Physical Media

Although it is possible to use several forms of wireless networking, such as radio and infrared, most networks communicate via some sort of cable. In this section, we'll look at three types of cables:

• Coaxial

• Twisted-pair Fiber-optic

• Coaxial Cable

Coaxial cable (or coax) contains a center conductor, made of copper, surrounded by a plastic jacket, with a braided shield over the jacket. A plastic such as PVC or Teflon covers this metal shield. The Teflon-type covering is frequently referred to as a plenum-rated coating. That simply means that the coating does not produce toxic gas when burned (as PVC does) and is rated for use in air plenums that carry breathable air. This type of cable is more expensive but may be mandated by electrical code whenever cable is hidden in walls or ceilings. Plenum rating applies to all types of cabling.

Coaxial cable is available in different specifications that are rated according to the RG Type system. Different cables have different specifications and, therefore, different RG grading designations (according to the U.S. military specification MIL-C-17). Distance and cost are also considerations when selecting coax cable. The thicker the copper, the farther a signal can travel- and with that comes higher costs and a less flexible cable.

Using Thick Ethernet

The original Ethernet cable is known as Thick Ethernet cable, or Thicknet. It is also called 10Base5 and is graded as RG-8. To the folks who installed the cable, it was more commonly called a 'frozen garden hose' because of its ½" diameter.

With Thick Ethernet, a station attaches to the main cable via a vampire tap, which clamps onto the cable. A vampire tap is so named because a metal tooth sinks into the cable, thus making the connection with the inner conductor. The tap is connected to an external transceiver that, in turn, has a 15-pin AUI connector (also called DIX or DB-15 connector) to which you attach a cable that connects to the station (shown in Figure 1.12). DIX got its name from the companies that worked on this format-Digital, Intel, and Xerox.

Figure 1.12: Thicknet and vampire taps

Not every Thick Ethernet cable connection type is a DIX. The other option that is found occasionally is the N-series connector. The N connector comes in a male/female screw-and-barrel configuration. A CB radio uses the PL-259 connector, and the N connector looks similar (as shown in Figure 1.13).

Figure 1.13: An N-series connector

Using Thin Ethernet

Thin Ethernet, also referred to as Thinnet or 10Base2, is a thin coaxial cable. It is basically the same as thick coaxial cable except that the diameter of the cable is smaller (about 1/4" in diameter). Thin Ethernet coaxial cable is RG-58. Figure 1.14 shows an example of Thin Ethernet.

Figure 1.14: A stripped-back Thinnet

With Thinnet cable, you use BNC connectors (see Figure 1.15) to attach stations to the network. It is beyond my province to settle the long-standing argument over the meaning of the abbreviation BNC. BNC could mean BayoNet Connector, Bayonet Nut Connector, or British Navel Connector. What is relevant is that the BNC connector locks securely with a quartertwist motion.

Figure 1.15: A male and female BNC connector

Tip 

The BNC connector can be attached to a cable in two ways. The first is with a crimper, which looks like funny pliers and has a die to hold the connector. Pressing the levers crimps the connector to the cable. Choice number two is a screw-on connector, which is very unreliable. If at all possible, avoid the screw-on connector!

Table 1.2 shows some of the specifications for the different types of coaxial cable.

Table 1.2: Coaxial Cable Specifications

RG Rating	Popular Name	Ethernet Implementation	Type of Cable
RG-58 U	N/A	None	Solid copper
RG-58 AU	Thinnet	10Base2	Stranded copper
RG-8	Thicknet	10Base5	Solid copper
RG-62	ARCnet	N/A	Solid/stranded

Note 

Although some great advantages are associated with using coax cable, such as the braided shielding that provides fair resistance to electronic pollution like electromagnetic interference (EMI) and radio frequency interference (RFI), all types of stray electronic signals can make their way onto a network cable and cause communications problems. Understanding EMI and RFI is critical to your networking success. For this reason, we'll go into greater detail in Chapter 6.

Signal Bounce

With coaxial cable, the signal travels up and down the entire length of the wire. When the signal reaches the end of the wire, the electrical change from copper to air prevents the conversation from simply falling out the end. Instead, the signal bounces back down the wire it just traversed. This creates an echo, just as if you were yelling into a canyon. These additional signals on the wire make communication impossible. To prevent this, you place a terminator on each end of the wire to absorb the unwanted echo.

Technically, proper termination also requires that one terminator be connected to a ground. Connecting both terminators to a ground can create a ground loop, which can produce all kinds of bizarre, ghostlike activity (for example, a network share that appears and disappears).

If you are not sure where to find a good ground point, connect one terminator to a screw holding a power supply inside a computer. This ensures that you are using the same ground as the PC. This does assume, however, that the outlet into which the PC is plugged is properly grounded.

Twisted-Pair Cable

Twisted-pair cable consists of multiple, individually insulated wires that are twisted together in pairs. Sometimes a metallic shield is placed around the twisted pairs. Hence, the name shielded twisted-pair (STP). (You might see this type of cabling in Token Ring installations.) More commonly, you see cable without outer shielding; it's called unshielded twisted-pair (UTP). UTP is commonly used in 10BaseT, star-wired networks.

Let's take a look at why the wires in this cable type are twisted. When electromagnetic signals are conducted on copper wires that are in close proximity (such as inside a cable), some electromagnetic interference occurs. In this scenario, this interference is called crosstalk. Twisting two wires together as a pair minimizes such interference and also provides some protection against interference from outside sources. This cable type is the most common today. It is popular for several reasons:

• It's cheaper than other types of cabling.

• It's easy to work with.

• It permits transmission rates considered impossible 10 years ago.

UTP cable is rated in the following categories:

Category 1  Two twisted-pair (four wires). Voice grade (not rated for data communications). The oldest UTP. Frequently referred to as POTS, or plain old telephone service. Before 1983, this was the standard cable used throughout the North American telephone system. POTS cable still exists in parts of the Public Switched Telephone Network (PSTN).

Category 2  Four twisted-pair (eight wires). Suitable for up to 4Mbps.

Category 3  Four twisted-pair (eight wires), with three twists per foot. Acceptable for 10Mbps. A popular cable choice since the mid-80s.

Category 4  Four twisted-pair (eight wires) and rated for 16Mbps.

Category 5  Four twisted-pair (eight wires) and rated for 100Mbps.

Category 6  Four twisted-pair (eight wires) and rated for 1000Mbps. Became a standard in December 1998.

Note 

Frequently, you will hear Category shortened to Cat. Today, any cable that you install should be a minimum of Cat 5. This is a minimum because some cable is now certified to carry a bandwidth signal of 350MHz or beyond. This allows unshielded twisted-pair cables to reach a speed of 1Gbps, which is fast enough to carry broadcast-quality video over a network. A common saying is that there are three ways to do things: the Right way, the Wrong way, and the IBM way. IBM uses types instead of categories when referring to TP (twisted-pair) cabling specifications. Even though a cabling type may seem to correspond to a cabling category (such as Type 1 and Category 1), the two are not the same; IBM defines its own specifications.

Category 5 Cabling Tips

If you expect data rates faster than 10Mbps over UTP, you should ensure that all components are rated to the category you want to achieve and be very careful when handling all components. For example, pulling too hard on Cat 5 cable will stretch the number of twists inside the jacket, rendering the Cat 5 label on the outside of the cable invalid. Also, be certain to connect and test all four pairs of wire. Although today's wiring usually uses only two pairs, or four wires, at the time of this writing the proposed standard for Gigabit Ethernet over UTP requires that all four pairs, or eight wires, be in good condition.

You should also be aware that a true Cat 5 cabling system uses rated components from end to end, patch cables from workstation to wall panel, cable from wall panel to patch panel, and patch cables from patch panel to hub. If any components are missing or if the lengths do not match the Category 5 specification, you don't have a Category 5 cabling installation. Also, installers should certify that the entire installation is Category 5 compliant.

Connecting UTP

Clearly, a BNC connector won't fit easily on UTP cable, so you need to use an RJ (Registered Jack) connector. You are probably familiar with RJ connectors. Most telephones connect with an RJ-11 connector.

The connector used with UTP cable is called RJ-45. The RJ-11 has four wires, or two pairs, and the network connector RJ-45 has four pairs, or eight wires, as shown in Figure 1.16.

Figure 1.16: RJ-11 and RJ-45 connectors

In almost every case, UTP uses RJ connectors. Even the now-extinct ARCnet used RJ connectors. You use a crimper to attach an RJ connector to a cable, just as you use a crimper with the BNC connector. The only difference is that the die that holds the connector is a different shape. Higherquality crimping tools have interchangeable dies for both types of cables.

Signaling Methods

The amount of a cable's available bandwidth (overall capacity, such as 10Mbps) that is used by each signal depends on whether the signaling method is baseband or broadband. Baseband uses the entire bandwidth of the cable for each signal (using one channel). It is typically used with digital signaling.

In broadband, multiple signals can be transmitted on the same cable simultaneously by means of frequency division multiplexing (FDM). Multiplexing is dividing a single medium into multiple channels. With FDM, the cable's bandwidth is divided into separate channels (or frequencies), and multiple signals can traverse the cable on these frequencies simultaneously.

FDM is typically used for analog transmissions. Another method, time division multiplexing (TDM), can also be used to further divide each individual FDM frequency into individual time slots. Additionally, TDM can be used on baseband systems.

Ethernet Cable Descriptions

Ethernet cable types are described using a code that follows this format: N<Signal>X. Generally speaking, N is the signaling rate in megabits per second, and <Signal> is the signaling type, which is either base or broad (baseband or broadband). X is a unique identifier for that specific Ethernet cabling scheme.

Let's use a generic example: 10BaseX. The two-digit number 10 indicates that the transmission speed is 10Mb, or 10 megabits. The value X can have different meanings. For example, the 5 in 10Base5 indicates the maximum distance that the signal can travel-500 meters. The 2 in 10Base2 is used the same way, but fudges the truth. The real limitation is 185 meters. Only the IEEE committee knows for sure what this was about. We can only guess that it's because 10Base2 seems easier to say than 10Base1.85.

Another 10Base standard is 10BaseT. The T is short for twisted-pair. This is the standard for running 10-Megabit Ethernet over two pairs (four wires) of Category 3, 4, or 5 UTP. The fourth, and currently final, 10Base is 10BaseF. The F is short for Fiber. 10BaseF is the standard for running 10-Megabit Ethernet over fiber-optic cable. Table 1.3, shown a bit later in this section, summarizes this data.

100BaseT

As network applications increased in complexity, so did their bandwidth requirements. Ten-megabit technologies were too slow. Businesses were clamoring for a higher speed standard so that their data could be transmitted at an acceptable rate of speed. A 100-megabit standard was needed. Thus the 100BaseT standards were developed.

The 100BaseT standard is a general category of standards for Ethernet transmissions at a data rate of 100Mbps. This Ethernet standard is also known as Fast Ethernet. There are two major standards for 100BaseT:

100BaseTX  The implementation of 100BaseT that is simply a faster version of 10BaseT. It uses two UTP pairs (four wires) in a Category 5 UTP cable (or Type 1 STP).

100BaseT4  The implementation of 100BaseT that runs over four pairs (eight wires) of Category 3, 4, or 5 UTP cable.

100BaseVG

This 100-Megabit Ethernet replacement came from HP, and in the popularity race, it lost. Even the name wasn't settled upon, so you may find it referred to as VG LAN, VGAnyLAN, or AnyLAN. Although it used UTP cable, it didn't follow the popular Ethernet standard. It attempted to improve on Ethernet by using collision avoidance as a method of controlling network traffic.

You will see details on Ethernet and several methods of handling traffic in chapters throughout this book. The point here is that the 100BaseVG standard was not compatible with 10BaseX and Ethernet. The combination of its incompatibility and its actually less than 100Mbps throughput (due to its media access method, which is discussed elsewhere in this book) ultimately spelled its demise. However, it was basically 100Mb, and it was out the door early in the game. Because some companies implemented this standard, you need to know about it.

Fiber-Optic Cable

Because fiber-optic cable transmits digital signals using light pulses rather than electricity, it is immune to EMI and RFI. You will find a complete discussion of these terms in Chapter 6, but you should know at this point that both could affect network performance. Anyone who has seen UTP cable for a network run down an elevator shaft would, without doubt, appreciate this feature of fiber. Light is carried on either a glass or a plastic core. Glass can carry the signal a greater distance, but plastic costs less. Regardless of which core is used, there is a shield wrapped around it, and it is surrounded by cladding, which is more glass that refracts the light back into the core. This is then wrapped in an armor coating, typically Kevlar, and then sheathed in PVC or Plenum.

Note 

For more information about fiber-optic cabling, see The Network Press Encyclopedia of Networking, published by Sybex.

Fiber-optic cables can use a myriad of different connectors, but the two most popular and recognizable are the straight tip (ST) and subscriber connector (SC) connectors. The ST fiber-optic connector, developed by AT&T, is probably the most widely used fiber-optic connector. It uses a BNC attachment mechanism similar to the Thinnet connection mechanism, which makes connections and disconnections fairly easy. Its ease of use is one of the attributes that makes this connector so popular. Figure 1.17 shows an example of an ST connector. Notice the BNC attachment mechanism.

Figure 1.17: An example of an ST connector

The SC connector (sometimes known also as a square connector) is another type of fiber-optic connector. As you can see in Figure 1.18, SC connectors are latched connectors. This makes it impossible for the connector to be pulled out without releasing the connector's latch (usually by pressing some kind of button or release).

Figure 1.18: A sample SC connector

SC connectors work with either single-mode or multimode optical fibers, and they will last for around 1,000 matings. They are seeing increased use, but aren't as popular as ST connectors for LAN connections.

Note 

If data runs are measured in kilometers, fiber-optic is your cable of choice, because copper cannot reach more than 500 meters (about 1500 feet) without electronics regenerating the signal. You may also want to opt for fiber-optic cable if an installation requires high security, because it does not create a readable magnetic field. Although fiber-optic technology was initially very expensive and difficult to work with, it is now being used in some interesting places, such as Gigabit Internet backbones. Also, some companies plan to bring fiber-optic speeds to the desktop. Ethernet running at 10Mbps over fiber-optic cable is normally designated 10BaseF; the 100Mbps version of this implementation is 100BaseFX.

Although fiber-optic cable may sound like the solution to many problems, it has pros and cons, just as the other cable types. On the pro side, fiber-optic cable:

• Is completely immune to EMI or RFI

• Can transmit up to 4 kilometers (about 2 miles)

On the con side, fiber-optic cable:

• Is difficult to install

• Requires a bigger investment in installation and materials

Table 1.3 summarizes the cable types discussed in this section.

Table 1.3: Common Ethernet Cable Types

Ethernet Name	Cable Type	Maximum Speed	Maximum Transmission Distance	Notes
10Base5	Coax	10Mbps	500 meters per segment	Also called Thicknet, this cable type uses vampire taps to connect devices to cable.
10Base2	Coax	10Mbps	185 meters per segment	Also called Thinnet, a very popular implementation of Ethernet over coax.
10BaseT	UTP	10Mbps	100 meters per segment	One of the most popular network cabling schemes.
100BaseT	UTP	100Mbps	100 meters per segment	One of the most popular network cabling schemes.
100BaseVG	UTP	100Mbps	213 meters (Cat 5); 100 meters (Cat 3)
100BaseT4	UTP	100Mbps	100 meters per segment	Requires four pairs of Cat 3, 4, or 5 UTP cable.
100BaseTX	UTP, STP	100Mbps	100 meters per segment	Two pairs of Category 5 UTP or Type 1 STP.
10BaseF	Fiber	10Mbps	Varies (ranges from 500 meters to 2000 meters)	Ethernet over fiber-optic implementation.
100BaseFX	Fiber	100Mbps	2000 meters	100Mbps Ethernet over fiber-optic implementation.
1000BaseT	Copper	1000Mbps	100 meters
1000BaseSX (Gigabit Ethernet)	Multimode Fiber	1000Mbps	260 meters	Uses SC fiber connectors.
1000BaseTX (Gigabit Ethernet)	Category 5 UTP	1000Mbps	100 meters	Uses same connectors as 10BaseT.
1000BaseLX	Multimode Fiber	1000Mbps	550 meters	Uses longer wavelength laser than 1000BaseSX.
FDDI	Multimode Fiber	100Mbps	10 kilometers	Uses MIC connector.