward.
On the day, the sign company brought tons of orange road pylons, and road-signs announcing the blockage. It was pretty thrilling, the idea of being responsible for blocking a street and re-routing traffic.
Once all the road markers were up, it was time to start opening manholes! A special tool is used, a type of crowbar with a hook on the end that lets you snag a lip on the bottom of the manhole lid.
What’s in a manhole? Typically a manhole is an opening to a vault. Sometimes as small as 4 feet square, sometimes as large as 20 feet by 40 feet, is a subterranean vault with concrete walls. Holes are drilled through the walls to allow the entry and exit of conduit, in which fiber or other cable is placed. Having vaults and conduit along the path allows for much easier repair of underground cable. If you simply buried the cable, you would have to potentially dig it up every time something went wrong with it. In this picture you can see some of the ducts coming out of the wall:
Sometimes manholes collect noxious gasses, methane and other such things that are not as good for you as breathing oxygen. Thus, positive air pressure is delivered into the manhole using a high-power fan and flexible ductwork, to prevent problems.
Once we identified which conduit was set aside for our use, we put the fiber on a spool and prepared to start pulling it into the manhole.
Since our run was relatively short and the fiber count (24 strand) relatively low, our spool is pretty small. Sometimes the spools are 10 feet across.
The vault this manhole opened into was one of the large ones. It is directly adjacent to the AT&T building downtown, and actually has a door from the building’s basement that opens into the vault. We didn’t know that at the time, so by going in through the manhole we were doing it the hard way.
Inside this vault was a literal ton of conduit, innerduct (flexible conduit inside the conduit), and fiber bundles.
The gear on the left edge is used to pressurize copper cable bundles, to keep water out of them. Remember, all of this is underground. The vault extended 20 t0 25 feet down, basically two stories under the street.
In this particular vault, our cable needed to come in and go back out to another vault. Here is our fiber cable (black), coming out of the blue conduit in the middle:
Now once the fiber was in end to end, we further had to run it upstairs inside our building. This was the hardest part, as the conduit inside our building was small for the cable. But we got it done.
The final step, is splicing this cable with the one we put in last time, joining the two cables together. Also, terminating the fiber, or putting ends on it.
The saga continues…