networkspide (2)

8/7/2019 networkspide (2)

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Computer Networks

Overview What is a network?

Types of network (LAN & WAN) Advantages & Disadvantages.

Hardware

Fileserver, Workstation, NIC, Router Software Operating Systems

Peer-to-Peer Client/Server

Cabling

UTP, Coaxial, Fiber Optic

Wireless

Protocols Ethernet

Token Ring

Topologies


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What is a Network?

A network consists of two or more

computers that are linked in order to

share resources (such as printers and CD-ROMs), exchange files, or allow

electronic communications. Thecomputers on a network may be linked

through cables, telephone lines, radio

waves, satellites, or infrared lightbeams.


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TYPES OF NETWORKSThe three basic types of networks include: LAN, MAN

andWAN.LOCAL AREA NETWORK (LAN)

A network is said to be Local Area Network (LAN) if it isconfined relatively to a small area. It is generally limited to a

building or a geographical area, expanding not more than amile apart to other computers.

LAN configuration consist of:

A file server - stores all of the software that controls the network,as well as the software that can be shared by the computers

attached to the network. A workstation - computers connected to the file server (Mac or

PCs).These are less powerful than the file server

Cables - used to connect the network interface cards in eachcomputer.


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METROPOLITAN AREA NETWORK (MAN)

Metropolitan Area Network (MAN) covers largergeographic areas, such as cities. Often used by locallibraries and government agencies often to connect tocitizens and private industries.

WIDE AREA NETWORK (WAN) Wide Area Networks (WANs) connect larger geographic

areas, such as London, the UK, or the world. In this type

of network dedicated transoceanic cabling or satelliteuplinks may be used.


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ADVANTAGES OF NETWORK

Speed. Sharing and transferring files within Networks are very rapid. Thussaving time, while maintaining the integrity of the file.

Cost. Individually licensed copies of many popular software programs canbe costly. Networkable versions are available at considerable savings.Shared programs, on a network allows for easier upgrading of the

program on one single file server, instead of upgrading individualworkstations.

Security. Sensitive files and programs on a network are passwordsprotected (established for specific directories to restrict access toauthorized users) or designated as "copy inhibit," so that you do not have

to worry about illegal copying of programs. Centralized Software Management. Software can be loaded on one

computer (the file server) eliminating that need to spend time and energyinstalling updates and tracking files on independent computersthroughout the building.


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ADVANTAGES OF NETWORK

Resource Sharing. Resources such as, printers, faxmachines and modems can be shared.

Electronic Mail. E-mail aids in personal andprofessional communication. Electronic mail on a

LAN can enable staff to communicate within thebuilding having tot to leave their desk.

Flexible Access. Access their files from computersthroughout the firm.

Workgroup Computing. Workgroup software (suchas Microsoft BackOffice) allows many users to workon a document or project concurrently


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DISADVANTAGES OF NETWORK

Server faults stop applications being available

Network faults can cause loss of data.

Network fault could lead to loss of resources

User work dependent upon network System open to hackers

Decisions tend to become centralized

Could become inefficient

Could degrade in performance

Resources could be located too far from users

Network management can become difficult


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DISADVANTAGES OF NETWORKS

Initial installation expense can be

costly

Fileserver may fail causing entirenetwork to go down

Proper administration requires

considerable time and expertise Cables may break


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HARDWARE

FileserverA file server stands at the heart of most

networks. It is a very fast computer with alarge amount of RAM and storage space,

along with a fast network interface card.The network operating system software

resides on this computer, along with any

software applications and data files thatneed to be shared.


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HARDWARE

WorkstationsAll of the computers connected to the fileserver on a network are called workstations. Atypical workstation is a computer that isconfigured with a network interface card,networking software, and the appropriate

cables. Workstations do not necessarily needfloppy disk drives or hard drives because filescan be saved on the file server. Almost anycomputer can serve as a network workstation.


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HARDWARE

Network Interface Cards

The network interface card (NIC) provides

the physical connection between thenetwork and the computer workstation.Most NICs are internal, with the card

fitting into an expansion slot inside thecomputer.


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HARDWARE

Concentrators/HubsA concentrator is a device that provides a centralconnection point for cables from workstations,

servers, and peripherals. Hubs are multi-slotconcentrators into which can be plugged anumber of multi-port cards to provide additionalaccess as the network grows in size. Most

concentrators are active, that is they electricallyamplify the signal as it moves from one device toanother.


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HARDWARE

RouterIf a LAN connects to the Internet a network

will need router. The router serves as thetranslator between the information on the

LAN and the Internet. It also determines

the best route to send the data over the

Internet.


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SOFTWARE

Network Operating Systems

Peer-to-PeerPeer-to-peer network operating systemsallow users to share resources and fileslocated on their computers and to accessshared resources found on othercomputers. However, they do not have afile server or a centralized managementsource. In a peer-to-peer network, all

computers are considered equal; they allhave the same abilities to use the resourcesavailable on the network. Peer-to-peernetworks are designed primarily for smallto medium local area networks


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SOFTWARE

Client/ServerClient/server network operating systems allow

the network to centralize functions andapplications in one or more dedicated fileservers. The file servers become the heart ofthe system, providing access to resources and

providing security. Individual workstations(clients) have access to the resources availableon the file servers.


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NETWORKS

CablingCable is the medium through which

information usually moves from onenetwork device to another.


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Unshielded Twisted Pair (UTP)

Cable

Twisted pair cabling comes in two varieties: shielded

and unshielded. Unshielded twisted pair (UTP) is the

most popular and is the type we use. The quality ofUTP may vary from telephone-grade wire toextremely high-speed cable. The cable has four pairs ofwires inside the jacket. Each pair is twisted with a

different number of twists per inch to help eliminateinterference from adjacent pairs and other electricaldevices.There are 5 categories recognized.


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UTP Connectors

The standard connector for UTP cabling is anRJ-45 connector. This is a plastic connectorthat looks like a large telephone-styleconnector. RJ stands for Registered Jack,implying that the connector follows a standardborrowed from the telephone industry. This

standard designates which wire goes witheach pin inside the connector.


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Coaxial Cable

Coaxial cabling has a single copper conductorat its center. A plastic layer provides insulationbetween the center conductor and a braidedmetal shield. The metal shield helps to blockany outside interference from fluorescentlights, motors, and other computers.


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Coaxial Cable Connectors

The most common type of connector usedwith coaxial cables is the BNC connector.Different types of adapters are available forBNC connectors, including a T-connector,barrel connector, and terminator. Connectorson the cable are the weakest points in any

network. To help avoid problems BNCconnectors should be crimped.


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Fiber Optic Cable

Fiber optic cabling consists of a center glass coresurrounded by several layers of protectivematerials. It transmits light rather than

electronic signals, eliminating the problem ofelectrical interference. Fiber optic cable has theability to transmit signals over much longerdistances than coaxial and twisted pair. It also

has the capability to carry information at vastlygreater speeds.


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Wireless LANs

Not all networks are connected with cabling; somenetworks are wireless. Wireless LANs use highfrequency radio signals or infrared light beams tocommunicate between the workstations and the file

server. Each workstation and file server on a wirelessnetwork has some sort of transceiver/ antenna tosend and receive the data. Information is relayedbetween transceivers as if they were physically

connected. For longer distance, wirelesscommunications can also take place through cellulartelephone technology or by satellite.


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Wireless networks are great for allowing laptopcomputers or remote computers to connect tothe LAN. Wireless networks are also beneficialin older buildings where it may be difficult or

impossible to install cables.

This includes all computers, peripherals,interface cards and other equipment needed to

perform data processing and communicationswithin the network.

Wireless LANs


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PROTOCOL

A protocol is a set of rules that governs the

communications between computers on a

network. These rules are guidelines that regulate

the access method, allowed physical topologies,types of cabling, and speed of data transfer.

The most common protocols are: Ethernet

LocalTalk

Token Ring

FDDI

ATM


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ETHERNETThis is the most widely used protocol. This protocol uses an

access method called CSMA/CD (Carrier Sense MultipleAccess/Collision Detection). In this system each computer listensto the cable for any transmitting node before sending anythingthrough the network. If the network is clear, the computer willtransmit. Else wait and try again when the line is clear.Sometimes, two computers attempt to transmit at the same

instant (causing a collision). Each computer then backs off andwaits a random amount of time before attempting to retransmit.The delay by collisions and retransmitting is very small and doesnot normally affect the speed of transmission on the network.

Topologies are bus star or tree and transmission is via twistedpair, coaxial, or fibre optic cable at a speed of 10 Mbps.


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Fast Ethernet

Support 100Mbps and are more expensivenetwork concentrators/hubs and networkinterface cards is requires for Fast Ethernet.Category 5 twisted pair or fibre optic cable is

necessary.

Gigabit Ethernet

The Ethernet has a standard protocol of 1Gbps

transmission speed but used primarily forbackbones on a network.


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LOCALTALK

Apple Computer developed LocalTalk forMacintosh computers. The method used byLocalTalk is called CSMA/CA (Carrier SenseMultiple Access with Collision Avoidance). It is

similar to CSMA/CD except that a computersignals its intent to transmit before it actuallydoes so. LocalTalk adapters and special twistedpair cable can be used to connect a series ofcomputers through the serial port.

LocalTalk protocol allows for linear bus, star, ortree topologies using twisted pair cable.


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TOKEN RING

This was developed by IBM in the mid 1980s. The

method used involves token-passing. Computers areconnected so that the signal travels around thenetwork from one computer to another in a logicalring. A single electronic token moves around the ring

from one computer to the next. If a computer doesnot have information to transmit, it simply passesthe token on to the next workstation. If a computerwishes to transmit and receives an empty token, it

attaches data to the token. The token then proceedsaround the ring until it comes to the computer forwhich the data is meant. At this point, the receivingcomputer captures the data.


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FIBRE DISTRIBUTED DATA INTERFACE

(FDDI)Access method of token-passing via a dual ring physicaltopology. Transmission on one of the rings; however, if a

break occurs, the system keeps information moving by

automatically using portions of the second ring to create anew complete ring. Transmission speed is100 Mbps over afiber optic cable, but expensive.


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ASYNCHRONOUS TRANSFER MODE (ATM)

Transmits data in small packets of a fixed sizeat a speed of 155 Mbps and higher. ATM

supports a variety of media such as video, CD-quality audio, and imaging. ATM employs a

star topology with fibre optic or twisted pair

cabling.


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NETWORKSTOPOLOGIES

Physical Topologies

The configuration of cables, computers,

and other peripherals.


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NETWORK TOPOLOGY

The term topologyrefers to the way in which the nodes of anetwork are connected.

The topology of a network will effect its performance (itterms of speed) and its cost (both short and long term).

Cost/resource considerations and the environment in which

the network is to be used often determines the choice oftopology.


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A network topology is the way in which a network is connectedup.

The simplest network topology you can have is a single link (i.e.cable, optical fibre, radio or any other means of transmittingdata) between two computers.

It may not be a very big network, but technically it is still anetwork. The computers can exchange data and they areautonomous.


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COMMON TOPOLOGIES

Some common topologies include:

MESH STAR

BUS

TREE

RING

BACKBONE


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STAR TOPOLOGY

If all the hosts are connected to a central hub (which can be a

switching device or a central server also acting as a router)then we have a star topology.

Hosts communicate by sending theirmessages to the hub. It thenforwards the messages to the

destination host. Star topologies are popular with

financial institutions who keep theirlatest records on a central server.Any changes to the records are

updated centrally.

Its easy to control and monitor access to a central server.

Cabling costs can still be high if network is spread out.

The central hub may be a bottleneck in busy networks.


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STAR NETWORKS

The hub switches messages through to theappropriate destination.

The hub may also provide a translation service fordevices with different protocols.

Star Networks are vulnerable, however. If the hub

fails then the network fails.

Star Networks may require a lot of cabling and canbe expensive to install.


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BUS NETWORKS

A bus network consists of a single medium (typically 5 pairtwisted-wire cable) to which all the host computers areconnected.

Packets are broadcasted on the medium to all nodes on thenetwork.

Network

Interface Units

Host

Computers

Terminator

Bi-directional

medium


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BUS TOPOLOGY

We can make a cheap network by connecting not just two but anumber of hosts to a single link. Such a network is said to havea bus topology. It is also called a multi-drop link.

Although physically simple, bus topologies need a complicatedprotocol in order to ensure that hosts get fair use of the linkand do not attempt to use the link at the same time. This canslow things down in a busy network.


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TREE NETWORKS

A tree network (as used in LANs) is a variant of the Bustopology.

Nodes are connected in a tree structure and messages arebroadcast across whole tree.

Communications Link

Root Node

ost Com uters


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TREE Networks

Tree topologies have the advantage that they areeasy to expand.

Furthermore, if a fault occurs, the effected branch

can be easily isolated so that the rest of the networkis not effected.

The disadvantage is that signals can be reflected

from the ends of branches and cause interference.For this reason, Tree Networks are usually run atlower speeds.


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TREE TOPOLOGY

A tree topology has a root and branches that gives it a

distinctive tree shape.

Messages pass up the tree until

they reach a branching point incommon with the destination

host. They are then passeddown the tree to thedestination.

Tree topology is easily extended.

There is a simple algorithm for routing messages.

A large number messages pass through the root, which maybecome a communications bottleneck.


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RING TOPOLOGY

A Ring network consists of nodes connected to each other toform a closed loop.

Nodes accept data from neighbouring nodes in the form ofpackets.

Data FlowHost

Computers

ACTIVE CABLE


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RING TOPOLOGY OPERATION

The NIUs (or, in some cases, the hosts themselves) act asrepeaters for the packets being forwarded.

This means that the Ring can be expanded to any size

(although more hops will be required to get the packets to theirdestinations).

One big advantage of Ring Topologies is that contention isavoided since each repeater knows if it has to forward an

existing packet or is free to accept a new one.


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RING TOPOLOGY

In a Ring Topology hosts are connected to their neighbours to

form a loop. Messages are passed from

one host to the next untilthey reach the destination

host.

Typically messages pass

the whole way around thering and are checked and

removed by the hosts thatsent them.

Less cabling is required because neighbouring hosts are notusually far apart.

A break in one of the links will stop the network from working(but failures can be quickly detected and fixed).


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BACKBONE NETWORK

A backbone network connects many smaller networks viadevices called bridges.

This type of network is easy to expand and isolates local traffic.


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BACKBONE NETWORK

Rather than connecting hosts directly, a network can be used

to connect other networks. Such a network is called abackbone network.

Most messages are sent betweennearby hosts which are usually

connected to the same local

networks. A message for a host on another

network is sent (via bridges) over the

backbone to the destination network.

Backbone networks are often used by institutions to connectlegacy networks.

Small networks of networks, such as those formed with abackbone network, are often called intranets.

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