unit 4

service and support
 for pc networks

        Unit 4 Service and Support for PC Networks

                        Learning outcomes – Service and S...
A summary of network media and LANs … 19
Network Interface Cards … 19
  Operation of the NIC … 19

How data moves around ...
unit 4                           service and support for pc networks

                                 Learning outcome...
service and support for pc networks                                                               unit 4

What is PC ne...
unit 4                           service and support for pc networks

                                 Why do we networ...
service and support for pc networks                                                                  unit 4

unit 4                           service and support for pc networks

                                 Network types
service and support for pc networks                                                                    unit 4

What is ...
unit 4                           service and support for pc networks

                                 Advantages of cl...
service and support for pc networks                                                                 unit 4

Advantages ...
unit 4                           service and support for pc networks

                                 Physical topolog...
service and support for pc networks                                                                 unit 4

A network t...
unit 4                           service and support for pc networks

                                 Token Ring
service and support for pc networks                                                               unit 4

Ethernet vs T...
unit 4                           service and support for pc networks

                                 Network media
service and support for pc networks                                                                    unit 4

unit 4                           service and support for pc networks

                                 Fibre-optic cabl...
service and support for pc networks                                                                 unit 4

A summary o...
unit 4                           service and support for pc networks

                                 How data moves a...
service and support for pc networks                                                               unit 4

What are netw...
unit 4                           service and support for pc networks

                                 The theoretical ...
service and support for pc networks                                                                    unit 4

How the ...
unit 4                           service and support for pc networks

                                 Layer 6: Present...
service and support for pc networks                                                          unit 4

 Remember: The OSI...
unit 4                           service and support for pc networks

                                 Server technolog...
service and support for pc networks                                                                unit 4

A device kno...
unit 4                           service and support for pc networks

                                 Drive arrays
service and support for pc networks                                                                  unit 4

RAID Level...
unit 4                           service and support for pc networks

                                 Level 3
service and support for pc networks                                                                    unit 4

unit 4                           service and support for pc networks

service and support for pc networks                                                               unit 4

unit 4                           service and support for pc networks

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service and support for pc networks unit

  1. 1. unit 4 service and support for pc networks
  2. 2. contents Unit 4 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 © e-skills NTO 2000
  3. 3. 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 © e-skills NTO 2000
  4. 4. 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: 1 describe the term network and the advantages and disadvantages of networking computers 2 describe peer-to-peer (workgroup) networks, server-based networks, combined networks and the common terms used to define them 3 describe Local Area Networks (LAN), Wide Area Networks (WAN), Enterprise Networks and the differences between them 4 identify the main functions of a Network Operating System, network connections and interconnectivity devices 5 describe the key operational features of both the Ethernet and Token Ring technologies and the basic principles for data movement in a network, including packet-switching 6 identify the seven layers of the OSI model 7 give examples of fault-tolerant hardware. 4 © e-skills NTO 2000
  5. 5. service and support for pc networks unit 4 What is PC networking? In the nineteen eighties, few PC computer users had access to a network. Instead, they were operating in stand-alone mode, or without being connected to any other computers. Networking is the concept of linking up PC computers via a cable or some other communication medium 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 corporations have networks. Education establishments are using PC networks to provide students and lecturers with access to information. Government institutions including the military and local authorities all take advantage of networking. Networks are utilised everywhere. PC networking definitions Networking can be defined as ‘the connecting of computers via a communication medium’. In simple terms, a network can be described as two or more computers connected to each other by a wire in order to exchange information. © e-skills NTO 2000 5
  6. 6. unit 4 service and support for pc networks Why do we network PCs? By networking computers, a business is able to take advantage of essential Information Technology and Information Systems. Reasons for networking computers are as follows: Sharing resources The ability to share resources is a major reason for networking computers. By having computers connected to a network, each computer is able to share other individual resources also connected to the network. These resources include hardware devices such as printers, scanners and storage devices, as well as software resources such as application programs and software utilities. 6 © e-skills NTO 2000
  7. 7. service and support for pc networks unit 4 e-work The ability to exchange information electronically between users is a major advantage of networking computers. This way of communicating (e-mail and e-workflow) is now standard. Rather than exchanging memos on paper, incurring printing costs and delays, networked users can instantly send messages to others and even check if they have been received and read. Work can be completed on an electronic form or soft copy, 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 work activities, it is common practice for users of a network to save their work on a shared network drive, a storage device in a central file server. In this way, a single data back up procedure carried out on the file server can preserve the information of numerous users located in various parts of an organisation. The task of securing an entire organisation’s work is made relatively straightforward, and in the event of work getting lost, recovery is from the one location. Protecting information Information being saved on a network drive is far safer than that saved on local drives, the hard drives in individual PCs. An additional layer of security is present when saving to network drives, as user login passwords are required to access any network information. © e-skills NTO 2000 7
  8. 8. unit 4 service and support for pc networks Network types In the world of network computing, there are various different industry standard network designs to choose. Each type of design has its particular advantages and disadvantages and the individual requirements of an organisation determine which design is best suited for their business. The various standards for network designs can be divided into several categories. Local Area Networks (LANs) are the most common type of computer network in use by organisations. LANs have radically changed the corporate information infrastructure. Corporations have moved from mainframe/terminal communication systems to client/server-based computer systems connected over LANs. Terminals have been replaced by PCs that can either emulate a terminal and in turn communicate with a mainframe or become an intelligent component of the network. Many mainframes are now obsolete and have been replaced by network servers. What is a LAN? A LAN is the term given to a particular network design for computer devices. This design concept is popular in Information Systems solutions. LANs provide high-speed, multi-user access to a wide variety of information sources residing on devices such as file-servers, mid-range computers and mainframes. Peripheral devices such as printers, scanners and DVDs can be connected via a LAN, making them accessible to multiple users. A LAN can be defined as a network of PCs and other computing devices connected in a client/server environment and in a locality that can range 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 spreading several square miles. The term local is generally regarded a single site for an organisation. 8 © e-skills NTO 2000
  9. 9. service and support for pc networks unit 4 What is a WAN? With the technological development of various inter-connectivity devices, LANs are now able to connect users to a wide variety of local, regional, and worldwide information networks. When an organisation connects a LAN from one of its sites to another site in order to exchange information, a Wide Area Network (WAN) is used. A WAN can be defined as two or more networks connected over a large geographical area. LANs connected to one another in this way form a WAN. A WAN that spans across several countries or continents is known as Enterprise networking. The Internet is the largest WAN in the world. It interconnects thousands of computer networks globally and is truly an enterprise network. Client/server networks Within a Local Area Network, PCs can be networked in various configurations. The most common configuration is the client/server. The term client/server is given to a particular networking design where a central file server communicates with individual PC workstations (clients). The file server is equipped with a Network Operating Software such as Novell NetWare, UNIX or Microsoft Windows NT Server to help manage the activities of the network. The server acts as a host to all the workstations on the network. End-user data, some utility and application software is centrally stored on the file server. The file server distributes programs and data to the workstations or clients in the network as they request them. Some tasks are performed by the workstation while other tasks are processed by the file server. Computers operating in this particular way are utilising client/server architecture. © e-skills NTO 2000 9
  10. 10. unit 4 service and support for pc networks 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 configured into a peer-to-peer network. A peer-to-peer network has no central file server and all the workstations communicate directly with each other using peer network operating system software such as Microsoft Windows 95/98 or AppleTalk. There is no central control managing a peer network. Users have the opportunity to share disk space and other local resources, such as printers, scanners, and DVD drives. Peer networks are organised into workgroups that have very little security control. There is no central login process. Logging into any peer on the network allows the user to gain access to any resource on the network that is not controlled by a password. Access to resources can be controlled by the use of a password for each resource. As there is no central security, peer users need to know the password for each secured and shared resource they wish to use. 10 © e-skills NTO 2000
  11. 11. service and support for pc networks unit 4 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 performance of the workstations low security features in comparison to client/server networks. Note: Although Microsoft Windows 95/98 workstation software can 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 a Workgroup and a client to a server. © e-skills NTO 2000 11
  12. 12. unit 4 service and support for pc networks Physical topologies The physical layout of a network is the physical topology of the network. Physical topology is how individual workstations connect to each other and the mapping or general physical shape of the network. There are three ways to connect computers in a network environment and each method has a name that corresponds with its mapping. A network that uses a physical bus topology consists of a single cable to which all the computers are attached. A network that uses a ring topology connects computers in a closed loop. In this topology, a cable connects the first computer to the second computer, another cable connects the second computer to the third, and so on, until a cable connects the final computer to the first. 12 © e-skills NTO 2000
  13. 13. service and support for pc networks unit 4 A network that uses a star topology arranges cables to run from all the computers to a central location where the cables are joined by a hub or switch. Ethernet Ethernet is the name of the most widely used LAN technology. The specifications and rights to build Ethernet solutions have been readily available since its introduction in 1980. This openness, combined with its ease of use and robustness, has resulted in a large Ethernet installed base. Operation of Ethernet Each workstation or server connected to an Ethernet is physically attached to a single shared medium (bus topology). These computers can operate independently, but may communicate with each other when required. Only one computer can transmit information on the Ethernet bus at a time. All the computers attached to the Ethernet bus listen to the traffic or data on the bus. They do not transmit their data until the line is quiet and no other computers are transmitting information. When the line is idle, transmission can be initiated. If two computers transmit simultaneously, each can detect the subsequent collision of transmissions and resend data at random intervals later. This data transmission process is known as Carrier Sense Multiple Access with Collision Detect (CSMA/CD). Ethernet LAN technology can incorporate a physical star topology while using a logical bus topology. The wiring of the computers appears like star topology but the wires run from each computer into a central hub and the electronics inside the hub emulate a single shared linear bus. © e-skills NTO 2000 13
  14. 14. unit 4 service and support for pc networks Token Ring Token Ring is a LAN technology that was introduced in 1984. IBM Corporation developed the Token Ring technology into a robust and highly reliable LAN system, which was branded the IBM Token Ring™. IBM owned the specification for Token Ring and did not allow free production by other manufacturers. Token Ring is known as a proprietary system, exclusively owned and developed by a particular manufacturer. Operation of Token Ring Each workstation or server connected to a Token Ring is physically attached to a single shared medium (ring topology). Unlike the Ethernet bus, which allows any workstation to send information to other workstations as long as no other workstation is transmitting data, Token Ring has a much fairer method for deciding when workstations can transmit. All workstations attached to Token Ring use a short data message called a Token to coordinate use of the ring. A single token travels around the ring from workstation to workstation. As each workstation on the ring receives this token, they have permission to send one frame, which is a small package of information. With the workstation holding the token, its transmitted frame travels the entire loop of the ring, passing from workstation to workstation. When the frame reaches its target, that workstation takes the frame, adds an acknowledgement of receipt and retransmits the frame onto the ring. When the frame arrives at the originating workstation, the acknowledgement is recognised and the workstation releases the token it has been holding back onto the ring. Note: Both Ethernet and Token Ring technologies are normally configured in a physical star topology, using centralised hubs to link the individual workstations to the LAN. However, Ethernet always uses logical bus topology and Token Ring always uses the logical ring topology. The electronics in the relevant hubs determine the logical topology. Ethernet hubs and Token Ring hubs are very different and care needs to be taken when configuring them. 14 © e-skills NTO 2000
  15. 15. service and support for pc networks unit 4 Ethernet vs Token Ring Although Ethernet and Token Ring LAN technologies operate in different ways both systems have advantages and disadvantages, and the choice of a LAN system can be influenced by a number of factors. As with the choice of network design type, the LAN technology will also be determined by how the network will be utilised by an organisation based 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. © e-skills NTO 2000 15
  16. 16. unit 4 service and support for pc networks Network media Network media is the connection between computers and networks. Signal cables are the main network medium, and the three basic cable types are described next. Computer networks can be configured to utilise various means of transmitting data including electrical signals in copper 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 is coaxial cable. Coax has a large bandwidth, meaning that it can handle sufficient amounts of data at high speeds between one point and another. Other advantages include the relative immunity to electromagnetic interference as compared to copper wire, the ability to transmit signals over a significant distance, and its familiarity to many suppliers 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. 16 © e-skills NTO 2000
  17. 17. service and support for pc networks unit 4 Unshielded twisted pair cable Unshielded twisted pair (UTP) cable is the most common networking cable. Telephone systems use UTP cable. UTP cable has significant benefits. UTP cable is light, thin, more flexible and easier to install than coaxial or fibre-optic cable, and inexpensive. UTP cabling is ideal for most business offices and workgroups in localities that are free from severe electromagnetic interference. Shielded twisted pair cable The difference between shielded twisted pair (STP) cable and UTP cable is that STP has a shield of aluminium/polyester between the outer 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 at both ends of the cables, called RJ45 for UTP. This type of solution permits the movement of computers from place to place without re-wiring the LAN. It allows a company to pre-wire a building for its phone and data services. Once this wiring is in place, people can move from office to office, and new cabling does not have to be installed. © e-skills NTO 2000 17
  18. 18. unit 4 service and support for pc networks Fibre-optic cable Fibre-optic cable has a very large bandwidth and can carry signals for long distances. By using light pulses as opposed to electricity as the method of transmitting data, fibre-optics is immune to all electromagnetic interference. It is more secure than copper cable, because an intruder cannot simply listen-in or eavesdrop on the signals, but must physically tap into the cable. To get at the data being transmitted, a device must be attached to the cable, and the light level will subsequently decrease. Installing fibre-optic cabling is a delicate operation because of the fine tolerances in the fibre connections. Splicing together and installing connectors on fibre-optic cables is more difficult than for copper cable. Expensive connector installation equipment and diagnostic tools are required to ensure a trouble free fibre-optic network. The illustration below shows that a single core fibre-optic cable has four 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. A detector receives the signals and converts them back to the electrical impulses that computers can understand. Data bits are encoded into light in various ways and the most popular method is to vary the intensity of the light. 18 © e-skills NTO 2000
  19. 19. service and support for pc networks unit 4 A summary of network media and LANs Before an Ethernet standard for unshielded twisted pair installations was approved in 1992, the majority of LANs used coaxial cable. Subsequent installations have used the modular design of the more flexible, less costly, higher bandwidth, UTP medium. Coaxial cabling for LAN technologies is now rarely seen in modern business environments. The use of fibre-optics is growing. Fibre is used on LAN backbones or those stretches of LAN that experience the busiest network traffic, and not to connect the desktop workstations. Up until the early 1990s, the wiring for LAN installations was governed by whichever access method was employed, or the way workstations decided when to transmit and receive data. The token-passing access method used by Token Ring had to use a twisted pair type of cable and the 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 shielded twisted pair and fibre-optic cabling. Network Interface Cards A Network Interface Card (NIC) is the term given to a hardware device that provides the physical interface from the computer system to a network. The NIC provides the physical connection to the network medium. Computers attached to a LAN require a network interface card or equivalent device. The NIC fits into one of the expansion slots on the motherboard, although motherboards can have on-board NIC. Operation of the NIC The NIC receives data from its computer, converts it into a format for sending onto the network and sends this formatted data over the network cable. The receiving NIC converts this formatted data back into the original format that its PC can understand. This process gets data from one computer to another computer. © e-skills NTO 2000 19
  20. 20. unit 4 service and support for pc networks How data moves around in networks LAN technologies are designed around the principle of network computers all sharing a common communication medium, usually the cabling. This cabling sharing principle does not mean that the workstations can use the cable all at once, but that all of them can use the same common cable to transmit information one at a time. The Medium Access Control (MAC) system used by the LAN technologies determines how the workstations decide whose turn it is to transmit information. For example, Ethernet uses CSMA/CD, and Token Ring uses Token Passing. Workstations connected to this type of network do not transmit information in a continuous stream but instead LAN technologies divide information up into packets of data that are sent individually. LANs are referred to as packet switching networks because of the use of packet technology. By forcing workstations to send their information in small, individual packets, the sending and receiving computers have an easier job in determining whether the information has arrived at its destination intact. Communicating computers have a better chance of gaining fair, prompt access to the shared medium, for the MAC system will only permit a workstation to transmit one packet of data across the network at a time, before allowing other workstations an opportunity to transmit. Note: Although the term packet refers to the general concept of a small block of data, there is no universal agreement on the exact format of a packet. Instead, each hardware technology defines the details of packets that can be transferred using that particular hardware device, and specifies how individual bits of the packet are transmitted. The term frame is often used to denote the definition of a packet used with a specific type of network (Ethernet or Token Ring). Therefore, reference is often made to the maximum amount of data a frame can hold or the order in which bits of the frame are transmitted across a network. 20 © e-skills NTO 2000
  21. 21. service and support for pc networks unit 4 What are network protocols? Network protocols are the agreed schemes that computers use when exchanging information. All parties involved in a communication must agree on a set of rules that will govern exchanging messages. This agreement outlines the language to be used and the rules for when messages can be sent. Diplomats call such an agreement a protocol. The same term is applied to computer communication as well. The set of rules that specify the format of messages and the appropriate action required for each message is known as a network protocol. Hardware protocols Hardware components communicate with one another via hardware protocols. Hardware protocols define how hardware devices operate and work together. The 10BaseT Ethernet protocol is a hardware protocol that specifies how two 10BaseT Ethernet devices will exchange information and what they will do if it is improperly transmitted or interrupted. Hardware protocols define signal voltage levels and which pairs of wires will be used for transmission and reception. There is no software program involved as all the rules govern electronic circuitry. Software protocols Software programs communicate with one another via software protocols. Network clients and servers have software protocol packages that must be loaded to allow communication with one another. The various network technologies have individual software protocol packages. Network Operating Systems come with a wide choice of network protocols that allow computers using different software protocols on the network to communicate with each another. An individual network protocol can be as simple as an agreement to use ASCII when transferring a text file, or very complex, such as an agreement 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 computers must use the TCP/IP protocol suit. TCP/IP is the choice of network protocol among network manufacturers. © e-skills NTO 2000 21
  22. 22. unit 4 service and support for pc networks The theoretical network With so many different technologies, products, and interconnecting schemes available, the task of networking individual computers that can also be of different platforms as the Intel PC and Apple Macintosh is really quite a science. Network hardware and software manufacturers have defined competing standards, and most networks incorporate components that use multiple standards. To help understand and regulate how the different components fit together, a theoretical model was developed. This model is not tangible. This model is a conceptual framework to allow better understanding of the complex interactions taking place between the devices in a network. This model was originally developed by the International Standards Organisation, and called the Open System Interconnection (OSI) Reference Model. This model, initiated in 1977, has become the most widely accepted reference model for understanding network communication. The diagram below illustrates the OSI Reference Model and shows how each 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 22 © e-skills NTO 2000
  23. 23. service and support for pc networks unit 4 How the OSI Reference Model works Like the software layers described in Unit 3, the OSI Reference Model describes one method the communications framework can be subdivided into layers. It is important to understand that the OSI model does not actually perform any functions in the framework. The work to achieve communication is done by the appropriate hardware and software. 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 their design. In so doing, protocol designers would succeed in getting their protocols to work. When computer networks were first being developed, manufacturers were unable to communicate properly with other manufacturers as they were writing protocols based on their own reference models. The OSI 7-layer model was embraced by many manufacturers when it was released in the mid-1980s as it provided this standard model for everyone to work to. The OSI model enables computers with different operating systems and hardware to successfully communicate with each other. What happens at what layer according to the model Layer 7: Application The application layer is the OSI layer closest to the user and manages the user interface to the network. It does not provide services to other OSI layers, but rather to application processes lying outside the scope of the OSI model. These services include common functions such as the protocols for providing remote file access services, file transfer services, message handling services for e-mail applications, global directory services to locate resources on a network and remote job execution. © e-skills NTO 2000 23
  24. 24. unit 4 service and support for pc networks Layer 6: Presentation The presentation layer ensures that data sent by the application layer of one system will be readable by the application layer of another system and if necessary, translates between multiple data representation formats by using a common data representation format. It manages security issues by providing services such as data encryption, and it compresses data to reduce the number of bits that need to be transferred on the network. Layer 5: Session The session layer establishes, manages, and terminates the sessions or links between applications. This layer establishes dialog control between the two computers in a session, regulating which side transmits, when, and for how long. Layer 4: Transport The transport layer is responsible for error recognition and recovery, ensuring the reliable delivery of messages. It also repackages long messages by dividing them into small packets for transmission. At the receiving end, it rebuilds the small packets into the original message. The transport layer sends an acknowledgment of receipt of messages. Layer 3: Network The network layer addresses messages and translates logical addresses and names into physical addresses. It creates packets and determines the 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 physical link and is involved with physical addressing, network topology, line discipline, error notification, encapsulating packets or user data into a bit-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 to another and regulates the transmission of a stream of bits over a physical medium. It defines how the cable is attached to the network adapter card and which transmission technique is used to send data over the cable, including the electrical signals, cable types and connectors. 24 © e-skills NTO 2000
  25. 25. service and support for pc networks unit 4 Remember: The OSI Reference Model does not specify any of these things, it simply states that whatever network protocol that is being written follows this sequence and specifies the information listed in the 7 layers, in this order. Note: In addition to hardware, network systems have complex protocol software that controls communication. Instead of interfacing directly with network hardware, most application programs and users interact with protocol software. The layering model is a fundamental tool that helps designers master the complexity of protocol design. © e-skills NTO 2000 25
  26. 26. unit 4 service and support for pc networks Server technology A file server is a combination of computer, internal hardware and software that allows LAN users to share file and print services across the LAN. An application server is a dedicated PC used to host a particular application program. Microsoft Exchange software is an e-mail application often processed by a dedicated server due to the huge demands made on the service and in turn the hardware. This demand placed on servers, due to clients simultaneously using the services, means they need a high performance processor, faster network interface, large memory capacity and larger data storage than most workstations. A file server must retrieve data rapidly and provide it to the requesting workstation with minimal delay. Factors affecting server performance include 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 Redundant Arrays of Independent Disks (RAID) technology and incorporate hot-swap drives. This is the ability to physically insert and remove drives while the system is up and running, which enhances the maintainability and reduces any downtime of the server. It is advisable that power is maintained to the server and is uninterrupted at all times. 26 © e-skills NTO 2000
  27. 27. service and support for pc networks unit 4 A device known as an Uninterruptible Power Supply (UPS) is a standard add-on feature to any server. Some servers have a UPS as an integral option within their hardware configuration. A UPS is a battery power pack positioned between the mains power supply and the server. If the mains power drops or fluctuates within a defined threshold, the UPS batteries activate and maintain power to the server. If the UPS function is selected in the operating system, a message from the UPS will inform the operating system of the power loss. A predetermined time will be flagged as remaining before the battery power runs out, if mains power is not restored. A UPS can range from a small, shoebox size unit capable of supplying reserve power for about 20 minutes, to a floor standing unit the size of the 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 a particular server, which may include multiple processors, memory and storage upgrades. The illustration below shows key features of a server system. © e-skills NTO 2000 27
  28. 28. unit 4 service and support for pc networks Drive arrays Connecting hard drives in an array is a method used by many servers in order to improve read/write performance and provide a level of hardware fault tolerance. A drive array is the term given to the hardware system of configuring multiple hard drives to a special controller called a RAID controller. The RAID controller connects all the drives in the array and allows data to be written to the drives in a special way. The RAID controller can stripe data across multiple physical drives whilst giving the software the impression there is only one logical drive as in the illustration below. The controller breaks down data to be written to the drives into smaller blocks and distributes these blocks across the multiple drives within the array. Striping data is detailed in the RAID Level 0 illustration later in this section. The RAID specification is divided into various levels. The five most common RAID levels used in servers are 0, 1, 3, 4 and 5. The method of data storage differs between RAID levels. The RAID levels are not indicative of hierarchy, but merely for differentiation. 28 © e-skills NTO 2000
  29. 29. service and support for pc networks unit 4 RAID Levels functionality The five most common RAID levels exhibit their own unique benefits and drawbacks. The following overview highlights their use in IT solutions. Level 0 RAID Level 0 is called disk striping. The data being written is broken down into strips and striped across the member disks of the array. This provides high I/O performance at low inherent cost but provides no redundancy. Level 1 RAID Level 1, or mirroring, has been in use longer than any other RAID. Level 1 provides redundancy by writing identical data to each member disk of the array, leaving a ‘mirrored’ copy on each disk. Mirroring is popular due to its simplicity and high level of data availability. Level 1 operates with two or more disks that may use parallel access for high data-transfer rates when reading, but more often operates independently to provide high I/O transaction rates. Level 1 provides high data reliability and improves the performance for read-intensive applications, but at relatively high cost. © e-skills NTO 2000 29
  30. 30. unit 4 service and support for pc networks Level 3 RAID Level 3 adds redundant data in the form of parity data to a parallel access striped array, permitting regeneration and rebuilding in the event of a single disk failure. One strip of parity data protects corresponding strips of user data on the remaining disks. RAID Level 3 provides high data transfer rates and high data availability, at a lower cost than mirroring. Its transaction performance is poor, however, because all RAID Level 3 array member drives operate in lockstep (spindle synchronisation). Level 4 Like Level 3, Level 4 uses parity concentrated on a single disk to protect data. Unlike Level 3, Level 4 member disks are independently accessible making it better suited to transaction I/O rather than large file transfers. Because the dedicated parity disk represents an inherent bottleneck, Level 4 is seldom used without accompanying technologies such as data caching. Level 5 By distributing parity across some or all of an array’s member disks, RAID Level 5 reduces (but does not eliminate) the write bottleneck inherent to Level 4. As with Level 4, the result is asymmetrical performance, with reads substantially outperforming writes. Level 5 is often used with data caching to reduce the asymmetry. 30 © e-skills NTO 2000
  31. 31. service and support for pc networks unit 4 Internetworking Internetworking is the term given to the technology of connecting individual networks to other networks, LANs being physically connected up to other LANs. The IT solutions of organisations have led to the installation of more than one local area network to satisfy its business requirements. This has led to the need for connecting discreet LANs together. It is probable that a user physically connected to a LAN requires access to resources located on another LAN within the company. This need has given rise to one of the fastest growing areas of the LAN industry, the internetworking device market. This market consists of repeaters, bridges, routers, gateways and advanced routing devices that perform a combination of these functions. These interconnectivity devices provide internetworking to users who are linked to large distributed groups of LANs. They also play a significant role in network management by allowing network administrators to segment, or divide, a single network into an assembly of smaller LANs called subnets. This subdivision can improve network performance by limiting the number of workstations on a network, and can reduce traffic over the segment wiring. It also facilitates security, as internetworking allows the control of the access of an individual or group to specified resources. Subnets can increase system reliability, for if 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 as switches. Switches provide the same functions as bridges and may perform local routing functions. Internetworking devices allow various levels of communication between individual networks. Each type of device also functions at a separate level within the OSI Reference Model. © e-skills NTO 2000 31
  32. 32. unit 4 service and support for pc networks Repeaters Repeaters offer the simplest form of interconnectivity. They regenerate, or repeat data packets between cable segments. Repeaters physically extend a network by amplifying the electrical data signals. Repeaters operate at the physical layer of the OSI model. Repeaters allow 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 the ability to connect multiple LAN cable segments into a large logical network. Bridges operate one layer higher than repeaters in the OSI model, in the data link layer. The diagram below depicts two bridged LAN segments. The bridge sends and receives complete frames from one LAN segment to the other. Filtering traffic Most bridges operate only between similar LAN technologies, between two Ethernets or two Token Rings segments, however some do offer cross-technology capabilities. Bridges regulate traffic by filtering data packets based on the destination address. When a destination address is local to the segment, the bridge does not forward it. When the destination address is remote to the local segment, the bridge forwards it. Bridges may automatically learn the addresses of the devices attached to their internetwork. Some bridges allow filtering of data on a variety of parameters, including packet size, source address and protocol type. Because filtering reduces network traffic, it can substantially increase overall network performance. 32 © e-skills NTO 2000
  33. 33. service and support for pc networks unit 4 Routers Routers operate at the network layer in the OSI Reference Model. Routers connect logically separate networks operating under the same protocol. Routers are protocol-dependent and must support the individual protocols being routed. These protocols could be TCP/IP and NetBEUI. A router allows multiple paths to exist in an enterprise-wide network and is intelligent enough to determine the most efficient path to send a particular data packet through the multiple loops. In a typical enterprise-wide network divided by routers, the separate networks are assigned unique ID numbers and each of these networks is managed separately. Routers automatically learn changes in the network configuration as bridges do, and are more complex than bridges because they determine these unique ID number when initialised. This requires network administrators to have an in-depth understanding of data communications to effectively manage router-based networks. Routers are particularly useful in organisations with large multiple networks connected to a single backbone. Because they must determine which protocol is being routed, routers were originally slower than bridges, but now routers are just as fast as any dedicated bridge device. Gateways Gateways act as translators between networks using incompatible network protocols, such as TCP/IP and NetBEUI. Depending on the level of incompatibility, gateways can operate at the transport layer through to the application layer of the OSI model. A gateway can be a communications link between a Local Area Network and the Internet, or between a Local Area Network and a mainframe or minicomputer, allowing LAN users to access the mainframe or minicomputer as a network server. © e-skills NTO 2000 33
  34. 34. unit 4 service and support for pc networks 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 coaxial cabling via a T-connector. The T-connector effectively allowed the computer to use the shared medium by physically tapping onto the network. An Ethernet hub provides a connecting technique where Ethernet equipped computer can use the 10BaseT system. 10BaseT is a hardware protocol that uses UTP cabling. The key feature of a hub is that the electronics simulates a single shared medium. Using hubs in this way allows both the bus and ring LAN technologies to take on the physical star topology, as the workstations in the network converge into these central hubs forming a physical star. The term hub has traditionally been used for Ethernet systems, however it is now associated with both Ethernet and Token Ring technologies. Switches Networking technology is described as being switched if the LAN hardware includes an electronic device that connects to one or more computers and allows the transfer of information between them. More specifically, a switched LAN consists of a single electronic device that transfers frames among many computers. Physically a switch resembles a hub. The difference is in the electronics. In an Ethernet hub, the electronics simulates a single shared medium, however in a switch it simulates a bridged LAN with only one computer connected per segment. This results in each computer connected to the switch having a simulated LAN segment to itself without contention for the shared medium from other workstations. Network performance is improved significantly with this configuration. 34 © e-skills NTO 2000