sábado, 17 de julio de 2010

Organizing optic fiber within a building

Out in the real world we may be asked to modify or repair an existing fiber layout. The first step is to see how the original installation was designed. This involves hope. We hope that the installers prepared records of what was actually installed in the building rather than what was intended when they placed their bid for the contract. There is always the possibility that modifications dreamt up on the back of an envelope during the lunch break were made but never recorded. We also have to hope that the installer followed good practice and adhered to the appropriate standards — especially if they have subsequently gone bust and disappeared. It is always a good start if we know roughly what to expect in a building, but before we look at the fiber layout, we will take a brief look at copper cables.

Copper cables

But this is a book on optic fibers. Yes, but we keep coming across copper cables so it may be worth a brief outline of their characteristics so at least we will know what we are dealing with. We will start with a look at the advantages and disadvantages.
Advantages of copper
  • Many technicians are happier when using copper cables because they are familiar and unless they have taken the trouble to get to grips with optic fiber, they seem so much simpler.
  • Connections are easier with hand tools and do not require expensive equipment and great precision.
  • A simple metal detector can find a buried cable. This is only half an advantage because many optic fiber cables use metallic foil for moisture protection and metallic armoring.
Disadvantages of copper

These have been explained in Chapter 8 as advantages of optic fibers but to save the trouble of finding them again they are listed below.

  • Electrical interference and crosstalk.
  • Care has to be taken in high voltage environments.
  • Reduced bandwidths.
  • Security.
  • Higher losses.
  • Size and weight
  • Two wires are used to send a single signal.
How does it work?

To send a telecommunication signal by copper cable requires an electric circuit. This means that we need two wires usually referred to as a single pair. See Figure 18.1.

The transmitter generates a voltage signal between the two wires at the input and this results in a current flowing along one wire, through the receiving circuitry and back along the other wire. Compared with fiber, this doubles the number of connections necessary.
One problem with any electrical signal is that whenever electric current flows, it causes a changing magnetic field. This is, in itself, not a worry unless there is another copper wire nearby. The changing magnetic field caused by the signal will cause a voltage to be induced into the copper wire causing a weak copy of the original signal. This effect is called 'crosstalk'.
The magnitude of the induced voltage decreases by the square of the distance away from the source of the interference so if we double the distance, we reduce the induced voltage by a factor of four. If two copper wires are running side by side, the one closest to the source of interference will pick up a larger signal than the one further away.

We can also reduce, but not entirely eliminate, this effect by twisting the copper wires.
In circuit A, the upper wire is closer to the source and hence has a higher voltage induced. This larger voltage is indicated by the larger arrows and, at the moment shown, is trying to push the current clockwise round the circuit while the other wire is trying to push the current counterclockwise. The big arrows will win and the overall effect will be to produce a small current that will be a copy of the interference signal.
In a communication system the interference signal may be a conversation on a nearby telephone or a crackle from electrical machinery or lightning or any other source of interference.
In circuit B, each wire changes position so that the total value of the induced voltage is equal in each wire. This would give the happy result of no overall effect and hence no interference.
To work efficiently, the two wires must be twisted very carefully to keep the two wires balanced. When we have several pairs close together in a single cable, the rate of twist is varied and this helps to reduce the likelihood of two wires accidentally running parallel with their twists in step.

Cable designs

The cables look very similar to an optic fiber cable. They have an outer cover of polyethylene to provide waterproof protection just like optic fiber. In fact, apart from markings on the outer cover, there is no way of telling them apart so we must be careful not to cut into an optic fiber cable only to find copper cores. This would not be popular with the owner of the copper.

Twisted pair cables

Just as in fiber optics, a range of copper cables is available for indoor use from just two pairs of conductors up to enormous armored underground cables containing up to 4200 pairs of conductors. Figure 18.3 shows a typical four-pair screened copper cable.

There are three similar versions of twisted pair cables all looking very similar to Figure 18.3. The differences are in the screening. The simplest is no screening. Then comes the grounded screen that helps to reduce high frequency interference signals from reaching (or leaving) the cable. The other alternative is to wrap each pair separately in aluminum foil and then added braid under the jacket to screen the whole cable. This also provides protection against low frequency interference.
Coaxial cable

These give improved protection against interference and are familiar cables at home where they provide the signal inputs to televisions and videos etc. Some minor variations occur. The outer braid sometimes has two layers, and some include aluminum foil wrapping around the center conductor, but they are all easily recognizable. See Figure 18.4.

Fiber in buildings

In this section we will have a look at a typical layout in a building or group of buildings. In reality, the installation may differ owing to local customs or special circumstances but the trend is towards using a standard layout of this form. Such harmonization will make life easier for everyone in the industry whether they are installing, extending or repairing a system.

Making an entrance

Getting the fiber into a building is usually a matter of balancing the immediate and long-term costs. We can have a low initial cost but accept higher costs when repairs or modifications are required or we can pay higher immediate costs but less in the future. Let's start with the cheap initial cost since this may win us the contract if the customer does not take the long-term view.

The dig a hole and throw it in method

This is easy. Get a trench digger or plow and prepare a hole, then pass a length of conduit through the wall of the building. Next we lay the copper and fiber cables in the trench, pass them through the conduit into the building and make the connections to the service provider in their manhole. Finally refill the trench. Job done. Out of sight, nice and cheap. There are some problems. What would the digger person do if they came across an obstruction? They would simply go around it, however long the detour, so long as they can finish at the conduit through the wall. The cables would be added and the trench refilled. Is the actual route of the cables going to be accurately recorded in the plans? What do you think? The cables are not protected and in years to come when the cable needs attention — are we going to be able to find it? Is it now under the new staff car park? Or the new building? Will it be cut by the utility company laying pipes?

The spend more and do it better method

If the cables are installed in conduit or other proper mechanical enclosure it takes a little extra time and money but the cable is better protected and is in a known place so extra cables can easily be installed. The access point of the conduit through the wall needs to be sealed to prevent water or gas gaining access to the building.

Making a start

In this example we have used an underground route to bring the cable from the service provider's manhole into our building. It is fed into an equipment room from which our cables can wander off around the building. This situation is shown in Figure 18.5.
The equipment room will be something between a proper room and a cupboard where the brushes are kept. Somewhere between the manhole in the road and the equipment room outlets,
the responsibility for the cabling is transferred to us from the service provider.

We need to be aware of where this transition occurs for all sorts legal reasons otherwise it may result in our lawyers getting richer and us getting poorer — not the idea at all.

Inside the building

So far we have got from the road to just inside our building. The cables, copper and fiber now go to another equipment room called the 'campus distributor' or 'main cross-connect'. This is the main interconnection point for the remainder of the system. The system may be a single room, a multistory building or several buildings like a business park or university campus. The bundles of cables going between equipment rooms are called 'backbone cables' as in Figure 18.6. The cabling may be hidden away in ceiling spaces or under the floors or in channels or conduit — wherever convenient.

Inside the room

To connect the 'campus distributor' or 'main cross-connect' to the individual work areas inside the room is the function of a junction box called a 'floor distributor' or 'horizontal cross-connect'. Despite the name, cables can be installed vertically as well as horizontally. By having an organized hierarchy of connections like this, faults can be traced back and isolated more systematically. The final connections within a room are shown in Figure 18.7.

Other buildings and other floors

To connect a communication system to another building or another floor in the same building, the cables are taken from the campus connector (main

cross-connector) to the building distributor (intermediate cross-connect) and from there to another floor distributor and then on to the work areas. See Figure 18.8.

Maximum length of optic fiber cable runs

These are governed by local regulations in each country but typical values are shown in the connection summary in Figure 18.9.

Kevin M Contreras H
CI 18.255.631


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