Module 1.0: Introduction

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Module 1.0 Introduction

• Networking Computer Networks
• Communication Model
• Type of networks
• WAN
• MAN
• LAN
• Layering Concept
• Protocols and Networking Issues
• TCP/IP Reference Model
• OSI Reference Model
• The difference

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What are Computer Networks?

• A communication network is a set of nodes that
  are interconnected by link or buses to permit the
  exchange of information.

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Network Topologies
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Networking

• Motivation Why needed?
• Point to Point Communication not Usually
  Practical
• Devices are too Far Apart
• Large Set of Devices would need Impractical
  Number of Connections
• Information access
• Interaction among cooperative application
  programs
• Resource sharing
• Practical Results
• E-mail
• File transfer/access
• Web browsing
• Remote login/execution
• The Internet

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What a Network Does

• Provides communication that is
• Reliable
• Fair
• Efficient
• From one application to another
• Automatically detects and corrects
• Data corruption
• Data loss
• Duplication
• Out-of-order delivery
• Automatically finds optimal path from source to
  destination

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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 Communications Model - Diagram
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Classes of Computer Networks

• Solution is a Communications Network
• Wide Area Network (WAN)
• Metropolitan Area Network (MAN)
• Local Area Network (LAN)

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Classification of Interconnections
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Wide Area Networks

• Large Geographical Area
• Rely in Part on Common Carrier Circuits (e.g., T1
  T3 by STC).
• Available Technologies
• Circuit Switching
• Packet Switching
• SMDS X.25
• Frame Relay
• Asynchronous Transfer Mode (ATM)
• Integrated Services Digital Network (ISDN)

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Switching (1)

• Circuit Switching
• Dedicated Communications Path Established for the
  Duration of the Conversation
• Example - Public Switched Telephone Network
• 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
• Note CO vs. CL
• Connection Oriented
• Modeled after the telephone system
• When PDUs are sequenced, I.e. logical connection
• Connectionless
• Modeled after the postal system
• When PDUs are not sequenced

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Switching (2)
Telecommunication networks
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SMDS X.25

• (SMDS) Switched Multimegabit Data Service
• Designed by Bellcore in the 1980s
• Connecting LAN to LAN through leased lines
• Connectionless protocol carrying packets
• Standard speed is 45Mbps.
• X.25
• Developed by CCITT in 1970s to specify the
  interface between public packet-switched networks
  and customer LANs.
• Connection-oriented protocol supporting both
  switched virtual circuit (SVC) and permanent
  virtual circuit (PVC).
• As opposed to SVC, PVC are never torn down.
• Operates at speed of 64 kbps

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Frame Relay

• Packet Switching Systems were built on unreliable
  communication lines, and thus had Large Overheads
  to Compensate for Errors
• Modern Systems Are More Reliable
• Errors Can Be Caught in End System (Applications)
• Most Overhead for Error Control is Stripped Out
• You can think of FR as permanent virtual circuit
• FR operates at 1.5 Mbps.
• The concept of committed rate and uncommitted
  rate.

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Asynchronous Transfer Mode

• ATM
• Evolution of Frame Relay
• Little Overhead for Error Control
• Fixed Packet Length (Called Cells)
• Anything From 10Mbps to Gbps
• Constant Data Rate Using Packet Switching
  Technique

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Integrated Services Digital Network

• ISDN
• Designed to Replace Public Telecom System
• Wide Variety of Services
• Entirely Digital Domain

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Local Area Networks

• Smaller Scope
• Building or Small Campus
• Usually Owned by Same Organization as Attached
  Devices
• Data Rates Much Higher
• Usually Broadcast Systems
• IEEE 802.2 Logical Link Control
• IEEE 802.3 - Ethernet
• IEEE 802.4 - Token bus
• IEEE 802.5 - Token ring
• IEEE 802.11 Wireless
• GigE 10GigE

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Monolithic vs. Structured

• Communications is a Complex Task
• Too Complex for Single Unit
• Structured Design Breaks Down Problem into
  Smaller Units
• Layered Structure Why?
• explicit structure allows identification,
  relationship of complex systems pieces
• modularization eases maintenance, develop,
  updating of system
• change of implementation of layers service
  transparent to rest of system
• e.g., change in gate procedure doesnt affect
  rest of system

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Layering Concept

• To reduce design complexity, network
  architectures are organized as a series of layers
  (principle of divide-and-conquer)

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The Need for Layering
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Protocols

• A protocol is a set of rules or conventions
  governing the ways in which two entities/layers
  cooperate to exchange information.
• A layer provides services to the layer above it.
• List of protocols (one protocol per layer) is
  called a protocol stack.
• The set of layers and protocols is called the
  Network Architecture
• Used for Communications Between Entities in a
  System
• Entities
• Layers
• User Applications
• E-mail Facilities
• DBMS
• Systems
• Computer
• Robot
• Remote Sensor

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Typical Functions of a Protocol

• Encapsulation
• Segmentation and reassembly
• Connection control
• Ordered delivery
• Flow control
• Error control
• Addressing
• Multiplexing
• Transmission services

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Encapsulation

• Addition of Control Information to Data
• Address Information
• Error-detecting Code
• Protocol Control

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Segmentation (Fragmentation)

• Application Layer Messages may be Large
• Network Packets may be Smaller
• Splitting Larger Blocks into Smaller Ones is
  Segmentation (or Fragmentation in TCP/IP)
• ATM Blocks (Cells) are 53 Octets Long
• Ethernet Blocks (Frames) Are up to 1526 Octets
  Long

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Why Fragment?

• Advantages
• More Efficient Error Control
• More Equitable Access to Network Facilities
• Shorter Delays
• Smaller Buffers Needed
• Disadvantages
• Overheads
• Increased Interrupts at Receiver
• More Processing Time

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Connection Control

• Connection Establishment
• Connection Indication
• Data Transfer
• Connection Termination
• May Be Connection Interruption and Recovery
• Sequence Numbers Used for
• Ordered Delivery
• Flow Control
• Error Control

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Ordered Delivery

• PDUs may Traverse Different Paths through
  Network
• PDUs may Arrive out of Order
• Sequentially Number PDUs used to Allow for
  Ordering

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Flow Control

• Done by Receiving Entity
• Limit Amount or Rate of Data
• Stop and Wait
• Credit Systems
• Sliding Window
• Needed at Application as well as Network Layers

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Error Control

• Guard Against Loss or Damage
• Error Detection
• Sender Inserts Error Detecting Bits
• Receiver Checks These Bits
• If OK, Acknowledge
• If Error, Discard Packet
• Retransmission
• If No Acknowledge in Given Time, Re-transmit
• Performed at Various Levels

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Addressing level

• Level in Architecture at which Entity is Named
• Unique Address for each End System (Computer) and
  Router
• Network Level Address
• IP or Internet Address (TCP/IP)
• Network Service Access Point or NSAP (OSI)
• Process Within the System
• Port Number (TCP/IP)
• Service Access Point or SAP (OSI)

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Addressing Mode

• Usually an Address Refers to a Single System
• Unicast Address
• Sent to One Machine or Person
• May Address all Entities within a Domain
• Broadcast
• Sent to All Machines or Users
• May Address a Subset of the Entities in a Domain
• Multicast
• Sent to Some Machines or a Group of Users

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Multiplexing

• Supporting multiple connections on one machine
• Mapping of multiple connections at one level to a
  single connection at another
• Carrying a number of connections on one fiber
  optic cable
• Aggregating or bonding ISDN lines to gain
  bandwidth
• The opposite of multiplexing is splitting,
  de-aggregating, de-multiplexing.

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Transmission Services

• Priority
• Certain traffic and certain messages (control)
  have higher priority.
• Quality of service
• Minimum acceptable throughput
• Maximum acceptable delay and loss
• Security
• Access restrictions
• Data encryption

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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
• Host to Host or Transport Layer (TCP)
• Internet Layer (IP)
• Network Access Layer
• Physical Layer

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TCP/IP Protocol Architecture Model
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Layering logical communication

• Each layer
• distributed
• entities implement layer functions at each node
• entities perform actions, exchange messages with
  peers

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Layering logical communication

• E.g. transport
• take data from app
• add addressing, reliability check info to form
  packet
• send packet to peer
• wait for peer to ack receipt
• analogy post office register mail.

transport
transport
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Layering physical communication
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Protocol layering and data

• Each layer takes data from above
• adds header information to create new data unit
• passes new data unit to layer below

source
destination
message
segment
packet
frame
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TCP/IP Model Layers

• Physical Layer
• Physical Interface Between Data Transmission
  Device (E.G. Computer) and Transmission Medium or
  Network
• Characteristics of Transmission Medium
• Signal Levels
• Data Rates
• Etc.
• Network Access Layer (Link)
• Exchange of Data between End System and Network
• Destination Address Provision
• Invoking Services Like Priority
• Internet Layer (IP)
• Systems may be Attached to Different Networks
• Routing Functions Across Multiple Networks
• Implemented in End Systems and Routers

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TCP/IP Model Layers (cont.)

• Transport Layer (TCP)
• CO
• Reliable Delivery of Data
• Ordering of Delivery
• End to End or Transport Layer (TCP/UDP/)
• End to End Transfer of Data
• May Include Reliability Mechanism (TCP)
• Hides Detail of Underlying Network
• Application Layer
• The layer where end-user applications live
• This is the highest level of abstraction and the
  level which is of primary importance (for most
  users)
• All the rest of the layers exist to support these
  applications
• Applications Email, Web, DBMS, Message System,
  Collaboration Software

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Some Protocols in TCP/IP Suite
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OSI - Model

• Open Systems Interconnection
• Developed by the International Organization for
  Standardization (ISO)
• Seven Layers
• A Theoretical System Delivered Too Late!
• TCP/IP Is the De Facto Standard
• 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
• Development of the model started in the
  mid-1970s
• Biggest Problems
• Very long time to complete the model and protocol
  standards
• Very hard to understand the detailed standards
• Difficult (expensive) to get the standards
  documents

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OSI Layers
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The OSI Environment
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Elements of Standardization

• Protocol Specification
• Operates Between the Same Layer on Two Systems
• May Involve Different Operating System
• Protocol Specification Must Be Precise
• Format of Data Units
• Semantics of All Fields
• Allowable Sequence of PDUs
• Service Definition
• Functional Description of What Is Provided
• Addressing
• Referenced by SAPs

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OSI Layers (1)

• 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 (2)

• 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 (3)

• 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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Standards Organizations

• ISO (International Standards Organization)
• voluntary, non-treaty organization whose members
  are designated standard bodies of participating
  nations
• ITU (International Telecommunications Union)
• United Nations treaty organization whose members
  are governments
• IEEE (Institute of Electrical and Electronics
  Engineers)
• largest professional organization in the world
• Open Software Foundation (OSF)
• Distributed Computing Environment (DCE) standards
• Object Management Group (OMG)
• Common Object Request Broker Architecture (CORBA)
• Department of Defense (DoD) Protocol Suite
• RFCs - Request for Comments, IETF - Internet
  Engineering Task Force