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Dr. L. Christofi 1

Local Local Local Local &&&& Metropolitan Metropolitan Metropolitan Metropolitan

Area NetworksArea NetworksArea NetworksArea Networks

ACOE322

Lecture 1

Introduction

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1. INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION

In this section we will cover :

1. Overview of data communications and networking

2. Definition and overview of LANs, MANs, WANs and the Internet

1. Transmission Media

2. LAN topologies

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Why Study Local and

Metropolitan Area Networks?

• Change the way we do business and the way we live

—Require immediate access to accurate information

• Database, online shopping

—Enable long distance communication

• Internet, IP phone

—Access variable of information such as text, voice and image

• Email, messenger, video conference

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The need for Local Networks

• Increased number of systems due to

—Continuing decrease in computer hardware costs

—Increase in computer hardware capability

—Development of applications (image processing, speech recognition, video-conferencing, multimedia authoring, etc)

—Businesses are relying on increasingly powerful servers to handle transaction and database processing and to support massive client/server networks

• So, there is a requirement to interconnect systems to

—Share and exchange data among systems

—Share expensive resources

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Overview of Data

Communications

• What is communication?

• What is data communication?

• Basic components of data a communication system

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What is Communication and

Data Communication? • Communications

—The exchange of information between individuals using a common set of symbols, signs, behavior or language.

—Objective: Ability to share information over long distances• High volume of information• No loss of accuracy (also protected from prying eyes)• Fast transmission rate

— Early attempts:• Voice Telegraph, Messengers / Runners, Signal Fires (in 1200 BC), Telegraph (first demonstrated in 1844 by Samuel Morse)

• Data— Information presented in whatever forms is agreed upon by the parties creating and using data

• Data communications—The exchange of data between two devices via some form of transmission medium such as a wire cable

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Overview of Data

Communications (Cont. )

• What is communication?

• What is data communication?

• Which are the basic components of a data communication system?

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A Data Communication Model

• Message

• Sender

• Receiver

• Medium

• Protocol

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Data Communication

Techniques

• Data representation— Bits and Bytes

— Character Codes

• Data transmission— Signal encoding

— Direction of data flow

— Transmission Mode• Parallel vs. Serial Transmission

• Asynchronous vs. Synchronous

— Line Topology

— Transmission media

— Transmission impairment and error detection

— Improve transmission efficiency

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Overview of Networking

• Point to point communication usually not practical—Senders and receivers are very far apart

— Large set of devices may require a link to many of the others at various times

• Solutions is to attach each device to a communication network

• Network—A set of devices connected by communication links

• Categories of networks—Wide-area Networks (WANs)

— Local-area Networks (LANs)

—Metropolitan-area Networks (MANs)

— LANs<MANs<WANs<Internet

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Wide Area Network

• Generally cover a large geographical area• Rely in part on common circuit carrier• Alternative technologies

—Circuit switching— Packet switching

• Frame Relay• ATM• IP/MPLS

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Local Area Networks (1)

• LAN is a communications network that interconnects a variety of devices and provides a means for information exchange among those devices

• Differences between LANs and WANs:

—LANs have a small footprint (typically a single building or a number of buildings)

—The LAN is a private or closed network usually owned by the same organization that owns the attached devices

—WAN is a public network

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Local Area Networks (2)

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Simple Local Area Network

• All devices are attached to a shared transmission medium through a hardware/software module that handles the transmission and medium access functions associated with the LAN.

• This module is a Network Interface Card (NIC) that is included in each device. – It contains logic for accessing the LAN and for sending and receiving data to and from the LAN.

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Metropolitan Area Networks (1)

• Occupies a middle ground between LANs and WANs

• MANs cover greater distances at higher data rates than LANs, although there is some overlap in geographical coverage

• Primary market for MANs is the customer that has high-capacity needs in a metropolitan area

• A MAN is intended to provide capacity at lower cost and greater efficiency than obtaining an equivalent service from the local telephone company

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Metropolitan Area Networks (2)

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Comparison of LANs, MANs,

and WANs (1)

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Comparison of LANs, MANs,

and WANs (2)

LANs: 1 – 1000 Mbps

MANs: 10 – 40 Gbps

WANs: Tbps

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The Internet

• intranet

—Communication in a closed network

—i.e. company or institution private network

• internet

—Communication between two or more private or public networks

• The Internet

—A collaboration of more than hundreds of thousands interconnected public networks

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Brief History of Internet• Mid-1960s

—Standalone devices— ARPA (Advanced Research Projects Agency) was interested in finding a way to connect computers to share information

— Backbones: None - Hosts: None • 1967

—ARPA presented its ideas for ARPANET—Backbones: None - Hosts: None

• 1969—The first physical network was constructed — Backbones: 50Kbps ARPANET - Hosts: 4

• 1972—The first e-mail program was created by Ray Tomlinson of BBN—Backbones: 50Kbps ARPANET - Hosts: 23

• 1973—Development began on the protocol later to be called TCP/IP (by Vint Cerf and Bob Kahn)

— Backbones: 50Kbps ARPANET - Hosts: >23

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LANs, WANs and Internet

• Private WANs

—Attractive option if there is a large volume of traffic between several sites, justifying the investment

—Constructed from leased lines from public carrier or use of wireless interconnections

• Alternatively, the Internet, or some other public network can be used to interconnect sites

—Provides cost savings over the use of private networks

—How secure is this solution?

• Set up Virtual Private Networks (VPNs)

• Apply a security protocol or encryption to data traffic

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LAN Connections to the

Outside World

• Traffic from the Internet must pass through firewall that regulates traffic to enforce security policies.

• Traffic from other LANs or the organization is allowed to pass directly to any host on this LAN.

• Traffic from other Internet users may only pass through the Web server on this LAN

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Internet

Today

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2. OVERVIEW OF LANs

• The key technology ingredients that determine the nature of a LAN or MAN are:

1. Transmission Media

2. LAN topologies

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2.1 Transmission media and

physical layer

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Transmission media

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Guided media

• Twisted-Pair Cable

• Coaxial Cable

• Fiber-Optic Cable

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Twisted-Pair Cable (1)

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Twisted-Pair Cable (2)

• Unshielded Twisted Pair (UTP)

—Cat 3

— Cheap

—Well understood

—Use existing telephone wiring in office building

— Low data rates

• High performance UTP

—Cat 5 and above

—High data rate for small number of devices

— Switched star topology for large installations

• Shielded Twisted Pair (STP)

—More expensive than UTP but higher data rates

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Twisted Pair - Pros and Cons

• Cheap

• Easy to work with

• Low data rate (usually up to 100Mbps, although some 1Gbps networks have been developed using twisted pair cabling)

• Short range (up to a few km)

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Twisted Pair - Transmission

Characteristics

• Analog

—Amplifiers every 5km to 6km

• Digital

—Use either analog or digital signals

— repeater every 2km or 3km

• Limited distance

• Limited bandwidth (1MHz)

• Limited data rate (100Mbps)

• Susceptible to interference and noise

—Because of easy coupling with electromagnetic fields

— Eg. impulse noise, 50Hz pick-up energy from AC power lines

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Twisted Pair - Applications

• Most common medium

• Telephone network

—Between house and local telephone exchange (subscriber loop)

• Within buildings

—For digital signaling to private branch exchange (PBX)

• For local area networks (LAN)

—10Mbps or 100Mbps

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Coaxial cable

50 ΩΩΩΩ

50 ΩΩΩΩ

75 ΩΩΩΩ

Impedance

Thick EthernetRGRG--1111

Thin EthernetRGRG--5858

Cable TVRGRG--5959

UseCategory

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Coaxial Cable - Transmission

Characteristics

• Better performance than twisted pair

—Superior frequency characteristics

—Much less susceptibility to interference and crosstalk

• For Analogue signals

—Amplifiers needed every few km

—Much less distance for higher frequencies

—Up to 1GHz of bandwidth

• For Digital signals

—Repeater needed every about 1 km

—Less distance for higher data rates

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Coaxial Cable - Applications

• Most versatile medium

• Television distribution—Arial to TV

—Cable, Satellite TV

• Long distance telephone transmission—Can carry 10,000 voice calls simultaneously

—Being replaced by fiber optic

• Short distance computer systems links

• Local Area Networks—Early days

—No longer used

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Optical fiber (1)

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Optical fiber (2)

• Optical fiber

—Electromagnetic isolation

—High capacity

—Small size

—High cost of components

—High skill needed to install and maintain

• Prices are coming down as demand and product range increases

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Optical fiber (3)

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Optical Fiber - Transmission

Characteristics

• Act as wave guide for 1014 to 1015 Hz

—Portions of infrared and visible spectrum

• Types of light sources in fiber optic systems

—Light Emitting Diode (LED)

• Cheaper

• Wider operating temperature range

• Last longer

—Injection Laser Diode (ILD)

• More efficient

• Greater data rate

• Wavelength Division Multiplexing (WDM)

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Optical Fiber - Applications

• Long-haul trunks in telephone networks— Circuit lengths of about 1500 km

— 20.000 to 60.000 voice channels

• Metropolitan trunks— Circuit lengths of about 12km

— May have 100.000 voice channels in a trunk group

• Rural exchange trunks— Circuit lengths of 40 – 160 km

— Typically fewer than 5.000 voice channels

• Subscriber loops— Fiber to the business, fiber to the home in the near future

• LANs— Support 100s and 1000s of stations at rates of about 10Gbps

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Unguided media

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Radio waves

Radio waves are used for multicast

communications, such as radio and television, and paging systems

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Microwaves

Microwaves are used for unicastcommunication such as cellular telephones, satellite networks, and wireless LANs.

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Infrared

• Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation

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Infrared transmission

techniques

• Direct beam infrared

— Can be used to create point-to-point links (up to a few km) ie cross-

building interconnection

— Range depends on the emitted power and on the degree of focusing

— Can be used to set up a token ring LAN

• Omnidirectional

— Involves a single base station that is within line of sight of all other stations on the LAN

— Typically, the station is mounted on the ceiling and it broadcasts an omnidirectional signal that can be received by all of the other IR transceivers in the area

• Diffused

— All IR transceivers are focused and aimed at the point at the ceiling

— IR radiation striking the ceiling is reradiated ominidirectionally and picked up by all of the receivers in the area

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Infrared LANs – Pros and Cons

• Advantages of infrared over other types of wireless LANs—Spectrum for infrared is virtually unlimited -> very high data rates

— Infrared spectrum is unregulated worldwide unlike some portions of the microwave spectrum

— Infrared light is diffusely reflected by light-colored objects, ie it is possible to use ceiling reflection to achieve coverage of an entire room

—Since infrared does not penetrate walls, it can be easily secured against eavesdropping than microwave

—A separate infrared installation can be operated in every room in a building without interference

— Equipment relatively inexpensive and simple

• Disadvantages —Many indoor environments experience intense background noise radiation from sunlight and indoor lighting

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Point-to-point connection

Wired connection

Wireless connection

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Point-to-multipoint connection

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2.2 LAN topologies

• The term topology refers to the way in which the end systems (or stations) attached to the network are interconnected

• Four common topologies for LANs

—Tree

— Bus (special case of Tree topology)

• One trunk, no branches

— Ring topology

— Star topology

• Also, mesh topology can be found in circuit switching carrier networks

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LAN/MAN Topologies

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Mesh and star topologies

Mesh topology Star topology

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Bus and ring topologies

Bus topology

Ring topology

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Bus and tree topologies

• Both topologies are characterized by the use of a multipoint medium

• For the bus—All stations attach directly to a linear transmission medium or bus

—Full duplex operation between the station and the medium allows data to be transmitted onto the bus and received from the bus

—A transmission from any station propagates the length of the medium in both directions and can be received by all other stations

—At each end of the bus, there is a terminator which absorbs any signal, removing it from the bus

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Tree topology (1)• Tree topology is a generalization of the bus topology

• Transmission medium is a branching cable with no closed loops

• Begins at a point known as the headend

• One or more cables start at the headend and each of these may have branches

• A transmission from any station propagates throughout the medium and can be received by all other stations

• However there are two problems here:

— Because the transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended

—A mechanism is needed to regulate transmission

• If two stations on the bus attempt to transmit at the same time, their signals will overlap and become garbled.

• One station could decide to transmit continuously for a long period of time, shutting off access to other stations

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Tree topology (2)

• To solve these problems stations transmit data in small blocks, known as frames

• Each frame consists of a portion of the data that a station wishes to transmit, plus a frame header that contains control information

• Each station on the bus is assigned a unique address and the destination address for a frame is included in its header

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Frame

Transmission

on Bus LAN

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Ring Topology

• Repeaters joined by point-to-point links in a closed loop— Receive data on one link and retransmit on another

— Links are unidirectional, ie data circulate around the ring in one direction

— Stations attach to the network at repeaters

• Data transmitted in frames— Circulate past all stations

— Destination recognizes address and copies frame

— Frame circulates back to source where it is removed

• Because multiple stations share the ring, Medium Access Control is needed to determine when each station can insert frames

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Frame

Transmission

Ring LAN

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Star topology

• Each station is directly connected to a common central node

• Each station attaches to a central node via two point-to-point links, one for transmission in each direction

• Two alternatives for the operation of the central node

—Broadcast fashion• A transmission of a frame from one station to the node is retransmitted on all of the outgoing links

• A transmission from any station is received by all other stations and

only one station at a time may successfully transmit

—Act as a frame-switching device• An incoming frame is buffered in the node and then retransmittedon one outgoing link to the destination station

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Choice of topology

• Depends on a variety of factors, including reliability, expandability and performance

• This choice is part of the overall task of designing a LAN

— Twisted pair: up to a few Mbps

—Baseband coaxial cable: makes use of digital signaling and was originally used by Ethernet

—Broadband coaxial cable: used in Cable TV systems with analog signaling. More expensive and more difficult to install than baseband coaxial cables

—Optical fiber: much higher data rates than copper wires

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Medium versus topology for

LANs and MANs

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Bus LAN

Transmission Media (1)

• Twisted pair

—Early LANs used voice grade cable

—Didn’t scale for fast LANs

—Not used in bus LANs now

• Baseband coaxial cable

—Uses digital signalling

—Original Ethernet

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Bus LAN

Transmission Media (2)

• Broadband coaxial cable— As in cable TV systems

— Analog signals at radio frequencies

— Expensive, hard to install and maintain

— No longer used in LANs

• Optical fiber— Expensive taps

— Better alternatives available

— Not used in bus LANs

• All hard to work with compared with star topology twisted pair

• Coaxial baseband still used but not often in new LAN installations

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Ring and Star Usage

• Ring

—Very high speed links over long distances

—Single link or repeater failure disables network

• Star

—Uses natural layout of wiring in building

—Best for short distances

—High data rates for small number of devices

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Choice of Medium

• Constrained by LAN topology

• Capacity

• Reliability

• Types of data supported

• Environmental scope

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References

W. Stallings, Local and Metropolitan Area Networks, 6th edition, Prentice Hall, 2000

B.A. Forouzan, Data Communications and Networking, 3rd edition, McGraw-Hill, 2004

W. Stallings, Data and Computer Communications, 7th edition, Prentice Hall, 2004