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.