We’re excited to announce that Craig Zacker’s CompTIA Network+Training Kit (Exam N10-005) (ISBN: 9780735662759; 704 pages) is now available for purchase!
You can find the book’s objective mapping and Introduction to understanding and preparing for the CompTIA Network+ Exam N10-005 in this previous post.
Craig helps you to ace your preparation for the CompTIA Network+ Exam N10-005 with this 2-in-1 Training Kit from Microsoft Press®. Work at your own pace through a series of concepts, tasks, and practical exercises, and then assess your network service/support technician skills with the online practice test—featuring multiple, customizable testing options to meet your specific needs. Your Training Kit includes:
· In-depth coverage of all 5 CompTIA Network+ domains in Exam N10-005
· Instructive real-world scenarios to enhance your performance on the job
· Hundreds of practice and review questions
· Online practice tests to help you assess your exam readiness
The entire Training Kit in searchable eBook format
In today’s post, please enjoy reading an excerpt from Chapter 2, “The Physical Layer.”
The physical layer of the OSI reference model defines the characteristics of the hardware that connects the network interfaces in computers and other network devices together. In the case of a typical local area network (LAN), this hardware consists of a series of cables, as well as the connectors and other components needed to install them. This chapter describes the types of cables that data networks use, the procedures for installing and maintaining them, and the types of cabling issues that network administrators often have to address while on the job.
Note Wireless LANs
Wireless LANs are an increasingly popular physical layer option in homes, in offices, and in public businesses. This chapter is devoted to cabled networks. For wireless network coverage, see Chapter 5, “Implementing Wireless LANs.”
Exam objectives in this chapter:
Objective 3.1 Categorize standard media types and associated properties.
· T1 Crossover
· Plenum vs. non-plenum
· Media converters:
· Singlemode fiber to Ethernet
· Multimode fiber to Ethernet
· Fiber to Coaxial
· Singlemode to multimode fiber
· Distance limitations and speed limitations
· Broadband over powerline
Objective 3.2 Categorize standard connector types based on network media.
· DB-9 (RS-232)
· Patch panel
· 110 block (T568A, T568B)
Objective 3.5 Describe different network topologies.
· Point to point
· Point to multipoint
Objective 3.6 Given a scenario, troubleshoot common physical connectivity problems.
· Cable problems:
· Bad connectors
· Bad wiring
· Open, short
· Split cables
· DB loss
· TXRX reversed
· Cable placement
Objective 3.8 Identify components of wiring distribution.
· Demarc extension
· Smart jack
Objective 4.2 Given a scenario, use appropriate hardware tools to troubleshoot connectivity issues.
· Cable tester
· Cable certifier
· Butt set
· Toner probe
· Punch down tool
· Protocol analyzer
· Loop back plug
· Environmental monitor
Network Administrators and Cables
When you are building a cabled LAN, there are two basic ways to proceed: you can purchase prefabricated components—such as packaged patch cables with the connectors already attached—and simply connect the network components together yourself, or you can purchase bulk cable on a spool, pull it through the walls and ceilings, and attach the ends to wall plates and patch panels containing the appropriate connectors.
For home networks, small businesses, or installations in which portability is more important than invisibility, the first option is inexpensive and easy to install without special tools or skills. For large networks, those involving many rooms or floors, or those requiring a nearly invisible installation, the bulk cable method is preferable.
Installing bulk cable is a specialized skill that network administrators typically outsource to a wiring contractor who has the appropriate tools and experience, often the same one who handles the office’s telephone wiring. Depending on the local building and fire codes, a licensed electrician and official inspection might be required. It is relatively rare for the average network administrator to perform large cable installations, but he or she might be expected to troubleshoot cabling problems and use basic cable testing tools as part of the troubleshooting process.
Cables and Connectors
As noted in Chapter 1, “Networking Basics,” a data network must have a network medium that connects the computers and other devices together. Until relatively recently, that network medium was a cable of some sort. Most of the cables used throughout the history of data networks have been copper-based. One or more strands of electrically conductive copper are encased within a non-conductive sheath. The computers transmit signals by applying electric voltages to the copper conductors in a pattern that the receiving systems interpret as binary data.
Key Term: attenuation
Although copper cables are relatively efficient transmitters of signals, they are prone to a few shortcomings that have occasionally made them troublesome in certain network situations. The first problem is that signals transmitted over copper cables are prone to attenuation; that is, the gradual weakening of the signal over distance. The longer the cable is, the weaker the signal gets, until it eventually becomes unviable. There are, therefore, always length limitations to copper-based network segments.
Key Term: electromagnetic interference (EMI)
Signals traveling over copper cables are also prone to disruption by noise and electromagnetic interference (EMI), such as that caused by other electrical equipment. When installing copper cables, contractors must be careful to avoid sources of EMI.
Key Term: fiber optic
To avoid these problems, it is also possible to build a network by using another type of cable, called fiber optic cable. A fiber optic cable consists of a glass or plastic core surrounded by a reflective sheath. The signals on a fiber optic network are not electrical; they are beams of light generated by a laser. Fiber optic networks are much less affected by attenuation than copper networks, so they can span much greater distances. Fiber optic networks are often utilized in connecting LANs or remote networks. They are also completely resistant to EMI, making them a viable medium in environments where copper networks cannot function.
Fiber optic is—and has always been—a marginal networking technology for local area network (LAN) installations, due its much greater cost when compared to copper, and the special skills and tools required to install and maintain it. Copper-based LAN cable installations have always vastly outnumbered fiber optic ones, not least because the copper cables used today are similar to those used for telephone systems.
However, fiber optic is the dominant cable technology for wide area network (WAN) infrastructures, such as those used to connect LANs together and deliver broadband services to residential and business customers. It is estimated by the Telecommunications Industry Association that the amount of installed fiber optic cable has more than doubled in the last seven years.
The average Network+ exam candidate has probably seen and worked with twisted pair cabling, but might not have experience with fiber optic cable. Be sure not to neglect this critical part of the exam objectives.
Copper Cable Types
For the most part, the history of copper-based data networks mirrors the history of the Ethernet LAN protocol. As Ethernet has evolved in the nearly four decades of its existence, it has increased in speed and changed the types of copper cables defined in its physical layer specifications. The following sections describe the various types of copper cables that Ethernet has supported over the years.
Twisted Pair Cable
Key Term: crosstalk
A twisted pair cable is a cluster of thin copper wires, with each wire having its own sheath of insulation. Individual pairs of insulated wires are twisted together, usually gathered in groups of four pairs, for a total of eight wires, and encased in another insulating sheath, as shown in Figure 2-1. The individual wire pairs in the sheath are twisted at differing rates of twists per foot, which is crucial to making the wires in the same cable resistant to crosstalk—that is, the bleeding of signals from one wire pair to another in the same cable.
Figure 2-1 A twisted pair cable.
There are many different types of twisted pair cables, designed to accommodate various environmental, legal, and performance requirements. In addition to the conductive wires, some types of twisted pair cable have an extra piece of fabric or other material to help strengthen the cable and to aid in removing the outer cover. These cable types are described in the following sections.
Unshielded Twisted Pair
Key Term: unshielded twisted pair (UTP)
Unshielded twisted pair (UTP) cable is the standard medium for copper-based Ethernet LANs these days. A typical UTP cable contains four wire pairs within a sheath approximately 0.21 to 0.24 inches in outside diameter. UTP grew to replace coaxial cable for Ethernet networks for two main reasons: first, the eight separate wires in a UTP cable make it much more flexible and easy to install within walls and ceilings. Second, voice telephone systems also tend to use UTP, which means that the same contractors who install telephone cables can often install data networking cables as well.
Note Unused Wire Pairs
Most UTP cables for Ethernet networks have four pairs (eight wires) in a single sheath, but in some cases the network nodes use only two of the four pairs for actual communication. Despite the fact that the other two pairs are left unused, it is not permitted to make use of them for other traffic, such as voice telephone communications. Leaving these additional wire pairs unused can also provide an avenue for a future upgrade to a four-pair technology.
Key Term: butt set
Voice telephone networks do not have performance requirements as strict as those for data networks, so they are less liable to suffer from crosstalk and other types of interference. As a result, installers often use larger UTP cables for telephone connections. UTP cables are available in configurations containing 25 wire pairs and 100 wire pairs in a single sheath, which enables installers to service multiple users with a single cable. Telephone company cable installers also have their own specialized tools, such as the ubiquitous butt set, a one-piece telephone with alligator clips that enables its operator to connect to a line anywhere that the cables are accessible.
UTP cables containing 25 or 100 wire pairs are suitable only for telephone connections, not for data networking. These types of telephony cables were at one time included in the Network+ exam objectives, but they have since been removed.
Shielded Twisted Pair
Key Term: shielded twisted pair (STP)
For environments with greater levels of EMI, there are also various types of shielded twisted pair (STP) cables. Some STP variants have metal shielding around each pair of wires, which provides greater protection against crosstalk. Some STP cables have shielding inside the sheath surrounding all of the pairs. This type of shielding is called screening and protects against outside EMI sources. Some cable types have both shielding and screening.
Some now-obsolete data-link layer protocols, such as Token Ring, had physical layer specifications that called for STP cables. Today, however, Ethernet networks only use them in special situations that require additional protection from EMI.
Stranded vs. Solid Conductors
As mentioned earlier, copper-based cables are subject to attenuation, and one of the factors affecting that subjectivity is the composition of the copper conductors within the individual wires. A twisted pair cable that has stranded conductors uses seven separate copper filaments within each of its eight wires. These separate filaments make the cable more flexible, and therefore easier to install and move without damage, but they also make it more expensive and more subject to attenuation, limiting its maximum length.
For cable segments 50 feet or longer, it is more common to see UTP cables with solid conductors; that is, with a single, thicker copper filament within each wire. The solid conductors make the cables stiffer, more prone to breakage, and more difficult to install, but they are also more resistant to attenuation.
Plenum vs. Non-Plenum
A plenum is an enclosed air space within a building, a passage through which HVAC equipment circulates breathable air, such as the space above a dropped ceiling or below a raised floor. When cables are to be installed in an existing building, using the plenums is often the most expedient way to run a cable from one location to another. The only problem with this is that in the event of a fire, the polyvinyl chloride (PVC) outer sheath of many cables can outgas toxic vapors when it burns. Releasing these vapors into an air space can obviously be hazardous to the people in the building. A plenum-grade cable is a cable with a sheath that produces less toxic smoke when it burns. Plenum-grade cables are typically much more expensive than non-plenum cables, but building codes in many cities require them when you install data network cable in air spaces.
Not every drop ceiling or raised floor is necessarily used for air handling. If you are uncertain, look for air vents, returns, or ducting, or check with the building engineer or an architect.
For the Network+ exam, be sure to understand what a plenum is and why it is a significant factor in cable selection.
Some manufacturers create UTP cables with special qualities that are designed to enable their use in special environments. For example, cables with UV-rated sheaths can survive exposure to direct sunlight without degradation.
A UTP cable also has an air space inside the sheath, where condensation can gather when the cable runs between indoor and outdoor environments or is buried directly in the ground. In these environments, installers might want to seal the ends of the cable or fill them with gel to eliminate the air space.
UTP cable comes in a variety of different grades, called categories in the cabling standards published by the Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA). The categories define the signal frequencies that the various cable types support, along with other characteristics, such as resistance to certain types of interference. The higher the category number, the higher the cable quality and, not surprisingly, the price. The cable categories that administrators are likely to encounter are as follows:
· Category 3 (CAT3) Long the standard for telephone communications, CAT3 cables were used by the first UTP-based Ethernet networks (called 10Base-T). CAT3 cables supported frequencies up to 16 megahertz (MHz). Insufficient for any of the faster Ethernet types, CAT3 is no longer supported for new installations.
· Category 5 (CAT5) Designed for 100Base-TX Fast Ethernet networks and supporting frequencies up to 100 MHz, CAT5 cabling was dropped from the latest version of the TIA/EIA cabling standards.
· Category 5e (CAT5e) Still rated for frequencies up to 100 MHz, CAT5e cable is designed to support full duplex transmissions over all four wire pairs, as on 1000Base-T Gigabit Ethernet networks. The standard calls for increased resistance to Near-End Crosstalk (NEXT) and Return Loss (RL) and also adds testing requirements for Power Sum Near-End Crosstalk (PS-NEXT), Equal-Level Far-End Crosstalk (EL-FEXT), and Power Sum Equal-Level Far-End Crosstalk (PS-ELFEXT).
· Category 6 (CAT6) Designed to support frequencies of up to 250 MHz, CAT6 cables easily handle 1000Base-T Gigabit Ethernet traffic and, with special installation considerations, 10Gbase-T.
· Augmented Category 6 (CAT6A) Created for 10Gbase-T installations with cable segments up to 100 meters long, CAT6a supports frequencies up to 500 MHz and includes an Alien Crosstalk (AXT) testing requirement. CAT6a cables use larger conductors and leave more space between the wire pairs, meaning that the outside diameter of the sheath is larger than a CAT6 cable, about 0.29 to 0.35 inches. CAT6a was added to the most recent version of the TIA/EIA standards in 2008.
· Category 7 (CAT7) Not officially ratified by the TIA/EIA, the CAT7 standard calls for a fully shielded and screened cable design, supporting frequencies up to 600 MHz. At the current time, this cable is recommended for high-bandwidth applications such as broadband video, environments with high levels of EMI, or as a lower-cost substitute for fiber optic segments.
· Category 7a (CAT7a) CAT7a, also not ratified by the TIA/EIA, is a fully shielded and screened cable that extends the frequency range to 1000 MHz. With full support for 10Gbase-T, CAT7a is expected to have a lifespan of 15 years or more, including the next iteration of Ethernet, running at 40 gigabytes per second.
Although CAT3 and CAT5 cables have been dropped from the official standards, this does not mean that administrators no longer encounter them. There are many CAT5 installations still in operation, and CAT5 cables are still available to maintain them.
important Cable Standards
If you are working in a CAT5 environment and you have no plans to install a higher grade of cable in the near future, it makes little sense to pay extra money for CAT5e or CAT6 patch cables. Like a chain, a network is only as strong as its weakest link, so CAT5 products are likely to remain available for a while.
CAT3 cable is still included in the Network+ objectives, but in today’s networks it is decidedly less prevalent in the installed base, because most CAT3 networks have long since been upgraded to at least Fast Ethernet (at 100 megabits per second), which requires a minimum of CAT5 cable.
At the time of this writing, the data networking cable marketplace is in a period of transition from CAT5e to CAT6 as the dominant cable type. Sales of CAT5 cable still dominate the market, but forward-thinking network designers and administrators who consider 10Gbase-T part of their future, or who run high-bandwidth applications such as streaming video, would be well advised to consider installing CAT6 or higher.
Twisted Pair Connectors
Key Term: RJ45
Twisted pair cables use modular connectors that are most commonly referred to by the telephone networking designation RJ45, but which should properly be called 8P8C. Network interface adapters, wall plates, patch panels, and other networking components, such as switches and hubs, all have female connectors. The patch cables used to connect everything together have male connectors, as shown in Figure 2-2.
Figure 2-2 A UTP patch cable with an 8P8C (often incorrectly referred to as RJ45) connector.
Key Term: RJ11
Although designed for telephones, the RJ45 connector was actually seldom used for that purpose. The smaller, four-pin RJ11 connector became the standard for telephone connections, and remains so to this day. When UTP came into use for data networks, the connectors were so similar in appearance to the RJ45 telephone company connector that network administrators adopted the designation for their own use.
Note Categorizing Connectors
Connectors for UTP cables are categorized, just as the cables are. When purchasing components for a modular cable installation, be sure that you select connectors of the same category rating as your cable. A cable installation must be rated according to its lowest-rated component. You might use all CAT6 cable for your network, but if you use CAT5 connectors, it is a CAT5 installation.
Key Term: 8P8C
The actual name for the UTP connector that Ethernet networks use is 8P8C, indicating that there are eight positions in the connector and eight electrical contacts in place in those positions. The RJ45 for telephone use is also known as an 8P2C, because though it has the same eight positions, there are only two conductors connected.
Although a network administrator might be technically correct in referring to an Ethernet cable as having 8P8C connectors, few (if any) other people will know what he or she is talking about. RJ45 is the colloquial term that the CompTIA exam and much of the networking literature (as well as the rest of this book) uses to refer to this connector.
Key Term: coaxial cable
Prior to the introduction of UTP cables, Ethernet networks called for coaxial cable of various types. A coaxial cable consists of a central copper conductor—which carries the signals—surrounded by a layer of insulation. Surrounding the insulation is a shielding, typically made of copper mesh – which functions as a ground—and the whole assembly is encased in a sheath, as shown in Figure 2-3. The mesh shielding in coaxial cables makes them quite resistant to EMI.
Figure 2-3 A coaxial cable.
Ethernet over Coaxial Cable
All of the coaxial cable types have been removed from the TIA/EIA network cabling standards, and you are unlikely to encounter any coaxial Ethernet networks in the field, but they are an important part of the history of data networking.
The physical layer specifications for the first Ethernet networks—referred to as thick Ethernet, thicknet, or 10Base5—called for a type of coaxial cable similar to RG-8, but with extra braided shielding. RG-8 is a 50-ohm cable, 0.405 inches in diameter. The large conductors, thick insulation, and frequently yellow sheath make the cable relatively inflexible, resulting in the nickname “frozen yellow garden hose."
Note Coaxial Cores
Coaxial cable designations appended with a /U suffix indicate that the cable has a solid core. Designations with an A/U suffix indicate that the core is stranded.
Key Term: vampire taps
Thick Ethernet cable was so inflexible that in had to be installed in a relatively straight run, usually along the floor. To connect it to computers and other devices, installers used thinner drop cables that connected to the coaxial by using external transceivers with metal teeth that pierced the insulation and made contact with the conductors. These connectors were called vampire taps. The other end of the transceiver cables had AUI connectors that plugged into an external transceiver or a network interface adapter.
Key term: BNC connector
Later iterations of Ethernet—called thin Ethernet, thinnet, or 10Base2—required RG-58 coaxial cable, still 50-ohm but much thinner than RG-8 (0.195 inches in diameter) and relatively flexible. This type of coaxial cable could run all the way to the individual computers, using a T configuration with three BNC connectors, as shown in Figure 2-4.
The Network+ objectives do not include a spellout for the acronym BNC, so you do not have to be concerned about remembering the correct one. Various sources cite several different meanings, including “British Naval Connector” and “Bayonet Nut Connector.” In fact, the Bayonet Neill-Concelman connector was invented by and named after two engineers, Paul Neill and Carl Concelman—neither of whom was British—who developed the connector in the late 1940s.
Figure 2-4 A coaxial cable with BNC connectors.
Cable TV over Coaxial Cable
Although coaxial cable is no longer used for Ethernet networks, it still has applications in the networking industry. Perhaps the application most familiar to people is cable television (CATV) networking. CATV networks often use fiber optic cables for the trunk lines, but in most cases, the service enters the subscribers' homes by using 75-ohm coaxial cable.
Key Terms: RG-59, RG-6
Early CATV networks used RG-59 coaxial, a cable with relatively light shielding that was sufficient for short runs. Today, this cable’s primary use is for closed-circuit television. With digital cable and Internet access nearly ubiquitous on cable systems, CATV providers switched to RG-6 coaxial, which has more shielding and is therefore slightly larger in diameter (0.27 inches versus 0.242 inches for RG-59).
Key Term: F connector
LANs that use a CATV provider for Internet access have a modem to which you connect the incoming RG-6 cable, using a screw-on F connector, as shown in Figure 2-5. The modem also usually has RJ45 and USB connectors, which you use to attach it to a router or a computer.
Figure 2-5 A coaxial cable with F connectors.