service and support for pc networks unit

service and support

for pc networks

unit 4

© e-skills NTO 2000

Service and Support for PC Networks

Learning outcomes – Service and Support for PC Networks … 4

What is PC networking? … 5

PC networking definitions … 5

Why do we network PCs? … 6

Sharing resources … 6

e-work … 7

Preserving information … 7

Protecting information … 7

Network types … 8

What is a LAN? … 8

What is a WAN? … 9

Client/server networks … 9

Peer networks … 10

Physical topologies … 12

Ethernet … 13

Operation of Ethernet … 13

Token Ring … 14

Operation of Token Ring … 14

Ethernet vs Token Ring … 15

Network media … 16

Cable types … 16

Coaxial cable … 16

Unshielded twisted pair cable … 17

Shielded twisted pair cable … 17

Fibre-optic cable … 18

Unit 4

contents

© e-skills NTO 2000

A summary of network media and LANs … 19

Network Interface Cards … 19

Operation of the NIC … 19

How data moves around in networks … 20

What are network protocols? … 21

Hardware protocols … 21

Software protocols … 21

The theoretical network … 22

How the OSI Reference Model works … 23

What happens at what layer according to the model … 23

Server technology … 26

Drive arrays … 28

RAID Levels Functionality … 29

Internetworking … 31

Repeaters … 32

Bridges … 32

Filtering traffic … 32

Routers … 33

Gateways … 33

Hubs … 34

Switches … 34

unit 4 service and support for pc networks

Learning outcomes – Service

and Support for PC Networks

It is expected that at the end of this unit the student will be able to:

describe the term network and the advantages and disadvantages of networking computers

describe peer-to-peer (workgroup) networks, server-based networks, combined networks and the common terms used todefine them

describe Local Area Networks (LAN), Wide Area Networks (WAN), Enterprise Networks and the differences between them

identify the main functions of a Network Operating System, network connections and interconnectivity devices

describe the key operational features of both the Ethernet and Token Ring technologies and the basic principles for datamovement in a network, including packet-switching

identify the seven layers of the OSI model

give examples of fault-tolerant hardware.7

6

5

4

3

2

1

4 © e-skills NTO 2000

What is PC networking?

In the nineteen eighties, few PC computer users had access to anetwork. Instead, they were operating in stand-alone mode, or withoutbeing connected to any other computers. Networking is the concept oflinking up PC computers via a cable or some other communicationmedium in order to share information.

PC networking is not only commonplace but also growing rapidly.Networking is used in every aspect of business, including research,advertising, production, shipping, selling, and legal. All corporationshave networks. Education establishments are using PC networks toprovide students and lecturers with access to information. Governmentinstitutions including the military and local authorities all takeadvantage of networking. Networks are utilised everywhere.

PC networking definitions

Networking can be defined as ‘the connecting of computers via acommunication medium’.

In simple terms, a network can be described as two or more computersconnected to each other by a wire in order to exchange information.

service and support for pc networks unit 4

5© e-skills NTO 2000


Why do we network PCs?

By networking computers, a business is able to take advantage ofessential Information Technology and Information Systems.

Reasons for networking computers are as follows:

Sharing resources

The ability to share resources is a major reason for networkingcomputers. By having computers connected to a network, each computeris able to share other individual resources also connected to thenetwork. These resources include hardware devices such as printers,scanners and storage devices, as well as software resources such asapplication programs and software utilities.


e-work

The ability to exchange information electronically between users is amajor advantage of networking computers. This way of communicating(e-mail and e-workflow) is now standard. Rather than exchangingmemos on paper, incurring printing costs and delays, networked userscan instantly send messages to others and even check if they have beenreceived and read. Work can be completed on an electronic form or softcopy, and posted electronically. In this way, many people can review,approve and send information at the click of a button.

Preserving information

Although PCs have ample storage capacity for most day-to-day workactivities, it is common practice for users of a network to save theirwork on a shared network drive, a storage device in a central fileserver. In this way, a single data back up procedure carried out on thefile server can preserve the information of numerous users located invarious parts of an organisation. The task of securing an entireorganisation’s work is made relatively straightforward, and in the eventof work getting lost, recovery is from the one location.

Protecting information

Information being saved on a network drive is far safer than that savedon local drives, the hard drives in individual PCs. An additional layer ofsecurity is present when saving to network drives, as user loginpasswords are required to access any network information.




Network types

In the world of network computing, there are various different industrystandard network designs to choose. Each type of design has itsparticular advantages and disadvantages and the individualrequirements of an organisation determine which design is best suitedfor their business.

The various standards for network designs can be divided into severalcategories. Local Area Networks (LANs) are the most common type ofcomputer network in use by organisations.

LANs have radically changed the corporate information infrastructure.Corporations have moved from mainframe/terminal communicationsystems to client/server-based computer systems connected over LANs.Terminals have been replaced by PCs that can either emulate aterminal and in turn communicate with a mainframe or become anintelligent component of the network. Many mainframes are nowobsolete and have been replaced by network servers.

What is a LAN?

A LAN is the term given to a particular network design for computerdevices. This design concept is popular in Information Systemssolutions.

LANs provide high-speed, multi-user access to a wide variety ofinformation sources residing on devices such as file-servers, mid-rangecomputers and mainframes. Peripheral devices such as printers,scanners and DVDs can be connected via a LAN, making themaccessible to multiple users.

A LAN can be defined as a network of PCs and other computing devicesconnected in a client/server environment and in a locality that canrange from a single office room through to an entire floor or building.The boundaries of a LAN can go as far as an entire campus spreadingseveral square miles. The term local is generally regarded a single sitefor an organisation.


What is a WAN?

With the technological development of various inter-connectivitydevices, LANs are now able to connect users to a wide variety of local,regional, and worldwide information networks. When an organisationconnects a LAN from one of its sites to another site in order toexchange information, a Wide Area Network (WAN) is used.

A WAN can be defined as two or more networks connected over a largegeographical area.

LANs connected to one another in this way form a WAN. A WAN thatspans across several countries or continents is known as Enterprisenetworking.

The Internet is the largest WAN in the world. It interconnectsthousands of computer networks globally and is truly an enterprisenetwork.

Client/server networks

Within a Local Area Network, PCs can be networked in variousconfigurations. The most common configuration is the client/server.The term client/server is given to a particular networking design wherea central file server communicates with individual PC workstations(clients). The file server is equipped with a Network Operating Softwaresuch as Novell NetWare, UNIX or Microsoft Windows NT Server to helpmanage the activities of the network. The server acts as a host to all the workstations on the network. End-user data, some utility andapplication software is centrally stored on the file server. The file serverdistributes programs and data to the workstations or clients in thenetwork as they request them. Some tasks are performed by theworkstation while other tasks are processed by the file server.Computers operating in this particular way are utilising client/serverarchitecture.




Advantages of client/server architecture:

safe and secure hosts for client data and other information

good central management of a large number of users

ability for servers to pool resources, lowering overall costs

dedicated servers, optimised to provide high performance resource sharing.

Disadvantages of client/server-based networks:

cost of high performance hardware and specialist software required to run servers

highly skilled staff required to support and manage the client/server systems.

Peer networks

In addition to client/server networks, computers can also be configuredinto a peer-to-peer network. A peer-to-peer network has no central fileserver and all the workstations communicate directly with each otherusing peer network operating system software such as MicrosoftWindows 95/98 or AppleTalk. There is no central control managing apeer network. Users have the opportunity to share disk space and otherlocal resources, such as printers, scanners, and DVD drives. Peernetworks are organised into workgroups that have very little securitycontrol. There is no central login process. Logging into any peer on thenetwork allows the user to gain access to any resource on the networkthat is not controlled by a password. Access to resources can becontrolled by the use of a password for each resource. As there is nocentral security, peer users need to know the password for eachsecured and shared resource they wish to use.


Advantages of peer networks:

ease of configuration

lower cost as no dedicated server equipment or administration staff required.

Disadvantages of peer networks:

no central file server; information is distributed and can be difficult to locate and manage

additional shared load on peers can degrade the overall performanceof the workstations

low security features in comparison to client/server networks.

Note: Although Microsoft Windows 95/98 workstation softwarecan be configured to operate as a peer network operating system,it can also operate as a client in a client/server environment. It can be configured to be both at the same time, being part of aWorkgroup and a client to a server.




Physical topologies

The physical layout of a network is the physical topology of thenetwork. Physical topology is how individual workstations connect toeach other and the mapping or general physical shape of the network.There are three ways to connect computers in a network environmentand each method has a name that corresponds with its mapping.

A network that uses a physical bus topology consists of a single cableto which all the computers are attached.

A network that uses a ring topology connects computers in a closedloop. In this topology, a cable connects the first computer to the secondcomputer, another cable connects the second computer to the third, andso on, until a cable connects the final computer to the first.


A network that uses a star topology arranges cables to run from all thecomputers to a central location where the cables are joined by a hub orswitch.

Ethernet

Ethernet is the name of the most widely used LAN technology. Thespecifications and rights to build Ethernet solutions have been readilyavailable since its introduction in 1980. This openness, combined withits ease of use and robustness, has resulted in a large Ethernetinstalled base.

Operation of Ethernet

Each workstation or server connected to an Ethernet is physicallyattached to a single shared medium (bus topology). These computerscan operate independently, but may communicate with each otherwhen required. Only one computer can transmit information on theEthernet bus at a time. All the computers attached to the Ethernet buslisten to the traffic or data on the bus. They do not transmit their datauntil the line is quiet and no other computers are transmittinginformation. When the line is idle, transmission can be initiated. If twocomputers transmit simultaneously, each can detect the subsequentcollision of transmissions and resend data at random intervals later.This data transmission process is known as Carrier Sense MultipleAccess with Collision Detect (CSMA/CD).

Ethernet LAN technology can incorporate a physical star topology whileusing a logical bus topology. The wiring of the computers appears likestar topology but the wires run from each computer into a central huband the electronics inside the hub emulate a single shared linear bus.




Token Ring

Token Ring is a LAN technology that was introduced in 1984. IBMCorporation developed the Token Ring technology into a robust andhighly reliable LAN system, which was branded the IBM Token Ring™.IBM owned the specification for Token Ring and did not allow freeproduction by other manufacturers. Token Ring is known as aproprietary system, exclusively owned and developed by a particularmanufacturer.

Operation of Token Ring

Each workstation or server connected to a Token Ring is physicallyattached to a single shared medium (ring topology). Unlike theEthernet bus, which allows any workstation to send information toother workstations as long as no other workstation is transmitting data,Token Ring has a much fairer method for deciding when workstationscan transmit.

All workstations attached to Token Ring use a short data messagecalled a Token to coordinate use of the ring. A single token travelsaround the ring from workstation to workstation. As each workstationon the ring receives this token, they have permission to send oneframe, which is a small package of information. With the workstationholding the token, its transmitted frame travels the entire loop of thering, passing from workstation to workstation. When the frame reachesits target, that workstation takes the frame, adds an acknowledgementof receipt and retransmits the frame onto the ring. When the framearrives at the originating workstation, the acknowledgement isrecognised and the workstation releases the token it has been holdingback onto the ring.

Note: Both Ethernet and Token Ring technologies are normallyconfigured in a physical star topology, using centralised hubs tolink the individual workstations to the LAN. However, Ethernetalways uses logical bus topology and Token Ring always uses thelogical ring topology. The electronics in the relevant hubsdetermine the logical topology. Ethernet hubs and Token Ring hubs are very different and care needs to be taken whenconfiguring them.


Ethernet vs Token Ring

Although Ethernet and Token Ring LAN technologies operate indifferent ways both systems have advantages and disadvantages, andthe choice of a LAN system can be influenced by a number of factors. Aswith the choice of network design type, the LAN technology will alsobe determined by how the network will be utilised by an organisationbased on their business needs.

Advantages of an Ethernet LAN

inexpensive to implement and upgrade

Ethernet equipment is readily available and serviceable, with many suppliers and service organisations to choose from

PCs now have motherboards with Ethernet on-board.

Disadvantages of an Ethernet LAN

performance degrades under heavy load

in a bus topology, diagnosis of network problems is very difficult.

Advantages of a Token Ring LAN

a fairer sharing of the network.

Disadvantages of a Token Ring LAN

complex and expensive to implement.




Network media

Network media is the connection between computers and networks.Signal cables are the main network medium, and the three basic cabletypes are described next. Computer networks can be configured toutilise various means of transmitting data including electrical signals incopper wire, light pulsed in fibre-optic cable, and wireless transmission.

Cable types

LAN users have three basic cable or wiring choices: coaxial, twisted pair, and fibre-optic.

Coaxial cable

Coaxial cable or coax has a long history. Television signal cable iscoaxial cable. Coax has a large bandwidth, meaning that it can handlesufficient amounts of data at high speeds between one point andanother. Other advantages include the relative immunity toelectromagnetic interference as compared to copper wire, the ability totransmit signals over a significant distance, and its familiarity to manysuppliers of network solutions.

The illustration below shows that coaxial cable has four parts:

the inner conductor of solid copper wire

an insulator between the inner and outer conductors

a thin, metal outer conductor or metal sheath surround

a plastic encasement which contains the other parts.


Unshielded twisted pair cable

Unshielded twisted pair (UTP) cable is the most common networkingcable. Telephone systems use UTP cable. UTP cable has significantbenefits. UTP cable is light, thin, more flexible and easier to install thancoaxial or fibre-optic cable, and inexpensive. UTP cabling is ideal formost business offices and workgroups in localities that are free fromsevere electromagnetic interference.

Shielded twisted pair cable

The difference between shielded twisted pair (STP) cable and UTPcable is that STP has a shield of aluminium/polyester between theouter plastic encasement and the individual wires.

The illustration below shows that UTP cable has three parts:

4x individual copper wire conductors

colour coded insulation coat for individual conductors

plastic encasement for the two twisted pairs.

UTP and STP installations provide a modular cabling solution,constructed with patch panels, wiring closets, and connector jacks atboth ends of the cables, called RJ45 for UTP. This type of solutionpermits the movement of computers from place to place without re-wiring the LAN. It allows a company to pre-wire a building for itsphone and data services. Once this wiring is in place, people can movefrom office to office, and new cabling does not have to be installed.




Fibre-optic cable

Fibre-optic cable has a very large bandwidth and can carry signals forlong distances. By using light pulses as opposed to electricity as themethod of transmitting data, fibre-optics is immune to allelectromagnetic interference.

It is more secure than copper cable, because an intruder cannot simplylisten-in or eavesdrop on the signals, but must physically tap into thecable. To get at the data being transmitted, a device must be attached tothe cable, and the light level will subsequently decrease.

Installing fibre-optic cabling is a delicate operation because of the finetolerances in the fibre connections. Splicing together and installingconnectors on fibre-optic cables is more difficult than for copper cable.Expensive connector installation equipment and diagnostic tools arerequired to ensure a trouble free fibre-optic network.

The illustration below shows that a single core fibre-optic cable hasfour parts:

a glass or plastic fibre inner core

a glass cladding to reflect the light back into the core

liquid gel or strength wires

plastic encasement.

Light Emitting Diodes (LEDs) send the signals down the cable. Adetector receives the signals and converts them back to the electricalimpulses that computers can understand. Data bits are encoded intolight in various ways and the most popular method is to vary theintensity of the light.


A summary of network media and LANs

Before an Ethernet standard for unshielded twisted pair installationswas approved in 1992, the majority of LANs used coaxial cable.Subsequent installations have used the modular design of the moreflexible, less costly, higher bandwidth, UTP medium. Coaxial cabling forLAN technologies is now rarely seen in modern business environments.

The use of fibre-optics is growing. Fibre is used on LAN backbones orthose stretches of LAN that experience the busiest network traffic, andnot to connect the desktop workstations.

Up until the early 1990s, the wiring for LAN installations was governedby whichever access method was employed, or the way workstationsdecided when to transmit and receive data. The token-passing accessmethod used by Token Ring had to use a twisted pair type of cable andthe Ethernet CSMA/CD method had to use coaxial type of cable.Ethernet now runs on coaxial cable, unshielded twisted pair and fibre-optic cabling while Token Ring runs on unshielded and shieldedtwisted pair and fibre-optic cabling.

Network Interface Cards

A Network Interface Card (NIC) is the term given to a hardware devicethat provides the physical interface from the computer system to anetwork. The NIC provides the physical connection to the networkmedium. Computers attached to a LAN require a network interface cardor equivalent device. The NIC fits into one of the expansion slots on themotherboard, although motherboards can have on-board NIC.

Operation of the NIC

The NIC receives data from its computer, converts it into a format forsending onto the network and sends this formatted data over thenetwork cable. The receiving NIC converts this formatted data back intothe original format that its PC can understand. This process gets datafrom one computer to another computer.




How data moves around in networks

LAN technologies are designed around the principle of networkcomputers all sharing a common communication medium, usually thecabling.

This cabling sharing principle does not mean that the workstations canuse the cable all at once, but that all of them can use the same commoncable to transmit information one at a time. The Medium Access Control(MAC) system used by the LAN technologies determines how theworkstations decide whose turn it is to transmit information. Forexample, Ethernet uses CSMA/CD, and Token Ring uses Token Passing.

Workstations connected to this type of network do not transmitinformation in a continuous stream but instead LAN technologiesdivide information up into packets of data that are sent individually.LANs are referred to as packet switching networks because of the useof packet technology.

By forcing workstations to send their information in small, individualpackets, the sending and receiving computers have an easier job indetermining whether the information has arrived at its destinationintact. Communicating computers have a better chance of gaining fair,prompt access to the shared medium, for the MAC system will onlypermit a workstation to transmit one packet of data across the networkat a time, before allowing other workstations an opportunity totransmit.

Note: Although the term packet refers to the general concept of asmall block of data, there is no universal agreement on the exactformat of a packet. Instead, each hardware technology defines thedetails of packets that can be transferred using that particularhardware device, and specifies how individual bits of the packetare transmitted. The term frame is often used to denote thedefinition of a packet used with a specific type of network(Ethernet or Token Ring). Therefore, reference is often made to themaximum amount of data a frame can hold or the order in whichbits of the frame are transmitted across a network.


What are network protocols?

Network protocols are the agreed schemes that computers use whenexchanging information.

All parties involved in a communication must agree on a set of rulesthat will govern exchanging messages. This agreement outlines thelanguage to be used and the rules for when messages can be sent.Diplomats call such an agreement a protocol. The same term is appliedto computer communication as well. The set of rules that specify theformat of messages and the appropriate action required for eachmessage is known as a network protocol.

Hardware protocols

Hardware components communicate with one another via hardwareprotocols. Hardware protocols define how hardware devices operateand work together. The 10BaseT Ethernet protocol is a hardwareprotocol that specifies how two 10BaseT Ethernet devices will exchangeinformation and what they will do if it is improperly transmitted orinterrupted. Hardware protocols define signal voltage levels and whichpairs of wires will be used for transmission and reception. There is nosoftware program involved as all the rules govern electronic circuitry.

Software protocols

Software programs communicate with one another via softwareprotocols. Network clients and servers have software protocol packagesthat must be loaded to allow communication with one another. Thevarious network technologies have individual software protocolpackages.

Network Operating Systems come with a wide choice of networkprotocols that allow computers using different software protocols onthe network to communicate with each another.

An individual network protocol can be as simple as an agreement touse ASCII when transferring a text file, or very complex, such as anagreement to use a complicated mathematical function to encrypt data.

Note: Computers on a network must use the same protocol order to communicate. To communicate with the Internet computersmust use the TCP/IP protocol suit. TCP/IP is the choice of networkprotocol among network manufacturers.




The theoretical network

With so many different technologies, products, and interconnectingschemes available, the task of networking individual computers thatcan also be of different platforms as the Intel PC and Apple Macintoshis really quite a science.

Network hardware and software manufacturers have definedcompeting standards, and most networks incorporate components thatuse multiple standards. To help understand and regulate how thedifferent components fit together, a theoretical model was developed.This model is not tangible. This model is a conceptual framework toallow better understanding of the complex interactions taking placebetween the devices in a network. This model was originally developedby the International Standards Organisation, and called the OpenSystem Interconnection (OSI) Reference Model. This model, initiated in 1977, has become the most widely accepted reference model forunderstanding network communication.

The diagram below illustrates the OSI Reference Model and shows howeach layer is numbered.

Application j Layer 7

Presentation j Layer 6

Session j Layer 5

Transport j Layer 4

Network j Layer 3

Data link j Layer 2

Physical j Layer 1


How the OSI Reference Model works

Like the software layers described in Unit 3, the OSI Reference Modeldescribes one method the communications framework can besubdivided into layers. It is important to understand that the OSI modeldoes not actually perform any functions in the framework. The work toachieve communication is done by the appropriate hardware andsoftware.

The OSI 7-layer model is saying:

there are seven main tasks that need to occur in order for computers to communicate and this is mapped in the seven individual layers

this is the order in which the main tasks follow

any network protocols designed for computers to communicate across networks can use this reference model as a basis for theirdesign. In so doing, protocol designers would succeed in getting their protocols to work.

When computer networks were first being developed, manufacturerswere unable to communicate properly with other manufacturers asthey were writing protocols based on their own reference models. TheOSI 7-layer model was embraced by many manufacturers when it wasreleased in the mid-1980s as it provided this standard model foreveryone to work to.

The OSI model enables computers with different operating systems andhardware to successfully communicate with each other.

What happens at what layer according to the model

Layer 7: ApplicationThe application layer is the OSI layer closest to the user and managesthe user interface to the network. It does not provide services to otherOSI layers, but rather to application processes lying outside the scopeof the OSI model. These services include common functions such as theprotocols for providing remote file access services, file transfer services,message handling services for e-mail applications, global directoryservices to locate resources on a network and remote job execution.




Layer 6: PresentationThe presentation layer ensures that data sent by the application layerof one system will be readable by the application layer of anothersystem and if necessary, translates between multiple datarepresentation formats by using a common data representation format.It manages security issues by providing services such as dataencryption, and it compresses data to reduce the number of bits thatneed to be transferred on the network.

Layer 5: SessionThe session layer establishes, manages, and terminates the sessions orlinks between applications. This layer establishes dialog controlbetween the two computers in a session, regulating which sidetransmits, when, and for how long.

Layer 4: TransportThe transport layer is responsible for error recognition and recovery,ensuring the reliable delivery of messages. It also repackages longmessages by dividing them into small packets for transmission. At thereceiving end, it rebuilds the small packets into the original message.The transport layer sends an acknowledgment of receipt of messages.

Layer 3: NetworkThe network layer addresses messages and translates logical addressesand names into physical addresses. It creates packets and determinesthe route along the network from the source to the destination.

Layer 2: Data Link

The data link layer provides reliable transit of data across a physicallink and is involved with physical addressing, network topology, linediscipline, error notification, encapsulating packets or user data into abit-stream organisation called frames, the ordered delivery of frames,and flow control.

Layer 1: Physical

The physical layer is responsible for getting bits from one computer toanother and regulates the transmission of a stream of bits over aphysical medium. It defines how the cable is attached to the networkadapter card and which transmission technique is used to send dataover the cable, including the electrical signals, cable types andconnectors.


Note: In addition to hardware, network systems have complexprotocol software that controls communication. Instead ofinterfacing directly with network hardware, most applicationprograms and users interact with protocol software. The layeringmodel is a fundamental tool that helps designers master thecomplexity of protocol design.

Remember: The OSI Reference Model does not specify any of thesethings, it simply states that whatever network protocol that isbeing written follows this sequence and specifies the informationlisted in the 7 layers, in this order.




Server technology

A file server is a combination of computer, internal hardware andsoftware that allows LAN users to share file and print services acrossthe LAN. An application server is a dedicated PC used to host aparticular application program. Microsoft Exchange software is an e-mail application often processed by a dedicated server due to thehuge demands made on the service and in turn the hardware. Thisdemand placed on servers, due to clients simultaneously using theservices, means they need a high performance processor, faster network interface, large memory capacity and larger data storage thanmost workstations.

A file server must retrieve data rapidly and provide it to the requestingworkstation with minimal delay. Factors affecting server performanceinclude the following:

CPU speed

speed of the network interface

amount of RAM available

type of hard disk and interface controller

cable type and length

network software efficiency

application programs

number of active clients.

Disk reliability is important. Servers may take advantage of RedundantArrays of Independent Disks (RAID) technology and incorporate hot-swap drives. This is the ability to physically insert and removedrives while the system is up and running, which enhances themaintainability and reduces any downtime of the server.

It is advisable that power is maintained to the server and isuninterrupted at all times.


A device known as an Uninterruptible Power Supply (UPS) is astandard add-on feature to any server. Some servers have a UPS as anintegral option within their hardware configuration. A UPS is a batterypower pack positioned between the mains power supply and the server.If the mains power drops or fluctuates within a defined threshold, theUPS batteries activate and maintain power to the server. If the UPSfunction is selected in the operating system, a message from the UPSwill inform the operating system of the power loss. A predeterminedtime will be flagged as remaining before the battery power runs out, ifmains power is not restored.

A UPS can range from a small, shoebox size unit capable of supplyingreserve power for about 20 minutes, to a floor standing unit the size ofthe server itself capable of supplying power for several hours.

Scalability is a further factor to consider with server configurations.Scalability defines the performance enhancement capabilities of aparticular server, which may include multiple processors, memory andstorage upgrades.

The illustration below shows key features of a server system.




Drive arrays

Connecting hard drives in an array is a method used by many servers inorder to improve read/write performance and provide a level ofhardware fault tolerance.

A drive array is the term given to the hardware system of configuringmultiple hard drives to a special controller called a RAID controller. TheRAID controller connects all the drives in the array and allows data tobe written to the drives in a special way. The RAID controller can stripedata across multiple physical drives whilst giving the software theimpression there is only one logical drive as in the illustration below.The controller breaks down data to be written to the drives into smallerblocks and distributes these blocks across the multiple drives within thearray. Striping data is detailed in the RAID Level 0 illustration later inthis section.

The RAID specification is divided into various levels. The five mostcommon RAID levels used in servers are 0, 1, 3, 4 and 5.

The method of data storage differs between RAID levels. The RAIDlevels are not indicative of hierarchy, but merely for differentiation.


RAID Levels functionality

The five most common RAID levels exhibit their own unique benefitsand drawbacks. The following overview highlights their use in ITsolutions.

Level 0RAID Level 0 is called disk striping. The data being written is brokendown into strips and striped across the member disks of the array. Thisprovides high I/O performance at low inherent cost but provides noredundancy.

Level 1RAID Level 1, or mirroring, has been in use longer than any other RAID.Level 1 provides redundancy by writing identical data to each memberdisk of the array, leaving a ‘mirrored’ copy on each disk. Mirroring ispopular due to its simplicity and high level of data availability. Level 1operates with two or more disks that may use parallel access for highdata-transfer rates when reading, but more often operatesindependently to provide high I/O transaction rates. Level 1 provideshigh data reliability and improves the performance for read-intensiveapplications, but at relatively high cost.




Level 3RAID Level 3 adds redundant data in the form of parity data to aparallel access striped array, permitting regeneration and rebuilding inthe event of a single disk failure. One strip of parity data protectscorresponding strips of user data on the remaining disks. RAID Level 3provides high data transfer rates and high data availability, at a lowercost than mirroring. Its transaction performance is poor, however,because all RAID Level 3 array member drives operate in lockstep(spindle synchronisation).

Level 4Like Level 3, Level 4 uses parity concentrated on a single disk to protectdata. Unlike Level 3, Level 4 member disks are independently accessiblemaking it better suited to transaction I/O rather than large filetransfers. Because the dedicated parity disk represents an inherentbottleneck, Level 4 is seldom used without accompanying technologiessuch as data caching.

Level 5By distributing parity across some or all of an array’s member disks,RAID Level 5 reduces (but does not eliminate) the write bottleneckinherent to Level 4. As with Level 4, the result is asymmetricalperformance, with reads substantially outperforming writes. Level 5 isoften used with data caching to reduce the asymmetry.


Internetworking

Internetworking is the term given to the technology of connectingindividual networks to other networks, LANs being physicallyconnected up to other LANs.

The IT solutions of organisations have led to the installation of morethan one local area network to satisfy its business requirements. Thishas led to the need for connecting discreet LANs together. It is probablethat a user physically connected to a LAN requires access to resourceslocated on another LAN within the company.

This need has given rise to one of the fastest growing areas of the LANindustry, the internetworking device market. This market consists ofrepeaters, bridges, routers, gateways and advanced routing devices thatperform a combination of these functions.

These interconnectivity devices provide internetworking to users whoare linked to large distributed groups of LANs. They also play asignificant role in network management by allowing networkadministrators to segment, or divide, a single network into an assemblyof smaller LANs called subnets. This subdivision can improve networkperformance by limiting the number of workstations on a network, andcan reduce traffic over the segment wiring. It also facilitates security, asinternetworking allows the control of the access of an individual orgroup to specified resources. Subnets can increase system reliability, forif one workgroup goes down, it does not affect the entire internetwork.

There are four primary types of internetworking devices:

repeaters

bridges

routers

gateways.

From approximately 1994, multi-port bridges began to be marketed asswitches. Switches provide the same functions as bridges and mayperform local routing functions. Internetworking devices allow variouslevels of communication between individual networks. Each type ofdevice also functions at a separate level within the OSI ReferenceModel.




Repeaters

Repeaters offer the simplest form of interconnectivity. They regenerate,or repeat data packets between cable segments. Repeaters physicallyextend a network by amplifying the electrical data signals.

Repeaters operate at the physical layer of the OSI model. Repeatersallow the extending of Ethernet network cable segments. In addition,they provide a level of fault tolerance by isolating networks electrically,so a problem on one cable segment does not affect other segments.

Bridges

Bridges isolate traffic to specific workgroups while still having theability to connect multiple LAN cable segments into a large logicalnetwork. Bridges operate one layer higher than repeaters in the OSImodel, in the data link layer.

The diagram below depicts two bridged LAN segments. The bridgesends and receives complete frames from one LAN segment to theother.

Filtering traffic

Most bridges operate only between similar LAN technologies, betweentwo Ethernets or two Token Rings segments, however some do offercross-technology capabilities. Bridges regulate traffic by filtering data packets based on the destination address. When a destinationaddress is local to the segment, the bridge does not forward it. Whenthe destination address is remote to the local segment, the bridge forwards it.

Bridges may automatically learn the addresses of the devices attachedto their internetwork. Some bridges allow filtering of data on a varietyof parameters, including packet size, source address and protocol type.Because filtering reduces network traffic, it can substantially increaseoverall network performance.


Routers

Routers operate at the network layer in the OSI Reference Model.Routers connect logically separate networks operating under the sameprotocol. Routers are protocol-dependent and must support theindividual protocols being routed. These protocols could be TCP/IP andNetBEUI. A router allows multiple paths to exist in an enterprise-widenetwork and is intelligent enough to determine the most efficient pathto send a particular data packet through the multiple loops.

In a typical enterprise-wide network divided by routers, the separatenetworks are assigned unique ID numbers and each of these networksis managed separately. Routers automatically learn changes in thenetwork configuration as bridges do, and are more complex thanbridges because they determine these unique ID number wheninitialised. This requires network administrators to have an in-depthunderstanding of data communications to effectively manage router-based networks.

Routers are particularly useful in organisations with large multiplenetworks connected to a single backbone. Because they mustdetermine which protocol is being routed, routers were originallyslower than bridges, but now routers are just as fast as any dedicatedbridge device.

Gateways

Gateways act as translators between networks using incompatiblenetwork protocols, such as TCP/IP and NetBEUI. Depending on the levelof incompatibility, gateways can operate at the transport layer throughto the application layer of the OSI model.

A gateway can be a communications link between a Local Area Networkand the Internet, or between a Local Area Network and a mainframe orminicomputer, allowing LAN users to access the mainframe orminicomputer as a network server.




Hubs

A hub is a device used for connecting multiple computers to a network.Before the modular design of UTP Ethernet LANs became common,Ethernet equipped computers would attach to the network coaxialcabling via a T-connector. The T-connector effectively allowed thecomputer to use the shared medium by physically tapping onto thenetwork. An Ethernet hub provides a connecting technique whereEthernet equipped computer can use the 10BaseT system. 10BaseT is ahardware protocol that uses UTP cabling.

The key feature of a hub is that the electronics simulates a singleshared medium. Using hubs in this way allows both the bus and ringLAN technologies to take on the physical star topology, as theworkstations in the network converge into these central hubs forming aphysical star. The term hub has traditionally been used for Ethernetsystems, however it is now associated with both Ethernet and TokenRing technologies.

Switches

Networking technology is described as being switched if the LANhardware includes an electronic device that connects to one or morecomputers and allows the transfer of information between them. Morespecifically, a switched LAN consists of a single electronic device thattransfers frames among many computers.

Physically a switch resembles a hub. The difference is in theelectronics. In an Ethernet hub, the electronics simulates a singleshared medium, however in a switch it simulates a bridged LAN withonly one computer connected per segment. This results in eachcomputer connected to the switch having a simulated LAN segment toitself without contention for the shared medium from otherworkstations. Network performance is improved significantly with thisconfiguration.


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