Computer Network

Bahçeşehir University Department of Computer Engineering
CMPE 2204 Introduction to Computer Networks Spring 2013

Chapter 1 Data Communications and Networks Overview and Protocol Layers Dr. Çağrı Güngör

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Basic Info about the Course
• Course Content (tentative):
— Computer networks; network architectures; protocol layers; TCP/IP; principles of Internet; wireless networks; introduction to network management, etc..

• Course Objectives:
— Understand widely known network protocols, communication architectures, and wireless networks, — Learn TCP/IP and protocol layers, — Get a basic understanding of network management, etc.

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Text Books
• J. Kurose, K. Ross, “Computer Networking: A Top Down Approach,” Addison-Wesley, 5th edition, March 2009.

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W. Stallings, “Data and Computer Communications,” Prentice Hall, 8th edition, 2007.
B. Forouzan, “Data Communications and Networking,” McGraw-Hill, 4th edition, 2007.
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Grading
• You will have:
— — — — 1 Midterm Quiz 1 Class Project 1 Final : : : : 25 10 25 40 % % % %

• NO MERCY ABOUT CHEATING !!!

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Internet Infrastructure

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Interesting Internet Appliances
Web-enabled toaster + weather forecaster

IP picture frame http://www.ceiva.com/

World’s smallest web server http://www-ccs.cs.umass.edu/~shri/iPic.html

Internet phones
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Networks are complex !!!
• Many “pieces”:
— Various applications, hosts, routers, protocols hardware, software

• The complexity of the communication task is reduced by using multiple protocol layers:
— Each protocol is implemented independently — Each protocol is responsible for a specific subtask — Protocols are grouped in a hierarchy

• A structured set of protocols is called a communications architecture or protocol suite
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Simplified Communications Model - Diagram

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A Communications Model
• Source
—generates data to be transmitted

• Transmitter
—Converts data into transmittable signals

• Transmission System
—Carries data

• Receiver
—Converts received signal into data

• Destination
—Takes incoming data
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Simplified Data Communications Model

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Examples of Communications Tasks
Transmission system utilization Interfacing Signal generation Addressing Routing Recovery

Synchronization
Exchange management Error detection and correction Flow control

Message formatting
Security Network management

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Networking
• Point to point communication is usually not practical
—Devices are too far apart —Large set of devices would need impractical number of connections

• Solution is a communications network • Network Types:
—Wide Area Network (WAN) —Local Area Network (LAN) —Metropolitan Area Networks (MAN)
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Data Flow Types

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Connection Types

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Categories of Topology

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Mesh Topology

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Star Topology

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Bus Topology

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Ring Topology

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Hybrid Topology: A star backbone with three bus networks

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Network Types
A. Wide Area Network (WAN) B. Local Area Network (LAN) C. Metropolitan Area Networks (MAN)

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A. Wide Area Networks
• • • • Covers large geographical area Circuits provided by a common carrier High-speed WANs use optical fiber Technologies:
1. 2. 3. 4. Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM)

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1. Circuit Switching
• Dedicated communications path established for the duration of the conversation (guaranteed service)
— e.g., telephone network

• Network resources (e.g., bandwidth) divided into “pieces”:
— Call setup required — Pieces allocated to users

• Dividing link bandwidth into “pieces”:
— frequency division (FDM) — time division (TDM)
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1. Circuit Switching: FDM and TDM
FDM
Example: 4 users frequency TDM time

frequency time
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2. 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
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2. Packet Switching
• Great for bursty data
—Resource sharing —Simpler, no call setup

• Excessive congestion: packet delay and loss
—Protocols needed for reliable data transfer and congestion control

• Challenge: How to provide performance QoS?
—Bandwidth guarantees needed for audio/video apps —Still an unsolved problem
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3. Frame Relay
• Packet switching systems have large overheads to compensate for errors

• Modern systems are more reliable
• Errors can be caught in end system • Most overhead for error control is stripped out

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4. Asynchronous Transfer Mode (ATM)
• Evolution of frame relay • Little overhead for error control • Fixed packet (called cell) length • Anything from 10Mbps to Gbps • Constant data rate using packet switching technique

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B. Local Area Networks (LAN)
• Smaller scope
—Building or small campus

• Usually owned by same organization as attached devices • Data rates much higher • Usually broadcast systems
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LAN Configurations
• Switched
—Switched Ethernet
• May be single or multiple switches

—ATM LAN —Fibre Channel

• Wireless
—Mobility —Ease of installation

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Differences between LANs and WANs
• Scope of a LAN is smaller — LAN interconnects devices within a single building or cluster of buildings

• LAN usually owned by organization that owns the attached devices — For WANs, most of network assets are not owned by same organization
• Internal data rate of LAN is much greater

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C. Metropolitan Area Networks (MAN)
• Middle ground between LAN and WAN • Private or public network • High speed • Large area

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The Need for MANs
• Traditional point-to-point and switched network techniques used in WANs are inadequate for growing needs of organizations • Need for high capacity and low costs over large area • MAN provides: —Service to customers in metropolitan areas —Required capacity —Lower cost and greater efficiency than equivalent service from telephone company
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Key Features of a Protocol
• Syntax —Concerns the format of the data blocks

• Semantics —Includes control information for coordination and error handling
• Timing —Includes speed matching and sequencing
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Standardized Protocol Architectures
• Required for devices to communicate • Vendors have more marketable products • Customers can insist on standards based equipment • Two standards:
—OSI Reference model
• Never lived up to early promises

—TCP/IP protocol suite
• Most widely used

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OSI Model - 1
• Open Systems Interconnection • Developed by the International Organization for Standardization (ISO) • Seven layers • A theoretical system delivered too late! • TCP/IP model is the de facto standard

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OSI Model - 2
• A layer model • Each layer performs a subset of the required communication functions • Each layer relies on the next lower layer to perform more primitive functions • Each layer provides services to the next higher layer • Changes in one layer should not require changes in other layers
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Layers of the OSI Model
• • • • • • • Application Presentation Session Transport Network Data link Physical

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OSI Layers
• Physical
—Physical interface between devices
• • • • Mechanical Electrical Functional Procedural

• Data Link
—Means of activating, maintaining and deactivating a reliable link —Error detection and control —Higher layers may assume error free transmission
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OSI Layers (Cont’ d.)
• Network
— Transport of information — Higher layers do not need to know about underlying technology — Not needed on direct links

• Transport
— Exchange of data between end systems — Error free — In sequence — No losses — No duplicates — Quality of service

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OSI Layers (Cont’ d.)
• Session
—Control of dialogues between applications —Dialogue discipline —Grouping —Recovery

• Presentation
—Data formats and coding —Data compression —Encryption

• Application
—Means for applications to access OSI environment
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Summary of OSI Layers

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Use of a Relay

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TCP/IP Protocol Architecture
• Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) • Used by the global Internet • No official model but a working one.
—Application layer —Transport layer (Host-to-host) —Routing layer (Internet) —Data link layer (Network Access) —Physical layer
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TCP/IP and OSI model

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Physical Layer
• Physical interface between data transmission device (e.g. computer) and transmission medium or network • Physical layer specifies:
—Characteristics of the transmission medium —The nature of the signals —The data rate

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Data Link Layer
• Concerned with the exchange of data between an end system and the network to which it's attached

• Software used depends on type of network —Circuit switching —Packet switching (e.g., X.25) —LANs (e.g., Ethernet)

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Routing Layer (IP)
• Uses internet protocol (IP) • Provides routing functions to allow data to traverse multiple interconnected networks • Implemented in end systems and routers

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Transport Layer (TCP)
• Commonly uses transmission control protocol (TCP)

• Provides reliability during data exchange
—Completeness —Order

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Application Layer
• Logic supports user applications • Uses separate modules that are peculiar to each different type of application • E.g., http, SMPT

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Common TCP/IP Applications
• Simple mail transfer protocol (SMTP)
—Provides a basic electronic mail facility

• File Transfer Protocol (FTP)
—Allows files to be sent from one system to another

• TELNET
—Provides a remote logon capability

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TCP/IP Architecture Dominance
• TCP/IP protocols matured quicker than similar OSI protocols —When the need for interoperability across networks was recognized, only TCP/IP was available and ready to go

• OSI model is unnecessarily complex —Accomplishes in seven layers what TCP/IP does with fewer layers
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TCP
• Usual transport layer is Transmission Control Protocol
— Reliable connection

• Connection
— Temporary logical association between entities in different systems

• TCP PDU
— Called TCP segment — Includes source and destination port (c.f. SAP)
• Identify respective users (applications) • Connection refers to pair of ports

• TCP tracks segments between entities on each connection
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UDP
• • • • • • Alternative to TCP is User Datagram Protocol Not guaranteed delivery No preservation of sequence No protection against duplication Minimum overhead Adds port addressing to IP

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Some Protocols in TCP/IP Suite

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Internetworking Terms
• Communication network – facility that provides a data transfer service among devices attached to the network • Internet – collection of communication networks, interconnected by bridges/routers • Intranet – internet used by an organization for internal purposes
— Provides key Internet applications — Can exist as an isolated, self-contained internet

• Bandwidth – the amount of data that can pass through a network interface over time.

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Internetworking Terms
• End System (ES) – device used to support end-user applications or services • Bridge – used to connect two LANs that use similar LAN protocols • Router – used to connect two networks that may or may not be similar

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