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Data Communications &

Computer Networks

Chapter 1

Data Communications and

Networks Overview

Fall 2008

Agenda

• Networking history

• Communications model

• Data communication networking

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Networking history• From a historical perspective, electronic communication has actually been

around a long time, beginning with Samuel Morse and the telegraph. — first telegraph message sent on May 24, 1844 from Washington DC to

Baltimore MD, 37 miles away

• 23 years later, Alexander Graham Bell invented the telephone— This led to the development of the ultimate analog network: the telephone

system.

• The first bit-oriented language device was developed by Emile Baudot –the printing telegraph. — By bit-oriented we mean the device sends pulses of electricity which were either

positive or had no voltage at all. — These machines did not use Morse code. — Baudot’s five-level code sent five pulses down the wire for each character

transmitted. — The machines did the encoding and decoding, eliminating the need for operators

at both ends of the wires. — For the first time, electronic messages could be sent by anyone.

Telephone Network

Analog Network

Telephone network

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Early computer networks• In the 1960’s and 1970’s, traditional

computer communications centered around the mainframe host. — The mainframe contained all the

applications needed by the users, as well as file management, and even printing.

— This centralized computing environment used low-speed access lines that tied terminals to the host.

• These large mainframes used digital signals — pulses of electricity or zeros and

ones, what is called binary to pass information from the terminals to the host.

— The information processing in the host was also all digital.

Mainframe Host

Computer Systems• Application Programs

• Database

• Printing

Low-Speed Access Lines

Digital NetworkDigital Network

Problem…

Anal

og

Anal

og

Dig

ital

Dig

ital

• This brought about a problem: — The telephone industry wanted to use computers to switch calls faster and

the computer industry wanted to connect remote users to the mainframe using the telephone service.

— But the telephone networks speak analog and computers speak digital.

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Analog and Digital Signals

Digital Transmission:Digital Transmission:

1’s and 0’s

On or OffComputer-speak

1 0 1 0 0 1 1 0 1“1” bit

“0” bit

Start

Bit

Stop

Bit

Analog Transmission:Analog Transmission:Wires or wireless,

Audio tones

Info conveyed throughsignal amplitude,

frequency, and phase

Solution—Modems

•• ModemModem (=(=Modulator/Demodulator)

Translates digital computer signals to analog signals which the telephone world can understand and vice versa

Modem Modem

POTS (Plain Old Telephone Service)

Mainframe

Host

POTS

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Another Solution—Multiplexing

BasebandBaseband—Carries only one signal at a time

BroadbandBroadband—Able to carry multiple signals simultaneously

MultiplexerMultiplexer——Allows multiple

signals to be carried across

a single physical medium

MainframeHost

Broadband—Wide-Area Network

(WAN)

Baseband—Local-Area Network(LAN)

Baseband versus Broadband

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A Communications Model

• Source

— generates information (called data) to be sent (i.e.transmitted)

• Transmitter (tx)

— Converts the data into transmittable signals

• Transmission System

— Carries the information (data)

• Receiver (rx)

— Converts received signal into data

• Destination

— Takes incoming data

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Simplified Communications

Model - Diagram

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Key Communications Tasks

• Transmission System Utilization

• Interfacing

• Signal Generation

• Synchronization

• Exchange Management

• Error detection and correction

• Addressing and routing

• Recovery

• Message formatting

• Security

• Network Management

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Key Communications Tasks (1/3)

• Transmission System Utilization

– Efficient use of tx facilities, eg multiplexing, congestion control

• Interfacing

– Electromagnetic signals propagated over the tx medium

• Signal Generation

– Signal must be capable of being propagated through the txsystem

– Signal must be interpretable as data at the rx

• Synchronization

– Between tx and rx

– Rx should determine when a signal begins to arrive, when it ends, and its duration

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Key Communications Tasks (2/3)• Exchange Management

— eg if data are exchanged in both directions

— Must be decided whether both devices may tx simultaneously or in turns, the amount and format of data, etc

• Error detection and correction

— Errors may occur, since tx signals may be distorted before reaching the rx

• Flow control

— To assure that the source does not overwhelm the destination by sending data faster than they can be processed by the rx

• Addressing and routing

— When more than two devices share a tx facility, a source system must indicate the identity of the intended destination

— The tx system must assure that only the destination system receives the data

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Key Communications Tasks (3/3)• Recovery

— To resume activity in case of interruption during info exchange

• Formatting

— Agreement to the form of data to be exchanged

• Security

— The sender of data wants to be assured that only the intended receiver actually receives the data

• Network Management

— For system configuration

— Monitoring the system status

— Planning

— Alarm reporting and troubleshooting

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Simplified Data

Communications Model

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Data Communication Networking

• Point-to-point communication not usually practical

—Devices are too far apart

—Large set of devices would need impractical number of connections

• Solution is a communications network

—Local Area Network (LAN)

—Metropolitan Area Network (MAN)

—Wide Area Network (WAN)

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Simplified Network Model

MAN or WAN

LAN

Local Area Network—LAN

• What is a LAN?

— A collection of computers, printers, modems, and other devices that can communicate with each other in a small area (< ~ 1km)

• What are the components?

— Computers

— Operating System (OS)

— Network Interface Card (NIC)

— Hubs/Switches/Routers

• How is a LAN controlled?

— Protocols

• Formal descriptions of sets of rules and conventions that govern how devices on a network exchange information

— Standards

• Sets of rules or procedures that are either widely used or officially specified

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Local-Area Networks

• LANs are designed to:

– Operate within a limited geographic area

– Allow multi-access to high-bandwidth media

– Control the network privately under local administration

– Provide full-time connectivity to local services

– Connect physically adjacent devices

Network Operating System (OS)

• Software that allows communicating and sharing of data and network resources

PC or WorkstationLoaded with network OS

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Connector Port

PC or WorkstationLoaded with network OS

Network Interface Card (NIC)

Network Interface Card

• Amplifies electronic signals

• Packages data for transmission

• Physically connects computer to transmission media (cable)

Printer(Also has a NIC)

PC or WorkstationLoaded with NOS

NIC

Wiring Hub

• Serves as center of network

• Contains multiple independent but connected modules where network equipment can be connected

WiringHub

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Printer(Also has a NIC)

NIC

PC or WorkstationLoaded with NOS

Wiring Hub

Cables or Transmission Media

• Physical environments through which transmission signals pass

— Twisted pair

— Coaxial cable

— Fiber-optic cable

• Connectors (RJ-11, RJ-45, etc.)

Cable

Connectors

RJ-45Connector

Network Cabling

• Cable is the actual physical path upon which an electrical signal travels as it moves from one component to another.

• Transmission protocols determine how NIC cards take turns transmitting data onto the cable.

• Media connecting network components— NIC cards take turns transmitting on the cable— LAN cables only carry one signal at a time— WAN cables can carry multiple signals simultaneously

• Three primary types of cabling— Twisted-pair (or copper)— Coaxial cable— Fiber-optic cable

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Twisted-Pair (UTP and STP)

Speed : 10/100 Mbps

Relative cost: Least costly

Media and connector size: Small

Maximum cable length: 100 m

RJ-45Connector

Color-CodedPlastic Insulation

Twisted-Pair

Outer Jacket

STP only: Shielded Insulation

to Reduce EMI

Coaxial Cable

Speed : 10/100 Mbps

Relative cost: More than UTP, but still low

Media and connector size: Medium

Maximum cable length: 200/500 m

OuterJacketBraided Copper Shielding

Plastic Insulation

Copper Conductor

BNC Connector

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Fiber-Optic Cable

Outer JacketKevlar ReinforcingMaterial

PlasticShield Glass Fiber

and Cladding

Single mode: One stream of laser-generated light (100 km)

Multimode: Multiple streams of LED-generated light (2 km)

Speed : 100+ Mbps

Average cost per node: Most expensive

Media and connector size: Small

Maximum cable length: Up to 2 km

MultimodeConnector

Throughput Needs !!

2,457,000 bits/screen30 screens/second73,728,000 bps

100,000 bits

64,000 bps

841,000 bits202,000,000 bits

7,300,000 bits/screen30 pictures/second224,000,000 bps!!!

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Throughput Rate and Bandwidth• Throughput rate

— The rate of information arriving at, and possibly passing through, a particular point in a network

• Bandwidth— The total capacity of a given network medium (twisted pair, coaxial, or

fiber-optic cable) or protocol— Bandwidth is also used to describe the difference between the highest

and the lowest frequencies available for network signals. This quantity is measured in hertz (Hz).

— The bandwidth of a given network medium or protocol is measured in bits per second (bps).

• Some of the available bandwidth specified for a given medium or protocol is used up in overhead, including control characters. — This overhead reduces the capacity available for transmitting data.

THROUGHPUT = BANDWIDTH - OVERHEAD

Throughput Rate

10,000 pages= 53 MB

(Megabytes)

NetworkingMade Easy

1 Byte = 8 bits1 Megabyte = 1MB1 Megabit = 1Mb

• This slide shows the tremendous variation in transmission time with different throughput rates. In years past, megabit (Mb) rates were considered fast. In today’s modern networks, gigabit (Gb) rates are possible. Nevertheless, there continues to be a focus on greater throughput rates.

• As seen in the last slide, throughput is dependent on the bandwidth of the medium and the transmission protocol.

Speed Transmit Time

9,600 bps = 12.27 hrs

24,000 bps = 4.91 hrs

56 Kbps = 2.1 hrs

1 Mbps = 7.1 min

10 Mbps = 42.4 sec

100 Mbps = 4.24 sec

1 Gbps = 0.42 sec

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Metropolitan Area Networks (MANs)

• Middle-ground between LAN and WAN

• Private or public network

• High speed

• Large area

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Wide Area Networks (WANs)

• Large geographical area

• Consists of a number of interconnected switching nodes

• Rely in part on common carrier circuits

• Alternative technologies

—Circuit switching

—Packet switching

• Frame relay

• Asynchronous Transfer Mode (ATM)

• IP

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Circuit Switching

• Dedicated communications path established for the duration of the conversation

• Path is a sequence of connected physical links between nodes

• Example: POTS telephone network

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Packet Switching

• Data sent out of sequence

• Small chunks (packets) of data at a time

• Packets passed from node to node between source and destination

• Used for terminal to computer and computer to computer communications

• Examples

—Frame Relay

—ATM

—IP

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Frame Relay

• Layer 2 technology (data link layer)

• Packet switching systems have large overheads to compensate for errors

• Data rates of 64kbps up to 2Mbps

• Modern systems are more reliable

• Errors can be caught in end system

• Most overhead for error control is stripped out

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Asynchronous Transfer Mode - ATM

• Layer 2 technology (data link layer)

• Evolution of frame relay

• Little overhead for error control

• Fixed packet (called cell) length of 53bytes

• Anything from 2Mbps to 2,5Gbps

• Constant data rate using packet switching technique

• ATM (Asynchronous Transfer Mode) was originally implemented to meet the transport requirements for local, metropolitan and wide-area network applications.

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Internet Protocol - IP

• Layer 3 technology (network or internet layer)

• IP allows voice, data, fax and video signals to share a common networking infrastructure.

• IP can run on top of ATM or Ethernet

• Data rates from as low as 64kbps up to 10Gbps or more

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Networking

Configuration

Example

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Further Reading

• W. Stallings, “Data and Computer Communications (7th edition)”, Prentice Hall,Chapter 1