Telecommunications Management 273-436/635 Lecture 5: Communication Networks - Part 1: Local Area Networks.

Telecommunications Management273-436/635

Lecture 5:

Communication Networks - Part 1: Local Area Networks

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Local Area Networks

Combination of hardware and software technologies that allow the interconnecting of computing and communications resources to support information and resource sharing.

LANs are usually confined to a single site or a small cluster of buildings.

LANs in one organization or site can be connected to LANs in other organizations using wide area network (WAN) services.

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Reasons for the popularity of LANs

Hardware sharing

Software sharing

Service enhancement

Local control

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Types of Local Area Networks

Personal Computer Local NetworksBackend NetworksHigh speed office networksBackbone local networks

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Types of Local Area Networks

Personal Computer Local NetworksDesigned to support personal computersAre usually implemented within individual department

or business unit. Connect user equipment to each other and to printers,

local servers which handle such applications as file sharing , faxing, and printing.

Are relatively low cost and support departmental applications such as word processing, spreadsheets, project management tools and Internet access.

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Types of Local Area Networks

Backend Networks Interconnecting of mainframes, supercomputers, and mass

storage devices can be done in a LAN configuration. Support high speed data transfer between devices in a

small area. Requires a very high level of reliability. Generally support distributed access among a limited

number of devices. Are usually found in large computer centres typically

associated with large companies or research centres.

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Types of Local Area Networks

High speed office networks New applications such as graphical and video image

processing have increased the need for high speed networks within the office environment.

Typical LAN operating at speeds of 10 Mbps while more than adequate for data processing, it does not have sufficient capacity to handle the huge demands associated with image processing.

A combination of technological advances in storage devices and LAN technology has led to the development of high speed LANs within the office.

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Types of Local Area Networks

Backbone local networks Used to connect local networks within single premises or cluster

of buildings. For example, while it is possible to connect all devices in a 6

floor building to a single LAN it might not be prudent to do so. The proliferation of devices on the single network will reduce its

capacity and reliability, and increase the cost of support. A more flexible, reliable and cost effective approach is to set up

smaller networks, say one for each floor, and then interconnect these with a backbone LAN.

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LAN configuration

Data processing equipment that is normally connected to LANs can be grouped into three categories.

Personal computers and workstationsServersMainframes and supercomputers

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LAN configuration

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LAN Technology

The cost, capacity, type of data transmitted, speed and efficiency of transmission, and the kinds of applications supported are all determined by the way various LAN technologies are combined.

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LAN Technology

The key elements of LANs are:Topology: bus and tree, ring, or starTransmission medium: twisted pair, coaxial cable,

or optical fibreLayout: linear or starMedium access control: CSMA/CD or token

passing

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LAN topology - Bus

Devices attach to the network in a linear fashion They are connected in sequence directly to the transmission

medium using the appropriate network interfacing device Typically this is a network interface card with the appropriate

connector. These are usually called tapstaps Data is transmitted simultaneously in both directions. The data is sent the whole length of the medium and can be

received by all the attached stations A terminator is attached at each end of the bus to remove all the

signals.

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LAN topology - Ring

With ring topology the network represents a closed loopConsists of a series of repeaters joined together by

point-to-point linksRepeaters are simple devices that receive and pass on

data from one link to another as fast as it is receivedThere is no buffering or reprocessing of the data at the

repeatersThe links go in one direction only, thus allowing data to

circulate around the ring (clockwise or counter-clockwise).

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LAN topology - Ring

Each station or device attach to the network at the repeater

Data is transmitted frames as with bus or tree networksAs a frame passes the destination station a copy of it is

absorb into the local bufferThe frame continues to circulate until it returns to the

originating stationSince there are many stations on the ring, media access

control is needed to determine at what time each station may insert a frame.

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Bus topology - issues

Two problems surface in bus arrangements.How does the network determine which station the

message is intended for.How can the network regulate the flow of data along the

transmission medium. Problems are solved by transmitting data in framesframes. Each frame contains a unique identifier and destination address

included in the header When the frame passes the intended address a copy is absorbed by the

device. To regulate the flow of data, each device takes turns in sending the

frames.

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LAN topology - Tree

A tree arrangement is similar to a bus arrangement except that the network in this configuration begin at a certain point called the headendheadend and then branches in bus formation.

Transmission sent from one station can be received by all stations on the bus.

Networks arranged in tree formation can have many branches.

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LAN topology - Bus

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LAN topology - Tree

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LAN topology - Ring

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LAN topology - Star

With star arrangement each station is connected directly to a common central node

Each station attaches to the central node, generally referred to as a star couplerstar coupler, via two point-to-point links, one for transmission in each direction

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LAN topology - Star

Data transmission in star networks can occur in two ways: broadcast or frame switching.

With broadcast data sent from one station is received by all

Only one station at a time can send framesThe star coupler can also act as a frame switching

device The incoming frame is retransmitted to the

destination station

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LAN topology - Star

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Choosing a topology

Choice of LAN will depend on a myriad of factors associated with the type of application, the requirements for reliability, flexibility, expandability, and performance.

It also depends on the physical set up transmission medium wiring layout, among other issues.

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Choosing a topology

In general, bus and tree topologies offer greater flexibility. can handle a wide range of devices and is easy to layout are best when data rates requirements are moderate and

confined to smaller area.

Ring topology is good for high speed transmission over long distances. However, if one link or repeater fails the whole network goes down.

Star network is good for connecting devices at small areas. If a device goes down it is simply unplugged.

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Transmission media

Most commonly used transmission media on LANS are twisted pair coaxial cable optical fiber.

Both shielded and unshielded twisted pair support digital digital signaling.

Fiber uses analoganalog signaling. Coaxial cable supports either digitaldigital and analog analog signaling. The two forms of signaling are referred to as basebandbaseband and

broadband.broadband.

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Baseband bus

A baseband bus uses digital signaling to transmit binary data The data is inserted directly into the medium as a series of voltage

pulses using the appropriate coding technique. Digital signals use the entire spectrum so there is no potential for

having multiple channels Data is propagated along the medium in both directions at the same

time Digital signaling therefore requires bus topology The distance over which data can travel before suffering from

attenuation is limited Repeaters are used to increase the distance over which digital signals

can be sent

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Broadband Bus

Broadband, in the context of LANs, refers to analog signaling

Analog signaling allows for multiple channels to be created using techniques such as frequency division multiplexing.

Separate channels can support separate and independent data traffic.

With broadband transmission signals can be sent over much greater distances than with baseband.

Broadband signaling is unidirectional Two separate data paths must be used for “inbound” and

“outbound” transmission

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Choice of transmission medium

The choice of medium depends on the topology chosen and other factors related to capacity, reliability, types of data supported, and environmental scope.

Generally, unshielded and shielded twisted pair are well understood and can be found pre-wired in most buildings.

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Choice of transmission medium

Unshielded twisted pair is most cost effective for a single building, low-traffic LAN.

Shielded twisted pair and baseband coaxial cable are more expensive than UTP but have greater capacity.

Broadband coaxial cable is even more expensive but also provide greater capacity.

Fiber is the most expensive medium and it also provides the highest capacity. It’s use in LANs is still not widespread.

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Choice of transmission medium

Commonly used network cable standardsName Type Data

rate(Mbps)

Distance(meters)

Often used by Cost(US$/foot)

Category 1 UTP 1 90 Modem .10Category 2 UTP 4 90 Token Ring-4 .05Category 3 UTP/STP 10 100 10 Base T

Ethernet.13

Category 4 UTP/STP 16 10 Token ring-16 .18Category 5 UTP/STP 100 200 100 Base T

Ethernet.25

RG-58 Coax -thin

10 185 10 Base2Ethernet

.30

RG-8 Coax -thick

10 500 10 Base5Ethernet

.85

X3T9.5 Fiber 100 2000 FDDI 1.00

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Structured cabling

To allow for the pre-planning of cable networks in commercial properties

Two structured cabling standards have been issued.

EIA/TIA-568 ISO 11801

These allow buildings to be generically wired in anticipation of future networking needs.

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Structured cabling

The cabling strategy is based on a hierarchical, star-wired layout. It consists of

External cables from the local telephone company or WANs to the equipment room.

Backbone cable linking each telecommunications closets on each floor or section.

Horizontal cables form the telecommunications closet (wiring closets) to the wall outlets

Horizontal cables should not exceed 90 meters regardless of the media type.

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Structured cabling

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LAN Standards

LAN technology is very complex and difficult to implement.

The cost of connecting LANs must be much less expensive than the devices that are attached to them.

If this is not the case then most people would not invest in the LANs.

Standards help make investment in developing the technology worthwhile

Widely accepted LAN standards will ensure that a large enough market will develop.

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LAN Standards

To develop standards for local area networks a committee was set up by the Institute of Electrical and Electronic Engineers (IEEE).

The IEEE 802 committee issued a set of standards that have been widely accepted and adopted by other standard making bodies such as ANSI and the International Organization on Standardization (ISO).

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Structure of LAN standards

LAN standards developed based on two criteria. the task of communication should be broken in several sub-

tasks. A variety of approaches was necessary because of the

variety of topologies, media, and access methods.

The standards are organized into as a three-layer protocol

Logical link control (IEEE 802.2) Media access control (IEEE 802.3, etc.) Physical media

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Structure of LAN standards

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LAN Hardware

Servers File, print, fax application, CD-ROM, DNS, etc.

Hubs For connecting devices to the network Act as repeaters or amplifiers

Switches Type of hub which acts as a switch for incoming frames

Bridges Connect two similar networks that use the same

protocol.

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LAN Hardware

Routers Connects two similar networks having the same protocol Chooses the best route between two networks when there

are multiple paths between them

Brouters Combines the function of a bridge and a router

Gateways A device that connects two dissimilar networks Allows different vendor networks to communicate by

translating one vendors protocol into another. E.g. X.400 and TCP/IP

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LAN Network Operating Systems (NOS)

NOS Server Software Novell UNIX Windows NT OS/2, etc.

NOS Client Software