Two of the primary challenges to building and operating a large network are crossing the legal boundaries to laying and access cable runs across personal and public property, called right-of- way, and the challenges of physically laying and managing long- distance cables. Each is considered in the following sections.
Right-of-Way
A story passes around the network engineering community of a native tribe in South America that controlled a particular area, including two mountains and a pass between them. A provider buried a cable through the region to connect two moderately sized cities. Within a week of bringing up the new circuit, however, the link failed.
Given this was a brand new fiber, it seemed unlikely there was some physical fault caused by a splice, failed electronics, or some other common problem. There were no reports ofconstruction anywhere along the fiber’s path, so it was unlikely a backhoe was responsible.
Note
Backhoe fade happens when a backhoe takes out acommunication cable.
Engineers used a time-domain reflectometer (TDR ) to discover the location of the cable break. The operation of a TDR is shown in Figure 12-1.
Figure 12-1 TDR Operation
The TDR transmits a signal down the cable, whether optical or electrical. If the cable is broken or damaged, some amount of signal will be reflected to the transmitting TDR. The TDR can determine where the break is by measuring the amount of time between transmitting the signal and receiving the reflection.
Following a cable map, technicians can get close enough to the break to find it through visual inspection.
In this particular case, when the technicians arrived at the break, they discovered a member of the local tribe standing over the spot—armed, preventing them from accessing the cable. After some conversation, they determined the local tribe wanted some form of payment for this cable to run across their land.
While a bit humorous, this situation highlights one of the challenges providers face when laying cables across long distances— right-of-way.
The idea of right-of-way originates in public paths: members of the public are granted permission to walk across privately owned land to follow paths that existed before the current owner. Courts and governments extended right-of-way, over the years, to an owner granting any use of a piece of land to some other person.
If your driveway crosses someone else’s land, you must gain a right-of-way to use it. If you want to place a sign in a farmer’s field, you must gain a right-of-way. If you want to bury a cable across someone’s land, or hang a cable above it from towers, you must gain a right-of-way.
Providers face a singular problem when trying to gain right-of-way to build networks: no one wants a provider digging up their property to cross their land.
Running a cable through hundreds of cities, tens of cities, and a few states requires getting right-of-way from hundreds of governments and private landowners.
Providers often work around this problem by purchasing rights-of-way from existing right-of-way holders, such as governments, railroads, electrical utilities, and water utilities.
Physical Plant
A provider’s physical plant is the cables, buildings, and other physical facilities they have built to carry data across long distances. Each kind of physical space presents different challenges to laying and maintaining cables. For instance, Figure 12-2 shows a typical cable installation in Nuuk, Greenland.
Figure 12-2 Cables Attached to Rock in Nuuk, Greenland Greenland presents many challenges for carrying data over long distances. Cities and villages are separated by hundreds of miles, and there are no roads between them, so the only access is via boat or helicopter. The ground is largely rock, so trenches must be cut by blasting and heavy machinery, or conduits must be affixed to the outside of the rock and cables strung through these conduits.
Another harsh environment for laying cables is under the sea. Figure 12-3 illustrates the undersea cable-laying process.
Figure 12-3 Undersea Cable-Laying Process
In Figure 12-3 a specially designed ship travels across the ocean surface, using highly accurate positioning technologies to ensure the cable is laid on a specific path. The ship has reels full of special cable for undersea use—shown larger than life. This cable is fed to a sled, which travels across the bottom of the ocean, pulled along by a tow line connected to the ship. The sled pulls the cable from the ship, digs a trench, and buries the cable a few feet under the sea floor.
Hundreds of undersea cables are used to carry traffic across oceans, through seas, and even along coastlines. Figure 12-4 is an undersea cable map.
Figure 12-4 Submarine Cable Map
The undersea cable map in Figure 12-4 shows the large number of cables now lying on or under the ocean floor. While many of these cables run along continental shelves, some of them cross oceans, including deep trenches and underwater mountains.
Another common way of installing WAN cables is by attaching them to utility poles. Providers face challenges attaching fiber-optic cables to existing poles because utilities and local telephone companies, which control these poles, often do not want the competition from a new provider in the area.
Providers might also need to replace poles if they cannot bear the additional weight of the fiber-optic cables.