K. Salah 1

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

Point-to-point Bus

RingStar

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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)

Telecommunicationnetworks

Circuit-switchednetworks

FDM TDM

Packet-switchednetworks

Networkswith VCs

DatagramNetworks

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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

system’s pieces– modularization eases maintenance, develop, updating of system

change of implementation of layer’s service transparent to rest of system

e.g., change in gate procedure doesn’t 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)

Layer N+1 Layer N+1

Layer N Layer N

Computer A Computer B

peer protocol

interfaceprotocol

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The Need for Layering

German speakingdiplomat peer-to-peer

communicationusing peerprotocol

French speakingdiplomat

German -Italian

interpreter

French -Italian

interpreter

Telephone Telephone

layer

layer

layer

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• 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

Protocols

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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

• PDU’s may Traverse Different Paths through Network

• PDU’s may Arrive out of Order

• Sequentially Number PDU’s 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

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

Each layer:

• distributed

• “entities” implement layer functions at each node

• entities perform actions, exchange messages with peers

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Layering: logical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

dataE.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.

data

transport

transport

ack

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Layering: physical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

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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

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

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-1970’s

• 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

Application

Presentation

Session

transport

Network

Data link

Physical

Application

Presentation

Session

transport

Network

Data link

Physical

Network

Data link

Physical

Source node Destination node

Intermediate node

Packets

Bits

Frames

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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)ISO (International Standards Organization)– voluntary, non-treaty organization whose members are designated standard

bodies of participating nations

• ITU (International Telecommunications Union)ITU (International Telecommunications Union)– United Nations treaty organization whose members are governments

• IEEE (Institute of Electrical and Electronics Engineers)IEEE (Institute of Electrical and Electronics Engineers)– largest professional organization in the world

• Open Software Foundation (OSF)Open Software Foundation (OSF)– Distributed Computing Environment (DCE) standards

• Object Management Group (OMG)Object Management Group (OMG)– Common Object Request Broker Architecture (CORBA)

• Department of Defense (DoD) Protocol SuiteDepartment of Defense (DoD) Protocol Suite– RFCs - Request for Comments, IETF - Internet Engineering Task Force