Coaxial Cable

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 polyvinyl chloride (PVC) or fluoroethylenepropylene (FEP, such as DuPont’s Teflon) covers this metal shield. The Teflon-type covering is frequently referred to as a plenum-rated coating. That simply means that the coating

doesn’t begin burning until a much higher temperature, doesn’t release as many toxic fumes as PVC when it does burn, and is rated for use in air plenums that carry breathable air, usually as

nonenclosed fresh-air return pathways that share space with cabling. This type of cable is more expensive but may be mandated by local or municipal fire code whenever cable is hidden in

walls or ceilings. Plenum rating applies to all types of cabling and is an approved replacement for all other compositions of cable sheathing and insulation, such as PVC-based assemblies.

Note:

As a certified Network+ technician, you no longer need to concern yourself with
the Thicknet and RG-58A/U (Radio Grade) types of coaxial cable, unless you
would like to do your own research for historical or nostalgic purposes. Today,
your focus should migrate from the 50ohm coax of early Ethernet to the 75ohm
coax of early (and modern, of course) cable television. The reason for this is
that while coax in the Ethernet world is all but a thing of the past, RG-6 or CATV
coax is alive and well in the world of broadband cable (cable modem) technology.
Chapter 7 will detail the location of 75ohm coaxial cable when used in a
cable-modem system. The connectors used with coax in this environment are
the same F-Type connectors used for standard cable television connectivity. In
fact, the data rides on the same medium, just over different frequencies.

Using Thin Ethernet

    Thin Ethernet, also referred to as Thinnet or 10Base-2, is a thin coaxial cable. It is basically the same as thick coaxial cable except that the diameter of the cable is smaller (about 1/4½ in diameter).

Thin Ethernet coaxial cable is RG-58. Figure 1.10 shows an example of Thin Ethernet. With Thinnet cable, you use BNC connectors (see Figure 1.11) 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. But it is most commonly referred to as the Bayonet Neill-Concelman connector. What is relevant is that the BNC connector locks securely with a quarter-twist motion.

Example:

A stripped-back Thinnet

Example:

A male and female BNC connector

    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 crimp 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!

In order to attach the backbone cable run to each station, a passive device, known as a T-connector, is used. Picture the uncut backbone cable extending to the back of each device. In order

to complete the connection, the cable needs to be cut at the point where the loop is closest to the interface. The two cut ends then need to be terminated with male BNC connectors and

plugged into the two female BNC interfaces of the T-connector, with the third, male connector attaching to the female BNC interface on the device’s NIC card. It is in violation of the standard

to have any sort of drop cable extending from the back of the device, unlike 10Base-5, where

such an attachment was customary. This requirement introduces a minimum of two caveats. The first is that any user that gains access to the back of their computer, and that wouldn’t be very hard, could disconnect the connectorized ends of the cut backbone, thus producing two unterminated LAN segments, neither one working properly. The second is that so many interconnections introduce failure points and opportunities for noise introduction.

Table 1.1 shows some of the specifications for the different types of coaxial cable.

Table 1.1:

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.

Using F-Type Connectors

The F-Type connector is a threaded, screw-on connector that differs from the BNC connector of early Ethernet mainly in its method of device attachment. Additionally, as alluded to earlier,

you typically find F-Type connectors with 75ohm coaxial media and BNC connectors with 50ohm applications. As with most other coax applications, the F-Type connector uses the center conductor of the coaxial cable as its center connecting point. The other conductor is the metal body of the connector itself, which connects to the shield of the cable. Again, due to the popularity of cable modems, the F-Type coaxial connector has finally made its way into mainstream data networking. Figure 1.12 shows an example of an F-Type coaxial connector.

Note:

There is also a twist-on F-Type connector used in fiber-optic cabling, known as
the FC connector.

Example: 

FIGURE 1 . 1 2 An example of an F-Type coaxial cable connector

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Physical Media

Physical Media

Although it is possible to use several forms of wireless networking, such as radio frequency and infrared, the majority of installed LANs today communicate via some sort of cable. In the following

sections, we’ll look at three types of cables:

•  Coaxial
• Twisted pair
• Fiber optic

Example:

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Selecting the Right Topology

Selecting the Right Topology

Each topology has its advantages and drawbacks. The process of selecting a topology can be much like buying a pair of shoes. It’s a matter of finding something that fits, feels right, and is
within your budget. Instead of asking what your shoe size is, ask questions such as, How much fault tolerance is necessary? and How often will I need to reconfigure the network? Creating a
simple network for a handful of computers in a single room is usually done most efficiently by using a wireless access point and wireless network cards because they are simple and easy to
install and don’trequire the running of cables. Larger environments are usually wired in a star because moves, adds, and changes to the network are performed more efficiently with a physical star than with any of the other topologies.

If you need up time to the definition of fault resistant (that is, 99.9-percent up time or less than 8 hours total downtime per year), you should seriously consider a partial mesh layout. While you are thinking about how fault tolerant a full mesh network is, let the word maintenance enter your thoughts. Remember that you will have n(n–1)/2 connections to maintain in a full mesh configuration and a subset of that for a partial mesh, which will quickly become a nightmare and could exceed your maintenance budget.
Generally speaking, you should balance the following  Considerations when choosing a physical topology for your network:

• Cost
• Ease of installation
• Ease of maintenance
• Cable fault tolerance

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Backbones and Segments

Backbones and Segments
With complex networks, we must have a way of intelligently identifying which part of the network we are discussing. For this reason, we commonly break networks into backbones and segments. Figure 1.9 shows a sample network and identifies the backbones and segments. You should refer to this figure when necessary as you read about backbones and segments.


Example:

Backbone and segments on a sample network

Understanding the Backbone

A backbone is the part of the network to which all segments and servers connect. A backbone provides the structure for a network and is considered the main part of any network. It usually uses a high-speed communications technology of some kind, such as Fiber Distributed Data Interface (FDDI) or 1 or 10 Gigabit Ethernet. All servers and all network segments typically connect directly to the backbone so that any segment is only one segment away from any server on that backbone. Because all segments are close to the servers, the network is more efficient. Notice in Figure 1.9 that the three servers and three segments connect to the backbone.

Understanding Segments

Segment is a general term for any short section of the network that is not part of the backbone. Just as servers connect to the  ackbone, workstations connect to segments. Segments are connected to the backbone to allow the workstations on them access to the rest of the network. Figure 1.9 shows three segments.

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Mesh Topology

Mesh Topology
In a mesh topology (as shown in Figure 1.8), a path exists from each station to every other station in the network, resulting in the most physical connections per node of any topology. While
not usually seen in LANs, a variation on this type of topology—the hybrid mesh—is used on the Internet and other WANs in a limited fashion. Hybrid mesh topology networks can have multiple
connections between some locations, but this is done only for redundancy. In addition, it’s called a hybrid because other types of toplogies might be mixed in as well. Also, it is not a full
mesh because there is not a connection between each and every node, just a few for backup purposes. Notice in Figure 1.8 how complex the network becomes with four connections.

Example:

A typical mesh topology

As you can see in Figure 1.8, a mesh topology can become quite complex as wiring and connections increase exponentially. For every n stations, you will have n(n–1)/2 connections. For example, in a network of 4 computers, you will have 4(4–1)/2 connections, or 6 connections. If your network grows to only 10 computers, you will have 45 connections to manage! Given this impossible overhead, only small systems can be connected this way. The  ayoff for all this work is a more fail-safe, or fault-tolerant, network, at least as far as cabling is concerned. 

      Today, the mesh topology is rarely used, and then only in a WAN environment and only because the mesh topology is fault tolerant. Computers or network devices can switch between

these multiple, redundant connections if the need arises. On the con side, the mesh topology is expensive and, as you have seen, quickly becomes too complex. Using what is known as a partial mesh is a workable compromise between the need for fault tolerance and the cost of a full mesh topology. With a partial mesh, the same technology can be used between all devices, but not all devices are interconnected. Strategy becomes the name of the game when deciding which devices to interconnect.

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Ring Topology

Ring Topology
In the ring topology, each computer is connected directly to two other computers in the network. Data moves down a one-way path from one computer to another, as shown in Figure 1.7. The good news about laying out cable in a ring is that the cable design is simple. The bad news is that, as with bus topology, any break, such as adding or removing a computer, disrupts the entire network. Also, because you have to “break” the ring in order to add another station, it is very difficult to reconfigure without bringing down the whole network. For this reason, the physical ring topology is seldom used.

Note:

Although its name suggests a relationship, Token Ring does not use a physical
ring topology. It instead uses a physical star, logical ring topology (and runs at
speeds of either 4Mbps or 16Mbps). You will learn more about logical topologies
later in this chapter.

Example:

A typical ring topology

 A few pros and many cons are associated with a ring topology. On the pro side, the ring topology is relatively easy to troubleshoot. A station will know when a cable fault has occurred because it will stop receiving data from its upstream neighbor.
On the con side, a ring topology has the following characteristics:

• Expensive, because multiple cables are needed for each workstation.
• Difficult to reconfigure.
• Not fault tolerant. A single cable fault can bring down the entire network.

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Star Topology

Star Topology
Unlike those in a bus topology, each computer in a star topology is connected to a central point by a separate cable or wireless connection. The central point is a device known by such names as
hub, MAU, concentrator, switch, and access point, depending on the underlying technology.

Real World Scenario
    A bus sounds good, but . . .

Despite the simplicity of the bus topology, there are some inherent disadvantages to this design. For example, what happens if the wire breaks or is disconnected? Neither side can communicatewith the other, and signal bounce occurs on both sides. The result is that the entire network is down. For this reason, bus topologies are considered to have very little fault tolerance. Sometimes, because a cable is inside a wall, you cannot physically see a break. To determine if a break has occurred, you can use a tool known as a Time Domain Reflectometer, or TDR (also called a cable tester). This device sends out a signal and measures how much time it takes to return. Any break in the cable will cause some portion of the signal to return prematurely, thus indicating the presence of, and the distance to, a break in the cable. Programmed with the specifications of the cable being tested, it determines where the fault lies with a high degree of accuracy.We’ll discuss cable testers in Chapter 6, “Wired and Wireless Networks.”

 Although this setup uses more cable than a bus, a star topology is much more fault tolerant than a bus topology. This means that if a failure occurs along one of the cables connecting to the hub, only that portion of the network is affected, not the entire network. Depending on the type of device at the other end of that cable, this may affect only a single device. It also means that you can add new stations just by running a single new cable. Figure 1.6 shows a typical star topology.

Example:

A typical star topology with a hub

The design of a star topology resembles an old wagon wheel with the wooden spokes extending from the center point. The center point of the wagon wheel would be considered the hub.
Like the wagon wheel, the network’s most vulnerable point is the hub. If it fails, the whole system collapses. Fortunately, hub failures are extremely rare.

As with the bus topology, the star topology has advantages and disadvantages. The increasing
popularity of the star topology is mainly due to the large number of advantages, which
include the following:

• New stations can be added easily and quickly.
• A single cable failure won’t bring down the entire network.
• It is relatively easy to troubleshoot.
• The disadvantages of a star topology include the following:
• Total installation cost can be higher because of the larger number of cables, but prices are constantly becoming more and more competitive.
• It has a single point of failure (the hub, or other central device). There are two subtle special cases for the star topology, the point-to-point link and the wireless

access point. If you think of a point-to-point connection as one spoke of a star-wired network,
with either end device able to play the role of the hub or spoke device, then you can begin
to see the nature of any star-wired topology. What about when there is no wire, though? It takes
a firm understanding of what the devices making up the wireless network are capable of to be
able to categorize the wireless topology. Wireless access points, discussed in detail in Chapter
6, are nothing more than wireless hubs or switches, depending on capability, that are able to act
as wireless bridges by establishing a wireless point-to-point connection to another wireless

access point. Either use is reminiscent of the wired star/point-to-point topologies they emulate.

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Bus Topology

Bus Topology
In a bus topology, all computers are attached to a single continuous cable that is terminated at both ends, which is the simplest way to create a physical network. Originally, computers were attached to the cable with wire taps. This did not prove practical, so drop cables were used to

attach computers to the main cable. In 10Base-2 Ethernet, no drop cables are used, but instead, a “T” is inserted in the main cable wherever a station needs to connect. Figure 1.5 shows an example of a bus network. Notice how the cable runs from computer to computer with several bends and twists.

Example:

An example of a physical bus topology

     When communicating on a network that uses a bus topology, all computers see the data on the wire. This does not create chaos, though, because the only computer that actually accepts the data

is the one to which it is addressed. You can think of a bus network as a small party. David is already there, along with 10 other people. David would like to tell Joe something. David yells out, “Joe! Will

you grab me a cup of coffee, please?” Everyone in the party can hear David, but only Joe will respond. A star network with a hub, which you’ll read about later, also operates in this manner.

As with most things, there are pros and cons to a bus topology. On the pro side, a bus

 topology has the following characteristics:

•  Is simple to install
• Is relatively inexpensive
• Uses less cable than other topologies

The following characteristics describe the con side of a bus topology:

• Is difficult to move and change
• Has little fault tolerance

 (a single fault can bring down the entire network)

• Is difficult to troubleshoot

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Physical Topologies

Physical Topologies:
A topology is basically a map of a network. The physical topology of a network describes the layout of the cables and workstations and the location of all network components. Often, physical
topologies are compared to logical topologies, which define how the information or data flows within the network. The topologies are usually similar. It is important to note, however, that a network can have one type of physical topology and a completely different logical topology. This was discussed earlier in the sidebar “Physical vs. Logical Concepts.” 

   The cables or connections in a physical topology are often referred to as network media (or physical media). Choosing how computers will be connected in a company’s network is critical.
A wrong decision in the physical topology makes the media difficult to correct because it is costly and disruptive to change an entire installation once it is in place. The typical organization changes the physical layout and physical media of a network only once about every 10 years, so it is important to choose a configuration that you can live with and that allows for growth.

      In the next section, we’ll look at physical media. In the following sections, we’ll look at the  five most common topologies:

• Bus
• Star
• Ring
• Mesh

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Peer-to-Peer vs. Client/Server Architecture

Peer-to-Peer vs. Client/Server Architecture
As you learned earlier in this chapter, the purpose of networking is to share resources. How this
is accomplished depends on the architecture of the network operating system software. The two
most common network types are peer-to-peer and client/server.

If you were to look at an illustration of a group of computers in a LAN, it would be impossible to determine if the network was a peer-to-peer or a client/server environment. Even a videotape of this same LAN during a typical workday would reveal few clues as to whether it is peer-to-peer or client/server. Yet, the differences are huge. Since you can’t see the differences, you might guess correctly that they are not physical but logical.

Physical vs. Logical Concepts
Throughout this book, you’ll see us refer to physical and logical networking topics. Generally speaking, when we’re referring to the physical aspects of a network, we’re referring to some aspect of the network that you can touch or that has physical substance (like electrons, electrical pulses, or the way cables are run). That is, they exist in the physical world. Logical concepts, on the other hand, are more imaginary and esoteric and deal with things like how data flows in a network. So, when we’re describing something as either physical or logical in nature, you’ll understand how those terms apply.

Peer-to-Peer Network
In peer-to-peer networks, the connected computers have no centralized authority. From an authority viewpoint, all of these computers are equal. In other words, they are peers. If a user
of one computer wants access to a resource on another computer, the security check for access rights is the responsibility of the computer holding the resource. Each computer in a peer-to-peer network can be both a client that requests resources and a server
that provides resources. This is a great arrangement, provided the following conditions are met: 
 

• Each user is responsible for local backup.
•   Security considerations are minimal.
• A limited number of computers are involved. 

Networks that run Windows 95/98 as their network operating system and networks using Windows NT, 2000, or XP in a workgroup are considered peer-to-peer networks. Figure 1.3 shows an example of a peer-to-peer network. Peer-to-peer networks present some challenges. For example, backing up company data becomes an iffy proposition. Also, it can be difficult to remember where you stored a file. Finally, because security is not centralized, users and passwords must be maintained separately on each machine, as you can see in Figure 1.3. Passwords may be different for the same users on different machines 

(or for different resources on Windows 9x machines).

Client/Server Network
In contrast to a peer-to-peer network, a client/server network uses a network operating system designed to manage the entire network from a centralized point, which is the server. Clients make requests of the server, and the server responds with the information or  ccess to a resource.

Example:

A peer-to-peer network

Client/server networks have some definite advantages over peer-to-peer networks. For one thing, the network is much more organized. It is easier to find files and resources because they are stored on the server. Also, client/server networks generally have much tighter security. All usernames and passwords are stored in the same database (on the server), and individual users can’t use the server as a workstation. Finally, client/server networks have better performance and can scale almost infinitely. It is not uncommon to see client/server networks with tens of thousands of workstations. Figure 1.4 shows a sample client/server network. Note that the server now holds the database of user accounts, passwords, and access rights.
Note that today’s networks are very often hybrids of the peer-to-peer model and the client/server model. Clients of early Novell NetWare networks, for example, had no ability to share  their resources, not that they had many worth sharing, for the most part. Conversely, today’s Microsoft and Apple networks, for example, have well-defined servers. They also allow the simultaneous sharing of resources from lesser devices that run what are considered workstation operating systems, which are capable of fewer inbound connections but are running the server service nonetheless. Purists shun the less organized mixture of this resource sharing among servers and clients alike, but the reality is that most networks would be worse off for losing this capability.

Example:

A client/server network

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Host, Workstation, and Server

Host, Workstation, and Server

Networks are made up of lots of different components, but the three most common network

entities are the host, workstation, and server. For the Network+ exam, you need a good understanding

of these three primary components of a network. Each one of these items can be found

on most networks.

Understanding Workstations

In the classic sense, a

workstation

is a powerful computer used for drafting or other math-intensive

applications. The term is also applied to a computer that has multiple central processing

units (CPUs) available to users. In the network environment, the term

workstation

normally

refers to any computer that is connected to the network and used by an individual to do work.

It is important to distinguish between workstations and clients. A

client

is any network entity

that can request resources from the network; a workstation is a computer that can request

resources. Workstations can be clients, but not all clients are workstations. For example, a

printer can request resources from the network, but it is a client, not a workstation.

Understanding Servers

In the truest sense, a server does exactly what the name implies: It provides resources to the clients

on the network (“serves” them, in other words). Servers are typically powerful computers

that run the software that controls and maintains the network. This software is known as the

network operating system.

Servers are often specialized for a single purpose. This is not to say that a single server can’t

do many jobs, but, more often than not, you’ll get better performance if you dedicate a server

to a single task. Here are some examples of servers that are dedicated to a single task:

•  File Server

Holds and distributes files.

•  Print Server

Controls and manages one or more printers for the network.

•  Proxy Server 

Performs a function on behalf of other computers. (Proxy means “on behalf of.”)

•  Application Server

Hosts a network application.

•  Web Server

Holds and delivers web pages and other web content using the Hypertext Transfer Protocol (HTTP).

• Mail Server

Hosts and delivers e-mail. It’s the electronic equivalent of a post office.

•   Fax Server 

Sends and receives faxes (via a special fax board) for the entire network without the need for paper.

•  Remote Access Server 

Listens for inbound requests to connect to the network from the outside.
Remote access servers provide remote users (working at home or on the road) with a connection
to the network, either via modems or an IP connection.

• Telephony Server

 Functions as a “smart” answering machine for the network. It can also perform

call center and call-routing functions.

Notice that each server type’s name consists of the type of service the server provides (remote

access, for example) followed by the word server, which, as you remember, means to serve.

Regardless of the specific role (or roles) these servers play, they should all have the following

in common:

_ Hardware and/or software for data integrity (such as backup hardware and software)

_ The capability to support a large number of clients

Figure 1.1, earlier in this chapter, shows a sample network. Physical resources, such as harddrive

space and memory, must be greater in a server than in a workstation because the server

needs to provide services to many clients. Also, a server should be located in a physically secure

area. Figure 1.2 shows a sample network that includes both workstations and servers. Note that

there are more workstations than servers because a few servers can serve network resources to

hundreds of users simultaneously.

Example:

A sample network including servers and workstations

Understanding Hosts

The term host covers pretty much every other networking device, but it can also refer to a workstation

and server and is most commonly used when discussing TCP/IP-related services and

functions. In fact, a host, in TCP/IP terms, is any network device that has an IP address. Workstations,

servers, and any other network device (as long as it has one or more IP addresses) can

all be considered hosts. In conversation, you may also hear the word host used to describe any

minicomputer or server. For the Network+ exam, however, you should stick to the classic definition

used here (i.e., workstations, servers, and other network devices).

The term host comes from the era when the only intelligent devices on the network were

mainframes, which were commonly referred to as hosts regardless of TCP/IP functionality.

Nearly all other devices were known as dumb terminals, but no other device had intelligence,

only the mainframe. As TCP/IP came into the picture, only the mainframes, or hosts, received

IP addresses. This is the same era that produced the term gateway to refer to any layer 3 intermediate

device, such as a router. Just as the term gateway remains in common use today, such

as in the very common term default gateway, the term host is still used, but its use is much

broader now that nearly every end and intermediate device is intelligent and has at least one IP

address, making them hosts.

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Wide Area Network

Wide Area Network:

Chances are you are an experienced wide area network (WAN) user and don’t even know it. If you have ever connected to the Internet, you have used the largest WAN on the planet. A WAN is any network that crosses metropolitan, regional, or national boundaries. Most networking professionals define a WAN as any network that uses routers and public network links. The Internet fits both definitions.

       WANs differ from LANs in the following ways:

_

                WANs cover greater distances.

_

                WAN speeds are slower.

_

                WANs can be connected on demand or permanently connected; LANs have permanent

                connections between stations.

_

                WANs can use public or private network transports; LANs primarily use private network

                transports.

_

                WANs can use either full- or half-duplex 

communications. LANs have typically used halfduplex

                communications, although many local area networks today use full-duplex communications

                (see the sidebar “Full-Duplex vs. Half-Duplex Communications”).

The Internet is actually a specific type of WAN. The Internet is a collection of networks that

are interconnected and, therefore, is technically an internetwork

( Internet is short for the word internetwork ).

                A WAN can be centralized or distributed. A centralized WAN consists of a central computer (at

a central site) to which other computers and dumb terminals connect. The Internet, on the other

hand, consists of many interconnected computers in many locations. Thus, it is a distributed WAN.

Full-Duplex vs. Half-Duplex Communications :

All network communications (including LAN and WAN communications) can be categorized as half-duplex or full-duplex. With half-duplex, communications happen in both directions, but in only one direction at a time. When two computers communicate using half-duplex, one computer sends a signal and the other receives; then, at some point, they switch sending and receiving roles. Chances are that you are familiar with half-duplex communications. If you have ever used a push-to-talk technology, such as a CB radio or walkie-talkie, you were communicating via half-duplex: One person talks, and then the other person talks. Full-duplex, on the other hand, allows communication in both directions simultaneously. Both stations can send and receive signals at the same time. Full-duplex communications are similar

to a telephone call, in which both people can talk simultaneously.

Example:

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