CT101_IntroductionToNetworking.ppt

7/27/2019 CT101_IntroductionToNetworking.ppt

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Introduction to Networking

CT101Computing Systems


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Contents

Network topologies Bus, star and ring

Cabling

Coaxial, UTP, Fiber Optic Cable

Media Access Methods CSMA/CD, CSMA/CA, Token Passing

LAN Architectures

Ethernet, Token Ring

Networking Devices

OSI Model. OSI layer functions. OSI versus TCP/IP


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What is a network

A networkas a "group of computers and associated devices that areconnected by communications facilities."

A network provides two principle benefits: the ability to communicateand the ability to share. A network supports communicationamong users in ways that other media

cannot. E-mail, the most popular form of network communication, provides low-

cost, printable correspondence with the capability for forwarding,acknowledgment, storage, retrieval, and attachments.

Sharinginvolves not only information (database records, e-mail, graphics, etc.),but also resources (applications, printers, modems, disk space, scanners, etc.)Through its ability to share, a network promotes collaboration

A network can consist of two computers connected together on a desk

or it can consist of many Local Area Networks (LANs) connectedtogether to form a Wide Area Network (WAN) across a continent.


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Network Classifications

Scope

Local area network (LAN)

Metropolitan area (MAN)

Wide area network (WAN)

OwnershipClosed versus open

Topology (configuration)

Bus (Ethernet)

Star (Wireless networks with central Access Point)Ring


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Network Topologies

A topologyrefers to the manner in which the cable is run toindividual workstations on the network.

the configurations formed by the connections between devices on alocal area network (LAN) or between two or more LANs

There are three basic network topologies (not countingvariations thereon): the bus, the star, and the ring.

It is important to make a distinction between a topology andan architecture.

A topologyis concerned with the physical arrangement of thenetwork components.

In contrast, an architectureaddresses the components themselvesand how a system is structured (cable access methods, lower level

protocols, topology, etc.). An example of an architecture is

10baseT Ethernet which typically uses the star topology.


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Bus Topology

A bus topologyconnects each computer (node) to a single

segment trunk. A trunk is a communication line, typically coax cable, that is

referred to as the bus. The signal travels from one end of thebus to the other.

A terminatoris required at each end to absorb the signal so itdoes not reflect back across the bus.

In a bus topology, signals are broadcast to all stations. Eachcomputer checks the address on the signal (data frame) as itpasses along the bus. If the signals address matches that of thecomputer, the computer processes the signal. If the addressdoesnt match, the computer takes no action and the signaltravels on down the bus.

Only one computer can talk on a network at a time. A mediaaccess method (protocol) called CSMA/CDis used to handlethe collisions that occur when two signals are placed on thewire at the same time.

The bus topology is passive. In other words, the computers onthe bus simply listen for a signal; they are not responsible formoving the signal along.

A bus topology is normally implemented with coaxial cable.


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Bus Topology

Advantagesof bus topology: Easy to implement and extend

Well suited for temporary networks that must be set up in a hurry

Typically the cheapest topology to implement

Failure of one station does not affect others

Disadvantagesof bus topology:

Difficult to administer/troubleshoot

Limited cable length and number of stations A cable break can disable the entire network; no redundancy

Maintenance costs may be higher in the long run

Performance degrades as additional computers are added


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Star Topology

All of the stations in a star topologyare connected to a central unit called a hub. The hub offers a common connection for all stations on the network. Each station has its

own direct cable connection to the hub. In most cases, this means more cable is requiredthan for a bus topology. However, this makes adding or moving computers a relativelyeasy task; simply plug them into a cable outlet on the wall.

If a cable is cut, it only affects the computer that was attached to it. This eliminatesthe single point of failure problem associated with the bus topology. (Unless, ofcourse, the hub itself goes down.)

Star topologies are normally implemented using twisted pair cable, specificallyunshielded twisted pair (UTP). The star topology is probably the most common formof network topology currently in use.


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Star Topology

Advantagesof star topology:Easy to add new stations

Easy to monitor and troubleshoot

Can accommodate different wiring Disadvantagesof star topology:

Failure of hub cripples attached stations

More cable required (more expensive to wire a buildingfor networking)


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Ring Topology

A r ing topologyconsists of a set of stations connected serially by cable. In other words, its a circle orring of computers. There are no terminated ends to the cable; the signal travels around the circle in aclockwise (or anticlockwise) direction.

Note that while this topology functions logically as ring, it is physically wired as a star. The centralconnector is not called a hub but aMul tistation Access UnitorMAU. (Dont confuse a Token Ring

MAU with a Media Adapter Unit which is actually a transceiver.) Under the ring concept, a signal is transferred sequentially via a "token" from one station to the next.

When a station wants to transmit, it "grabs" the token, attaches data and an address to it, and then sends itaround the ring. The token travels along the ring until it reaches the destination address. The receivingcomputer acknowledges receipt with a return message to the sender. The sender then releases the tokenfor use by another computer.

Each station on the ring has equal access but only one station can talk at a time.


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Ring Topology

In contrast to the passive topology of the bus, the ring employs anactivetopology. Each station repeats or boosts the signal before

passing it on to the next station.

Rings are normally implemented using twisted pair or fiber-opticcable

Advantagesof ring topology: Growth of system has minimal impact on performance

All stations have equal access

Disadvantagesof ring topology:

Most expensive topology

Failure of one computer may impact others

Complex


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Choosing a Topology The following factors should be considered when choosing a topology:

Installation Maintenance and troubleshooting

Expected growth

Distances

Infrastructure

Existing network

As a general rule, a bus topology is the cheapest to install, but may be moreexpensive to maintain because it does not provide for redundancy.

Various topologies can be mixed on the same network.

One very common example is a large Ethernet network with multiple hubs. Usuallythe hubs are located on different floors in a building or perhaps outside in another

building. Each hub is wired in the typical star configuration. However, the hubs are

connected together along a bus, typically referred to as a backbone. The backbone between hubs might consist of fiber optic cable while the

workstations are wired to each individual hub with UTP (unshielded twisted pair)cable.


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Cabling

Coaxial Cable

Thinnetlooks like regular TV cable. It is about 1/4 inch in diameter and is veryflexible and easy to work with.

Thicknetis about 1/2 inch in diameter and not very flexible. Thicknet is olderand not very common anymore except as a backbone within and between

buildings. Coax transmits at 10 Mbps..

Twisted Pair . Twisted pair looks like telephone wire and consists ofinsulated strands of copper wire twisted together. There are twoversions of twisted pair cable:

Shielded Twisted Pair (STP). STPis commonly used in Token Ring networks

Unshielded Twisted Pair (UTP). UTPis used in Ethernetnetworks. Transmission rates vary between 10-100-1000-10000 Mbps.

F iber-Optic Cable. Fiber-optic cable consists of a thin cylinder ofglass surrounded by glass cladding, encased in protective outersheath. Fiber-optic cable is very fast (over 1Gbps). It can transmit

over long distances (2 km +) but is expensive.


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Cabling

Top: Unshielded Twisted Pair and Shielded Twisted Pair Cable

Bottom: Coaxial and Optical Fiber Cable


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Media Access Methods

A media access methodrefers to the manner in which a computergains and controls access to the networks physical medium (e.g.,cable).

Common media access methods include the following:

CSMA/CD

CSMA/CA Token Passing

One of the primary concerns with media access is how to preventpackets from colliding when two or more computers try to transmitsimultaneously. Each of the methods listed above takes a different

approach to this problem. Data transmitted over a network is sent one bit at a time. A bit is

either a 1 or a 0 represented by a voltage change (on or off) or a lightpulse. If two stations are transmitting at the same time, it is possiblethat the signals may overlap, resulting in garbled data. Suchoverlapping is referred to as a "collision."


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CSMA/CD

CSMA/CDstands forCarr ier-Sense Mul tiple Access with Coll ision Detection. It is

a media access method which means it defines how the network places data on thecable and how it takes it off.

CSMA/CD specifies how bus topologies such as Ethernet handle transmissioncollisions. A collisionoccurs when two or more computers transmit signals at thesame time. "Carrier Sense"means that each station on the LAN continually listens to (tests) the

cable for the presence of a signal prior to transmitting. " Mul tiple Access"means that there are many computers attempting to transmit and

compete for the opportunity to send data (i.e., they are in contention).

" Coll ision Detection"means that when a collision is detected, the stations will stoptransmitting and wait a random length of time before retransmitting.

CSMA/CD works best in an environment where relatively fewer, longer data framesare transmitted. This is in contrast to token passing which works best with arelatively large amount of short data frames.

Because CSMA/CD works to controlor manage collisions rather than preventthem,network performance can be degraded with heavy traffic. The greater the traffic, thegreater the number of collisions and retransmissions.

CSMA/CD is used on Ethernet networks.


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CSMA/CD Operation

In its most simple form it operates as follows:A station that wishes to transmit on the network checks

to see if the cable is free.

If the cable is free, the station starts transmitting.

However, another station may have detected a free cableat the same instant and also start transmitting. The resultis a "collision."

Once the collision is detected, all stations immediately

stop transmitting.

Stations then wait a random length of time beforechecking the cable and then retransmit


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CSMA/CA

CSMA/CA stands forCarr ier-Sense Mul tiple Access with Coll isionAvoidanceand is a media access method very similar to CSMA/CD.

The difference is that the CD (collision detection) is changed to CA(collision avoidance). Instead of detecting and reacting to collisions,CSMA/CA tries to avoid them by having each computer signal its

intention to transmit before actually transmitting. In effect, thetransmitting computer gives a 'heads up' prior to transmitting.

Although CSMA/CA can prevent collisions, it comes with a cost inthe form of the additional overhead incurred by having eachworkstation broadcast it's intention prior to transmitting. Thus,

CSMA/CA is slower than CSMA/CD. CSMA/CA is used on Apple networks and on WiFi (IEEE 802.11)

networks.


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The hidden terminal problem


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Token Passing

Token passingis a media access method by which collisions areprevented.

Collisions are eliminated under token passing because only acomputer that possesses a f ree token(a small data frame) is allowedto transmit. The token passing method also allows different priorities

to be assigned to different stations on the ring. Transmissions from astations with higher priority take precedence over stations with lowerpriority.

Token passing works best in an environment where a relatively largenumber of shorter data frames are being transmitted. (As opposed to

CSMA/CD which works best in an environment where relativelyfewer, longer data frames are being transmitted.)

Token passing is used on Token Ring networks


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Token Passing Operation

In its most simple form it operates as follows: A station that wishes to transmit on the network waits until it will

receive a free token.

The sending station transmits its data with the token.

The data travels to the recipient without stopping at other stations(it is just relayed).

The receiving station receives the data and returns the token to the

sender as an acknowledgment.

The sender receives acknowledgment and releases the token tonext station.

The token continues being passed along the ring until it is seized"

by the next station that wants to transmit.


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LAN Architectures

Network archi tecturerefers to the manner in whichthe hardware and software is structured. Thearchitecture includes the cable access method(transmission), topology, and lower level protocols.

The most common types ofLAN archi tecturesareEthernet and Token Ring

These architectures are sometimes referred to as

" lower-level protocols"because they represent thespecifications for the IEEE802 model whichencompasses the physical (1st) and data link (2nd)layers of the OSI model (to be discussed latter)


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Ethernet

Ethernetis a popular, relatively inexpensive, easy-to-install LAN architecture with

the following characteristics: Uses the CSMA/CDmedia access control.

Data transmission normally occurs at 100 Mbps (10Mbps in the early forms and 10Gbpsin the most recent forms).

Typically implemented in a star topology (early versions used bus topology as well).

Ethernet LANs are normally distinguished by the type of cable they use Twisted Pair

(Thinnet or Thicknet were also used in the past). The Ethernet architecture conforms to most but not all of the I EEE 802.3

specification (the physical layers are identical but the MAC layers are somewhatdifferent).

An Ethernet LAN is often described in terms of three parameters: transmission rate,transmission type, and segment distance or cable type.

" 100baseT"means: 100 - transmission rateor through put of 100Mbps base - transmission typeis baseband rather than broadband network (i.e., the signal is placed

directly on the cable, one signal at a time)

Tthe cable type (e.g. Twisted pair)

Few types of Ethernet: 10Base2, 10Base5, 10BaseT and 10BaseF, 100BaseT,100BaseF, etc..


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Token Ring

Token ringis a relatively expensive LAN architecture that is strongly

influenced by IBM. It is very stable and can be expanded without asignificant degradation in network performance.

Token ring uses the token passingmedia access control. Datatransmission normally occurs at 4 or 16 Mbps depending on the cable.

Token ring is normally implemented in a logical ring/physical star

topology with a MAU (Multistation Access Unit) as the hub. Themaximum number of stations on one ring is 260 for shielded twisted

pair and 72 for unshielded twisted pair (UTP). There can be up to 33MAUs per ring.

Token Ring LANs normally use shielded twisted pair (STP) but may

also use unshielded twisted pair (UTP) or fiber-optic cable. Themaximum distance to the MAU from the workstation depends on thecable and varies from 45 meters for UTP to 100 meters for STP.

The Token Ring architecture conforms generally to theIEEEs 802.5specification


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Ethernet vs. Token Ring

Ethernetis generally less expensive and easier to installthan Token Ring.

Token Ringis generally more secure and more stable thanEthernet, but not used anymore in typical LANconfigurations.

It is usually more difficult to add more computers on aToken RingLAN than it is to an EthernetLAN. However,as additional computers are added, performance degradationwill be less pronounced on the Token RingLAN than it will

be on the EthernetLAN.

Ethernetuses CSMA/CD media access control and TokenRinguses token passing. This makes Ethernetbetter suitedin a situation where there are a large number of computerssending fewer, larger data frames. Token Ringis better

suited for small to medium size LANs sending many,


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Connecting Networks

Repeater: Extends a network Bridge:Connects two compatible networks, doesnt

necessarily pass all the messages across the connection

Switch: Connect several compatible networks, allowing it

to connect several busses rather than two.


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Building a large bus network from

smaller ones


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Connecting Networks

When networks are connected via repeaters, bridges or switches, the

result is a single large network.

The entire system operates in the same way as the original smaller

networks

Sometimes the networks to be connected have incompatible

characteristics (e.g. WiFi network to be connected with Ethernet

network, etc..).

When building networks of networks, the system is known as internet

(note the small i, term that is distinct from the Internetwhich

refers to a particular world wide internet).

Router: Connects two incompatible networks resulting in a network

of networks - internet

i i i k


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Routers connecting two WiFi networks

and an Ethernet network to form an

internet


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Routers and internet addressing

Routers purpose is to route(forward messages) in their proper

directions.

The forwarding process is based on an internet wide addressing

system which all the machines in the internet (including the machines

in the original networks as well as the routers) are assigned unique

addresses.

Thus each machine in an internet has two addresses: its original local address

within its own network and the internet address

A machine wanting to send a message to a machine in a distant

network, it will attach the internet address of the destination and willdirect the message to its local router. From there it is forwarded to the

proper direction (based on a forwarding table maintainedby the

router).


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Client and Server

The terms "client" and "server" are used to describe individual

computers that are part of a network where computing resources andworkload are shared.

A serveris a computer that makes its resources available to thenetwork and responds to the commands of a client. The serversshared resources can be files (a file server); printers (a print server);

processing power (an application server); etc A clientis a computer that uses the resources made available by a

server. The client must have sufficient processing power on its own torun applications that interact with the resources on the server.

It is possible, and quite common, for an individual computer to

function as both a client and a server. For example, if Bill queries a SQL Server database from his workstation for the

data he needs to create an Excel spreadsheet, then his workstation is functioningas a client. However, if Sue then connects to Bills workstation from hercomputer and copies the spreadsheet, then Bills workstation is functioning as aserver.


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ISO/OSI Model

The International Standards Organization (ISO) Open Systems Interconnect (OSI) is

a standard set of rules describing the transfer of data between each layer in anetwork operating system. Each layer has a specific function (i.e. the physical layerdeals with the electrical and cable specifications)

The OSI Model clearly defines the interfaces between each layer. This allowsdifferent network operating systems and protocols to work together by having eachmanufacturer adhere to the standard interfaces. The application of the ISO OSI

model has allowed the modern networks that exist today. There are seven layers inthe OSI model.


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OSI Model

The Physical Layer

Establishes the physical characteristics of the network (e.g., the type of cable,connectors, length of cable, etc.)

Defines the electrical characteristics of the signals used to transmit the data (e.g.signal voltage swing, duration of voltages, etc.)

Transmits the binary data (bits) as electrical or optical signals depending on themedium.

The Data L ink Layer Defines how the signal will be placed on or taken off the NIC. The data frames

are broken down into individual bits that can be translated into electric signalsand sent over the network. On the receiving side, the bits are reassembled intoframes for processing by upper levels.

Error detection and correction is also performed at the data link layer. If anacknowledgement is expected and not received, the frame will be resent.Corrupt data is also identified at the data link layer.

Because the Data-Link Layer is very complex, it is sometimes divided intosublayers (as defined by the IEEE 802 model). The lower sublayer providesnetwork access. The upper sublayer is concerned with sending and receiving

packets and error checking.


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OSI Model

The Network Layer

Primarily concerned with addressing and routing. Logical addresses (e.g., an IPaddress) are translated into physical addresses (i.e., the MAC address) fortransmission at the network layer. On the receiving side, the translation processis reversed.

It is at the network layer where the route from the source to destination

computer is determined. Routes are determined based on packet addresses andnetwork conditions. Traffic control measures are also implemented at thenetwork layer.

The Transport Layer

On the sending side, messages are packaged for efficient transmission and

assigned a tracking number so they can be reassembled in proper order. On thereceiving side, the packets are reassembled, checked for errors andacknowledged.

Performs error handling in that it ensures all data is received in the propersequence and without errors. If there are errors, the data is retransmitted.


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OSI Model

The Session Layer

Is responsible for establishing, maintaining, and terminating a connection called a'session'.

A session is an exchange of messages between computers (a dialog). Managing thesession involves synchronization of user tasks and dialog control (e.g., who transmits andfor how long). Synchronization involves the use of checkpoints in the data stream. Inthe event of a failure, only the data from the last checkpoint has to be resent.

Logon, name recognition and security functions take place at the Session Layer.

The Presentation Layer It is responsible for data translation (formatting), compression, and encryption.

The Presentation Layer is primarily concerned with translation; interpreting andconverting the data from various formats. For example, EBCIDIC characters might beconverted into ASCII. It is also where data is compressed for transmission anduncompressed on receipt. Encryption techniques are implemented at the Presentation

Layer. The redirector operates at the presentation layer by redirecting I/O operations across the

network.

The Application L ayer Provides the operating system with direct access to network services.

It serves as the interface between the user and the network by providing services that

directly support user applications.


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OSI Model


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OSI Model

Each layer may add a Header and a Trailer to its Data (which consists of the nexthigher layer's Header, Trailer and Data as it moves through the layers). The Headerscontain information that specifically addresses layer-to-layer communication. Forexample, the Transport Header (TH) contains information that only the Transportlayer sees. All other layers below the Transport layer pass the Transport Header as

part of their Data.


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OSI vs. TCP/IP


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References

Andrew S. TanenbaumComputer Networks,ISBN: 0-13066102-3

J Glenn Brookshear Computer Science An

Overview, ISBN: 0-321-54428-5 Eugene Blanchard Introduction to Networking and

Data Communications

Documents

CT101_IntroductionToNetworking.ppt