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Lesson 3 - The Physical Network

B. Cable types


Cable types


Each network allows some flexibility in choosing cable. For example, Ethernet supports coaxial cable, UTP or fiber optic. Token Ring , on the other hand, supports UTP, STP and fiber optic cable. It is up to the network designer to decide which of these cables to choose.

Many factors affect this decision including the length of the cable runs, throughput of the network, possible interference and security needs. In the following pages, you will examine the cable types along these parameters and at the end, decide for yourself which cable type is appropriate in various situations.

Coaxial cable

Coaxial cable was the first major cable used for Local Area Networks. The cable was already extensively used in mainframe systems and its characteristics made it ideal for early LANs. It has now fallen out of favor and is being replaced with cable that is more flexible and less expensive, such as unshielded twisted pair or by cable which can offer greater distances or more capacity such as fiber optic cable.

Coax
Figure 19: Coaxial cable

Construction
Coaxial cable is composed of a center conductor, either solid or stranded, copper or maybe aluminum. Surrounding it is an insulation, called the dielectric. It is normally foam. Around the dielectric is a copper wire mesh or aluminum foil, which is the second conductor. This layer also give the wire some protection from interference. Finally, there is a covering called the cladding. It may be made from PVC (PolyVinyl Chloride) or Teflon.

Notice that the two conductors run parallel to each other or on the same axis, hence the name “coaxial” cable.

Coaxial cable varieties

Ethernet has two kinds of coaxial cable available. Thinnet is less expensive, hence its nickname “cheapernet”, easier to work with and install. Thicknet has a larger diameter, is very expensive and hard to work with. However, it has much greater resistance to interference and can extend to a 500 meter distance. Thinnet has a maximum length of 185 M.

Pros and cons
Coaxial cable can go longer distances and has better protection from EMI than twisted pair. On the negative side, it is harder to work with and does not lend itself to the modern practice of running cable from every office to a central location, usually the wiring closet. Nevertheless, when you need to go just that extra distance, coax may solve the problem and it does a good job of connecting networks on different floors together.


Twisted Pair Cable


Twisted Pair cable

The most popular network cable is twisted pair. It developed from the cable used in telephone systems. That cable, however, wasn’t twisted, nor did it need to be because voice signals traveled along it fine. When early developers attempted to put digital signals on it, it was deemed a failure because of the crosstalk. Twisting the conductors around each other alleviated the crosstalk and the modern twisted pair data cable was born. Early unshielded twisted pair (UTP) cable was restricted to low throughput networks such as ARCnet and 4Mbps Token Ring. There was despair that high speed networks could ever use UTP. With advances in quality construction and by increasing the number of twists per foot, UTP can now successfully be used for networks with throughput as high as 1Gbps.

Construction
Each conductor is composed of a copper core, either solid or stranded, surrounded by an insulating sheath. Two of these conductors are twisted about each other to form a twisted pair. A varied number of pairs are bundled together to make the final cable. Modern LANs have standardized on a four pair cable.



UTP
Figure 20: UTP cable

Shielded or unshielded
Twisted pair cable is available in two varieties, shielded or unshielded. Shielded twisted pair (STP) cable has a shield made of copper braid, aluminum braid or aluminum foil surrounding the inner conductors. This gives it much greater resistance to electrical interference than unshielded twisted pair (UTP) cable. STP can go three times the distance of UTP because of this characteristic. Unfortunately, STP, like coaxial cable, is much heavier than UTP and is harder to work with. More to the point, it has a difference impedance than UTP and therefore cannot be substituted without making special provisions. Ethernet uses UTP while Token Ring can use either.

Plenum or patch cable
The plenum is the space above a dropped down ceiling in a modern building. It is the natural space in which to run cabling between rooms. UTP plenum cable is stiff and holds its shape when bent. The construction of the cable is the reason. It uses a solid, copper core for each conductor. This type of cable would break if handled excessively, which is the case for patch cables. These are used to connect the computer to the wall plates in a room and are expected to be flexed many times in their life. Patch cables use stranded copper wire as their conductors. Therefore always use the correct cabling for the job: patch cables to attach computers to the wall, plenum cable for the interior of walls and ceilings.

Twisted pair specifications
Originally, there was no standard for twisted pair cables used for data transmission. However, when IBM designed their Token Ring network, they also created specifications for the cable which would be used. Enough cables were specified so that all important wiring conditions would be met. These included shielded and non-shielded cable, patch cables and cables for both the plenum (in the ceiling) as well as under carpet. These cable types are summarized in the following table. Subsequently, the Electrical Industry Association and the Telephone Industry Association (EIA/TIA) codified the specifications for UTP into categories called the EIA/TIA 568 Commercial Building Wiring Standard. The most modern practice is to wire buildings with category 5 (Cat 5e) UTP cable which is suitable for transmission up to 100Mbps.

American Wire Gauge (AWG)
When reading specifications, you will often run across the term AWG. This represents the thickness of the metal conductor. Smaller numbers represent thicker wire. Telephone wire is typically 22 AWG, 14 is thicker, 26 is thinner.

The conductors in the cable only carry signals in one direction, therefore two conductors are used, each carrying signals in the opposite direction. The two cables are connected together with Kevlar webbing.

Pros and cons
Fiber optic cable has the longest range and should be used when long distance is a requirement. It has extremely high bandwidth, in the Gbps range. It is also immune to electrical interference, nor does it give off any of its own. This would be a factor in dangerous environments where an electrical spark could set off an explosion, say in an explosives factory. Finally, it is extremely difficult, but not impossible, to tap into a fiber optic system.

On the negative side, fiber optic cable is the most expensive to install. The cable and connectors are more expensive plus the installation requires highly skilled craftsmen. This is particularly true when two fibers need to be spliced together.

Connectors used for fiber cable are SC connectors which are a push and pull connector and ST connectors which are a bayonet mount.

Exercise 3-3: Cable exercise


Exercise: Which cable?
Your are the network architect and need to specify the most appropriate cable. For the following situations, which cable would you recommend?

1. Cable for an explosive’s factory
______________________________________________

2. A network which will be running Ethernet at 100Mbps
______________________________________________

3. Cable for a building which is being used by a national security agency
______________________________________________

4. A cable must be run between two buildings facing each other across a courtyard. The distance between the wiring closet in the basement of one building to the wiring closet in the basement of the other building is 150M. The network is a 10Mbps Ethernet.
______________________________________________

5. Cable which must connect switches in the wiring closets of each floor of a 10 story building. The cable will be routed down the elevator shaft.
______________________________________________

6. Wiring for a test network that will be using Ethernet running at 1,000Mbps.
______________________________________________

7. Cable that will run between a signal controller and the control room in a locomotive switching yard. The distance is 2km. ______________________________________________


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Figure 21: The structure of fiber optic cable

Single-mode cable
Single mode cable only has one light beam in it. The cable has a small diameter core and is made of the purest glass. A laser is usually the light source and the cable can extend several kilometers without a repeater.

Multimode cable
Multimode cable has greater capacity because several light beams are sent down it. The beams enter at slightly different angles and therefore don’t interfere with each other. The core is wider than used in single-mode and often made of fine plastic. Instead of lasers, the light source is usually light emitting diodes (LEDs). The system has greater capacity and is less expensive than single-mode. On the other hand, multimode doesn’t have the same reach, often restricted to hundreds of meters instead of kilometers. For this reason, multimode is usually used inside buildings, whereas single-mode is used between buildings and long distances. If the cable needs to be extended, optical repeaters are used.

The conductors in the cable only carry signals in one direction, therefore two conductors are used, each carrying signals in the opposite direction. The two cables are connected together with Kevlar webbing.

Pros and cons
Fiber optic cable has the longest range and should be used when long distance is a requirement. It has extremely high bandwidth, in the Gbps range. It is also immune to electrical interference, nor does it give off any of its own. This would be a factor in dangerous environments where an electrical spark could set off an explosion, say in an explosives factory. Finally, it is extremely difficult, but not impossible, to tap into a fiber optic system.

On the negative side, fiber optic cable is the most expensive to install. The cable and connectors are more expensive plus the installation requires highly skilled craftsmen. This is particularly true when two fibers need to be spliced together.

Connectors used for fiber cable are SC connectors which are a push and pull connector and ST connectors which are a bayonet mount.

Exercise 3-3: Cable exercise


Exercise: Which cable?
Your are the network architect and need to specify the most appropriate cable. For the following situations, which cable would you recommend?

1. Cable for an explosive’s factory
______________________________________________

2. A network which will be running Ethernet at 100Mbps
______________________________________________

3. Cable for a building which is being used by a national security agency
______________________________________________

4. A cable must be run between two buildings facing each other across a courtyard. The distance between the wiring closet in the basement of one building to the wiring closet in the basement of the other building is 150M. The network is a 10Mbps Ethernet.
______________________________________________

5. Cable which must connect switches in the wiring closets of each floor of a 10 story building. The cable will be routed down the elevator shaft.
______________________________________________

6. Wiring for a test network that will be using Ethernet running at 1,000Mbps.
______________________________________________

7. Cable that will run between a signal controller and the control room in a locomotive switching yard. The distance is 2km. ______________________________________________


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Cable Chart
. Impedance Used By