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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