5/7/2003

Chapter 1 – Introduction

COSC 3213Summer 2003

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Course Objectives

� Introduce communication networks and understand how different components work including the underlying technology including hardware and software used.

3 components:

� Data communications: Review basic concepts of digital communications including signal transmission, signal encoding, multiplexing, error detection and error correction schemes.

� Networking:

� Understand different topologies used in local area networks (LAN)

� Comprehend how an internet (wide area network or WAN) is formed

� Communication protocols: Present various networking standards

� Transmission Control Protocol / Internet Protocol (TCP/IP)

� Open Systems Interconnection Reference model (OSI)

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Data Communications

Signal transmission, signal encoding, multiplexing, data link control (flow control, error detection and error correction)Signal transmission, signal encoding, multiplexing, data link control (flow control, error detection and error correction)

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What is a Communication Network?

� Communication network is a set of equipment, software, and facilities that allows transfer of information between geographically distant users

� Why Communication networks:

Resource Sharing

High Reliability: Provide alternative routes and sources of data

Parallel Processing: Use multiple computers for a single application

Scalability: Ability to increase system size based on demand without degrading system performance

� Examples:

Radio and TV broadcasting: Single source transmits to multiple users; Real-time (low latency), Unidirectional (simplex).

Telephone connection: One to one connection (connection-oriented), Real-time (low latency), Bidirectional (Duplex), Fixed location

Cellular Telephone: One to one connection through microwave (roaming), similar in service to telephone connection

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What is a Communication Network? (cont.)

� Examples (contd):

Electronic mail (Email): Not real-time, Not connection-oriented, Reliable

World wide web (WWW): accessed through a uniform resource locator (URL) identifying a home page developed using a hypertext language.

Video-on-demand: Not real-time

Streamed audiovisual services: real-time

Audio and video conferencing: real-time

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Block Representation: Communication Network

Source: generates dataTx: Converts data into tx signalsTransmission System: Carries signalsRx: Converts rx signal back into dataDestination: Accepts incoming data

Tx: TranmistterRx: Receiver

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Communication Tasks

Tx. System Utilization: Efficient sharing of tx. resources between multiple usersInterfacing: Interface info. source / destination to communication networkMessage formatting: Decide on the format of data that is transmittedSignal Generation: Generate tx. signals that are correctly interpretable by rx.Synchronization: Synchronize the start and end of transmission btw. tx. & rx.Exchange Management: Manage conventions of data transmissions Error detection / correction: Detect and eliminate errors resulting from channel distortionsAddressing : Indicate the transmitting source and the intended destinationRouting: Specify a route between source and destination Recovery: Resume activity after an interruptionSecurity: Guarantee communication, prevent eavesdroppingFlow control: Control rate of transfer constrained by network conditionsHeterogeneous networks: Provide connectivity across different networksFirewall: Prevent trouble makers from accessing the networkNetwork Management: Configure system, respond to failures, and plan for future

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Network (1)

� Is a medium (channel) to transfer information between a source and destination.

t0t1

Network

From S1 To D1

S2 S3 S4 S5 D2 D3 D4 D5

Network

� Depending upon the route, propagation delay varies in between different packets.

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Network (2)

� To transfer information, networks must provide connectivity between source and destination.

� Two possible way of connectivity: Pairwise interconnection and Access network

� Pairwise interconnection is not sustainable with increasing number of users (N)

� Number of lines = N x (N - 1) / 2; N = 5, no. of lines = 10; N = 100, no. of lines = 4950

Pairwise Interconnection Access network

Switch

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Network (3)

� Different access networks can be connected via multiplexers and demultiplexers.

� The concept can be extended to form a metropolitan area network (MAN).

Mux Demux

Access network bAccess network a

Trunk

A

1*

a

c

b

d

2

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MAN A consists of accesssubnetworks a, b, c, and d.

A

γ

α

β

National network (WAN) consists of Several regional subnetworks α, β, γ.

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Types of Networks (1)

� Networks are typically classified in three types: LAN, MAN, and WAN

1. Local area networks (LAN)

Small networks confined to a few kilometer (<= 1 km).

Speeds confined to 100 Mbps. Newer LANs run at up to 10 Gbps.

Uses the principal of broadcasting (one transmits, others listen)

Various Topologies including Token bus and Token ring are possible

Terminator

Token Bus Token Ring

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Types of Networks (2)

2. Metropolitan area networks (MAN)

Covers up to a city (<= 10 km), Example: Cable TV network, IEEE 802.16

Cable network were initially designed for TV and later extended to Internet

Internet is fed into a head for subsequent re-distribution

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Types of Networks (3)

3. Wide area networks (WAN)

Spans a continent (<= 10000 km), Example: Internet

Interconnects various LAN using switches (routers) and transmission lines

Uses either: (1) circuit switching to establish a route between the tx. and rx. or (2) packet switching in conjunction with the store and forward technology

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Switching

� Switching refers to the technology used for transmission of data between two hosts

1. Circuit switching (based on telephone companies)

End-to-end path is established between the transmitter and receiver .

Complete block of data is transmitted and circuit terminated.

Tx and Rx are inaccessible for the duration of the connection .

2. Message switching (based on telegraphic networks):

No physical path is established between Tx and Rx.

Connection is established between the Tx and first switching office (router).

Entire block of data is transmitted to the switching office.

Block is forwarded one hop at a time.

No limit on block size, switching stations inaccessible for duration of transfer.

3. Packet switching (used in Internet):

A tight limit is placed on maximum block size.

Data is broken in different sub-blocks and each sub-block is transmitted one hop at a time, one after the other.

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The caller picks up the phone triggering the flow of current in wires that connect to the telephone office.

TelephoneOffice

1.

The current is detected and a dial tone is transmitted by the telephone office to indicate that it is ready to receive the destination number.

TelephoneOffice

2.

The caller sends the number by pushing keys on the telephone set. Each key generates a pair of tones that specify a number. (In older sets the user dials a number which generates corresponding number of pulses.)

TelephoneOffice

3.

The equipment in the telephone office uses the telephone network to attempt a connection. If the destination telephone is busy, then a busy tone is returned. If the destination telephone is idle, then ringing signals are sent to both the originating and destination telephones.

4. TelephoneOffice

The ringing signals are discontinued when the destination phone is picked up and communication can then proceed.

TelephoneOffice

5.

Either of the users terminate the call by putting down a receiver.

6. TelephoneOffice

Circuit Switching (Connection-oriented Service)Dedicated communication path is established between the source and receiver communication follows the same path. Example is the telephone network.

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Message Switching

The message is queued at the city office and forwarded on its turn to the country office. The process is repeated till the destination regional office is reached

5. Network

Regional office acknowledges indicating it is ready to accept message2. Network

Sender (client) sends the complete message with the receiver address.3. Network

Regional office forwards message to next hop possibly to city office

4. Network

The telegram application establishes a connection with the regional office

1. Network

The regional office contacts the telegram application and notifies the destination of reception of a message.

6. Network

A complete communication path is not established between the source and receiver. Message is transmitted one hop at a time. Example is the telegraphic service.

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Packet Switching (Connectionless Service)

1. No dedicated communication path is established between the source and receiver. 2. Data is divided into packets.3. Each packet contains destination address and reaches rx. Independently.4. Example is the postal service.

Other switching techniques used are Frame relay, ATM, ISDN, and Broadband ISDN (pp. 10-12).

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Switching: Comparison

Timing of events for: (a) circuit switched; (b) message switched; (c) packet switched networks

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Switching: Comparison (cont.)

Same as for message switching

Intelligent adaptive routing algorithms are required at each switching station

Complete path is established once at initialization of connection

Routing

Each packet may use a different path and hence arrive out of order

No problemNo problemSynchronization

A big issue since numerous packets from different sources are roaming in the network.

Can be an issue.No problemCongestion

Highest probability of overflow and data loss

A router will be accepting data from various sources and may overflow.

No problem until a connection is dropped in the middle of transfer

Data Loss

Best under normal working conditions

Improvement over circuit switching by dividing the connection in smaller segments

Highly inefficient due to long setup time

Efficiency

Packet SwitchingPacket SwitchingMessage SwitchingMessage SwitchingCircuit SwitchingCircuit Switching

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Protocols: Basic Definitions

1. Protocol: Set of rules and conventions used by two communicating parties. These rules dictate how a communication will be initiated and terminated, how data and control information are arranged in a packet, what control information is included, etc. Examples include Hypertext transfer protocol (HTTP), File transfer protocol (FTP), Simple mail transfer protocol (SMTP), and Transmission control protocol (TCP).

2. Client/Server configuration: A server is a powerful machine that houses data and other shared resources. Clients are smaller machines that connect to the server to retrieve shared information. The two communicate based on the following protocol: Client makes a request over the network to the server Client waits When server gets the request, it performs the requested job and returns a reply

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Protocols: Basic Definitions (cont.)

3. Port (Service Access Points): A server process waits for incoming requests by listening to a port. Analogous to a wall socket in a telephone network.

4. Networks consist of two components: Hardware that forms the infrastructure connecting the computers, e.g., twisted pair wire /

optical fibre / cable, routers, switches, servers, etc. Software that forms a cohesive connection such that the user sees the entire network as a

single coherent system. The design of software is highly structured typically broken in layers. There are two standards available: OSI (7-layered model) and TCP/IP (5-layered model).

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Example: A Three-Layer Model

� Transfer of a file between two computers connected via a network using a hypothetical 3-layer model.

Application Layer:1. Format translation to provide compatibility between different file formats used by 2 computers.2. Ensure that file transfer application in destination application module is ready to accept data.Transport Layer:1. Ensure that the destination computer is ready to accept data.2. Confirms that data is received correctly with no errors.Network Access Layer:1. Activates a connection between the source and destination computers.

Application Layer

Transport Layer

Network Access Layer

Computer X

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Example (2)

� Each module communicates virtually except for the lowest module with its peer module on the other machine.

� Each layer has specific tasks to do that are accomplished by adding headers

Application Module

Transport Module

Network Module

Computer X

Application Module

Transport Module

Network Module

Computer Y

Network

Application Protocol

Transport Protocol

Network Access ProtocolService Access Point (SAP)

Application Module

Transport Module

Network Module

Application Data

Application Data ITH

Application Data ICHNH

Application Data 2TH

Application Data 2CHNH

Application PDU

Transport PDU

Network PDU

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Example (3)

� Each layer in Source X adds a header while the corresponding layer in destination Y removes it before passing on the PDU to the upper layer.

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Reference Models: OSI vs. TCP/IP

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History of Networks

1. SAGE (Semi-Automatic Ground Environment System):

First computer network developed in 1950 for air-defense purposes

2. SABRE: an airline reservation system introduced in 1964

3. ARPANET:

Developed in mid 1960s at height of cold war to survive a nuclear war.

Based on packet switching technology s. t. part of the network is working under any circumstances.

4. NSFNET:

Developed in mid 1970s to connect research institutions and universities in U.S.

5. ARPANET and NSFNET connected in 1983.

6. Internet was the glueing technology for (5). Growth continued exponentially with the size of networks doubling almost every year.

7. Other networks connected to Internet include Aurora (MIT, IBM, UPenn, Bell core); Blanca (resulting from XUNET project in AT&T), CASA (a network for supercomputers based in CA), Nectar (CMU to UPitt), Vistanet (Univ. in North Caroliona), and many others

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ARPANET as of 1972

UCLA RAND TINKER

USC

NBS

UCSB

HARV

SCD

BBN

STAN

AMES

AMES McCLELLAN UTAH BOULDER GWC CASE

CARN

MITRE

ETAC

MIT

ILL

LINC

RADC