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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.

>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). However, PVC gives off poisonous gas when burned. Therefore, coax which is installed in walls or ceilings often has a Teflon covering instead.

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

Impedance

Impedance is a broad measure of how much resistance the cable has to the electrons flowing through it. It is measured in “Ohms” (W). Impedance varies among cables and the correct cable is required for the proper functioning of the network.

Coaxial cable varieties

There are many variations in coaxial cable, and some have already been mentioned. However, the network designer needs to be aware of two in particular.

Thinnet vs. thicknet

Ethernet has two kinds of coaxial cable available, thinnet and thicknet. Thinnet has a smaller diameter, about .25 inches. Therefore it is is less expensive, hence its nickname “cheapernet”, easier to work with and install. Thicknet has a larger diameter, about .5 inches. This makes it very expensive and hard to work with. However, it has much greater resistance to interference and can extend to a longer distance. Under Ethernet, thinnet has a maximum length of 185 meters before needing a repeater. Thicknet can be installed as a cable run of 500 meters before needing a repeater. Thicknet is was popular as a backbone cable.

Common coaxial cable are listed in the following table.

Coaxial Cable Types Cable Description Impedance Network RG58 /U Solid copper core 50 W Ethernet RG58 A/U Stranded wire core 50 W Ethernet RG58 C/U Military spec of RG58 A/U 50 W Ethernet Thinnet RG58 50 W Ethernet Thicknet Custom cable, styled after RG8 50 W Ethernet RG59 Baseband systems, cable TV 75 W RG62 ARCnet, mainframe systems 92 W ARCnet 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 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. 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, cables for both the plenum (in the ceiling) as well as under carpet and finally fiber optic cable. 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 5 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. Cable Chart Impedance Used By Bandwidth Category 1 100 ohm Telephone, RS-232C only two pair < 1 Mbps Category 2 IBM Type 3 100 ohm ARCnet, 4 Mbps Token Ring < 10 Mbps Category 3 100 ohm Ethernet/802.3, 10BaseT 100Base4 10 Mbps 100 Mbps Category 4 100 ohm 16 Mbps Token Ring, 802.5 20 Mbps Category 5, 5e 100 ohm CDDI, FastEthernet, ATM 100 Mbps Category 6 100 ohm Gigabit Ethernet 1000 Mbps Shielded Twisted Pair IBM Type 1 IBM Type 2 IBM Type 6 IBM Type 8 IBM Type 9 150 ohm 16 Mbps Token Ring, 802.5 2 pairs shielded 2 pairs shielded, 2 pairs not shielded (data+voice) Data patch cable Under carpet grade Plenum grade 16 Mbps Pros and cons Unshielded twisted pair cable has become the most popular LAN cabling because it is inexpensive and it is easy to install. It lends itself to modern topologies in which offices are prewired and the cable is terminated in a central location, the wiring closet. Because it can be used by both Ethernet and Token Ring, it is common practice to install CAT5 wiring during construction without worrying about which LAN will eventually be used. UTP cable segments are limited to 100 meters or about 328 feet. Since it is estimated that 90% of all cable runs fall within this distance, UTP is sufficient. If however, a longer cable segment is required, other cable must be used. With Ethernet, coaxial cable or fiber optic cable can be used. With Token Ring, shielded twisted pair or fiber optic cable is an option. UTP is also the most prone to interference and is the easiest to tap into. If these are concerns, fiber optic cable maybe the best choice although shielded cable may be considered. Because of the different categories of UTP, it may be possible to have the wrong kind, necessitating an expensive retrofitting of the cable system. An organization with 10 Mbps Ethernet running on CAT3 cable may desire to upgrade to fast Ethernet at 100Mbps. Since this requires CAT5 cable, the old cabling will need to be replaced. Since 1995, CAT5 cable has enhanced performance characteristics, hence the CAT5e designation. Fiber optics A data transmission medium consisting of glass or plastic fibers. Light-emitting diodes (LEDs) or lasers send light through the fiber to a detector, which then converts the light back into electrical signals. Fiber-optics offer immense bandwidth, as well as protection from eavesdropping, electromagnetic interference, and radioactivity. Fiber-optic cabling employs one or more optical fibers, consisting of a central glass or plastic fiber, glass cladding, and a plastic outer sheath. Fiber-optic cable carries information as light instead of electricity, and can carry much more information over greater distances than copper cabling. Optical principles Fiber can transmit light a long distance because it keeps it in a channel. This is accomplished by surrounding the inner fiber with a cladding that has a different refractive index. This keeps the light bouncing along from side to side until the end. 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 a 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 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 a Kevlar jacket. 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.

Attenuation

Attenuation simply means that the signal becomes weaker the further it travels. Resistance in the wire or outside interference degrade the signal until its message can no longer be decoded. Each cable type has a maximum distance which is defined in the specifications for the network. The maximum distances are documented in the latter parts of this chapter and it is important to memorize them in preparation for your exam. Attenuation

If the cable run is insufficient to reach the destination, a repeater or amplifier is required to extend it. A repeater is used if the signal is digital; an amplifier is used for an analog signal.

Impedance

Impedance is the opposition to a current flowing dynamically in a transmission media. It is a complex measurement made up of inductance, resistance and capacitance. Impedance is measured in Ohms and the symbol representing Ohms is Ω.

Why bring up this rather technical topic? Because cable is rated by it and it must match the other components on the network. For example, the coaxial cable used by Ethernet is rated at 50Ω whereas the unshielded twisted pair cable used by the same network is rated at 100Ω.

What happens when you want to use cable with the wrong impedance on your network or connect two mismatched cables together? The device you need is a BALancing/UNbalancing resistor, called a BALUN for short.

Electromagnetic Interference

Electromagnetic interference (EMI) affects electrical signals whether traveling in a cable or being broadcast through the atmosphere. EMI can disrupt the signal or degrade it sufficiently so that it becomes unintelligible. It can be produced by many sources including electric motors, flourescent lights, other broadcasting equipment and even sunspot activity.

Some cables are shielded to protect against EMI. Typically the shielding takes the form of a copper or aluminum braid or foil which surrounds the conductor used to transmit the signal. Both coaxial and shielded twisted pair cable take this approach. If total immunity from EMI is required, fiber optic cable is used. Since it uses light instead of electricity, it is not effected by EMI.

Crosstalk

Cross talk is interference from adjacent cables that are close together and running side by side. Twisted pair cable has this problem. In fact, the twists in the conductors are put there specifically to try to counter this problem. As the cables twist about each other, the polarity of the signal reverses, nullifying the crosstalk to some extent but not eliminating it fully. You may have experienced crosstalk yourself on a telephone line if during a call, you overheard an unrelated conversation.

The measurement for crosstalk is called NEXT (Near End Crosstalk). It is so named because the test instrument makes the measurement where it is attached to the wire, the near end.

Exercise 3-1: Cable concepts

Answer the following questions.
1. _______________ is a measure of crosstalk.
2. _______________is a loss of signal.
3. _______________ can disrupt electrical signals.
4. _______________ is a measurement of the complex resistance to current flow.
5. If the electrical signal becomes too weak and you have to extend the cable, you would use a __________ or an ____________.
6. If your electrical signals were being disrupted by interference, you could switch to ___________ copper cable or non-metallic _____________ cable.
7. One measure which minimizes crosstalk is _________ the conductors.
8. If you need to connect two cables which have different impedances, you would use a ___________.

Digital vs. analog

Definition: Digital

Digital On and Off signaling: There can only be two conditions in a digital system, on or off, 1 or 0, positive or negative, black or white. One is represented by a 1 and off is represented by a 0. Because there can only be two conditions, a digital system is also binary. All communications with and within a computer are in a digital form. If they originate in some other form they must be converted before communications is possible. One unit of information is stored in a computer as a BInary digiT or a “BIT”. The opposite of digital is analog.

Definition: Analog

Analog Refers to the content of a transmission being determined by the strength, amplitude or frequency of a signal. Far from having only two conditions, an analog signal can have an infinite number of conditions. Voice is an example of an analog signal.

Sometimes the digital and analog worlds must interface with each other. Than a signal conversion must take place. A good example is sending digital computer data over the analog voice telephone system. The form of the analog carrier wave on the telephone system must be changed (modulated) so that it carries the digital encoding. In fact, this is exactly what a modem does.

Baseband vs. broadband

Baseband transmission

Baseband Baseband signaling uses the complete bandwidth of the cable system. Therefore, only one signal is ever found on the cable at one time. The signal is digital and can be electrical or light. The system is simple and is well suited to local area networks.

Broadband transmission

Broadband If the cable must carry multiple signals simultaneously, a broadband system must be used. How is this accomplished? For one thing, the signals are analog, not digital. This means they can exist as different frequencies and multiple signals, at different frequencies, can coexist on the cable. Broadband systems are more sophisticated and more complex than baseband and not considered suitable for most LANs. However, they do have their uses. Your personal experience with a broadband system will be in the form of cable TV, 200 channels but not a decent movie on Saturday night.

Broadband transmission is only in one direction, which explains why the cable TV companies have had to massively re-engineer their systems in order to deliver the Internet via cable. Two way communications is achieved by assigning one frequency to transmit and a different one to receive (mid-split broadband) or using a dual cable configuration.

Transmission Media

Bounded vs. unbounded

One key difference between transmission media is whether the signal is constrained in a channel. If it is, it is bounded and is propagated through a copper or fiber optic cable. If it is unbounded, the signal is propagated in all directions, or even if tightly focused, still spreads out. Good examples of the latter include radio waves, infrared, microwaves or satellite.

Cable types

Cable used in modern communication systems falls into three categories, coaxial cable, twisted pair or fiber optic.

Wireless communications

Wireless communications generally makes use of one of four technologies, radio, infrared, microwave or satellite transmissions.

Each of the above technologies has strengths and weaknesses and each will be examined in turn.

Exercise 3-2: Cable concepts crossword

Crossword

Fill in the crossword puzzle with these clues.

1. Across: System in which multiple signals can coexist on the cable.
2. Down: Signal which travels inside a channel.
3. Down: System in which the signal propagates through the atmosphere
4. Down: Signal with an infinite number of data points.
5. Across: Signaling system which allows only one signal on the system at one time.
6. Across: disrupts electrical signals.
7. Down: measurement of resistance to an electrical flow.
8. Across: measurement of crosstalk.
9. Down: Device which allows the connection of two cables of different impedance.
10. Across: Signals which can have only two conditions.

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B. Cable types C. Wireless D. Topologies E. Cable hardware F. Troubleshooting Review/Exercise