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1 Network Hardware Concepts In this chapter you will learn about The concepts of gateways, routers, and bridges The role of client/server computing Firewalls and proxy servers Database servers Application servers Mail servers FTP servers File and print servers Fax servers Web servers How failover, clustering, scalability, and high availability relate to a network server The role of network interface cards (NICs) and the concepts of Adaptive Fault Tolerance, Adapter Load Balancing,and Adapter Teaming The characteristics of Ethernet, Fast Gigabit Ethernet, and Token Ring networks CHAPTER 1 While individual computers can be quite powerful, they are still “individual.” Sharing files and resources typically meant copying a file to a diskette, then manually walking that diskette to other systems—for example, working on a document after work, then returning that updated document to work the next day in order to print it. Obviously, this is a cumbersome and time-consuming process. If there were a means of “connect- ing” two or more computers, you could access your work from another location
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Network HardwareConceptsIn this chapter you will learn about

• The concepts of gateways, routers, and bridges

• The role of client/server computing

• Firewalls and proxy servers

• Database servers

• Application servers

• Mail servers

• FTP servers

• File and print servers

• Fax servers

• Web servers

• How failover, clustering, scalability, and high availability relate to anetwork server

• The role of network interface cards (NICs) and the concepts ofAdaptive Fault Tolerance, Adapter Load Balancing, and Adapter Teaming

• The characteristics of Ethernet, Fast Gigabit Ethernet, and Token Ringnetworks

CHAPTER 1

While individual computers can be quite powerful, they are still “individual.” Sharingfiles and resources typically meant copying a file to a diskette, then manually walkingthat diskette to other systems—for example, working on a document after work, thenreturning that updated document to work the next day in order to print it. Obviously,this is a cumbersome and time-consuming process. If there were a means of “connect-ing” two or more computers, you could access your work from another location

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(i.e., a computer in your home), finish the work that night, and then send the work toa printer located back at the office. This is the underlying premise behind a network—two or more computers connected together in order to share files, resources, and evenapplications. This chapter introduces you to the basic concepts and terminologyneeded to understand the tangible elements of common networks and servers.

A Network PrimerA networked computer that provides resources is called a server. The computer access-ing those resources is referred to as a workstation or client. Servers are usually the mostpowerful computers on the network because they require the added processing powerto service the many requests of other computers sharing their resources. By compari-son, workstations or clients are usually PCs that are cheaper and less powerful. As arule, a computer may be a server or a workstation, but rarely both (this separationgreatly simplifies the management and administration of the network). Of course, allthe computers on a network must be physically connected, and such connections aretypically established with network interface card (NIC) adapters and copper (or fiber-optic) cabling.

Advantages of a NetworkWith individual computers, applications and resources (such as printers or scan-ners) must be duplicated between PCs. For example, if two data analysts want towork on an Excel spreadsheet and print their results each day, both computers willneed a copy of Excel and a printer. If the users needed to share data, it would haveto be shuttled between the PCs on diskette or CD-RW. And if users needed to sharecomputers, they would have to wade through the other user’s system—each with itsown desktop setup, applications, folder arrangement, and so on. In short, it wouldbe a wasteful, frustrating, and error-prone process. As more users become involved,it wouldn’t take long before the whole process would be impossible to handle.However, if those two computers in our example were networked together, bothusers could use Excel across the network, access the same raw data, and then outputtheir results to a single “common” printer attached to the network. If more userswere then added to the network, all users could share the application, data, andresources in a uniform fashion. More specifically, computers that are part of a net-work can share:

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• Documents (memos, spreadsheets, invoices, and so on)

• E-mail messages

• Word-processing software

• Project-tracking software

• Illustrations, photographs, videos, and audio files

• Live audio and video broadcasts

• Printers

• Fax machines

• Modems

• CD-ROM drives and other removable media drives (such as Zip and Jaz drives)

• Hard drives

Because many computers can operate on one network, the entire network can be effi-ciently managed from a central point (a network administrator). Consider the previousexample and suppose that a new version of Excel became available to our data analysts.With individual computers, each system would have to be upgraded and checked sepa-rately. That’s not such a big deal with only two systems, but when there are dozens (evenhundreds) of PCs in the company, individual upgrades can quickly become costly andinefficient. With a network, an application only needs to be updated on its server once—then all the network’s workstations can use the updated software immediately. Centralizedadministration also allows security and system monitoring to take place from one location.

Network SizesComputer networks typically fit into one of three groups depending on their size andfunction. A local area network (LAN) is the basic classification of any computer network.LAN architecture can range from simple (two computers connected by a cable) to com-plex (hundreds of connected computers and peripherals throughout a major corpora-tion). The distinguishing feature of a LAN is that it is confined to a limited geographicarea such as a single building or department. If the computers are connected over sev-eral buildings across a large metropolitan area, the network is sometimes termed a met-ropolitan area network (MAN). By comparison, a wide area network (WAN) has nogeographical limit. It can connect computers and peripheral devices on opposite sidesof the world. In most cases, a WAN is made up of a number of interconnected LANs—perhaps the ultimate WAN is the Internet.

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Network TypesNetworks are generally divided into two distinct categories: peer-to-peer and server based.This is an important distinction because these two categories are vastly different andoffer different capabilities to the users. Peer-to-peer networks are simpler and lessexpensive networks that appear in small organizations (such as SOHO or small work-group applications). Server-based networks are found in mid-sized and larger organi-zations where security, centralized administration, and high traffic capacity areimportant. Let’s look a bit closer at these network types.

Peer-to-Peer NetworksThis is a simple and straightforward networking approach that simply connects com-puters to allow basic file sharing. There are no dedicated servers, and there is no hierar-chy among the computers. Since all the computers are equal, they are known as peers.Each computer serves as both a client and a server, and no administrator is responsiblefor the entire network—the user at each computer determines what data on that com-puter is shared on the network. All users can share any of their resources in any mannerthey choose. This includes data in shared directories, printers, fax cards, and so on.Peer-to-peer networks are also commonly called workgroups (which implies a smallgroup of people) because there are typically 10 or fewer computers in a peer-to-peernetwork. As a result of this simplicity, peer-to-peer networks are often less expensivethan server-based networks.

In a peer-to-peer network, the networking software does not require the same stan-dard of performance or security as the networking software designed for dedicatedserver systems. In fact, peer-to-peer networking capability is built into many popularoperating systems (such as Windows 98/Me). This means you can set up a peer-to-peernetwork without any additional network operating system.

Security is a real weakness in peer-to-peer environments. Generally speaking, security(i.e., making computers—and the data stored on them—safe from harm or unauthorizedaccess) consists of setting a password on a resource (i.e., a directory) that is shared on thenetwork. All peer-to-peer network users set their own security, and shared resources canexist on any computer, so centralized control is very difficult to maintain. This has a bigimpact on network security because some users may not implement any security mea-sures at all. In summary, a peer-to-peer network is often the best choice when

• There are 10 (or fewer) users.

• Users share resources (such as files and printers) but no specialized servers exist.

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• Security is not an issue.

• The organization (and the network) is expected to experience only limited growth.

NOTE: Because every computer in a peer-to-peer environment can act as botha server and a client, users generally need additional training before they canact as both users and administrators of their computers.

Server-Based NetworksIn most network situations, the duality of peer-to-peer networks is simply not adequate.Limited traffic capability and security/management issues often mean that networksneed to use dedicated servers. A dedicated server is a computer that functions only as aserver to provide files and manage resources; it is not used as a client or workstation.Servers are optimized to handle requests from numerous network clients quickly andensure the security of files and directories. Consequently, server-based networks havebecome the standard models for modern business networking. Server-based networksare also known as client/server networks (sometimes denoted as two-tier architectures).

NOTE: Servers provide specific resources and services to the network, andthere may be several (perhaps many) servers available in a given network.

Server TypesAs networks increase in size (i.e., as the number of connected computers increases, andthe physical distance and traffic between them grows), more than one server is usuallyneeded. Spreading the networking tasks among several servers ensures that each taskwill be performed as efficiently as possible. Servers must perform varied and complextasks, and servers for large networks have become specialized to accommodate theexpanding needs of users. Some examples of different server types included on manylarge networks are listed here:

• File and print servers File and print servers manage the user’s overall access anduse of file and printer resources. For example, when you’re running a word-processing application (such as Microsoft Word), that application runs on your

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workstation. The document stored on the file and print server is loaded into yourworkstation’s memory so that you can edit or use it locally. In other words, file andprint servers are used for file and data storage. If you wish to print the document,the file and print server manages the transfer of that file to the network printer.

• Database servers In most cases, a database server is a server that runs an SQL-based database management system (DBMS). Client computers send the SQLrequests to the database server. The server accesses the stored database to processthe request, and then returns the results to the client computer. When referring toa database server, the term “server” may refer to the computer itself or the DBMSsoftware that manages the database (such as Microsoft SQL Server).

• Application servers Where file and print servers will download a file to therequesting client PC, an application server does not—only the results of a requestare sent to the client PC. For example, you might search the employee database forall employees who were born in November. Instead of the entire database beingdownloaded to your PC so that you can search it, the search is performed on theapplication server itself, and only the result of your query is sent from the server toyour computer. This subtle but powerful difference makes application servers(such as Lotus Domino) ideal for maintaining vast quantities of data and effi-ciently providing that data to clients.

• Mail servers E-mail is an important part of modern communication, so mailservers (such as Microsoft Exchange Server) handle the flow of e-mail and messag-ing between network users. In most cases, mail servers are similar to applicationservers because the e-mail typically remains on that server. When you check your e-mail, you only see the e-mail intended for your screen name. Storing e-mail in acentral fashion such as this allows for better security and e-mail management (i.e.,old e-mails can be purged after so many days in a system-wide fashion).

A variation of this is the mailing list server (a.k.a., list server), which is needed forcreating, managing, and serving mailing lists. Stand-alone list servers (such asMajordomo) generally offer more features and better performance than their inte-grated counterparts. Uses for mailing lists and list servers include the distributionof e-zines, newsletters, product updates, technical support documents, classroomschedules, and product brochures, along with discussion forums for clubs andgroups, electronic memos, and so on.

• Fax and communication servers Networks rarely exist in a vacuum, and thereare generally several ways to access the network from outside. Two popular meansof external network access are faxes and dial-up. A fax server (such as FaxMaker)

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manages fax traffic into and out of the network using one or more fax/modemcards. This allows network users to send faxes outside of the network (and viceversa). Communication servers handle data file and e-mail transfers between yourown networks and other networks, mainframe computers, or remote users whodial in to the servers over modems and telephone lines. For example, a networkuser may access the Internet through a communication server.

• Audio/video servers Audio and video servers deliver multimedia capabilities toWeb sites by giving users the ability to listen to sound or music and watch movieclips through Web browser plug-ins. While the use of traditional formats like.WAV, .MIDI, .MOV, or .AVI on Web sites doesn’t really demand a specializedserver, the recent emergence of streaming audio and video content has made theaudio/video server a necessity in many cases (with tools such as RealServer Plus).New streaming technologies mark an important transition for multimedia on theWeb, and will undoubtedly become one of the Internet’s most exciting technolo-gies as it evolves.

• Chat servers It is common practice for two or more users to exchange real-timemessages. This is called a chat, and chat servers (using tools like MeetingPoint) pro-vide the management for real-time discussion capabilities for a large number ofusers. Potential chat uses include teleconferences, private meeting areas, help sup-port forums, and employee recreational get-togethers. The three major types ofcommunications servers are Internet Relay Chat (IRC), conferencing, and commu-nity servers. The most advanced chat servers have recently started augmenting thetext-based medium of conversation with dynamic voice (and even video) support.It is common for IRC-based chat to use dedicated IRC servers (with software likeIRCPlus).

• FTP servers From downloading the newest software to transferring corporatedocuments, a significant percentage of Internet traffic consists of file transfers. FileTransfer Protocol (FTP) servers make it possible to move one or more files betweencomputers with security and data integrity controls appropriate for the Internet(using tools like ZBServer Pro). FTP is a typical client/server arrangement. The FTPserver does the main work of file security, file organization, and transfer control.The client (sometimes part of a browser and sometimes a specialized programsuch as FTP Voyager) receives the files and places them onto the local hard disk.

• News servers News servers function as a distribution and delivery source for over20,000 public newsgroups currently accessible over the USENET news network(the largest news and discussion group-based network on the Internet). Newsservers use tools (like INN News Server) that employ the Network News Transport

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Protocol (NNTP) to interface with other USENET news servers and distribute newsto anyone using a standard NNTP newsreader (such as Agent or Outlook Express).News servers also make it possible to serve your own news and discussion groupspublicly over the Internet—or privately over your own local network.

• Gateway servers A gateway is a translator that allows differing networks to com-municate. For example, one common use for gateways is to act as translatorsbetween personal computers and minicomputer or mainframe systems. In a LANenvironment, one computer is usually designated as the gateway computer. Specialapplication programs in the desktop computers access the mainframe by commu-nicating with the mainframe environment through the gateway computer, andusers can access resources on the mainframe just as if those resources were on theirown desktop computers.

• Firewalls and proxy servers Simply stated, a firewall is a feature designed to pre-vent unauthorized access to or from a private network (i.e., a corporation’s LAN),and is generally considered to be a first line of defense in protecting private infor-mation. Firewalls can be implemented in both hardware and software (and ofteninvolve a combination of both). When properly implemented, firewalls preventunauthorized Internet users from accessing private networks that are connected tothe Internet—especially intranets. All messages entering or leaving the intranetpass through the firewall, which examines each message and blocks those that donot meet the required security criteria. There are numerous firewall techniquesincluding packet filters, application gateways, circuit level gateways, and proxyservers. The proxy server is perhaps the most popular form of firewall. In actualpractice, a proxy server sits between a client program (i.e., a Web browser) andsome external server (usually another server on the Web). The proxy server effec-tively hides the true network address, then monitors and intercepts any requestsbeing sent to the external server, or that come in from the Internet connection.This allows the proxy server to filter messages, improve performance, and shareconnections.

Filtering is a security feature. Proxy servers can inspect all traffic (in and out) overan Internet connection and determine if there is anything that should be deniedtransmission, reception, or access. Since this filtering works both ways, a proxyserver can also be used to keep users out of particular Web sites by monitoring forspecific URLs, or restrict unauthorized access to the internal network by authenti-cating users. Since proxy servers are handling all communications, they can logeverything the user does. For HTTP (Web) proxies, this includes logging every URL.

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For FTP proxies this includes tracking every downloaded file. A proxy server canalso examine the content of transmissions for “inappropriate” words or scan forviruses. Proxy servers can also improve performance by proxy server caching. Theproxy server analyzes user requests and determines which (if any) should have thecontent stored temporarily for immediate access. One example might be a com-pany’s home page located on a remote server. If many employees visit this pageseveral times a day, the proxy server can cache it for immediate delivery to the Webbrowser. Some proxy servers—particularly those targeted at small business—provide a means for sharing a single Internet connection among a number ofworkstations. While this has practical limits in performance, it can still be a veryeffective and inexpensive way to provide Internet services (such as e-mail) to anentire office.

• Web servers Web servers allow you to provide content over the Internet using theHypertext Markup Language (HTML). A Web server (with software like MicrosoftPWS) accepts requests from browsers like Netscape and Internet Explorer, and thenreturns the appropriate HTML document(s) to the requesting computer. A numberof server technologies can be used to increase the power of the server beyond itsability to simply deliver standard HTML pages—these include CGI scripts, SSLsecurity, and Active Server Pages (ASPs).

• Telnet/WAIS servers Telnet servers give users the ability to log on to a host com-puter and perform tasks as if they’re actually working on the remote computeritself. Users can access the host system through the telnet server from anywhere inthe world using a telnet client application. Before the arrival of the Web, Wide AreaInformation Server (WAIS) servers were critical for allowing users to performsearches for keywords in files. While telnet and WAIS are really not that populartoday, network developers looking to broaden their selection of Internet servicesmay consider supporting telnet or WAIS services.

Server SoftwareOne major issue that separates servers from peer computers is the use of software. Nomatter how powerful a server may be, it requires an operating system (i.e., WindowsNT/2000 or Novell NetWare) that can take advantage of the server’s resources. Serversalso require their specific server applications in order to provide their services to thenetwork. For example, a Web server may use Windows NT and Microsoft PWS. It’s notimportant for you to fully understand software issues at this point. Chapter 2 coversnetwork protocols and operating systems in more detail.

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Client/Server Advantages There is little doubt that server-based networks are morecomplicated to install and configure, but there are some compelling advantages overpeer-to-peer networks:

• Sharing Servers allow for better resource organization and sharing. A server isintended to provide access to many files and printers while maintaining perfor-mance and security for the user. A server’s data and resources can be centrallyadministered and controlled. This centralized approach makes it easier to find filesand support resources than would otherwise be possible on individual computers.

• Security In a server-based environment, one administrator can manage networksecurity by setting network policies and applying them to every user.

• Backups Backup routines are also simplified because only servers need to bebacked up (client/workstation PCs do not). Server backups can be scheduled tooccur automatically (according to a predetermined schedule) even if the servers arelocated on different parts of the physical network.

• Fault tolerance Because data is mainly held on servers, fault-tolerant data stor-age (i.e., RAID) can be added to the servers to prevent data loss due to drive fail-ures or system crashes. This creates a more reliable server subject to less downtime.

• Users A server-based network can support thousands of users. Such a large net-work would be impossible to manage as a peer-to-peer network, but current mon-itoring and network-management utilities make it possible to operate aserver-based network for large numbers of users.

Server Reliability Reliability is basically the notion of dependable and consistentoperation—the probability that a component or system will perform a task for a speci-fied period of time. This includes the server as well as the network, and is often mea-sured as a function of the time between system failures using the term mean timebetween failure (MTBF). Data integrity and the capability to warn of impending hard-ware failures before they happen are two other aspects of reliability. Servers frequentlyinclude reliability features such as redundant power supplies and fans, predictive fail-ure analysis for hard drives (SMART), and redundant array of independent disks(RAID) systems to ensure that a server continues to function and protect its data evenwhen trouble occurs. Other reliability features include the memory self-test at boottime where the system detects and isolates bad memory blocks, as well as error check-ing and correcting (ECC) memory to improve data integrity.

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NOTE: Reliability is a critical server issue and is absolutely vital to long-termnetwork operation. Large networks typically strive for 99.999 percent reliabilityor better.

Server High Availability A server must constantly be “up” and ready for immediateuse, allowing a user to access the resources they need in real time. This is the issue of highavailability. Another aspect of highly available servers is the capability to quickly recoverfrom a system failure (i.e., use a “hot spare” RAID disk to recover data from a failed drive).Highly available systems may or may not use redundant components (such as redundantpower supplies), but they should support the hot swapping of key components. Hotswapping is the ability to pull out a failed component and plug in a new one while thepower is still on and the system is operating. A highly available system has the capabilityto detect a potential failure and transparently redirect or failover the questionableprocesses to other devices or subsystems. For example, some SCSI drives can automati-cally move data from marginal sectors (i.e., sectors that produce occasional read errors)to spare sectors without the operating system or the user being aware of the change.

In general, availability is measured as the percentage of time that a system is func-tioning and usable. For instance, a system that provides 99-percent availability on a 24hours/day, 7 days/week basis would actually experience the loss of 88 processing hoursa year (unacceptable to many users). However, a 99.999 percent level of availabilitytranslates to about 5.25 minutes of unscheduled downtime per year (though this levelof availability may be quite costly to achieve).

Server Scalability Computer customers of the past often bought mainframes twicethe size they needed in anticipation of future growth, knowing that they would eventu-ally “grow into” the machine. Today it’s possible to select computers to fit the task now,then add more equipment as needs demand—this is known as scalability. A scalable PChas the capability to grow in size (capacity) and speed. Some machines offer limitedscalability by design, while some can grow to virtually any size needed. Scalabilityincludes the ability to add memory (RAM), add additional processors (i.e., for multi-processing platforms), add storage (hard drives), and still work within the limitationsof the network operating system.

There is a subtle difference between upgrading and scaling. An upgrade is the replace-ment of an existing component with a faster or better component. Scaling a PC is the

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addition of more components for added capacity. For example, an ordinary PC may usea single processor. It may be possible to upgrade that processor to a faster model, but itis not possible to scale up the processing capacity with more processors—you’d need aserver with a multiprocessing motherboard for that. By contrast, virtually all PCs canscale memory (RAM) simply by adding more DIMMs or RIMMs to the system. Thesame concept holds true for your disk space—you can upgrade to a larger or faster disk,or you could scale up the drive capacity by adding additional hard drives.

SMP and Parallel Processing Since processors are a key element of server perfor-mance and scalability, it is a good time to cover multiprocessing in a little more detail.A symmetric multiprocessing (SMP) machine is a computer that utilizes two or moreprocessors. Each processor shares memory and uses only one copy of the operating sys-tem. SMP machines can scale by starting small (with only two processors), then addingmore processors as business needs and applications grow. Beyond CPUs, such comput-ers typically have the ability to scale memory, cache, and disks. Currently, SMPmachines are designed to scale from 2-32 processors.

There are limiting factors to consider when dealing with SMP systems. While it mayseem possible to scale far more than 32 processors, that is often not the case. If you wereto start with two processors and add two more, a near 100-percent improvement mayresult, but because there is only one operating system and all memory is shared, a dimin-ishing return on performance will be realized as more processors are added. Most SMPsystems will show worthwhile improvements until they scale above eight processors (thediminishing return also varies based on the operating system and the applications in use).While UNIX systems with 16 or more processors are not uncommon today, Windows NTscalability is commonly thought to be limited to about four CPUs. In addition, manyoperating systems or database applications can only utilize the first 2GB of memory.

By comparison, some of the largest and most scalable systems in the world utilizeparallel processing technology. Parallel processing takes SMP a step further by combiningmultiple SMP nodes. These nodes can work in parallel on a single application—usuallya database that is fully parallel-capable. Because each node has its own copy of theoperating system, and the nodes communicate through a specialized interconnectionscheme, adding additional nodes does not increasingly tax a single OS. This means par-allel processing can scale to much higher levels than SMP alone.

Server Clustering Years ago, only a single processor was needed to run a server andoperate all its applications. With the advent of multiprocessing, two or more processorsshared a pool of memory, and the server could handle more and larger applications.Later on, multiple servers were organized into groups with each server performing a

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specific task (i.e., file server, application server, and so on). Today, many high-end net-works employ server clusters, where two or more server PCs act like a single server—providing higher availability and performance than a single could handle. Applicationscan move from one server to another, or run on several servers at once, and all transac-tions are transparent to the users.

Clustering provides higher availability and scalability than would be possible if thecomputers worked separately. Each node in the cluster typically has its own resources(processors, I/O, memory, OS, storage, and so on), and is responsible for its own set ofusers. The high availability of a server cluster is provided by failover capability. When onenode fails, its resources can “failover” to one or more other nodes in the cluster. Once theoriginal node is restored to normal operation, its resources can be manually (or auto-matically) switched back. Server clusters are also easily scalable without an interruptionof service. Upgrades can be performed by proactively failing over the functions of a serverto others in the cluster, bringing that server down to add components, and then bringingthe server back up into the cluster and switching back its functions from the other servers.

Server clustering is not really a new idea, but they have generally been proprietary inboth hardware and software. IS managers are looking at clusters more seriously now asthey become more accessible using mass-produced, standards-based hardware likeRAID, SMP systems, network and I/O adapters, and other peripherals. While clusters arepoised to gain more sophistication in the future, a growing number of cluster optionsare available today, and formal standards for clustering are still being developed.

Network TopologyIn order to create a network, two or more PCs (and other peripheral devices) must beconnected together. However, there are several different ways to arrange these connec-tions, and each connection scheme is known as a network topology. Each topology offersits own unique capabilities and limitations. Unfortunately, topologies aren’t as simpleas plugging one computer into another; each topology requires certain cabling, NICadapters, network operating systems, and other devices. For example, a particular topol-ogy can determine not only the type of cable that is used, but also how the cabling runsthrough floors, ceilings, and walls. While most network topologies use physical cablesto connect one computer to another, a growing number of networks use wireless trans-ceivers for at least some connections. Topology can also determine how computerscommunicate on the network. This part of the chapter introduces you to three tradi-tional network topologies: bus, star, and ring.

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Bus TopologyThe bus is the simplest and most straightforward type of network topology, and is com-monly used with Ethernet networks. With a bus (see Figure 1-1), computers are con-nected to each other in a straight line along a single main cable called a trunk (a.k.a.backbone or segment). Bus networks are easy to connect and inexpensive to imple-ment, and a computer failure won’t impair the entire network. However, overall busperformance is limited, and cable breaks can shut down the entire network.

Bus OperationComputers on a bus network communicate by addressing data to a particular computerand sending out that data to all computers on the cable. Only the computer whoseaddress matches the address encoded in the original signal will accept the information;all other computers simply ignore the data. Since data goes out to all computers simul-taneously, only one computer at a time can send messages. As you might expect, thisalso means the number of computers attached to the bus will affect network perfor-mance. The more computers there are on a bus, the more computers will be waiting toput data on the bus, and the slower the network will be. Bus performance is difficult to

Terminator

NIC

PC 1

Terminator

Cable Segment

NIC

PC 2

NIC

PC 4

NIC

PC 3

Figure 1-1 Typical bus topology

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judge because numerous other factors affect bus performance (in addition to the num-ber of computers) such as

• Hardware capabilities (i.e., NIC type) of computers on the network

• Total number of PCs waiting to be sent data (a.k.a. the network traffic)

• Types of applications (i.e., file system sharing) being run on the network

• Types of cable used on the network

• Distances between computers on the network (the overall length of the trunk)

The electronic signals that represent data are sent to the entire network, and travelfrom one end of the cable to the other. If the signal is allowed to continue uninterrupted,it will keep bouncing back and forth along the cable. This signal bounce can prevent othercomputers from sending data. The signal must be stopped after it has reached the properdestination address. To stop a signal from bouncing, a simple device called a terminatoris placed at each end of the network cable to absorb the signals. This clears the cable sothat other computers can send data. When using a bus topology, both ends of each cablesegment must be plugged into something. For example, a cable end can be plugged intoa computer or connector to extend the cable’s length. Any open cable ends not pluggedinto something must be terminated to prevent signal bounce.

NOTE: Terminator problems are a common issue with bus networks, and theyshould be checked and verified whenever network transmission problems arise.

Bus DisruptionsComputers on a bus topology will either transmit data to other computers or listen fordata from other computers on the network—they are not responsible for moving datafrom one computer to the next. As a result, if one computer fails, it does not affect therest of the network. This is a main advantage of the bus topology. Unfortunately, bus-type networks are extremely sensitive to cable breaks. A break in the cable will occur ifthe cable is physically separated into two pieces (i.e., accidentally cut), or if at least oneend of the cable becomes disconnected (i.e., someone fiddles with a cable connectionbehind the PC). In either case, one or both ends of the cable will not have a terminator,the signal will bounce, and all network activity will stop, causing the network to godown. The individual computers on the network will still be able to function as stand-alone PCs, but they will not be able to communicate with each other or access shared

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resources as long as the cable remains broken. The computers on the down segmentwill continually attempt to establish a connection, and this will slow the workstations’performance until the problem is resolved.

Expanding the BusIt is fairly easy to expand the bus topology to accommodate more users and peripheraldevices. Simply remove a terminator from one end of the network trunk, add a cable toanother PC’s T connector, then replace the terminator at that last T connector (refer toFigure 1-1). If you need to extend a given cable length to make it longer, you can fastentwo cable lengths together using a barrel connector. However, connections tend todegrade signal strength, so it should be used only when absolutely necessary. Too manyconnectors can prevent the signal from being received correctly. It’s preferable toremove the shorter cable length and attach a more suitable length instead—one con-tinuous cable is preferable to connecting several smaller ones with connectors. As analternative, a repeater can be used to connect two cable lengths. A repeater actuallyboosts the signal strength, so the signal remains stronger across multiple connectors ora longer piece of cable.

Star TopologyThe star topology is slightly more sophisticated than a bus approach because all PCs onthe network are tied to a central connection point called a hub (see Figure 1-2). A star net-work is a bit more robust than the bus approach because connections are direct from thePC to the hub. It’s an easy matter to add clients to the network simply by connectingthem to an available port in the hub (multiple hubs can be ganged together for larger net-works with additional users). Because each connection is independent, you don’t need toworry about terminators. A cable problem or PC fault will only affect that particularworkstation; it won’t disable the entire network. On the negative side, more cabling isoften required because each PC needs its own cable to the hub. Also, a hub failure candisable all the PCs attached to it (though this is a fairly easy issue to troubleshoot).

Star OperationComputers on a star network communicate by addressing data to a particular computerand sending out that data through the hub to all computers on the network. Only thecomputer whose address matches the address encoded in the original signal will acceptthe information; all other computers simply ignore the data. Because data goes out toall computers simultaneously, only one computer at a time can send messages. Thismeans the number of computers attached to the star will affect network performance.The more computers there are on a star network, the more computers will be waiting to

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send data to the hub and the slower the network will be. Star performance is difficult tojudge because numerous other factors affect network performance (in addition to thenumber of computers), such as

• Hardware capabilities (i.e., NIC type) of computers on the network

• Performance and capabilities of the hub

• Total number of PCs waiting to be sent data (a.k.a. the network traffic)

• Types of applications (i.e., file system sharing) being run on the network

• Types of cable used on the network

• Distances between computers on the network

Unlike the bus topology, star network connections are not bothered by signalbounce, so no special termination is needed. You simply connect the PC’s NIC adapterport to the corresponding hub port.

Cable segment

NIC

PC 4

NIC

PC 3

NIC

PC 2

NIC

Hub

Ports

PC 1

Figure 1-2 Typical star topology

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Star DisruptionsComputers on a star topology will either transmit data through the hub to other com-puters or listen for data from the hub; they are not responsible for moving data fromone computer to another. As a result, if one computer fails, it does not affect the rest ofthe network. This is an important advantage of the star topology. Also, the fact that allof the network’s PCs must come together to a single point (the hub) means that thehub(s), server(s), and other key network devices can all be conveniently located andserviced in one place. This improves network troubleshooting and administration. If abreak or disconnection occurs with a cable, only that PC is effected, and the remainderof the network can continue on normally. However, since the hub serves as a centralcommunication point in the star topology, a hub failure will quickly disable all the PCsattached to it.

Expanding the StarIt is fairly easy to expand the star topology to accommodate more users and peripheraldevices. Additional users can simply be connected to an available port on an existinghub. However, the added wiring becomes problematic. When a nearby PC is added toa bus-type network, you only need to attach the new PC in-line with the existing trunkwiring. When a new PC is added to a star-type network, you may need to run an entirelynew cable from the PC to the hub. This might require dozens (maybe hundreds) of feetof additional wiring, which may need to be routed through floors, walls, and ceilingsdepending on what’s between the user and the hub.

NOTE: In actual practice, network designers often mix topologies (i.e.,bus/star) to make a more efficient use of cabling and equipment.

Ring TopologyThe ring topology (usually called token ring) is a bit more sophisticated than a busapproach because the trunk cable that connects all PCs on the network basically forms aloop (see Figure 1-3). Computers are connected in a continuous network loop in whicha key piece of data (called a token) is passed from one computer to the next. The token isa data frame (or packet) that is continuously passed around the ring. In actual practice,token ring networks are physically implemented in a star configuration but managed log-ically as a loop. Workstations on a token ring network are attached to a specialized hubcalled a multistation access unit (MAU). It’s an easy matter to add clients to the network

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simply by connecting them to an available port in the MAU (several MAUs can be gangedtogether for larger networks with additional users). Since the overall effect is that of aloop, you don’t need to worry about terminators. The token-passing approach ensuresthat all PCs have equal access to the network, even when there are many users. On thenegative side, more cabling is often required because each PC needs its own cabling to theMAU. Also, each computer must pass a token to the next, so a PC failure (or a MAU fault)can impair the entire network. This can easily complicate the troubleshooting process.

Ring OperationThe most popular method of transmitting data around a ring is called token passing. Thetoken itself is little more than a short sequence of data bits that travel around a tokenring network, and each network has only one token. The token is passed (received andretransmitted) from computer to computer. An advantage of this retransmission is thateach PC in the loop acts as a repeater—boosting the data signal to the next workstation.This process of token passing continues until the token reaches a computer that has

Cable segment

Token

NIC

PC 4

NIC

PC 3

NIC

PC 2

NIC

MAU

Ports

PC 1

Figure 1-3 Typical ring (token ring) topology

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data to send. The sending computer modifies the token, puts an electronic address onthe data, and reinserts this new data package into the ring.

This data package passes by each computer until it finds the one with an address thatmatches the address on the data. The receiving computer takes the data and attaches averification message to the token, which is re-addressed to the sender and returned tothe ring. The sending computer eventually receives the verification message, indicatingthat the data has been received. After verification, the sending computer creates a newtoken and inserts it on the network. The token continues to circulate within the ringuntil another workstation needs it to send data. Token ring performance is difficult tojudge because numerous other factors affect network performance (in addition to thenumber of computers), such as

• Hardware capabilities (i.e., NIC type) of computers in the ring

• Performance and capabilities of the MAU

• Total number of PCs waiting to be send data (a.k.a. the network traffic)

• Types of applications (i.e., file system sharing) being run on the network

• Types of cable used on the network

• Distances between computers on the network (the overall size of the ring)

Unlike the bus topology, ring network connections are not bothered by signalbounce, so no special termination is needed. You simply connect the PC’s NIC adapterport to the corresponding MAU port to add that PC to the loop.

Ring DisruptionsComputers in a token ring topology are constantly receiving and retransmitting tokensfrom one computer to the next. As a result, if one computer fails or a cable breaks, itinterrupts the rest of the network. Since token rings also use MAUs to pass data fromone PC to the next (refer to Figure 1-3), a MAU failure can also disable the network.These are important disadvantages of token ring topology, and can present a technicianwith serious troubleshooting problems when faced with locating the break in a tokenring. On the plus side, a MAU provides a centralized communication point for networkadministration and maintenance.

NOTE: The idea of a “ring” is only from a logical perspective. From a practicalstandpoint, the network is wired as a “star”—an MAU is used to provide thering feature.

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Expanding the RingBecause a token ring is physically structured very similarly to a star network, it’s fairlyeasy to expand the ring topology in order to accommodate more users and peripheraldevices. Additional users can simply be connected to an available port on an existingMAU. As with star clients, however, the added wiring can be problematic. When anearby PC is added to a bus-type network, you only need to attach the new PC in-linewith the existing trunk wiring. When a new PC is added to a token ring network, youmight need to run an entirely new cable from the PC to the MAU. This may requiredozens (maybe hundreds) of feet of additional wiring, which may need to be routedthrough floors, walls, and ceilings depending on what’s between the user and net-work’s MAU.

Network HardwareNow that you’ve had a chance to learn about server types and network topologies, it’stime to learn a bit more about the various hardware elements involved with the imple-mentation of a network. Network hardware has a profound impact on the speed, qual-ity, and overall performance of the network. For the purposes of this book, networkhardware includes hubs, repeaters, bridges, routers, gateways, network interface cards,and cabling.

HubsSimply stated, a hub is a central connection device that joins computers in a star topog-raphy. A variation of the hub is a Multistation Access Unit (or MAU, sometimes calleda token ring hub) used to connect PCs in a token ring topology. Hubs are now standardequipment in modern networks, and are typically classified as passive or active. A pas-sive hub does not process data at all; it’s basically just a connection panel. By compari-son, active hubs (sometimes called repeaters) regenerate the data in order to maintainadequate signal strength. Some hubs also have the capability to handle additional taskssuch as bridging, routing, and switching. Hub-based systems are versatile and offer sev-eral advantages over systems that do not use hubs. For example, with an ordinary bustopology, a break in the cable will take the network down. But with hubs, a break in anyof the cables attached to the hub affects only that limited segment of the network.

Most hubs are active—that is, they regenerate and retransmit signals in the same waythat a repeater does. Since hubs usually have eight to twelve ports for network comput-ers to connect to, they are sometimes called multiport repeaters. Active hubs always

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require electrical power to run. Some hubs are passive (examples include wiring panelsor punch-down blocks). They act only as connection points, and do not amplify orregenerate the signal. The signal just passes through the hub. Passive hubs do notrequire electrical power to run. An emerging generation of hubs will accommodate sev-eral different types of cables. These are called hybrid hubs.

CAUTION: Be careful when connecting hubs. Crossover cables are wired differ-ently than standard patch cables, and one will not work correctly in place ofthe other. Check with the hub manufacturer to determine whether you need astandard patch cable or a crossover cable.

RepeatersAs electrical signals travel along a cable, they degrade and become distorted. This effectis called attenuation. As cable lengths increase, the effects of attenuation worsen. If acable is long enough, attenuation finally will make a signal unrecognizable, and thiswill cause data errors in the network. Installing a repeater enables signals to travel far-ther by regenerating the network’s signals and sending them out again on other cablelengths. The repeater takes a weak signal from one cable, regenerates it, and passes it tothe next cable. As you saw earlier, active hubs frequently act as repeaters, but stand-alone repeaters might be needed to support very long cable lengths.

NOTE: Broadband systems will use amplifiers rather than repeaters.

It is important to realize that repeaters are simply signal amplifiers. They do nottranslate or filter the network signals from one cable to another. For a repeater to workproperly, both cables joined by the repeater must use the same packets, logical proto-cols, and access method. The two most common access methods are carrier sense mul-tiple access with collision detection (CSMA/CD) and token passing. A repeater cannotconnect a segment using CSMA/CD to a segment using the token-passing accessmethod. In effect, a repeater will not allow an Ethernet network to talk to a token ringnetwork—there are other more sophisticated devices used for that type of translation.However, repeaters can move packets from one kind of physical media to another. Forexample, a repeater can take an Ethernet packet coming from a thin coaxial cable and

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pass it on to a fiber-optic cable (provided that the repeater is capable of accepting thephysical connections).

Because repeaters simply pass data back and forth between cables, you should real-ize that problem data (such as malformed packets) also will be processed by therepeater. Bad data will not be filtered out, and excessive network traffic will not be man-aged. As a rule, avoid the use of repeaters when there is heavy network traffic or whendata filtering features are needed.

BridgesA bridge offers more features for a busy network. A bridge can act like a repeater toextend the effective length of a network cable. However, a bridge has more “intelli-gence,” and can also divide a network to isolate excessive traffic or problem data. Forexample, if the volume of traffic from one or two computers (or a single department) isflooding the network with data and slowing down the entire operation, a bridge couldisolate those computers (or department). Rather than distinguish between one proto-col and another, bridges simply pass all protocols along the network. Because all pro-tocols pass across bridges, it is up to the individual computers to determine whichprotocols they can recognize. Bridges can also link different physical media such astwisted-pair cable and thin coaxial cable.

Routing DataA bridge also offers superior data-handling capabilities not provided by hubs andrepeaters. Bridges “listen” to all traffic, check the source and destination address of eachpacket, and build a routing table (as information becomes available) so that they cansort data to different parts of the network efficiently. Bridges actually have the capabil-ity to learn how to forward data. As traffic passes through the bridge, information aboutthe computer addresses is stored in the bridge’s memory. The bridge uses this informa-tion to build a routing table based on source addresses. Initially, the bridge’s memoryis empty and so is the routing table. As packets are transmitted, the source address iscopied to the routing table. With this address information, the bridge eventually learnswhich computers are on which segment of the network.

When the bridge receives a packet, the source address is compared to the routingtable. If the source address is not there, it is added to the table. The bridge then com-pares the destination address with the routing table database. If the destination addressis in the routing table and is on the same network segment as the source address, thepacket is discarded (because it’s assumed that another PC on the same part of the net-work has received the data). This filtering helps reduce network traffic and isolate dif-ferent parts of the network. If the destination address is in the routing table and not in

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the same segment as the source address, the bridge forwards the packet out of theappropriate port to reach the destination address. If the destination address is not inthe routing table, the bridge forwards the packet to all its ports except the one on whichit originated.

Reducing TrafficRemember that many PCs on a network may need to send data, but not all PCs mayneed to receive that data. Often, all PCs must receive data to see whether the informa-tion is intended for that workstation, then each must wait for an opportunity to senddata itself. In a large network, this can significantly reduce network performance. How-ever, large networks often group PCs into departments, and the data sent betweendepartments is often far less than the traffic sent between PCs within the same depart-ment. By using bridges to separate the overall company network into several smallerdepartmental groups, it is possible to reduce the traffic going out to the entire network,and thus improve the network’s overall performance.

Organizing Traffic with a BridgeLet’s look at an example. Consider a company with five major departments: Sales,Accounting, Shipping, Manufacturing, and Design. In an “open” network, traffic sentfrom a PC in sales would eventually reach every other PC on the network (i.e., Account-ing, Shipping, and so on). Since traffic from one department is most commonlyintended for other PCs in the same department, it’s often a waste of network time tohave all those other PCs check all that traffic. If a bridge is used to segregate the networkinto five different areas, traffic sent from one PC in Design to another PC in Designwould not go out to the other areas of the network. This would reduce traffic becauseall the other PCs would not need to check that traffic to see if it was intended for them.If a PC in Design wanted to send traffic to another PC in Sales, the bridge would know(through its routing table) which segment to relay that traffic to, and the other seg-ments would not need to deal with that traffic. This controlling (or restricting) of theflow of network traffic is known as segmenting network traffic. A large network is not lim-ited to one bridge. Multiple bridges can be used to combine several small networks intoone large network.

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Remote ConnectionsBridges are often used to join smaller networks that are separated by large physical dis-tances. For example, when two separate LANs are located at a great distance from eachother, they can be joined into a single network using two remote bridges connectedwith synchronous modems to a dedicated data-grade telephone line.

Routers and BroutersWhen you’re working in more complex network environments that use several differentnetwork segments—each with different protocols and architectures—a bridge is ofteninadequate to handle fast and efficient communication between diverse segments.Such a complex network demands a sophisticated device that knows the address ofeach segment, determines the best path for sending data, and filters broadcast traffic tothe local segment. This type of device is called a router. As with a bridge, routers can fil-ter and isolate network traffic and also connect network segments. Further, routers canswitch and route packets across multiple networks. They do this by exchanging specificprotocol information between separate networks. Routers have access to more packetinformation than bridges; routers use this additional information to improve packetdeliveries. Routers are used in complex networks because they provide better trafficmanagement. For example, routers can share status and routing information with oneanother and use this information to bypass slow or malfunctioning connections.

There are two principal router types: static and dynamic. A static router is sometimescalled a manual router because all routes must be configured manually by the networkadministrator. Routing tables are fixed, so the static router always uses the same route(even if network activity changes). This means there’s no guarantee that the router isusing the shortest routes. By comparison, dynamic routers must be configured initially,but they will adapt to changing network conditions automatically—using lower cost orlower traffic routes as needed.

Routing DataRouters maintain their own routing tables, which usually consist of network addresses(though host addresses can also be kept if the network needs it). To determine the des-tination address for incoming data, the routing table includes all known networkaddresses, logical instructions for connection to other networks, knowledge of the pos-sible paths between routers, and even the costs of sending data over each path. Thus, arouter uses its routing table to select the best route for data transmission based on costsand available paths. You should understand that the “routing tables” used for bridgesand routers are not the same thing. Routers require specific addresses. They understand

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only the network numbers that allow them to communicate with other routers andlocal NIC addresses, so routers don’t talk to remote computers.

When routers receive packets destined for a remote network, they send them to therouter that manages the destination network. The use of routers allows designers to sep-arate large networks into smaller ones and offers an element of security between thesegments. Unfortunately, routers must perform complex functions on each packet, sothey are slower than most bridges. For example, as packets are passed from router torouter, source and destination addresses are stripped off and then re-created. Thisenables a router to route a packet from a TCP/IP Ethernet network to a server on aTCP/IP Token Ring network—a feature unattainable with a bridge.

Reducing TrafficRouters do not look at the destination node address. Instead, they look only at the networkaddress and will pass information only if the network address is known. Routers will notallow corrupted (i.e., nonaddressed) data to be passed onto the network. This capability tocontrol the data passing through the router reduces the amount of traffic between networksand allows routers to use these links more efficiently than bridges. Consequently, routers cangreatly reduce the amount of traffic on the network and the wait time experienced by users.Remember that not all protocols are routable (you’ll see more about protocols in Chapter2). Typical routable protocols include DECnet, Internet Protocol (IP), and InternetworkPacket Exchange (IPX), while protocols such as Local Area Transport Protocol (LATP) orNetBIOS Extended User Interface (NetBEUI) are not routable. Routers are available that canaccommodate multiple protocols (such as IP and DECnet) in the same network.

Selecting a RouteOne distinct advantage enjoyed by routers is that they can support numerous activepaths between LAN segments and select redundant paths if necessary. Since routers canlink segments that use completely different data packaging and access schemes, thereare usually several possible paths available for a router to use. For example, if one routerdoes not function, data can still be sent using alternative routes. This also applies tonetwork traffic. If one path is very busy, the router identifies an alternative path andsends data over that one instead. Routers use powerful algorithms such as OSPF (OpenShortest Path First), RIP (Routing Internet Protocol), or NLSP (NetWare Link ServicesProtocol) to determine an appropriate transmission path for a data packet.

BroutersThe functional distinctions between bridges and routers are blurring as technologyadvances. Some bridges have advanced intelligence that allows them to handle tasks

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that would normally require a router. These advanced bridges are called brouters. Abrouter can act as a “router” for one protocol and “bridge” for all the others. A broutercan route selected routable protocols, bridge nonroutable protocols, and provide morecost-effective and manageable internetworking than separate bridges and routers.

GatewaysA gateway acts as a powerful interpreter designed to connect radically different networks.Although slower than a bridge or router, a gateway can perform complex functions suchas translating between networks that speak different languages (using techniques suchas protocol and bandwidth conversion). For example, a gateway can convert a TCP/IPpacket to a NetWare IPX packet (and vice versa). Gateways enable communicationbetween entirely different architectures and environments. They effectively repackageand convert data going from one type of network to another so that each can under-stand the other’s data. A gateway repackages information to match the requirements ofthe destination system, and changes the format of a message so that it conforms to theapplication running at the receiving end of the transfer. In most cases, gateways are task-specific, which means that they are dedicated to a particular type of transfer. They areoften referred to by their task (i.e., Windows NT Server-to-SNA gateway).

Network Interface Cards (NICs)NICs (also known as LAN adapters) function as an interface between the individualcomputer (server or client) and the network cabling (see Figure 1-4). Internally, the NICmust identify the PC on the network and buffer data between the computer and thecable. When sending data, the NIC must convert the data from parallel bytes into serialbits (then back again during reception). On the network side, an NIC must generate theelectrical signals that travel over the network, manage access to the network, and makethe physical connection to the cable. Every computer on the network must have at leastone NIC port installed. Modern NICs increase their effective throughput usingadvanced techniques of adapter teaming such as adapter fault tolerance (AFT), which pro-vides automatic redundancy for your adapter. If the primary adapter fails, the secondarytakes over. Adaptive load balancing (ALB) allows balancing the transmission data flowbetween two to four adapters. You’ll see much more about NICs in Chapter 10.

CablingFinally, networks of all sizes and configurations depend on the physical cabling thatconnects all the PCs and other hardware together. Cabling (also referred to as networkmedia) comes in many different configurations, but common cabling used for everyday

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networking includes unshielded twisted pair (UTP), coaxial cable, shielded twisted pair(STP), and fiber-optic (FO) cable. As a technician, you should understand the threemain considerations for cabling:

• Resistance to crosstalk (electrical currents between pairs of wires in the same cable)

• Resistance to interference from outside electrical fields (noise created by electricmotors, power lines, relays, and transmitters)

• Ease of installation

These are important issues because cables resistant to crosstalk and interference canbe run longer and support higher data transmission rates. For example, coaxial andshielded twisted-pair cable have a thin metal foil outer layer that offers good resistanceto electrical noise, but the extra foil creates a larger, thicker cable that is more difficultto pull through conduit and walls during installation. Unshielded twisted pair is thin-ner and easier to install, but offers less resistance to electrical noise. By comparison,fiber-optic cable carries light signals instead of electrical pulses, so it is impervious toelectrical interference. This allows fiber-optic cable to carry signals faster and fartherthan any other type of cable. Unfortunately, FO cable is often far more expensive than

Figure 1-4 The Symbios SYM22915 network interface card (Courtesy of LSI Logic Corp.)

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other cable types, and proper installation demands specialized tools and training. Thefollowing section explains these major cable types in detail.

Understanding Network MediaEvery computer in any kind of network must ultimately be connected to one another.These connections are responsible for transmitting vast amounts of informationbetween the computers and peripheral devices. Although wireless networking is grow-ing in popularity, the vast majority of network connections are made physically using avariety of cable types—each intended for a specific type of network architecture. Weusually refer to this interconnecting wiring as network media. While there are more than2,000 different types of cabling, most network applications use only three differentcable types: coaxial, twisted pair, and fiber-optic cable.

Signal TransmissionIn order to understand the importance and characteristics of cabling, you shouldunderstand the idea of “bandwidth,” and the two approaches used to transmit datasignals: baseband and broadband. Bandwidth is simply the amount of data that can behandled by a cable or device over a given time (sometimes called a data transfer rate ortransmission rate). For example, coaxial cable can typically support from 4Mbps–100Mbps (depending on the quality and length of the cable), while fiber-optic cablecan handle up to 1Gbps. Of course, the network architecture and network interfacecards must also support the same bandwidth if you’re going to use the cabling to itsoptimum capacity.

Baseband transmission employs digital signaling to use the entire bandwidth of thecable. The serial cable that connects your PC’s COM port to an external modem is a verysimple example of baseband transmission. Twisted pair and fiber-optic cables are typi-cally used in baseband systems, though coaxial cables are sometimes used that wayalso. Baseband systems normally use repeaters to receive incoming signals and retrans-mit them at their original strength to increase the practical length of a cable.

By comparison, broadband transmission uses analog signaling across a wide range offrequencies. Your everyday cable service that brings 200 channels to your TV and high-speed Internet access to your PC is an example of broadband transmission. With broad-band, each signal is allocated a part of the total bandwidth. Every device associatedwith the system (i.e., all computers connected to the LAN cable) must then be tuned sothat they use only the frequencies that are within the allocated range. Broadband sys-tems use amplifiers (rather than repeaters) to regenerate analog signals to their original

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strength. As with baseband signals, broadband signals flow in one direction only, sothere must be two paths for bi-directional data. The obvious solution is to use twocables: one for transmission and the other for reception. However, since broadbandalso allows numerous signals on the same cable, bandwidth is often split, with half ofthe bandwidth assigned to “transmit” channels and half of the bandwidth allocated to“receive” channels.

Coaxial CableCoaxial cable (coax) is an inexpensive, flexible, and rugged type of transmission cable.Coaxial cables use a single copper wire at the center of an internal insulating layer, cov-ered by a finely braided metal shield, and covered by a protective outer jacket (see Fig-ure 1-5). Its light weight and flexibility make coaxial cable easy to install in a wide rangeof office environments. That wire in the middle of the coaxial cable is what actually car-ries the signal. It is often a solid copper wire, but might sometimes be strandedaluminum. A fairly thick dielectric insulating layer surrounds the core, and this sepa-rates the core from the metal shielding. A braided wire mesh acts as an electrical groundand protects the core from electrical noise and crosstalk. The shielding also protectstransmitted data from electrical noise. For additional protection, a coaxial cable mayincorporate one layer of foil insulation and one layer of braided metal shielding (dualshielding), or two layers of foil insulation and two layers of braided metal shielding(quad shielded). Additional shielding adds greatly to the cable’s cost and weight.Finally, a protective outer cover of rubber, Teflon, or PVC plastic is used to jacket thecable. You’ll generally find two types of coaxial cable used in networking: thin and thick.

All coaxial cables are attached using specialized quick-twist connectors called BNCconnectors. A BNC T connector is an adapter used to attach two lengths of cable to your

Core

Insulator

Braided ShieldOuter Jacket

Figure 1-5 Diagram of a typical coaxial cable

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NIC. If you need to adapt two lengths of cable to make one longer run, use a BNC bar-rel connector. Finally, you’ll need a BNC terminator to cap each end of the cable run(usually attached to the unused port of the last BNC T connectors).

Thinnet CableAs the name implies, thinnet cable is thin—roughly 0.25 in. (diameter)—and can carryelectrical signals for over 600 ft. The cable industry refers to this common type of cableas RG-58. Thinnet cable presents a 50� impedance (signal resistance) to the data sig-nals flowing through it. The cable’s small diameter makes it flexible and easy to installjust about anywhere.

Thicknet CableThicknet cable (sometimes called standard Ethernet cable because of its use with earlyEthernet networks) offers a diameter of 0.5 in.—twice the diameter of thinnet cable. Thecopper core wire is also thicker, and this allows thicknet cable to transfer signals well over1,500 ft. This capability to carry signals a great distance makes thicknet an ideal choice fora backbone cable that’s capable of connecting several smaller thinnet network segments.Unfortunately, thicknet cable does not bend easily, so it is considerably harder to install.

The transition from thicknet to thinnet cable is made with a transceiver device. Thetransceiver’s sharp points pierce the thicknet cable (referred to as a vampire tap) in orderto contact the cable’s core and shielding. An output cable from the transceiver attachesto the computer’s corresponding NIC port. In many cases, the NIC adapter requires anattachment unit interface (AUI) port connector (also known as a Digital Intel Xerox[DIX] connector) to accommodate the transceiver.

Cable GradesChances are that you’ll be running coaxial cable through walls, in ceilings, under floors,and in or through other odd locations throughout your office. It’s important to remem-ber that ordinary coaxial cable uses a jacket of PVC or other synthetic material thatmakes it easy to pull and route. However, building fire codes generally prohibit the useof everyday coaxial cable in a building’s plenum (the shallow space in many buildingsbetween the false ceiling and the floor above). During a fire, PVC jackets will burn andgenerate poisonous gases. Coaxial cable rated for plenum-grade use employs insulationand jacket materials that are certified to be fire resistant and produce a minimumamount of smoke. This reduces poisonous chemical fumes in the event of a fire.Plenum cable can also be used in the plenum area and in vertical runs (i.e., up a wall)without conduit. Be sure to review and understand the fire safety codes for your loca-tion when building, servicing, or expanding your network.

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Twisted-Pair CableAnother popular cable type that is commonly used with current networks is calledtwisted pair. As the name suggests, a twisted pair is little more than two insulated lengthsof copper wire twisted around each other—though a typical twisted pair cable carriestwo, three, or even four pairs of wire contained in a single plastic, PVC, or Teflon jacket(see Figure 1-6). The physical twisting of the wires works to cancel out electrical noisefrom adjacent pairs, as well as other noise sources such as motors, relays, and trans-formers. Twisted-pair cable is either shielded or unshielded, and the choice betweenthese two may have a profound impact on the reliability of your data (especially if youmust carry data over a distance).

Twisted-pair cabling uses RJ-45 telephone connectors. At first glance, these connec-tors look like the RJ-11 telephone connectors that attach your telephone cord to the wall.The RJ-45 connector is slightly larger, and will not fit into an RJ-11 telephone jack. TheRJ-45 connector handles eight cable connections, while the RJ-11 supports only four.This means you can’t accidentally exchange your telephone and network connectors.

Unshielded Twisted Pair (UTP)When there are one or more pairs of twisted wire, but none of the pairs (nor the fullcable) contain additional metal foil or braid for shielding, the twisted-pair cable is saidto be unshielded twisted pair (UTP). UTP is an inexpensive and versatile cable that has

Four Pairs

RJ-45 ConnectorTwisted Pair Cable

Figure 1-6 A typical twisted pair cable

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been popular with 10BaseT networks. The maximum cable length for a UTP networksegment is about 328 ft. National standards organizations have specified the type ofUTP cable that is to be used in a variety of building and wiring situations. These stan-dards include five distinct categories for UTP:

• Category 1 This is traditional UTP telephone cable that was intended to carryvoice but not data. Most telephone cable prior to 1983 was Category 1 cable.

• Category 2 This type of UTP cable is designed for data transmissions up to4Mbps (megabits per second) with four twisted pairs of copper wire.

• Category 3 This category includes UTP cable for data transmissions up to16Mbps with four twisted pairs of copper wire.

• Category 4 This is UTP cable intended for medium-speed data transmissions upto 20Mbps with four twisted pairs of copper wire.

• Category 5 This category certifies UTP cable for high-speed data transmissionsup to 100Mbps with four twisted pairs of copper wire.

One reason why UTP is so popular is because many buildings are prewired fortwisted-pair telephone systems using a type of UTP. In fact, extra UTP is often installedto meet future cabling needs as part of the facility’s prewiring process. If preinstalledtwisted-pair cable meets the category requirements to support data transmission, it canbe used in a computer network directly. However, common telephone wire (Category 1wire) might not have the twisting and other electrical characteristics required for clean,secure, computer data transmission.

Shielded Twisted Pair (STP)In order to avoid degradation of the data because of crosstalk and noise, twisted pairsof wire are often shielded with a wrap of thin metal foil. A fine copper braid then sur-rounds all the pairs, and a thick protective jacket of plastic, Teflon, or PVC is applied.These metal shields reduce signal errors and allow the cable to carry data faster over agreater distance. Other than the shielding, UTP and STP cable is identical.

Fiber-Optic CableTraditional wire cable carries data in the form of electrical signals (i.e., voltage and cur-rent). Fiber-optic cable is fundamentally different in that it uses specialized opticalmaterials to carry data as pulses of light. This makes fiber-optic cable uniquely immuneto electrical noise and crosstalk, and allows FO cable to carry a high data bandwidthover several miles with surprisingly effective security—that is, the FO cable cannot be

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tapped without interrupting the data. Fiber-optic cable transmissions are extremelyfast, easily handling 100Mbps, and with demonstrated data rates to 1Gbps.

An optical fiber consists of an extremely thin fiber of glass (called the core) sur-rounded by another layer of glass with slightly different optical characteristics (knownas the cladding). The cladding effectively keeps light signals in the core material as itpasses down the cable. Since each fiber only passes signals in one direction, a completecable includes two strands in separate jackets: one strand transmits and the otherreceives. A coating of plastic surrounds each glass strand, and Kevlar fibers providestrength. Plastic (rather than glass) is sometimes used as the optical material because itis cheaper and easier to install, but plastic is not as optically clear as glass and cannotcarry light signals over the same long distance.

IBM Cabling SystemIf you work with network cabling for any length of time, chances are that you’llencounter the IBM cabling system. IBM introduced its cabling system in 1984 to ensurethat network cabling and connectors would meet the specifications of their own equip-ment. The IBM cabling system classifies cable into “types” rather than categories. Forexample, Category 3 cable (voice-grade UTP cable) is denoted as Type 3 cable in theIBM system. The major types of IBM cabling are listed here:

• Type 1 This is standard shielded twisted-pair (STP) cable used for computers and multistation access units (MAUs).

• Type 2 This is considered STP voice and data cable.

• Type 3 This is conventional UTP voice-grade cable.

• Type 4 Undefined.

• Type 5 This is industry-standard fiber-optic (FO) cable.

• Type 6 This is STP cable used for data patch applications.

• Type 7 Undefined.

• Type 8 This is referred to as carpet cable—STP cable housed in a flat jacket for use under carpets.

• Type 9 This is plenum-grade (fire safe) STP cable.

One element unique to the IBM cabling system is the cable connector. These IBMType A connectors (commonly known as universal data connectors) are different fromstandard BNC or other connectors. They are neither male nor female—you can connect

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one to another by flipping either one over. These IBM connectors require special face-plates and distribution panels to accommodate their unique shape.

Network ArchitecturesThe architecture of a network is basically the way it is designed, and the way informationis exchanged. There are three basic types of network architectures that you should befamiliar with: Ethernet, token ring, and ARCnet. This part of the chapter examines thesearchitectures in more detail and explains the impact of cabling and access techniques.

Understanding the PacketTo the novice, it might seem that networks exchange information as a continuousstream of data between computers. This is not the case. Sending large amounts of dataat one time causes other computers to wait idly while the data is being moved. Thismonopolizes the network and wastes the time of other users waiting to use the network,especially if a transmission error requires the data to be retransmitted. Rather thanexchange entire files at one time, data is broken down into much smaller chunks. Eachchunk is wrapped with the essential details needed to get the data to its correct destina-tion without errors. These organized chunks are called packets (or frames), and mayrequire many packets to transfer an entire file from one network computer to another.

By transferring data in small packets, wait times seen by other computers on the net-work are much shorter because numerous computers on the network take turns send-ing packets. Should a packet arrive at a destination computer in a damaged orunreadable state (because of signal attenuation), it is much easier and faster to retrans-mit that packet rather than the entire file. Packet data typically contains information(such as e-mail messages or files), but many other types of data can be exchanged inpackets, such as command and control data or session control codes (i.e., feedback thatindicates a packet was received properly, or requires retransmission).

Packet OrganizationA packet is basically made up of three parts: header, data, and trailer. Data is precededby a header, which includes a signal that indicates a packet is being transmitted, asource address, a destination address, and clock information to synchronize the trans-mission. The actual data being sent is included after the header. The header of thepacket may vary greatly in size depending on the particular network, but most networksinclude from 512 bytes to 4KB. Remember that most files are much larger than this, so

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it may take many packets to transmit a complete file. A trailer follows the data. The exactcontent of a trailer may vary, but a trailer usually contains error-checking informationcalled a cyclical redundancy check (CRC). The CRC is a number produced by a mathe-matical calculation performed on the packet at its source. When the packet arrives at itsdestination, the calculation is made again. If the results of both calculations are thesame, the data in the packet has remained intact. If the calculation at the destinationdiffers from that at the source, the data has changed during the transmission, and aretransmission is requested.

NOTE: The exact formation and length of a packet will depend on the net-work’s communication protocol—the set of rules or standards that enable computers to connect with one another and exchange information with as little error as possible.

Understanding Access MethodsOf course, the computers on a network can’t just start spewing packets at any point.While network traffic may seem to be moving simultaneously, a closer look will revealthat computers are actually taking turns placing their data on the network. If two com-puters place their data on the network at the same time, both data packets would “col-lide” and be destroyed. The flow of network traffic must be carefully regulated. Therules that govern how data is sent onto (or taken from) a network are called the accessmethod. An access method provides the traffic control needed to organize data trans-missions on the network. It is also important to realize that all computers on the net-work must use the same access method. Otherwise, network problems would occurbecause some access methods would monopolize the cable. There are three majoraccess methods: CSMA, token passing, and demand priority.

CSMA/CDIn the carrier sense multiple access with collision detection method (CSMA/CD), eachcomputer on the network (clients and servers alike) checks to see that the cable is freebefore it sends a packet. If data is currently on the cable, the computer will not send; itwill wait and check the cable again. Once a computer has transmitted data on the cable,no other computer can transmit data until the original data has reached its destinationand the cable is free again. This is often known as a contention method because two ormore computers are contending for the network cable. If two or more computers happento send data at exactly the same time, there will be a data collision. The two computers

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involved will stop transmitting for random periods of time, and then attempt to retrans-mit. The CSMA/CD technique is only useful up to about 1.5 miles. Beyond that, it mightnot be possible for a computer at one end to sense that a computer at the other end istransmitting. CSMA/CD can be frustratingly slow when network traffic is heavy.

CSMA/CAThe carrier sense multiple access with collision avoidance method (CSMA/CA) is simi-lar to CSMA/CD, but allows each computer to signal its intention to transmit databefore the packet is actually sent. This enables other computers to sense when a datacollision might occur, and thus avoid transmissions that might result in collisions. Theproblem with this approach is that broadcasting the intent to transmit actually adds tothe network traffic and can result in even slower network performance. This makesCSMA/CA the least popular access method.

Token PassingWith the token-passing method, a special type of packet (called a token) is circulatedaround a cable ring from computer to computer. In order for any computer on the ringto send data across the network, it must wait for a free token. When a free token isdetected, the computer waiting for the token will take control of it. The sending com-puter then modifies the packet to include appropriate headers, data, and trailers, andsends the new packet on its way. The receiving computer accepts the packet and its data,and then creates another token for the sending computer indicating that the packet hasbeen received. When the sending computer receives this token, it creates a new freetoken and passes it back onto the ring. When a token is in use by a computer, othercomputers cannot transmit data. Because only one computer at a time can use thetoken, no contention (or collision) takes place, and no time is spent waiting for com-puters to re-send tokens because of network traffic.

Demand PriorityThe demand-priority method is a fairly new approach intended to service the 100MbpsEthernet standard (IEEE 802.12 or 100VG-AnyLAN) based on the star (or star/bus)topology. Hubs manage network access by doing round-robin searches for requests tosend from all nodes on the network. As with CSMA/CD, two computers using demandpriority can cause contention by transmitting at exactly the same time. With demandpriority, however, it is possible to decide which types of data will be given priority ifcontention occurs. If a hub receives two requests at the same time, the highest-priorityrequest is serviced first. If the two requests are of the same priority, both requests areserviced by alternating between the two.

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Demand priority offers several powerful advantages over CSMA/CD. First, commu-nication only takes place between the sending computer, hub, and destination com-puter. This means transmissions are not broadcast to the entire network. Second,demand priority uses twisted-pair cabling (four pairs), which allows computers on thenetwork to receive and transmit at the same.

EthernetEthernet can trace its origins back to the late 1960s when the University of Hawaiideveloped a network that would connect computers across its large campus. This earlynetwork employed a bus topology, baseband transmission, and a CSMA/CD accessmethod. Xerox built upon this scheme, and by 1975 introduced the first Ethernet net-working products intended to operate over 2.5Mbps and connect more than 100 com-puters across a 1km trunk. This early implementation of Ethernet proved so popularthat Xerox, Intel, and Digital (DEC) collaborated on the 10Mbps Ethernet standard(now one of several specifications allowing computers and data systems to connect andshare cabling). Ethernet has become one of the most popular network architectures forthe desktop computer, and is used in network environments of all sizes. Today, Ether-net is considered to be a nonproprietary industry standard that is widely supported bynetwork hardware manufacturers. Ethernet can be summarized as follows:

• Topologies Bus or star

• Transmission Baseband

• Access method CSMA/CD

• Bandwidth 10Mbps or 100Mbps

• Cable type(s) Coaxial (thicknet or thinnet) or UTP

• IEEE specification IEEE 802.3

Ethernet PacketsAn Ethernet packet (commonly called a frame among Ethernet users) is between 64and 1,518 bytes long (512–12,144 bits), and every packet includes control information.For example, the Ethernet II packet format used for Transmission ControlProtocol/Internet Protocol (TCP/IP) is the standard for data transmission over net-works (including the Internet). This packet includes six distinct areas. The preamblemarks the start of the packet (similar to the start bit used in serial communication). Theaddresses denote the destination and source addresses for the packet. A type entry is usedto identify the network layer protocol—usually either IP (Internet Protocol) or IPX

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(Novell’s Internetwork Packet Exchange). The packet’s data then follows, and the packetis concluded by error checking (CRC) information.

Ethernet Performance NotesEthernet performance can be improved by dividing a crowded segment into two less-populated segments then joining them with either a bridge or a router. This reduces thetraffic on each segment; because fewer computers are attempting to transmit onto thesegment, the apparent access time improves. You might consider dividing segments ifnew users are quickly joining the network or if new bandwidth-intensive applications(i.e., database or video software) are added to the network.

Ethernet architecture is also quite versatile, and can use multiple communicationprotocols or connect mixed computing environments such as NetWare, UNIX, Win-dows, or Macintosh. Ethernet will work with most popular network operating systems,including

• Microsoft Windows 95, Windows 98, and Windows 2000

• Microsoft Windows NT Workstation and Windows NT Server

• Microsoft Windows 2000 Professional and Windows 2000 Server

• Microsoft LAN Manager

• Microsoft Windows for Workgroups

• Novell NetWare

• IBM LAN Server

• AppleShare

• UNIX

10BaseT (IEEE 802.3)10BaseT is an Ethernet standard designed to support 10-Mbps baseband data transmis-sion over Category 3, 4, or 5 twisted-pair cable (UTP). UTP cable is more common, butSTP can be substituted without difficulty. Cables are connected with RJ-45 connectors.Each computer uses two pairs of wire: one pair to receive and the other pair to transmit.While Ethernet LANs are traditionally configured in a bus topology, a growing numberare set up as a star topology (using bus signaling and access methods). The hub of a10BaseT network typically serves as a multiport repeater. The maximum length of a10BaseT segment is 328 ft, though repeaters can be used to extend this maximumlength. The minimum cable length between computers is 8 ft. A 10BaseT Ethernet LANcan serve up to 1,024 computers.

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NOTE: Although twisted-pair cable is used to connect computers to a hub, coaxial cable or fiber-optic cable may serve as a backbone between10BaseT hubs.

10Base2 (IEEE 802.3)10Base2 is an Ethernet standard designed to support 10Mbps baseband data transmis-sion over thin coaxial (thinnet) cable. Cables are connected with BNC connectors(including barrel connectors, T connectors, and terminators). 10Base2 Ethernet LANsare traditionally configured in a bus topology. The maximum length of a 10Base2 seg-ment is 607 ft., though repeaters can join up to five segments to create an effective buslength of over 3,000 ft. The minimum cable length between computers is 2 ft. A10Base2 Ethernet LAN will only serve up to 30 computers per segment, but this is oftenideal for small department and workgroup situations.

NOTE: BNC barrel connectors can be used to connect thinnet cable lengthstogether. However, the use of barrel connectors should be kept to a minimumbecause each connection in the cable reduces the signal quality and adds to thedanger of cable separation and accidental disconnection, which can effectivelyshut down the network.

10Base5 (IEEE 802.3)10Base5 is an Ethernet scheme (called standard Ethernet) designed to support 10Mbpsbaseband data transmission over thick coaxial (thicknet) cable. 10Base5 EthernetLANs are traditionally configured in a bus topology, and the maximum length of a10Base5 segment is 1,640 ft., though repeaters can join up to five segments to createan effective bus length of over 8,200 ft. The backbone (or trunk) segment is the maincable from which transceiver cables are connected to stations and repeaters. The min-imum cable length between transceivers is 8 ft. A 10Base5 Ethernet LAN will only serveup to 100 computers per segment, and this is often ideal for small to mid-sized net-work situations.

Cabling a 10Base 5 network can be a bit more involved than other Ethernet configu-rations. The thicknet cabling includes transceivers that provide communicationsbetween the computer and the main LAN cable, and are attached to the main cable withvampire taps. Once a transceiver is placed on the main cable, a transceiver cable (a.k.a.,a drop cable) connects the transceiver to the NIC. A transceiver cable attaches to an NIC

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through an AUI (or DIX) connector. Other cabling is attached with N-series connectors,including barrel connectors and terminators.

NOTE: A thicknet network can combine as many as five cable segments connected by four repeaters, but only three segments can have computersattached. This means two segments are untapped and often known as “inter-repeater links.” This is known as the 5-4-3 rule. Remember that thelength of the transceiver cables is not used to measure the distance of the thicknet cable—only the end-to-end length of the thicknet cable segmentitself is used.

10BaseFLIt is also possible to run an Ethernet network over fiber-optic cable. 10BaseFL isdesigned to support 10Mbps baseband data transmission over fiber-optic cablebetween computers and repeaters. The main reason for using 10BaseFL is to accommo-date long cable runs between repeaters, such as between buildings. The maximum dis-tance for a 10BaseFL segment is about 6,500 ft.

100BaseVGOriginally developed by Hewlett-Packard, the 100BaseVG (a.k.a. voice grade) AnyLANscheme is an emerging networking technology that combines elements of both Ether-net and token ring architectures. This type of architecture is known by several terms:100VG, AnyLAN, 100BaseVG, or simply VG. 100BaseVG supports a minimum data rateof 100Mbps in a star (or cascaded star) topology across Category 3, 4, and 5 twisted-pair (as well as fiber-optic) cable. Because 100BaseVG is compatible with existing10BaseT cabling systems, it is a simple matter to upgrade from existing 10BaseT instal-lations (though new hubs and NIC adapters will be required). 100BaseVG uses thedemand-priority access method that allows for two priority levels (low and high), andsupports both Ethernet frames and token ring packets. While data transmission ratesare higher, the cable distances of 100BaseVG are limited when compared to otherimplementations of Ethernet. A cable run from the 100BaseVG hub to a computer can-not exceed about 820 ft.

100Base“X”There are several variations of the 100Base”X” family depending on the media beingused. 100BaseT4 uses four-pair Category 3, 4, or 5 UTP cable; 100BaseTX uses two-pairCategory 5 UTP or STP cable; and 100BaseFX uses two-strand fiber-optic cable. But all

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are referred to as Fast Ethernet because of their 100-Mbps transmission speeds.100Base”X” also uses CSMA/CD in a star-wired bus topology (similar to 10BaseT whereall cables are attached to a hub).

Token RingIBM introduced the token ring architecture in 1984 for personal, midrange, and main-frame (i.e., SNA) computers. The main objective behind token ring was to establish asimple and reliable wiring method using twisted-pair cable, which could connect indi-vidual workstations to a central location. The architecture of a token ring network istechnically a physical ring. However, rather than cabling the network PCs in an actualcircle (which could make upgrades and workstation additions a real nightmare), thetoken ring approach uses a star topology where all PCs are connected to a central hubcalled a multistation access unit (MAU). In effect, the ring is provided by the MAU ratherthan by the physical cabling.

Cable segments can range from 148 ft–656 ft (depending whether the cable isshielded or unshielded), and requires a minimum of 8 ft between computers. A seg-ment will support up to 72 computers using unshielded cable, though up to 260 com-puters can be supported on a segment with shielded cable. Rings can be connectedthrough the use of bridges. Although Ethernet is more popular, many large companiesare selecting token ring architecture to support mission-critical applications. Token ringarchitectures can be summarized as follows:

• Topologies Star

• Transmission Baseband

• Access method Token passing

• Bandwidth 4Mbps or 16Mbps

• Cable type(s) Shielded and unshielded twisted pair (IBM Types 1, 2, and 3)

• IEEE specification IEEE 802.5

Token Ring PacketsThe token ring packet is a bit more involved than an Ethernet packet, but contains thesame essential information. A start delimiter indicates the start of a packet, and accesscontrol information describes the packet as a token (being passed around the network)or data (having a specific destination). Packet control information will carry details forall computers or only for one computer. The packet is directed with a destination addressand source address, and then the data to be transferred is included. Data might also

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include network commands or status information. A packet check sequence will provideCRC error-checking information, and an end delimiter marks the end of the packet.Packet status information is tagged onto the packet that tells whether the packet was rec-ognized or copied, or if the destination address was even available. This information ispassed back to the sending computer.

Token Ring OperationNow is a good time to review token ring operation. When the network initializes, atoken is generated that then travels around the ring and polls each computer until oneof the computers wants to transmit data—that computer then takes control of thetoken. After a computer captures the token, it sends a data packet out to the network.The packet proceeds around the ring until it reaches the computer with the address thatmatches the destination in that packet. The destination computer copies the frame intoa receive buffer and updates the packet’s Packet Status field to indicate that the infor-mation was received. The updated packet continues around the ring until it arrives backat the sending computer. The sending computer acknowledges the successful transmis-sion, and then removes the packet from the ring and transmits a new token back to thering. It is important to remember that a computer cannot transmit unless it has posses-sion of the token, and no other computer can transmit data while the token is in use bya computer. Only one token at a time can be active on the network, and the token cantravel in only one direction around the ring.

System Monitoring and Fault ToleranceOne major advantage of the token ring architecture is its self-monitoring (or self-diag-nosing) capability. Normally, the first computer to come online in a token ring networkis assigned to monitor network activity. The monitoring computer verifies that packetsare being delivered and received correctly by checking for packets that have circulatedthe ring more than once (and ensuring that only one token is on the network at a time).This monitoring process is called beaconing. A beacon announcement is produced everyseven seconds. The beacon is passed from computer to computer throughout the entirering. If a station does not receive an expected announcement from a PC upstream, ittries to notify the network. It sends a message of the neighbor that did not respond andattempts to diagnose the problem without disrupting the entire network. If a correctioncannot be made automatically, service will be required.

In addition, MAUs incorporate a certain amount of fault tolerance. When one com-puter fails in a “true” token passing network, the token cannot be passed and the net-work fails. MAUs are designed to detect a NIC failure and disconnect that computerfrom the network. This bypasses the failed PC so that the token can continue on to the

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next subsequent computer. This means a faulty computer or connection will not affectthe rest of the token ring network.

ARCnetARCnet (Attached Resource Computer Network) is a simple, inexpensive, flexiblescheme designed for workgroup-size networks. Datapoint Corporation developed theARCnet network architecture in 1977, and the first ARCnet cards shipped in 1983. ARC-net uses a token-passing access method in a star-bus topology, passing data packetswith up to 508 bytes of data at rates approaching 2.5Mbps. ARCnet Plus (a successor tothe original ARCNet) uses data packets with up to 4,096 bytes and supports data trans-mission rates up to 20Mbps.

As a star topology, each computer is connected by cable to a hub. The standardcabling used for ARCnet is 93� RG-62 coaxial cable, though RG-58 coaxial, twisted-pair, and fiber-optic cables are also supported. When using coaxial cable, ARCnetallows a maximum cable distance of about 2,000 ft from a workstation to the hub. Themaximum distance for a bus segment is only 1,000 ft. When using unshielded twisted-pair cable, ARCnet supports a maximum cable distance of 800 ft.

• Topologies Star-bus

• Transmission Baseband

• Access method Token passing

• Bandwidth 2.5Mbps or 20Mbps

• Cable type(s) RG-62 and RG-59 coaxial cable, UTP, or fiber cable

• IEEE specification None (closest to IEEE 802.4)

Server BenchmarksNetwork designers and technicians are always concerned with server and network per-formance. By measuring performance, changes in network performance can be evalu-ated (i.e., determining the impact of adding new users). Performance is measured withbenchmarks—test software that can be run on servers and/or workstations. In manycases, benchmarks are run to establish a baseline for server/network operation, andthen run again when repairs or changes are made to measure differences in operation(and possibly identify problem areas). This part of the chapter discusses TPC, Net-Bench, and ServerBench benchmarks.

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TPCThe Transaction Processing Performance Council (TPC) is a well-known benchmarkorganization founded in the early 1980s, and is represented by a wide range of vendors.TPC members have cooperatively designed a series of tests to measure the power,throughput, and performance of computer systems intended primarily for point-of-sale(POS) applications. Here is a summary of the TPC benchmarks:

• The TPC-A benchmark was developed in November 1989, and measures the per-formance in update-intensive database environments for OLTP (online transac-tion processing) applications. TPC-A measures the number of transactions persecond a system can perform when driven from multiple terminals. In actual prac-tice, TPC-A is rarely used today.

• The TPC-B benchmark was developed in August 1990, and is designed as a data-base stress test. It tests disk I/O (input/output), system and application executiontime, and transaction integrity. TPC-B measures throughput in transactions persecond. TPC-B is not used widely today.

• The TPC-C benchmark appeared in July 1992, as a successor to TPC-A, and isdesigned to measure OLTP performance. TPC-C benchmarks are different thanTPC-A benchmarks because they’re considered “complex queries.” TPC-C mea-sures multiple transaction types, more complex databases, and overall executionstructures, along with a mix of five concurrent transactions measured in transac-tions per minute (TPM). These tests are designed to test OLTP applications thatmanage, sell, or distribute a product or service. This benchmark is used widely intoday’s computer environment.

• The TPC-D benchmark appeared in April 1995, and is designed to measure deci-sion support systems (DSS). The TPC-D benchmark is used widely in today’s DSSenvironment and measures applications that use sophisticated long-runningqueries against large complex databases. There are 17 complex query tests, includ-ing two update tests. The TPC sets data volume points of 1GB, 10GBs, 30GBs,100GBs, 300GBs, 1TB, 3TBs, and 10TBs. Each data volume point is considered adifferent benchmark.

ServerBenchServerBench 4.1 is the latest version of Ziff-Davis’ standard benchmark for measuring theperformance of servers in a true client/server environment. This differs from NetBench inthat ServerBench measures the performance of application servers, while NetBench mea-sures the performance of file servers. The ServerBench setup places data and applications

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on the server, and uses client PCs as front ends to provide access to the applications.ServerBench clients make requests of an application that runs on the server, and theserver’s capability to service those requests is reported in transactions per second (pro-ducing an overall score for the server). ServerBench 4.1 runs on IBM’s OS/2 Warp Server,Microsoft’s Windows NT Server (for both Digital Alpha and x86-compatible processors),Novell’s NetWare, Sun’s Solaris (32-bit SPARC and x86), Linux, and SCO’s OpenServerand UnixWare 2.1. To test network file servers, use the NetBench utility instead.

NetBenchNetBench 6.0 is the official Ziff-Davis benchmark test for checking the performance ofnetwork file servers. NetBench provides a way to measure, analyze, and predict how a fileserver will handle network file I/O requests from 32-bit Windows clients. It monitors theresponse of the server as multiple clients request data and reports the server’s totalthroughput. The clients access the server with requests for network file operations. Eachclient keeps track of how many bytes of data it moves to and from the server and howlong the process takes. This information is used by the client to calculate throughput forthat particular test mix. NetBench totals all the client throughputs together to producethe overall throughput for a server. To test application servers, you should use the Server-Bench utility instead. Version 6.0 of NetBench supports new response time measures forNetBench clients that show how long a server takes to respond to each client’s requests.There is also support for the 32-bit Windows client only for Windows 95, Windows 98,or Windows NT (there is no support for 16-bit Windows, DOS, or Mac OS clients).

Avoiding Benchmark ProblemsOne of the most serious problems encountered with benchmarks is the integrity oftheir numbers. You’ve probably heard that “statistics can lie,” and the same thing is trueof benchmarks. In order for benchmarks to provide you with reliable results, there aresome precautions that you must take:

• Note the complete system configuration When you run a benchmark andachieve a result, be sure to note the entire system configuration (i.e., CPU, RAM,cache, OS version, and so on). The benchmark may yield vastly different numberson different configurations of the same system.

• Run the same benchmark on every system Benchmarks are still software, andthe way in which benchmark code is written can impact the way it produces resultson a given computer. Often, two different versions of the same benchmark will

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yield two different results. When you use benchmarks for comparisons betweensystems, be sure to use the same program and version number.

• Minimize hardware differences between hardware platforms A computer is anassembly of many interdependent subassemblies (i.e., motherboard, drive con-trollers, drives, CPU, and so on), but when a benchmark is run to compare a dif-ference between systems, that difference can be masked by other elements in thesystem. For example, suppose you’re using a benchmark to test the hard drive datatransfer on two systems. Different hard drives and drive controllers will yield dif-ferent results—that’s expected. However, even if you’re using identical drives andcontrollers, other differences between the systems (such as BIOS versions, TSRs, OSdifferences, or motherboard chipsets) can also influence different results.

• Run the benchmarks under the same load The results generated by a benchmarkdo not guarantee that same level of performance under real-world applications. Thiswas one of the flaws of early computer benchmarking—small, tightly written bench-mark code resulted in artificially high performance, but the system still performedpoorly when real applications were used. Use benchmarks that make use of (or sim-ulate) actual programs, or otherwise simulate your true workload.

Chapter ReviewNetworks connect computers together in order to share files, resources, and even appli-cations. A networked computer that provides resources is called a server. The computeraccessing those resources is referred to as a workstation or client. Server-based networksallow resources, security, and administration to be handled from a single central loca-tion. Software is needed to support particular server features. Reliability is basically thenotion of dependable and consistent operation. Availability means that a server mustconstantly be “up” and ready for immediate use, allowing a user to access the resourceshe or she needs in real time. Hot swapping is the ability to pull out a failed componentand plug in a new one while the power is still on and the system is operating. Scalabil-ity allows administrators to select computers to fit the task now, and then add moreequipment as needs demand. Clustering allows more than one server to take on redun-dant roles in the network and improve performance. To improve server performance,more than one processor can be used to perform additional tasks simultaneously.

Bus, star, and ring are the three major topologies used in current networks. A bustopology connects all PCs in a single line (or “trunk”). Bus networks use terminators toprevent signal bounce across the cabling. A star topology connects all PCs to a single

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central hub without the use of terminators, but a hub failure can disable the entirenetwork. The ring topology connects all PCs in a logical “loop,” and uses a token topass control of the network from system to system.

A bridge can act like a repeater to extend the effective length of a network cable, butit can also divide a network to isolate excessive traffic or problem data. A router knowsthe address of each segment, determines the best path for sending data, and filtersbroadcast traffic to the local segment. A gateway can perform complex functions such astranslating between networks that speak different languages (using techniques such asprotocol and bandwidth conversion). The network interface card (NIC, also known asa LAN adapter) functions as an interface between the individual computer (server orclient) and the network cabling.

Cabling (or network media) comes in many different configurations, includingunshielded twisted pair (UTP), coaxial cable, shielded twisted pair (STP), and fiber-optic (FO) cable. Bandwidth is simply the amount of data that can be handled by a cableor device over a given time. Baseband transmission employs digital signaling to use theentire bandwidth of the cable, while broadband transmission uses analog signalingacross a wide range of frequencies. Coaxial cables are available in thinnet and thicknetversions. There are five categories of unshielded twisted pair (UTP) cable. IBM cablingis separated into nine categories.

Networks break down data into small packages called packets. A packet is basicallymade up of three parts: header, data, and trailer. A trailer usually contains error-checking information called a cyclical redundancy check. Access methods regulate theflow of traffic on the network. There are three major access methods: CSMA, token pass-ing, and demand priority. There are several types of Ethernet: 10BaseT, 10Base2,10Base5, 10BaseFL, 100BaseVG, and 100BaseX. Token Ring passes control from PC toPC through the use of special packets (called tokens). ARCnet uses a token-passingaccess method in a star-bus topology with data at rates approaching 2.5Mbps. Perfor-mance is measured with benchmarks—test software that can be run on servers and/orworkstations. ServerBench is the Ziff-Davis standard benchmark for measuring the per-formance of servers in a true client/server environment. NetBench is the Ziff-Davisbenchmark test for checking the performance of network file servers.

Questions

1. A networked computer that provides resources is called a . . .a. Peerb. Nodec. Serverd. Client

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2. A document that is loaded into your workstation’s memory so that you can editor use it locally is typically stored on a . . .a. Database serverb. File and print serverc. Web serverd. Telnet server

3. The notion of dependable and consistent server operation is termed . . .a. Scalabilityb. Availabilityc. Reliabilityd. Redundancy

4. Grouping more than one server to perform the same job in the network is called . . .a. Clusteringb. Failoverc. Redundancyd. Scalability

5. Which topology connects computers to each other in a straight line along a singlemain cable called a trunk?a. Lineb. Starc. Ringd. Bus

6. Which topology connects all PCs on the network to a central connection pointcalled a hub?a. Lineb. Starc. Ringd. Bus

7. Which topology/architecture will shut down if the MAU fails?a. Lineb. Starc. Ringd. Bus

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8. What kind of network hardware can also divide a network to isolate excessivetraffic or problem data?a. Repeaterb. Amplifierc. Bridged. Patch panel

9. RG-58 is a type of . . .a. Coaxial cableb. Shielded twisted pairc. Unshielded twisted paird. Fiber-optic cable

10. 10BaseT is a form of . . .a. Token ringb. ARCnetc. Gigabit Ethernetd. 10Mbps Ethernet

Answers

1. c. Server

2. b. File and print server

3. c. Reliability

4. a. Clustering

5. d. Bus

6. b. Star

7. c. Ring

8. c. Bridge

9. a. Coaxial cable

10. d. 10Mbps Ethernet

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