Sams Teach Yourself MCSE Networking Essentials in 14 Days

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The Teach Yourself MCSE in 14 Days series is designed from the ground up to make it easy for readers to learn and study the material. Each chapter begins with a list of the MCSE objectives the material will cover, followed by a listing of Fast Facts : important facts covered in the chapter. This gives the reader an overview of the chapter and acts as a ready reference for any last-minute studying. Each chapter ends with review questions and an exercise section. This book includes a Technical Glossary and an ...
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Overview

The Teach Yourself MCSE in 14 Days series is designed from the ground up to make it easy for readers to learn and study the material. Each chapter begins with a list of the MCSE objectives the material will cover, followed by a listing of Fast Facts : important facts covered in the chapter. This gives the reader an overview of the chapter and acts as a ready reference for any last-minute studying. Each chapter ends with review questions and an exercise section. This book includes a Technical Glossary and an Employable MCSE appendix that shows readers how to best utilize their certification. Teach Yourself MCSE Networking Essentials in 14 Days is organized in a concise, easy-to-read manner to give readers the most valuable information efficiently, saving them countless hours and thousands of dollars in courses.
  • Teach Yourself in 14 Days time component provides readers with a built-in method for organizing their study time to order to pass the Networking Essentials Exam 70-58
  • For every one MCSE, there are three job openings! (ATEC)
  • Covers the networking knowledge and skills common to both Windows 95 and BackOffice products
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Product Details

  • ISBN-13: 9780672311758
  • Publisher: Sams
  • Publication date: 2/28/1998
  • Series: Sams Teach Yourself Series
  • Edition number: 1
  • Pages: 448
  • Product dimensions: 7.37 (w) x 9.10 (h) x 1.06 (d)

Table of Contents

Introduction 2
1 How to Become an MCSE 10
2 Introduction to Network Models and Components 20
3 Network Types and Topologies 42
4 Transmission Media 64
5 Media Access 96
6 LAN Architectures 114
7 Network Protocols 152
8 Network Design 180
9 Network Server and Client Setup 202
10 Network Administration 232
11 Remote Access 260
12 Expanding the Network 272
13 Wide Area Networks (WANs) 296
14 Network Troubleshooting 318
A: Glossary 338
B: Answers to Review Questions 360
C The Employable MCSE 378
D Protocol Definitions and Standards 390
Index 400
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First Chapter









[Figures are not included in this sample chapter]


Teach Yourself MCSE Networking Essentials in 14 Days






Day 3


Network Types and Topologies


TEST DAY FAST FACTS


The following list of facts is a concise picture of the information this chapter
presents. It acts both as an overview for the chapter and as a study aid to help
you do any last-minute cramming.



  • A network's topology describes the physical arrangement of its wiring and hardware.



  • There are three types of LANs: peer-to-peer, server-based, and a hybrid of those
    two.



  • Network topologies include bus, ring, star, and a star bus (a.k.a. switched).

3.1. Overview


Local area networks (LANs) have become ubiquitous in today's business environment.
Despite their familiarity, they remain little more than a mystery to most people.
It's easy to point to the hubs and switches and declare them to be "the network,"
but that's just one piece of one type of network.


As explained in Chapter 2 "Introduction to Network Models and Components,"
LANs can be best understood by dissecting them into their physical components. Often,
these components are stratified into layers, as defined by the Open Systems
Interconnect (OSI) Reference Model
. Each layer supports a distinct functionality
set.


A necessary prerequisite to this layered dissection of a LAN is an exploration
of two other attributes: resource access methodology and topology. A LAN's resource
access methodology describes the manner in which network-attached resources are shared.
Frequently, this aspect of a network is referred to as its "type."
The two prevailing types are peer-to-peer and client/server, although the functional
distinctions between them are rapidly disappearing.





TIP: Client/server networks are sometimes also refered to as being server-based
networks.



A LAN's topology refers to its physical arrangement of hubs and wiring. The basic
topologies include bus, star, ring, and switched.


Together, these attributes will help form the context for your detailed exploration
of a LAN's functional layers. This chapter explores all the possible permutations
of both LAN types and topologies. Their benefits, limitations, and possible uses
are also presented.

3.1.1. Objectives


Microsoft publishes preparation guides to use while studying for certification
exams. The following list points out the recommended guidelines from Microsoft concerning
network design. After reading this chapter, you should be able to:


  • Identify common LAN topologies.



  • Recognize the different resource access methodologies, known as LAN types,
    as well as the benefits and limitations of each.

3.1. LAN-Attached Devices


Before you delve into LAN types and topologies, it's beneficial to first examine
some of the basic resources that can be found on a LAN. The three most common primary
devices are clients, servers, and printers. A primary device is one that can
either directly access other devices or be accessed by other devices. A server
is any LAN-attached computer that controls resources that are shared by other LAN-attached
devices. A client is any computer that accesses resources controlled by servers,
via the LAN. Printers, of course, are output devices that produce hard copies of
files.


Numerous other devices, like CD-ROM drives and tape archives, can also be accessed
via a LAN, but they tend to be secondary resources (that is, they are connected to
a primary device). Subordination of a device to another device, such as a CD-ROM
drive to a server, is known as slaving. Printers can also be slaved off a
primary device, or they can be primary devices that are directly connected to the
network. Figure 3.1 illustrates the primary resources found in a LAN, as well as
the relationship of secondary resources to those primaries.


Figure 3.1. Primary and secondary LAN resources.


The word server is frequently used generically to describe all multiuser computers.
It is important to note, however, that there are many types of servers that fall
under the "server" group. They are frequently specialized by function,
such as file servers, print servers, application servers, and many others.

3.1.1. File Servers


One of the most basic and familiar of the specialized servers is the file server.
The file server is a centralized storage mechanism for files that are needed by a
user group. Placing these files in one centralized location, rather than scattered
across numerous client-level machines, imparts several benefits. These benefits include:


  • Centralized location. All users enjoy a single, constant repository for
    shared files. This provides a dual benefit. Users don't have to search multiple potential
    storage locations to find a file; files are stored in one place. Users also are relieved
    of the burden of maintaining separate logon credentials to multiple machines. One
    logon provides them with access to all the files they require.





WARNING: The server types described in this chapter are presented in an intentionally
generic manner. This enables you to examine their functionality without becoming
mired in the peculiarities of any given operating system.




  • Electric power conditioning. The use of a centralized server for file
    storage also enables the introduction of many techniques that can provide protection
    for data from inconsistencies in electrical power. Fluctuations in the frequency
    of electricity, or even the sudden loss of power, can damage a computer's data and
    hardware. Power filtration and battery backup through an Uninterruptible Power
    Supply
    (UPS) is cost-effective on a single server. Similar protection
    in a peer-to-peer network might be cost-prohibitive because of the numbers of computers
    that would need protection.



  • Consistent data archiving. Storing all shared files in a common, centralized
    location greatly facilitates backups, because only a single output device and routine
    is required. Decentralized storage of data, such as at every desktop, means that
    every desktop's data must be backed up separately. Backups are an essential protection
    against lost or damaged files. Suitable devices for backups include tape drives,
    writeable optical drives, and even hard drives. Multiple hard drives can also be
    used in a technique known as striping. Striping involves multiple, simultaneous
    writes to different hard drives. Although this is primarily done to provide faster
    reading of data, it can also be used to create redundancy with every write operation,
    provided parity is enabled.



  • Speed. The typical server is a more robust and fully configured platform
    than the typical client computer. This directly translates into a noticeable performance
    gain relative to retrieving files in a peer-to-peer network, assuming that the performance
    bottleneck is not in the network.





NOTE: The use of a file server does not always yield an increase in speed.
You can always access files stored locally more quickly than files stored on a remote
computer and retrieved over a LAN. The speed increase discussed here is relative
to the speed with which files can be retrieved from other client-level machines in
a peer-to-peer network, not the speed with which files can be retrieved from a local
hard drive.



Not all files are good candidates for storage on a file server. Files that are
sensitive, proprietary, or not suitable for access by the user community are best
left on a local hard drive. Storing them on a file server still imparts some of the
benefits of a file server (such as automated backups) but may pose an unnecessary
risk.

3.1.2. Print Servers


Servers can also be used to share printers among the users of a LAN. Although
the costs of printers, especially laser printers, have decreased considerably since
their introduction, most organizations would be hard-pressed to justify one for every
desktop. Instead, servers are used to share one or more printers among the user population.


A print server's only functions are to accept print requests from all networked
devices, put them in a queue, and spool (send) them to the appropriate printer
(see Figure 3.2).





NOTE: Although the word spool has become a verb synonymous with printing,
it is an acronym. SPOOL originally meant Simultaneous Peripheral Operations On Line.
It's the temporary storage of programs and data in the form of output streams on
a magnetic media, for later output or execution.



Figure 3.2. Simple print server arrangement.


Each printer connected to a print server has its own queue, or waiting
list. These queues represent the pecking order of all requests that are temporarily
stored and awaiting their turns to print. Requests are generally processed in the
order they are received. Microsoft's client operating systems Windows 95 and Windows
NT Workstation both enable a printer to be shared.


An alternative to using a print server is to directly attach the printer to the
LAN. Many printers can be configured with a Network Interface Card (NIC) that
enables them to be directly attached to a LAN. Such printers are still controlled
by a print server; they just aren't directly tied to its parallel port.

3.1.3. Application Servers


Servers are also frequently used as central repositories of application software.
Application servers, although superficially similar to file servers, are unique
creatures. An application server hosts executable application software. To run that
application software, a client must establish a connection across a network to that
server. The application actually runs on that server. Servers that enable clients
to download copies of an application for execution on their desktop computer are
file servers. Their files are actually application software files, but they are functioning
as a file server.


Application servers can enable an organization to reduce its overall cost of application
software. Purchase prices and maintenance of a single, multiuser copy of an application
is usually much less than the costs of acquiring and maintaining separate copies
for each desktop.





NOTE: Installing a commercially purchased single-user application software
package on an application or file server might violate the terms and conditions set
forth in its Copyright Agreement. In much the same way as an individual user might
pass around the original install media of an application, a server makes a single
copy of a program available to any and all network users. This constitutes software
piracy.
Always ensure that any software package you install on a server has been
purchased through some form of multiuser agreement.



Although it's usually desirable to separate application software from its data
files by using separate servers (for example, an application server and a file server),
there is one important exception to this rule. Some applications build and maintain
large relational databases. These applications and their databases should reside
together on the application server.


The reason for this is simple: the mechanics of retrieving data from a database
are different from simply pulling down a Word or Excel file. The relational database
application only releases the data that was requested, keeping everything else in
its database. Office automation applications, like Word and Excel, store information
in standalone files that, typically, are not interdependent with other data in a
complex structure. The relational database application is directly responsible for
the integrity of the database and its indexes. Managing the database across a network
increases the risk of corrupting the indexes and disabling the application.

3.2. Network Types


The network's type describes the manner in which attached resources can
be accessed. Resources can be servers, devices, files, and so on that reside on,
or are controlled by, a network-attached computer. These resources can be accessed
in one of two ways: peer-to-peer and client/server.

3.2.1. Peer-to-Peer Networks


A peer-to-peer network supports unstructured access to network-attached resources.
Each device in a peer-to-peer network can be a client and a server simultaneously.
All devices in the network are capable of accessing data, software, and other network
resources directly. In other words, each networked computer is a peer of every
other networked computer; there is no hierarchy. Figure 3.3 illustrates a peer-to-peer
network.


Figure 3.3. A peer-to-peer network. There are four main benefits
to having a peer-to-peer network.


  • Peer-to-peer networks are relatively easy to implement and operate. They are
    little more than a collection of client computers with a network operating system
    that permits peer-to-peer resource sharing. Thus, establishing a peer-to-peer network
    requires only the procurement and installation of the computers, network interface
    cards, network protocols and drivers, wiring, and an operating system that permits
    this resource access methodology. Peer-to-peer networks may, or may not, be developed
    using repeating hubs. If they are in close enough proximity, they may be cabled directly
    together using a bus or ring topology.





TIP: Technically, a hub is not a requirement for a peer to peer network as
it may exist on a bus. However, the price of hubs relative to their functionality
has all but rendered such hubless LANs obsolete.




  • Peer-to-peer networks are also inexpensive to operate. They lack expensive, sophisticated,
    dedicated servers that require special administrative care and climate conditioning.
    The lack of dedicated servers also eliminates the attendant expenses of staffing
    and training, as well as the additional real estate costs for developing a climate-controlled
    room just for the servers. Each machine resides, at least in theory, on a desktop
    and is cared for by its primary user.



  • A peer-to-peer network can be established with familiar operating systems such
    as Windows 95, Windows NT, and Windows for Workgroups.



  • The lack of a hierarchical dependence makes peer-to-peer networks much more fault-tolerant
    than client/server networks. In theory, a server in a client/server network is a
    single point of failure. Single points of failure are vulnerabilities that can impact
    the entire network. In a peer-to-peer network, the failure of any given machine results
    in the unavailability of only a subset of the network's attached resources.

Peer-to-peer networking is not without its risks or faults. Some of the more serious
of these limitations are in the areas of security, performance, and administration.
The peer-to-peer network suffers from numerous security weaknesses:


  • Users must maintain multiple passwords, typically one for each machine they need
    to access.



  • The lack of a central repository for shared resources imposes the burden of finding
    information squarely on each user. This difficulty can be overcome with methods and
    procedures, provided each member of the workgroup complies. Users tend to devise
    creative means of coping with an excess of passwords. Most of these ways directly
    compromise the security of every machine in the peer-to-peer network.



  • Like the network-attached resources, security is distributed evenly throughout
    the peer-to-peer network. Security in this form of network usually consists of user
    authentication via an ID and password, coupled with specific access permissions for
    specific resources. It's up to the "administrator" of each networked computer
    to define these permission structures for all other users in the network.





TIP: Although each machine's user in a peer-to-peer network can be considered
that machine's administrator, it is rare that these users will have the knowledge
and skill sets to be proficient at their administrative duties. It's even rarer for
the administrative skill levels to be consistent across even a small workgroup. This
is one of the pitfalls of peer-to-peer networking.




  • Unfortunately, technical proficiency is usually not uniformly distributed. Consequently,
    the security of the entire network is established on the skills and abilities of
    the least technically proficient member. A chain is only as strong as its weakest
    link, and in a peer-to-peer network, security is only as strong as its weakest peer.

Although the administrative burden is less in a peer-to-peer network than a server-based
network, this burden is spread across users. This creates some logistical issues.
The two most serious issues are:


  • Uncoordinated--and probably highly inconsistent--backups of data and software.
    Each user is responsible for his own machine, so it's possible, and even likely,
    that each will perform backups at his own leisure.



  • Decentralized responsibility for enforcing file-naming conventions and storage
    locations.
    Given that there is no central repository for stored information,
    or any other logic by which LAN-attached resources are organized, keeping current
    with what information is stored where can be quite challenging. As with everything
    else in a peer-to-peer network, the effectiveness of the whole is directly dependent
    upon the degree to which methods and procedures are adhered to by all participants.

Lastly, performance also suffers. An integral aspect of a peer-to-peer network
is that each machine is a multiuser machine. The typical machine is better suited
for use as a single-user, client-only computer than for multiuser support. Consequently,
the performance of any given machine suffers noticeably, as perceived by its primary
user, whenever remote users log on and share its resources.


The availability of files, and any other resources that a given peer may host,
corresponds with the availability of the host. In other words, if a machine's primary
user is out of the office and left the computer powered down, its resources are unavailable
to the rest of the networked computers. This can be circumvented by leaving all machines
powered on all the time, but doing so raises questions about other issues, like security.


Another, more subtle aspect of performance is scalability. The peer-to-peer methodology
is inherently non-scalable. The more peers are networked together, the more unmanageable
the network becomes.


Despite these limitations, peer-to-peer networking remains a useful resource access
methodology. It has two primary uses. First, it is ideally suited for small organizations
with a limited budget for information technologies, and limited need for information
sharing. Alternatively, workgroups within larger organizations can also use this
methodology for a tighter sharing of information within that group.

3.2.2. Client/Server Networks


Client/server networks introduce a hierarchy that is designed to improve the manageability
of a network's various supported functions as the size of the network scales upward.
Often, client/server networks are referred to as server-based networks. Figure
3.4 illustrates this hierarchy of clients and servers.


Figure 3.4. A client/server network.


In a client/server network, frequently shared resources are consolidated onto
a separate tier of computers, known as servers. Servers, typically, do not
have a primary user. Rather, they are multiuser machines that regulate the sharing
of their resources across the base of clients. In this type of network, clients are
relieved of the burden of functioning as servers to other clients.


Many benefits are inherent in the client/server approach to accessing network
resources. These benefits directly correspond to the limitations of a peer-to-peer
network. The areas of benefit are security, performance, and administration.


Client/server networks can be made, and kept, much more secure than peer-to-peer
networks due to multiple factors. First, security is managed centrally. Networked
resources are no longer subjected to the "weakest link in the chain" theory
that is an integral part of a peer-to-peer network.


Instead, all user accounts (also known as IDs) and passwords are centrally
managed and verified before any user is granted access to requested resources. Coincidentally,
this also makes the lives of the users better, by diminishing the need for multiple
passwords.


Another benefit of this centralization of resources is that administrative tasks,
like backups, can be done consistently and reliably.


Client/server networks offer improved performance for networked computers in several
ways. First, each client is relieved of the burden of processing requests from other
clients for its stores. Each client in a client/server network need only keep up
with the requests generated by its primary (and only) user.


More significantly, this processing is offloaded onto a server whose configuration
is optimized for that service. Typically, a server contains more processing power,
more memory, and larger, faster disk drives than a client computer. The net effect
is that users' client computers are able to better satisfy their own requests, and
requests for resources centralized on a server are fulfilled much more effectively.


Users are also spared the effort that would otherwise be required to learn which
resources are stored where in a network. In a client/server network, the possible
"hiding places" are reduced to just the number of servers on the network.
In a Windows NT server environment, client/server resources can be linked to a logical
drive. After the network drive linkage is established, remote resources stored on
the server can be accessed as easily as any that are locally resident on a user's
PC.


A client/server network is also very scalable relative to peer-to-peer networks.
Regardless of how many clients are connected to the network, the resources are always
centrally located. In addition, these resources are always centrally managed and
secured. Consequently, the performance of the aggregate network isn't compromised
by increases in scale.


The client/server network has one limitation: it costs much more to implement
and operate than a peer-to-peer network. There are many facets of this significant
cost difference.


First, the hardware and software costs are significantly increased because of
the need for a separate networked computer that services the clients. Servers can
be fairly sophisticated--which translates into "expensive"--machines.


The costs of operating a client/server network are also much higher. A trained
professional is needed to administer the network and its servers. In a peer-to-peer
network, each user is responsible for the maintenance of his or her own machine;
no individual needs to be dedicated to this function.


The last cost consideration is the potential cost of downtime. In a peer-to-peer
network, the loss of any given peer translates into only a modest decrement in the
available resources on the LAN. In a server-based LAN, the loss of a server can directly,
and significantly, impact virtually all the users of the network. This increases
the potential business risks of a client/server network. Numerous approaches, including
clustering servers for redundancy, can be used to combat this risk. Unfortunately,
every one of these approaches only further drives up the cost of a server-based network.


Client/server networks are extremely useful in large organizations. They can also
be useful in any circumstances that warrant tighter security or more consistent management
of network-attached resources. The added cost of client/server networks, however,
might place them beyond the reach of smaller organizations.

3.2.3. Combination Networks


The distinctions between peer-to-peer and client/server networking aren't quite
so clear as the preceding sections might suggest. They were presented as distinct
types intentionally, and for academic purposes. The distinctions between them have
been blurred through the capabilities of numerous operating systems, like Microsoft's
Windows for Workgroups, Windows 95, and Windows NT.


The norm today is a combination of peer-to-peer and client/server resource access
in a single network. An example of this is a network with a client/server architecture
that centralizes resources that are universally needed. Within this context, local
workgroups can optionally provide peer-based access amongst themselves.

3.3. LAN Topologies


Local area network topologies can be described by using either a physical or a
logical perspective. A physical topology describes the geometric arrangement
of components that make up the LAN. The topology is not a map of the network; it's
a theoretical construct that graphically conveys the shape and structure of the LAN.


A logical topology describes the possible connections between pairs of
networked endpoints that can communicate. This is useful in describing which endpoints
can communicate with which other endpoints, and whether those pairs capable of communicating
have a direct physical connection to each other. This chapter focuses only on physical
topological descriptions. The three basic topologies are bus, ring,
and star. The implications of LAN-switching on topology is explained in the
"Switches and Topology" section later in this chapter.

3.3.1. Bus Topology


A bus topology features all networked nodes directly interconnected using
a single, open-ended cable. This topology, also known as a daisy-chain, can
support only a single channel on the bus. The cable is called the bus. Some
bus-based technologies use more than a single cable. These technologies can support
more than one channel, although each cable remains limited to just one transmission
channel.


Both ends of the bus must be terminated with a resistive load, known as a terminating
resistor
that serves to prevent signal bounce. Whenever a station transmits,
the signal it puts on the wire automatically propagates in both directions. If a
terminating resistor is not encountered, the signal reaches the end of the bus and
reverses direction. As a result, a single transmission can completely usurp all available
bandwidth and prevent any other stations from transmitting. An example of bus topology
is illustrated in Figure 3.5.


Figure 3.5. Typical bus topology.


The typical bus topology features a single cable, not supported by external electronics,
that interconnects all networked nodes in a daisy-chain. All connected devices listen
to the bussed transmissions and accept those packets addressed to them. The lack
of reliance on any external electronics, like repeaters, makes bus LANs simple and
inexpensive. The downside is that it also imposes severe limitations on functionality
and scalability.


One example of an industrial-strength bus topology LAN is the IEEE's 802.4 Token
Bus LAN specification. This technology was fairly robust, deterministic, and bore
many similarities to a Token Ring LAN. Deterministic LANs offer the administrator
a high degree of control in determining the maximum amount of time that a frame of
data can be in transmission. The primary difference, obviously, was that Token Bus
was implemented on a bus topology.


Token Bus found extremely limited market support. Its implementation tended to
be limited to factory production lines. Bus topologies, in general, prospered in
myriad other forms. Two early forms of ethernet--10Base2 and 10Base5--used a bus
topology and coaxial cabling. Linear buses also became a critical technology for
interconnecting system-level components and peripheral devices within the internal
architectures of computers.

3.3.2. Ring Topology


The ring topology started out as a simple peer-to-peer LAN topology. Each
networked workstation had two connections, one to each of its nearest neighbors (see
Figure 3.6). The interconnection had to form a physical loop, or ring. Data was transmitted
unidirectionally around the ring. Each workstation acted as a repeater, accepting
and responding to packets addressed to it, and forwarding on the other packets to
the next workstation on the ring.


Figure 3.6. Peer-to-peer ring topology.


The original LAN ring topology featured peer-to-peer connections between workstations.
These connections had to be closed; that is, they had to physically form a ring.
The benefit of such LANs was that response time was fairly predictable. The more
devices in the ring, the longer the network delays. The drawback was that early ring
networks could be completely disabled if one of the workstations failed.


These primitive rings were made obsolete by IBM's Token Ring Network, which was
later standardized by the IEEE's 802.5 specification. Token Ring enables a departure
from the peer-to-peer interconnection by instituting a repeating hub. This eliminated
a ring network's vulnerability to workstation failure by eliminating the peer-to-peer
ring construction. Token Ring networks, despite their name, are implemented with
a star topology and a circular access method, as shown in Figure 3.7.


Figure 3.7. Star-wired ring topology.


The star topology of the LAN's wiring does not affect its circular access method.
The Token Ring Network shown in Figure 3.7 demonstrates the virtual ring formed by
the round robin access method. The solid lines represent physical connections and
the dashed line represents the logical flow of regulated media access.


Functionally, the access token passes in a round robin fashion among the networked
endpoints in a circular sequence, even though they are all interconnected to a common
hub. This may cause some confusion in classifying Token Ring's topology. Many people
succumb to the temptation of describing Token Ring networks as having a "logical"
ring topology, within a physical star topology. There is nothing logical about the
ring; at the electronics level, Token Ring implements a physical ring. Its wire topology
may resemble a star, but the LAN itself remains a somewhat contorted ring.


This physical ring is implemented in Token Ring's hardware. The Token Ring hub,
known more properly as a Multi-Station Access Unit (MSAU), provides a physical
ring at the electronics level.

3.3.3. Star Topology


Star topology LANs actually use a bus, but have connections to networked
devices that "radiate" out from a common point. This common point is the
hub, as shown in Figure 3.8. Unlike ring topologies--physical or virtual--each
networked device in a star topology can access the media independently. These networked
devices have to share the hub's available bandwidth. An example of a LAN with a star
topology is 10BaseT ethernet.


Figure 3.8. Star topology.


A small LAN with a star topology features connections that radiate out from a
common point. Each connected device can initiate media access independent of the
other connected devices. The risk inherent in the star topology is that the concentrator
is a single point of failure. If it fails, all the devices connected to it lose connectivity
with the LAN.


Star topologies have become the dominant topology type in contemporary LANs. Star
topologies are flexible, scalable, and relatively inexpensive compared to more sophisticated
LANs with strictly regulated access methods. Stars have all but made buses and rings
obsolete in LAN topologies and have formed the basis for the final LAN topology:
switched.

3.3.4. Switches and Topology


A switch is a multiport, Data Link Layer (OSI Reference Model Layer 2)
device. A switch "learns" Media Access Control (MAC) addresses and stores
them in an internal lookup table. Temporary, switched paths are created between the
frame's originator and its intended recipient, and the frames are forwarded along
that temporary path.


Switched LANs, regardless of their architecture, feature a star bus topology (see
Figure 3.9). The star bus topology features multiple connections to a switching hub.
Each port, and the device to which it connects, has its own dedicated bandwidth.


Although originally switches forwarded frames based upon their MAC address, technological
advances are rapidly changing this. Switches are available that can process cells,
frames, and even packets that use a Layer 3 address like IP.





NOTE: A frame is a variable length structure that contains data, source,
and destination addresses, and other data fields required for its carriage and forwarding
in Layer 2 of the OSI Reference Model. Cells are similar to frames, except
that they feature a fixed, not variable, length. Packets are a construct of
protocols that operate at Layer 3 of the OSI Reference Model. IP and IPX are two
examples of Layer 3 protocols that use packets to encapsulate data for transport
to foreign domains.



Figure 3.9. A switched LAN uses the star bus topology.


Switches can improve the performance of a LAN in two important ways. First, they
increase the aggregate bandwidth available throughout that network. For example,
a switched ethernet hub with 8 ports contains 8 separate collision domains of 10Mbps
each, for an total of 80Mbps of bandwidth.


Switches also improve LAN performance by reducing the number of devices forced
to share each segment of bandwidth. Each switch-delineated collision domain is inhabited
by only two devices: the networked device and the port on the switching hub to which
it connects. These are the only two devices that can compete for the 10Mbps of bandwidth
on their segment. In networks that do not utilize a media access method based on
competition for bandwidth, like Token Ring or FDDI, the tokens circulate among a
much smaller number of networked machines than are typically supported in competition-based
networks.


One area for concern with large switched implementations is that switches do not
isolate broadcasts. They bolster performance solely by segmenting collision, not
broadcast, domains. Excessive broadcast traffic can significantly and adversely impact
LAN performance.


Switches actually implement a star bus topology, regardless of which Data
Link Layer protocol they are designed for. Given that the word "switch"
has become readily understood (thanks to the tireless marketing campaigns of switch
manufacturers), it has become more descriptive than star bus. Consequently, switching
could be regarded as a new, fourth topology. The MCSE exam, however, does not regard
switching as a discrete topology. The term that Microsoft uses to describe switched
LANs is star bus, regardless of the LAN's architecture.

Lab


This lab aids your learning by testing you on the information presented in this
chapter as well as giving you exercises to hone your skills. Answers can be found
in Appendix B, "Answers to Review Questions."

Questions



1. A __________ is a LAN-based computer with software that acts as a controlling
device for controlling access to at least part, if not all, of a local area network
and its available resources.


A. Novell PC


B. Client


C. Server


D. Network PC



2. What is the biggest disadvantage of the Star topology?



A. There isn't one


B. If one node goes down, it brings down the entire ring


C. If the hub goes down, it brings down all the nodes on that section


D. If the hub goes down, it brings down all the nodes on all of the rings



3. The __________ of the network concerns how network devices are physically
and electrically (or optically) interconnected.



A. Physiology


B. Topology


C. Both A and B


D. None of the above



4. The principal topologies used with LANs are:



A. Bus


B. Star


C. Ring


D. All of the above


5. What are the various types of bus architectures?


A. Star-wired


B. Linear


C. Parallel


D. Both A and B


E. Both A and C



6. What are the various types of ring architectures?



A. Star-wired


B. Linear


C. Circular


D. Both A and B


E. Both A and C


7. In a __________, each workstation is directly connected to a common
communications channel.


A. Ring Topology


B. Bus Topology


C. Star Topology


D. None of the above



8. In a __________, each workstation attaches to a common backplane via
its own physical cable that terminates at the hub.



A. Ring Topology


B. Bus Topology




C. Star-wired Bus Topology

D. None of the above



9. In a __________, the cable system forms a loop with workstations attached
at various intervals around the loop.



A. Ring Topology


B. Bus Topology


C. Star-wired Bus Topology


D. None of the above



10. The Media Access Control (MAC) is a sublayer of the:



A. Network Layer


B. Transport Layer


C. Physical Layer


D. Data Link Layer









© Copyright, Macmillan Computer Publishing. All rights reserved.



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