Lan Project 1

8/3/2019 Lan Project 1

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INDEX

Introduction to LAN

LAN Protocols and OSI Reference Model

LAN Media-Access Methods

LAN Transmission Methods

LAN Design Goals and Components

Contention issues with Ethernet

Network Design Methodology

The Three Components of a Network

OSI ModelTransmission Media

Logical Network Topologies

Bibliography

Introduction to LAN


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A LAN is a high-speed data network that covers a relatively small geographic area. It isrestricted to a small area such as home, office or college. It typically connectsworkstations, personal computers, printers, servers, and other devices. LANs offercomputer users many advantages, including shared access to devices and applications,

file exchange between connected users, and communication between users viaelectronic mail and other applications.

LAN Protocols and OSI Reference Model


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LAN protocols function at the lowest two layers of the OSI reference model, between thephysical layer and the data link layer. The following figure illustrates how several popularLAN protocols map to the OSI reference model.

Popular LAN Protocols Mapped to the OSI Reference Model

LANMedia-Access

Methods

Media

contention occurs when two or more network devices have data to send at the sametime. Because multiple devices cannot talk on the network simultaneously, some type ofmethod must be used to allow one device access to the network media at a time. This isdone in two main ways: carrier sense multiple access collision detection (CSMA/CD) andtoken passing.

In networks using CSMA/CD technology such as Ethernet, network devices contend forthe network media. When a device has data to send, it first listens to see if any otherdevice is currently using the network. If not, it starts sending its data. After finishing itstransmission, it listens again to see if a collision occurred. A collision occurs when twodevices send data simultaneously. When a collision happens, each device waits arandom length of time before resending its data. In most cases, a collision will not occuragain between the two devices. Because of this type of network contention, the busier anetwork becomes, the more collisions occur. This is why performance of Ethernetdegrades rapidly as the number of devices on a single network increases.

In token-passing networks such as Token Ring and FDDI, a special network packet calleda token is passed around the network from device to device. When a device has data tosend, it must wait until it has the token and then sends its data. When the datatransmission is complete, the token is released so that other devices may use thenetwork media. The main advantage of token-passing networks is that they aredeterministic. In other words, it is easy to calculate the maximum time that will passbefore a device has the opportunity to send data. This explains the popularity of token-passing networks in some real-time environments such as factories, where machinerymust be capable of communicating at a determinable interval.

For CSMA/CD networks, switches segment the network into multiple collision domains.This reduces the number of devices per network segment that must contend for themedia. By creating smaller collision domains, the performance of a network can beincreased significantly without requiring addressing changes.


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Normally CSMA/CD networks are half-duplex, meaning that while a device sendsinformation, it cannot receive at the time. While that device is talking, it is incapable ofalso listening for other traffic. This is much like a walkie-talkie. When one person wantsto talk, he presses the transmit button and begins speaking. While he is talking, no oneelse on the same frequency can talk. When the sending person is finished, he releasesthe transmit button and the frequency is available to others.

When switches are introduced, full-duplex operation is possible. Full-duplex works much

like a telephoneyou can listen as well as talk at the same time. When a network deviceis attached directly to the port of a network switch, the two devices may be capable ofoperating in full-duplex mode. In full-duplex mode, performance can be increased, butnot quite as much as some like to claim. A 100-Mbps Ethernet segment is capable oftransmitting 200 Mbps of data, but only 100 Mbps can travel in one direction at a time.Because most data connections are asymmetric (with more data travelling in onedirection than the other), the gain is not as great as many claim. However, full-duplexoperation does increase the throughput of most applications because the network mediais no longer shared. Two devices on a full-duplex connection can send data as soon as itis ready.

Token-passing networks such as Token Ring can also benefit from network switches. Inlarge networks, the delay between turns to transmit may be significant because thetoken is passed around the network.

LAN Transmission Methods

LAN data transmissions fall into three classifications: unicast, multicast, and broadcast.In each type of transmission, a single packet is sent to one or more nodes.

In a unicast transmission, a single packet is sent from the source to a destination on anetwork. First, the source node addresses the packet by using the address of thedestination node. The package is then sent onto the network, and finally, the networkpasses the packet to its destination.

A multicast transmission consists of a single data packet that is copied and sent to aspecific subset of nodes on the network. First, the source node addresses the packet byusing a multicast address. The packet is then sent into the network, which makes copiesof the packet and sends a copy to each node that is part of the multicast address.

A broadcast transmission consists of a single data packet that is copied and sent to allnodes on the network. In these types of transmissions, the source node addresses the

packet by using the broadcast address. The packet is then sent on to the network, whichmakes copies of the packet and sends a copy to every node on the network


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LAN Design Goals and Components

LAN Design Goals

Designing a network can be a challenging task, andinvolves more than just connecting computers together. Anetwork requires many features in order to be scalable andmanageable. To design reliable, scalable networks,network designers must realize that each of the majorcomponents of a network has distinct design requirements.Even a network that consists of only fifty nodes can pose

complex problems that lead to unpredictable results.Attempting to design and build networks that containthousands of nodes can pose even more complexproblems.

The first step in designing a LAN is to establish anddocument the goals of the design. These goals are

particular to each organization or situation. However, thefollowing requirements tend to show up in most networkdesigns:

Functionality-The network must work. That is, itmust allow users to meet their job requirements. Thenetwork must provide user-to-user and user-to-application

connectivity with reasonable speed and reliability. Scalability-The network must be able to grow.

That is, the initial design should grow without any majorchanges to the overall design.


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Adaptability-The network must be designed withan eye toward future technologies, and it should include noelement that would limit implementation of newtechnologies as they become available.

Manageability-The network should be designed

to facilitate network monitoring and management toensure ongoing stability of operation.

Critical Components of LAN Design

With the emergence of high-speed technologies such asAsynchronous Transfer Mode (ATM) and more complex LANarchitectures that use LAN switching and VLANs over thepast several years, many organizations have beenupgrading existing LANs or planning, designing, andimplementing new LANs. To design LANs for high-speed

technologies and multimedia-based applications, networkdesigners should address the following critical componentsof the overall LAN design:

The function and placement of servers

Collision detection

Segmentation

Bandwidth versus broadcast domains

Contention issues with Ethernet

We should decide carefully on the selection and placement

of networking devices to be used in the LAN in order todecrease the collision detection and media contention on anetwork. Contention refers to excessive collisions onEthernet caused by too many devices, each with a great


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demand for the network segment. The number ofbroadcasts becomes excessive when there are too manyclient packets looking for services, too many serverpackets announcing services, too many routing tableupdates, and too many other broadcasts dependent on the

protocols, such as Address Resolution Protocol (ARP).

An Ethernet node gets access to the wire by contendingwith other Ethernet nodes for the right to do so. When yournetwork grows to include more nodes on the sharedsegment or wire, and these nodes have more and moremessages to transmit, the chance that a node will contend

successfully for its share of the wire gets much worse, andthe network bogs down. The fact that contention mediaaccess does not scale or allow for growth is Ethernet'smain disadvantage.

As traffic increases on the shared media, the rate ofcollisions also increases. Although collisions are normal

events in Ethernet, an excessive number of collisions will(sometimes dramatically) reduce available bandwidth. Inmost cases, the actual available bandwidth is reduced to afraction (about 35% to 40%) of the full 10 Mbps. Thisreduction in bandwidth can be remedied by segmentingthe network by using bridges, switches, or routers.

Network Design Methodology

Gathering and analyzing requirements

For a LAN to be effective and serve the needs of its users,it should be designed and implemented according to aplanned series of systematic steps, which include the

following:

Gathering the users' requirements andexpectations


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Analyzing requirements Designing the Layer 1, 2, and 3 LAN structure (thatis, topology)

Documenting the logical and physical networkimplementation

The first step in designing a network should be to gatherdata about the organizational structure. This informationincludes the organization's history and current status,projected growth, operating policies and managementprocedures, office systems and procedures, and theviewpoints of the people who will be using the LAN. We

need to answer the following questions: Who are thepeople who will be using the network? What is their level ofskill, and what are their attitudes toward computers andcomputer applications? Answering these and similarquestions will help determine how much training will berequired and how many people will be needed to supportthe LAN.

Next, we should determine who in the organization hasauthority over addressing, naming, topology design, andconfiguration. Some companies have a centralManagement Information Systems (MIS) department thatcontrols everything. Some companies have very small MISdepartments and, therefore, must delegate authority to

departments. Focus on identifying the resources andconstraints of the organization. Organization resources thatcan affect the implementation of a new LAN system fallinto two general categories: computer hardware/softwareand human resources. An organization's existing computerhardware and software must be documented, andprojected hardware and software needs identified. How

these resources are currently linked and shared? Whatfinancial resources does the organization have available?Documenting these types of things helps you estimatecosts and develop a budget for the LAN. You should make


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sure you understand performance issues of any existingnetwork.

Factors that affect network availability

Availability measures the usefulness of the network. Manythings affect availability, including the following:

Throughput

Response time

Access to resources

Every customer has a different definition of availability. For

example, there may be a need to transport voice and videoover the network. However, these services require morebandwidth than is available on the network or backbone.

You can increase availability by adding more resources,but resources drive up cost. Network design seeks toprovide the greatest availability for the least cost.

After considering availability, the next step in designing anetwork is to analyze the requirements of the network andits users that were gathered in the last step. Network userneeds constantly change. For example, as more voice- andvideo-based network applications become available, thepressure to increase network bandwidth will becomeintense.

Another component of the analysis phase is assessing theuser requirements. A LAN that is incapable of supplyingprompt and accurate information to its users is of little use.

Therefore, we must take steps to ensure that theinformation requirements of the organization and itsworkers are met.

The Three Components of a Network


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In order to have full access to a network (local or wide)from our workstation, three components are required.

The first component is hardware.

Our workstation must have an Ethernet card or token ringboard installed and a cable running from this card to thedata jack in your office.

The data jack must be wired from your office through thebuilding to the campus broadband. Once this hardwarewiring connection is made, you have the infrastructure inplace to access the network.

The second component is network softwarethat recognizes the hardware and will use it.Different software is required depending on thenetwork access we want.

For a Local Area Network (LAN), we will need networkoperating system software (i.e., Novell or Windows NT). If

we want to access the Wide Area Network and the LocalArea Network, we will need both kinds of software.

The third component is application softwarerunning on the Local Area Network. Examples ofthese would be any network version of wordprocessors (i.e., Microsoft Word, WordPerfect),databases (Paradox, Dbase), spreadsheets (Lotus,Excel), etc. These packages are designed toprovide multiple access to files and records and tolock files and records so that a particular documentcan be edited by only one person at a time.


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OSI Model

[A Layered Approach to Networking]

Transmission Media

Data is transmitted over copper wires, fiber optic cable,radio and microwaves. The term 'media' is used togenerically refer to the physical connectors, wires ordevices used to plug things together.


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Basic Communications Media Types

Coppero Unshielded Twisted Pair (3,5,5e,6,7)

o Shielded Twisted Pair

o Coaxial Cable (Thinnet, Thicknet)o Heliax

Fiber Optico Single-mode

o Multi-mode

Infrared

Radio & Microwave

COPPER

Coaxial Cabling

Coaxial cabling is used in bus-style Ethernet networks.Coaxial cable consists of a copper wire core surrounded by

a plastic cladding sheathed in a wire mesh. Coaxial cablecomes in two sizes which are called thinnetand thicknet.

Unshielded Twisted Pair (UTP)

If we use two pairs of wires to enable two communicationscircuits, one for transmit, and one for receive and we twistthe wires of each pair, we can place them much closer

together. There are several grades of coaxial cable withcategory ratings. There are Category 3 (


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fiberglass fibers wrapped in a plastic cladding. Single modetypically has much longer reach, but a larger bend radiusthan multi-mode.

Dispersion Shifted

Non-Dispersion Shifted Non-Zero Dispersion-Shifted

Multi-Mode

Multi-mode fiber can carry multiple wavelengths, is madeof special clear plastic materials and has a much smallerbend radius than single mode fiber. Multi-mode does not

have as long a reach as single mode fiber.

Step Index Graded-Index

INFRARED

There are many systems today using infra-red

communications. This is usually a directional infrared lightsignal transmitted into the air and received by nearbydevices. Such systems came into use in the early 90's foruse with laptops, printers and later in the 90's withcameras and handhelds.

Duplex vs. Simplex

SIMPLEX

Simplex communication is permanent unidirectionalcommunication. Some of the very first serial connectionsbetween computers were simplex connections. Forexample, mainframes sent data to a printer and neverchecked to see if the printer was available or if the

document printed properly since that was a human job.Simplex links are built so that the transmitter (the onetalking) sends a signal and it's up to the receiving device(the listener) to figure out what was sent and to correctly


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do what it was told. No traffic is possible in the otherdirection across the same connection.

We must useconnectionless protocols with simplex circuitsas no acknowledgement or return traffic is possible over a

simplex circuit. Satellite communication is also simplexcommunication. A radio signal is transmitted and it is up tothe receiver to correctly determine what message hasbeen sent and whether it arrived intact. Since televisionsdon't talk back to the satellites (yet), simplexcommunication works great in broadcast media such asradio, television and public announcement systems.

HALF DUPLEX

A half duplex link can communicate in only one direction,at a time. Two way communication is possible, but notsimultaneously. Walkie-talkies and CB radios sort of mimicthis behavior in that you cannot hear the other person ifyou are talking. Half-duplex connections are more common

over electrical links. Since electricity won't flow unless youhave a complete loop of wire, you need two pieces of wirebetween the two systems to form the loop. The first wire isused to transmit, the second wire is referred to as acommon ground. Thus, the flow of electricity can bereversed over the transmitting wire, thereby reversing thepath of communication. Electricity cannot flow in both

directions simultaneously, so the link is half-duplex.FULL DUPLEX

Full duplex communication is two-way communicationachieved over a physical link that has the ability tocommunicate in both directions simultaneously. With mostelectrical, fiber optic, two-way radio and satellite links, this

is usually achieved with more than one physicalconnection. Your telephone line contains two wires, one fortransmit, the other for receive. This means you and yourfriend can both talk and listen at the same time.


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Half or Full-Duplex is required for connection-orientedprotocols such as TCP. A duplex circuit can be created byusing two separate physical connections running in halfduplex mode or simplex mode. Two way satellitecommunication is achieved using two simplex connections.

Reliable vs. Unreliable

The terms reliable and unreliable don't refer to whether itworks or not. It refers to whether something is done toguarantee or not.

RELIABLE

End stations running reliable protocols will work together toverify the transmission of data to ensure accuracy andintegrity of the data. A reliable system will set up a

connection and verify that: all data transmitted iscontrolled in an orderly fashion, is received in the correctorder and is intact. Reliable protocols work best overphysical medium that loses data, and is prone to errors.

The error correction, ordering and verification mechanismsrequire overhead in the data packets and increase the totalamount of bandwidth required to transmit data.

Transmission Control Protocol (TCP) is a typical reliableprotocol.TCP often usually adds an average of 42-63 bytesof overhead to datagrams. For a Telnet connection whichtransmits each keystroke individually, this is horriblyinefficient because up to 64 bytes of data are transmittedto communicate just 1 byte of useful information.

UNRELIABLE

Unreliable protocols make no effort to set up a connection,they don't check to see if the data was received andusually don't make any provisions for recovering from
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errors or lost data. Unreliable protocols work best overphysical medium with low loss and low error rates. UserDatagram Protocol (UDP) is an example of an unreliableprotocol. UDP makes no provisions for verifying whetherdata arrived or is intact. However, UDP adds a minimum of

overhead when compared toTCP and is thus much fasterfor data transfers over high quality physical links that arehigh speed and exhibit little or no errors in communication.

Serial vs. Parallel

The two most basic types of communication are serial andparallel. They are so common that even the cabling bearsthe name serial cable and parallel cable. Since electricitybehaves according to the laws of physics, it is impossibleto get the electrical signal to go any faster. There are twoways to get the data from one place to the other faster.

The first is to squish the data bits tighter together (leave

less distance between them when they travel down thewire). The second way is to transmit more bitssimultaneously.

Keep in mind that the information below is very generaland not exactly correct from an engineering standpoint.We're just focusing on getting you to understand concepts

here.

SERIAL

When information is sent across one wire, one data bit at atime, its called serial. Every computer on the face of theearth has some form of serial communications connectoron it, whether internally or externally. Most people arefamiliar with the 'D' shaped 9-pin connector on the back oftheir computer. This is a serial connector. The typical 9-pin'D' shaped connector on the back of your computer uses 2
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loops of wire (1 in each direction) for data communication,plus additional wires to control the flow of information.However, in any given direction, data is still flowing over asingle wire.

PARALLELInstead of squishing bits together, bits are sent over morewires simultaneously. In the case of a 25-pin parallel port,you have eight data-carrying wires so that eight bits can besent simultaneously. Because there are 8 wires to carry thedata, the data finishes being transferred eight times fasterthan a serial connection.

Logical Network Topologies

Peer-to-Peer

A peer-to-peernetwork is composed of two or more self-

sufficient computers. Each computer handles all functions,logging in, storage, providing a user interface etc. Thecomputers on a peer-to-peer network can communicate,but do not need the resources or services available fromthe other computers on the network. Peer-to-peer is theopposite of the client-server logical network model.

A Microsoft Windows Workgroup is one example of a peer-

to-peer network. UNIX servers running as stand-alonesystems are also a peer-to-peer network. Logins, servicesand files are local to the computer. You can only accessresources on other peer computers if we have logins on thepeer computers.

Client - Server

The simplest client-server networkis composed of a serverand one or more clients. The server provides a service thatthe client computer needs. Clients connect to the serveracross the network in order to access the service. A server


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can be a piece ofsoftware running on a computer, or it canbe the computer itself.

One of the simplest examples of client-server is a FileTransfer Protocol (FTP) session. File Transfer Protocol (FTP)

is a protocol and service that allows your computer to getor put files to a second computer using a networkconnection. A computer running FTP software opens asession to an FTP server to download or upload a file. TheFTP server is providing file storage services over thenetwork. Because it is providing file storage services, it issaid to be a 'file server'. A client software application isrequired to access the FTP service running on the fileserver.

Most computer networks today control logins on allmachines from a centralized logon server. When we sitdown to a computer and type in your username andpassword, your username and password are sent by thecomputer to the logon server. UNIX servers use NIS, NIS+

or LDAP to provide these login services. Microsoft Windowscomputers use Active Directory and Windows Logon and/oran LDAP client.

Users on a client-server network will usually only need onelogin to access resources on the network.
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Lan Project 1