What is a computer network? 9.1 The Need for a Computer Network A system of interconnected computers, peripherals and communication devices that can exchange information and share resources The function becomes more extensive when the computer is networked

Advantages of networking 9.1 The Need for a Computer Network Better communication Users can communicate with each other by , instant messaging or file transfer. Communication through network

Advantages of networking 9.1 The Need for a Computer Network Hardware sharing Networks allow users to share hardware resources such as hard disks, printers and scanners. The shared equipment can be better utilized and saves the cost of repetitive hardware. Sharing of hardware on a network

Advantages of networking 9.1 The Need for a Computer Network Centralized data management Users can share centralized data files stored on a network server. Each user can access up-to-date and identical information all the time. Sharing of centralized file on a network

Advantages of networking 9.1 The Need for a Computer Network Software sharing Application software can be shared among computers on a network simultaneously. A network licence makes multiple access to the software legally.

There is no need to carry out the installation on each computer. Advantages of networking 9.1 The Need for a Computer Network Software sharing Features of network licenses Some may set a limit on the number of users who use the program simultaneously. Some are based on the number of computers on which the licensed program can run. The cost of a network licence is usually lower than the total cost of individual software licences.

Advantages of networking 9.1 The Need for a Computer Network Internet access service sharing Users can access the Internet via a computer network with a single Internet service account. Sharing of Internet access

Advantages of networking 9.1 The Need for a Computer Network Efficient backup The information on individual computers can be backed up and stored on a server via the network. The backup files can be used to recover lost information stored originally in a computer which is found out-of-order.

Advantages of networking 9.1 The Need for a Computer Network Efficient backup The information on individual computers can be backed up and stored on a server via the network. The backup files can be used to recover lost information stored originally in a computer which is found out-of-order.

9.2 LANs and WANs Networks are categorized according to the area they cover. wide area network (WAN): a network that spans a wide geographical area local area network (LAN): a network that connects computers within a relatively limited area

Local Area Network (LAN) 9.2 LANs and WANs Consists of networked computers, or workstations, shared peripheral devices, communication devices and links Peer-to-peer and client/server are two common architectures of LANs

Two examples of peer-to-peer network 9.2 LANs and WANs Sharing printing service Sharing Internet access

Example of client/server network 9.2 LANs and WANs A client/server network

Comparison between peer-to-peer network and client/server network 9.2 LANs and WANs Peer-to-peer networkClient/server network It is a network of workstations without a server, where all the workstations can communicate directly with each other. Client workstations are connected to a central server, or a host computer. Each computer can access the resources shared by all the other peer workstations. The clients can access the resources stored on the server.

Comparison between peer-to-peer network and client/server network 9.2 LANs and WANs Peer-to-peer networkClient/server network A large and heavily loaded network will result in noticeable drop in performance at each workstation. It provides better security controls and network services in a large-scale network environment. The set-up cost is relatively low. The set-up cost is relatively high. It is usually found in small companies and homes. It is ideal for medium or large organizations.

Wide Area Network (WAN) 9.2 LANs and WANs A communication network that consists of two or more remote LANs, which are connected via public communication channels The Internet is the largest WAN in the world LANs are interconnected to form a WAN.

Wide Area Network (WAN) 9.2 LANs and WANs A communication network that consists of two or more remote LANs, which are connected via public communication channels The Internet is the largest WAN in the world LANs are interconnected to form a WAN.

9.3 Connect to a Computer Network Network Interface Card (NIC) The hardware component needed to connect the computer to a network Each NIC has a medium access control (MAC) address which is used to identify the computer on the network. Can be classified by the types of connectors, expansion buses or the size of its bandwidth. Criterion for classification Examples Bandwidth10 Mbps, 100 Mbps and 1000 Mbps Expansion busISA, PCI, PCMCIA and USB ConnectorAUI, BNC and RJ-45

9.3 Connect to a Computer Network Different types of connectors BNC AUI RJ-45

9.3 Connect to a Computer Network Different types of expansion bus ISA PCI PCMCIA USB

9.3 Connect to a Computer Network Five common types of connecting devices Hub It provides a central connection point for cables on a network. It can be a low cost substitution for a switch in a network which has a low transmission traffic. Switch It provides a central connection point for cables on a network. Typically, one or more computers are connected to a switch with network cables. The switch in turn connects to a server.

9.3 Connect to a Computer Network Five common types of connecting devices Repeater It amplifies the received signals, removes unwanted noises and then sends the amplified signals to the connected device. It overcomes the problem of signal attenuation due to the signal transmission over long distance.

Router It translates signals from different networks. It is responsible for the connectivity between different types of LANs and between a LAN and the Internet. Bridge It is used to interconnect two distinct LANs of the same type. 9.3 Connect to a Computer Network Five common types of connecting devices

9.3 Connect to a Computer Network Hardware components of networking

9.3 Connect to a Computer Network Network cables In a network, computers and other network devices are usually linked together by network cables. Common network cables include: coaxial cable optical fibre cable twisted-pair cable

Unshielded twisted-pair (UTP) cable It does not have metal shielding. It is a low-cost substitution for the STP cable. 9.3 Connect to a Computer Network Twisted-pair cable Widely used in local area networks Looks similar to a telephone cable and has a RJ-45 connector on both ends Two types of twisted-pair cables: Shielded twisted-pair (STP) cable It has a metal wrapper around each wire to reduce noise.

9.3 Connect to a Computer Network Twisted-pair cable Several types of UTPs: Category1 (Cat1) Category 2 (Cat2) Category3 (Cat3) Category4 (Cat4) Category6 (Cat6) maximum data transfer rate: about 1,000 Mbps within 100 m distance limit commonly used network cable in a LAN construction Category6 (Cat6) maximum data transfer rate: about 1,000 Mbps within 100 m distance limit commonly used network cable in a LAN construction Category5 (Cat5)

9.3 Connect to a Computer Network Coaxial cable It is commonly used for VCR and cable television connections. It has a BNC connector on both ends.

9.3 Connect to a Computer Network Advantages of the coaxial cable over the twisted-pair cable Has a higher ability to retain the strength of a transmitting signal over a long distance Consists of a copper wire, a sheath of insulation, a wire mesh that conducts and shields against interference, and an outside coating. This strengthened insulation allows coaxial cable to transmit more information

9.3 Connect to a Computer Network Optical fibre cable Made of a bundle of glass threads that transmit light beams

9.3 Connect to a Computer Network Optical fibre cable Advantages of optical fibre cables over twisted-pair cables and coaxial cables: greater signal- carrying capacity faster data transmission better security for signal transmission reduced susceptibility to noise or interference from other devices thinner lighter in weight more durable

9.3 Connect to a Computer Network Optical fibre cable Have replaced coaxial cables in telephone networks Disadvantages of optical fibre cables: The cost is very high. The installation process is much more difficult than that of other network cables. It is inflexible to changes once installed. A special device is needed to convert digital signals to optical light beams and vice versa.

9.3 Connect to a Computer Network Characteristics of different types of cables Twisted- pair cable Coaxial cable Optical fibre cable Transmission speed 10 Mbps, 100 Mbps up to 200 Mbps 100 Mbps to several Gbps Transmission signal electrical optical Costcheapexpensivemore expensive Topologystar, ringbusstar, ring

9.3 Connect to a Computer Network Wireless networks Wireless local area network (WLAN) uses high-frequency radio waves to transmit and receive data without the limitations of cables Access point: a device that allows a wireless device to communicate to other wireless devices by radio frequencies or to a wired network

9.3 Connect to a Computer Network Wireless networks A WLAN connects to a wired LAN by means of an access point.

9.3 Connect to a Computer Network Features of WLANs Provide mobility. A wireless device must equip with a wireless network interface card or a wireless node Become an alternative for wired LANs Set-up cost for a large-scale wireless LAN is high and it poses security risks on the whole network Wireless network interface card

9.3 Connect to a Computer Network Network topology The logical layout in which the computers and other devices are connected together Each computer or device on the network is called a node Common network topologies include: bus topology star topology ring topology

9.3 Connect to a Computer Network Bus topology All computers and other devices connect to a single central cable known as the bus Data is transmitted between the nodes through the bus Bus topology

9.3 Connect to a Computer Network Features of bus topology Set-up is simple and inexpensive Widely used in LAN constructions Very flexible as they allow computers and other devices to be attached or detached from the network, without disturbing the rest of the network (at any point) Failure of one device does NOT affect the rest of the bus network If the bus fails to work, the whole network will not function properly.

9.3 Connect to a Computer Network Star topology All devices on the network are connected to a central computer or a switch to form a star All data transferred from one computer to another passes through the central computer Star topology

9.3 Connect to a Computer Network Features of star topology Each device connects directly to the central computer If one device of the system fails, only that device will fail to work while other devices remain unaffected If the central computer fails, the whole network will become inoperative

9.3 Connect to a Computer Network Ring topology All devices on the network are connected on a circular path that allows data to flow in one direction only A computer transmits data packets to the computer next to it until all data packets arrive at their destination Ring topology

9.3 Connect to a Computer Network Features of ring topology Control data transmission with a token- passing scheme Token: A control signal passing from one device to the next Only the token-holding device is allowed to transmit data Advantages of the token-passing scheme: Ensures only one device can transmit data at a time Ensures no data collisions occur If any device fails, no device can transmit data properly as the token cannot be passed around the ring any longer.

9.3 Connect to a Computer Network Intranet and Extranet Intranet: a local area network built with Internet technology, forming a private, small-scale Internet inside an organization. Features of the intranet: Applications such as internal systems, message boards, web sites and video- conferencing may be available. Users can use the web browser, FTP application or any Internet access tools to retrieve information from the intranet.

9.3 Connect to a Computer Network Intranet and Extranet Extranet :a network connection that allows external users to access the intranet through the Internet and the public telecommunication channels Connection is usually protected by a firewall ensures the access is secure and under control Extranet

9.3 Connect to a Computer Network Network Operating Systems Functions of a network operating system (NOS): Two components of a NOS: to manage network resources to control the flow of data to maintain security network server software network client softwar e

9.3 Connect to a Computer Network Basic functions of a network server software control the access to the server’s resources such as hard disks, printers, Internet access and application software track and record the activities of users with log files manage user accounts gather login information access the shared resources on the network Basic functions of a network client software

9.3 Connect to a Computer Network Some examples of network operating systems developed by different companies Company Network client software Network server software AppleMac OS XMac OS X Server IBMOS 2 WarpOS 2 Warp Server Microsoft Windows XP Professional Windows.NET Server NovellNetware 6

9.3 Connect to a Computer Network Server and client software components The server and client software components do not have to match each other Network client software Windows 98 NT Workstation XP Professional Network client software Windows 98 NT Workstation XP Professional Network server software Windows NT Server Windows 2003 Server Linux UNIX Novell Netware Network server software Windows NT Server Windows 2003 Server Linux UNIX Novell Netware allow to access

9.3 Connect to a Computer Network Server and client software components The server and client software components do not have to match each other Network client software Windows 98 NT Workstation XP Professional Network client software Windows 98 NT Workstation XP Professional Network server software Windows NT Server Windows 2003 Server Linux UNIX Novell Netware Network server software Windows NT Server Windows 2003 Server Linux UNIX Novell Netware allow to access

9.4 Common Services Available in a Network Environment Services available on different servers Different server computers provide different services on a client/server LAN. The servers include: File servers Print servers Application servers

9.4 Common Services Available in a Network Environment File server Shares programs and data files stored on its hard disk with other workstations Workstations access the network hard disk just as they do for their local hard disks process data or execute programs using their own processor and main memory File server shares programs and data with workstations in a LAN

9.4 Common Services Available in a Network Environment Print server Manages the printing services Receives print jobs from workstations and then forwards them to a network printer Functions of a print server: Maintains the print queue which stores print jobs until they are processed successfully Able to keep track of the printing activities and bill the users accordingly It can be a file server, or a workstation in the network

9.4 Common Services Available in a Network Environment Print server A print server manages print jobs by a print queue.

9.4 Common Services Available in a Network Environment Application server It runs a specific application software package and forwards the processed results to the requested workstations. The interaction cycle of an application server and its client

9.4 Common Services Available in a Network Environment Application server It runs a specific application software package and forwards the processed results to the requested workstations. The interaction cycle of an application server and its client