• Different concepts covered include:• transmission of data• protocols• handshaking• networks and their topologies• network access methods• Transmission of data occurs in two ways:• parallel transmission• serial transmission
• Parallel transmission is the transmission ofdata simultaneously using separatechannels.• Serial transmission transmits the data onebit after the other along one channel.• Parallel is faster, but can only be used overshorter distances.• The direction of data flow can be:• simplex• half-duplex• full-duplex
• Simplex mode allows transmission in onedirection only.• Half-duplex allows transmission in eitherdirection, but only in one direction at atime.• Full duplex allows transmission in bothdirections simultaneously.
• Serial transmission can be eitherasynchronous or synchronous.• Synchronous transmission requires that allof the data is sent at the same transfer rate.• The same number of bits is sent eachsecond.• Both computers are synchronized throughthe use of an internal clock.• It is faster and more efficient, but quiteexpensive as it requires very stableconnections.
• Asynchronous transmission is sent atvarying rates and the flow is regulatedthrough the use of start and stop bits.• One of the protocols agreed upon duringhandshaking is how many start and stop bitswill be used.• [Diagram 3.10, p.88]• Protocols are a set of rules that govern thetransfer of data between two computers.
• They define:• how a link is established• how data is transmitted• how errors are detected and corrected• Numerous protocols have been developedfor different technologies.• [Table 3.1, p.89 – Do the following: IPX,X.25, ATM, Kermit, TCP/IP, HTTP, FTP,POP, MIME]
• A network is any number of computers andtheir peripheral devices connected togetherin some way.• Each device in a network is called a node.• Terminals are devices that send data to andreceive data from another computer.• If the terminal has memory and processingcapabilities it is called an intelligentterminal.• Personal computers are often calledworkstations on a network.
• Networks can be either LAN’s, WAN’s orvery rarely, GAN’s• Local Area Networks (LAN’s) – are linkedcomputers at one physical location• Wide Area Network (WAN’s) – connectcomputers over several geographicallocations• Global Area Networks (GAN’s) –computers and devices connected all overthe world
• Network topology is the physicalarrangement of the devices in a network.• Three common types of topologies are:• bus• star• ring
• The bus topology is where all devices areattached to a direct line called a bus.• Each devices checks that the line is free andsends its signal.• If there is a data collision then the sendingterminal waits a random time then attemptsresending.• If one node fails the system will still work.• A star topology has a central computer witheach device connected directly to it.
• The central computer receives messagesand sends them to the destination device.• It requires extra cabling because eachdevice must be connected to the centralcomputer.• If one devices fails, the network will stillfunction unless it is the central computerthat is broken.• Its speed is determined by the processingpower of the central computer.
• A ring topology is an arrangement where alldevices are attached so that the connectionis a continuous loop.• Data flow is one direction moving fromdevice to device.• A ‘token’ is passed along the network anddata packets are attached to the token andreleased when they reach the appropriatenode.
• There are a number of ways to deal withmultiple users wanting to access thenetwork.• The two most common methods are:• Ethernet• token ring• Ethernet is based upon a bus network.• Ethernet allows data to be sent in bothdirections simultaneously.• This means there will be data collisions anderrors
• To overcome this problem, Ethernet uses asystem called Carrier Sense MultipleAccess and Collision Detection(CSMA/CD)• In CSMA/CD all nodes have the ability tosense signals on a network.• When a node wishes to transmit, it ‘listens’for a signal on the bus.• When no signals are detected, it transmits.• If two nodes transmit at the same time thena data collision can occur.
• When a collision is detected, each devicestops transmitting, waits a randomlydetermined time, then transmits again.• Token ring is a LAN access protocol basedupon ring topologies.• Special data packets, called tokens, arecontinually passed between nodes on thenetwork.• Workstations with data to send attach it to afree token and transmit it.• A busy token cannot be used by othernodes.
• When the data is received it is replaced withan acknowledgement and sent back to theoriginal sending node.• There are several special purpose hardwaredevices used in a successful network.• They include:• a network interface card• servers• routers and switches• bridges and gateways• hubs
• A network interface card (NIC) is anexpansion card that links the network to thecomputer motherboard.• The type of NIC depends upon the type ofnetwork.• A server is a dedicated computer thatprovides services to other computers.• Workstations log on to the server to givethem access to files, applications orperipheral devices.
• Types of servers include:• file server• print server• mail server• Internet server• Networks are often linked to other networksand this requires paths to be established andsignal strength to be boosted.• Devices used to determine the path betweennetworks include routers and switches.
• Routers maintain a table of available routesand their conditions.• It uses this table to determine the best routefor any given data packet.• Data packets often travel through severalnetworks and routers before they reach theirdestination.• A switch is similar, but it does not maintaina table, therefore making it a simpler andfaster mechanism for passing the messagealong.
• However, they do not always choose thefaster route for the packet to travel along.• The actual connection between networks isachieved through bridges and gateways.• A bridge is a combination of hardware andsoftware used to link two similar networks.• Bridges examine each data packet andforward any that are addressed to aconnected LAN.• Gateways are combinations of hardwareand software used to link two differentkinds of network.
• This usually involves converting differentprotocols.• A repeater may also be used to increase adiminishing signal.• Repeaters are less intelligent than bridges orgateways.• A hub is a central connecting device withina network.• A hub can be considered passive if it justchannels data or it can contain a router toimprove traffic flow.
• As well as the devices connected to anetwork, we need to consider thetransmission medium.• Data is transferred along a transmissionmedium.• The capacity of a transmission medium iscalled the bandwidth.• Bandwidth is generally measured in bps• They can be categorised as wire or wireless.• Wire transmission involves wires andcables.
• Twisted pair cable consists of two insulatedcopper wires twisted around each other to forma spiral.• There are two types:• shielded twisted pair (STP)• unshielded twisted pair (UTP)• STP is more often used in ‘noisy’environments.• Twisted pair is the slowest medium withtransmission speeds up to 60 Kbps.
• Coaxial cable consists of a single copper wiresurrounded by insulation, shielding , then moreinsulation.• It allows transmission with little interference.• Generally used for under 2kms.• Bandwidth is around 10Mbps.• Fibre-optic cables are made from glass fibresthe width of a human hair, that are oftenbundled together.
• They are:• free from interference• very secure• able to transmit at high speeds(> 400 Mbps)• Most LAN’s use twisted pair or coaxial cableas fibre-optic is too expensive but this ischanging and fibre-optic is becoming muchmore common.
• There are two types of transmission used:• baseband• broadband• Baseband uses the entire capacity of the cable totransmit one signal.• Broadband networks divide the cable so thatseveral signals can be transmitted simultaneously.• Most LAN’s are baseband.• Wireless transmission moves data without aphysical connection.
• Wireless includes:• microwaves• radio frequencies (RF)• infra-red (IR)• satellites• A microwave is a high frequency radio signalthat requires antennae no further than 50 kmapart.• It is faster than coaxial or regular telephonelines and relatively error-free.
• Radio frequencies are used for many home andbusiness wireless networks, usually in the 2.4GHz range.• Can have an access point (or hot-spot) or bedone in an ad-hoc manner.• Satellites can transmit large amounts of dataover large distances.• Used by major corporations, televisionnetworks and telecommunications.• IR is used by some devices in LAN’s
• Each computer in a network must haveappropriate network software.• This software is usually organised by anetwork administrator.• Their responsibilities include:• network security• software installations and updates• monitoring daily activity• enforcing licensing agreements• developing a storage solution• performing routine backups
• A network operating system (NOS) is anoperating system designed to supportnetworked computers.• One part resides on each node with anotherpart on the server.• Tasks carried out by a network OS include:• administration• file management• applications• resource management• security
• An intranet is a private network that uses asimilar interface to the Web.• It consists of many interlinked LANs.• It provides access similar to the Internet, butonly to a select group of users (e.g. employees,students)• Firewalls monitor the flow of data in bothdirections to maintain the security of theorganisation.
• An extranet is an intranet that is accessible tocustomers, suppliers or others outside theorganisation.• It provides such information as productdescriptions, answers to frequently askedquestions, warranties and how to contactcustomer service.• Extranets are still limited to accessing selectcomputer paths and files.