3.3) Transmitting and 3.3) Transmitting and ReceivingReceiving
• 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 of data simultaneously using separate channels.
• Serial transmission transmits the data one bit after the other along one channel.
• Parallel is faster, but can only be used over shorter distances.
• The direction of data flow can be:• simplex• half-duplex• full-duplex
• Simplex mode allows transmission in one direction only.
• Half-duplex allows transmission in either direction, but only in one direction at a time.
• Full duplex allows transmission in both directions simultaneously.
• Serial transmission can be either asynchronous or synchronous.
• Synchronous transmission requires that all of the data is sent at the same transfer rate.
• The same number of bits is sent each second.
• Both computers are synchronized through the use of an internal clock.
• It is faster and more efficient, but quite expensive as it requires very stable connections.
• Asynchronous transmission is sent at varying rates and the flow is regulated through the use of start and stop bits.
• One of the protocols agreed upon during handshaking is how many start and stop bits will be used.
• [Diagram 3.10, p.88]
• Protocols are a set of rules that govern the transfer 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 developed for 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 and their peripheral devices connected together in some way.
• Each device in a network is called a node.
• Terminals are devices that send data to and receive data from another computer.
• If the terminal has memory and processing capabilities it is called an intelligent terminal.
• Personal computers are often called workstations on a network.
• Networks can be either LAN’s, WAN’s or very rarely, GAN’s
• Local Area Networks (LAN’s) – are linked computers at one physical location
• Wide Area Network (WAN’s) – connect computers over several geographical locations
• Global Area Networks (GAN’s) – computers and devices connected all over the world
• Network topology is the physical arrangement of the devices in a network.
• Three common types of topologies are:
• bus
• star
• ring
Bus Network
Star Network
Ring Network
• The bus topology is where all devices are attached to a direct line called a bus.
• Each devices checks that the line is free and sends its signal.
• If there is a data collision then the sending terminal waits a random time then attempts resending.
• If one node fails the system will still work.
• A star topology has a central computer with each device connected directly to it.
• The central computer receives messages and sends them to the destination device.
• It requires extra cabling because each device must be connected to the central computer.
• If one devices fails, the network will still function unless it is the central computer that is broken.
• Its speed is determined by the processing power of the central computer.
• A ring topology is an arrangement where all devices are attached so that the connection is a continuous loop.
• Data flow is one direction moving from device to device.
• A ‘token’ is passed along the network and data packets are attached to the token and released when they reach the appropriate node.
• There are a number of ways to deal with multiple users wanting to access the network.
• The two most common methods are:
• Ethernet
• token ring
• Ethernet is based upon a bus network.
• Ethernet allows data to be sent in both directions simultaneously.
• This means there will be data collisions and errors
• To overcome this problem, Ethernet uses a system called Carrier Sense Multiple Access and Collision Detection (CSMA/CD)
• In CSMA/CD all nodes have the ability to sense 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 then a data collision can occur.
• When a collision is detected, each device stops transmitting, waits a randomly determined time, then transmits again.
• Token ring is a LAN access protocol based upon ring topologies.
• Special data packets, called tokens, are continually passed between nodes on the network.
• Workstations with data to send attach it to a free token and transmit it.
• A busy token cannot be used by other nodes.
• When the data is received it is replaced with an acknowledgement and sent back to the original sending node.
• There are several special purpose hardware devices 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 an expansion card that links the network to the computer motherboard.
• The type of NIC depends upon the type of network.
• A server is a dedicated computer that provides services to other computers.
• Workstations log on to the server to give them access to files, applications or peripheral devices.
• Types of servers include:
• file server
• print server
• mail server
• Internet server
• Networks are often linked to other networks and this requires paths to be established and signal strength to be boosted.
• Devices used to determine the path between networks include routers and switches.
• Routers maintain a table of available routes and their conditions.
• It uses this table to determine the best route for any given data packet.
• Data packets often travel through several networks and routers before they reach their destination.
• A switch is similar, but it does not maintain a table, therefore making it a simpler and faster mechanism for passing the message along.
• However, they do not always choose the faster route for the packet to travel along.
• The actual connection between networks is achieved through bridges and gateways.
• A bridge is a combination of hardware and software used to link two similar networks.
• Bridges examine each data packet and forward any that are addressed to a connected LAN.
• Gateways are combinations of hardware and software used to link two different kinds of network.
• This usually involves converting different protocols.
• A repeater may also be used to increase a diminishing signal.
• Repeaters are less intelligent than bridges or gateways.
• A hub is a central connecting device within a network.
• A hub can be considered passive if it just channels data or it can contain a router to improve traffic flow.
• As well as the devices connected to a network, we need to consider the transmission medium.
• Data is transferred along a transmission medium.
• The capacity of a transmission medium is called the bandwidth.
• Bandwidth is generally measured in bps
• They can be categorised as wire or wireless.
• Wire transmission involves wires and cables.
• Twisted pair cable consists of two insulated copper wires twisted around each other to form a 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 with transmission speeds up to 60 Kbps.
• Coaxial cable consists of a single copper wire surrounded by insulation, shielding , then more insulation.
• It allows transmission with little interference.
• Generally used for under 2kms.
• Bandwidth is around 10Mbps.
• Fibre-optic cables are made from glass fibres the width of a human hair, that are often bundled together.
• They are:
• free from interference
• very secure
• able to transmit at high speeds(> 400 Mbps)
• Most LAN’s use twisted pair or coaxial cable as fibre-optic is too expensive but this is changing and fibre-optic is becoming much more common.
• There are two types of transmission used:
• baseband
• broadband
• Baseband uses the entire capacity of the cable to transmit one signal.
• Broadband networks divide the cable so that several signals can be transmitted simultaneously.
• Most LAN’s are baseband.
• Wireless transmission moves data without a physical connection.
• Wireless includes:
• microwaves
• radio frequencies (RF)
• infra-red (IR)
• satellites
• A microwave is a high frequency radio signal that requires antennae no further than 50 km apart.
• It is faster than coaxial or regular telephone lines and relatively error-free.
• Radio frequencies are used for many home and business wireless networks, usually in the 2.4 GHz range.
• Can have an access point (or hot-spot) or be done in an ad-hoc manner.
• Satellites can transmit large amounts of data over large distances.
• Used by major corporations, television networks and telecommunications.
• IR is used by some devices in LAN’s
• Each computer in a network must have appropriate network software.
• This software is usually organised by a network 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 an operating system designed to support networked computers.
• One part resides on each node with another part 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 a similar interface to the Web.
• It consists of many interlinked LANs.
• It provides access similar to the Internet, but only to a select group of users (e.g. employees, students)
• Firewalls monitor the flow of data in both directions to maintain the security of the organisation.
• An extranet is an intranet that is accessible to customers, suppliers or others outside the organisation.
• It provides such information as product descriptions, answers to frequently asked questions, warranties and how to contact customer service.
• Extranets are still limited to accessing select computer paths and files.
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