1

OPERATING SYSTEMS &

Network

OVERVIEW

1: OS & Network Overview

1: OS & Network Overview 2

WHAT IS AN OPERATING SYSTEM?

• An interface between users and hardware - an environment "architecture”

• Allows convenient usage; hides the tedious stuff

• Allows efficient usage; parallel activity, avoids wasted cycles

• Provides information protection

• Gives each user a slice of the resources

• Acts as a control program.

OPERATING SYSTEM

OVERVIEW

1: OS & Network Overview 3

OPERATING

SYSTEM OVERVIEW

The Layers Of

A System

Program Interface

Humans

User Programs

O.S. Interface

O.S.

Hardware Interface/

Privileged Instructions

Disk/Tape/Memory

1: OS & Network Overview 4

A mechanism for scheduling jobs or processes. Scheduling can be as simpleas running the next process, or it can use relatively complex rules to picka running process.

A method for simultaneous CPU execution and IO handling. Processing isgoing on even as IO is occurring in preparation for future CPU work.

Off Line Processing; not only are IO and CPU happening concurrently, butsome off-board processing is occurring with the IO.

OPERATING SYSTEM

OVERVIEWComponents

1: OS & Network Overview 5

The CPU is wasted if a job waits for I/O. This leads to:

• Multiprogramming ( dynamic switching ). While one job waits for aresource, the CPU can find another job to run. It means that several jobsare ready to run and only need the CPU in order to continue.

All of this leads to:

• memory management

• resource scheduling

• deadlock protection

OPERATING SYSTEM

OVERVIEWComponents

1: OS & Network Overview 6

OPERATING SYSTEM

OVERVIEWCharacteristics

Interrupts:

• Interrupt transfers control to the interrupt service routine generally, through the

interrupt vector, which contains the addresses of all the service routines.

• Interrupt architecture must save the address of the interrupted instruction.

• Incoming interrupts are disabled while another interrupt is being processed to prevent

a lost interrupt.

• A trap is a software-generated interrupt caused either by an error or a user request.

• An operating system is interrupt driven.

1: OS & Network Overview 7

OPERATING SYSTEM

OVERVIEWHardware

Support

These are the

devices that make

up a typical system.

Any of these

devices can cause

an electrical

interrupt that grabs

the attention of the

CPU.

1: OS & Network Overview 8

OPERATING SYSTEM

OVERVIEWHardware

Support

Sequence

of events

for

processing

an IO

request.

Comparing

Synchronous

and

Asynchronous

IO Operations

1: OS & Network Overview 9

Very fast storage is very expensive. So the Operating System manages a hierarchy ofstorage devices in order to make the best use of resources. In fact, considerable effortgoes into this support.

OPERATING SYSTEM

OVERVIEWStorage

Hierarchy

Fast and Expensive

Slow an Cheap

1: OS & Network Overview 10

Performance:

OPERATING SYSTEM

OVERVIEWStorage

Hierarchy

1: OS & Network Overview 11

Caching:

•Important principle, performed at many levels in a computer (in hardware,operating system, software)

•Information in use copied from slower to faster storage temporarily

•Faster storage (cache) checked first to determine if information is there

• If it is, information used directly from the cache (fast)

• If not, data copied to cache and used there

•Cache smaller than storage being cached

• Cache management important design problem

• Cache size and replacement policy

OPERATING SYSTEM

OVERVIEWStorage

Hierarchy

Process != Program

1: OS & Network Overview 12

Extensions

• OS – Centralized Version

• Two Trends

• System Virtualization: Allow multiple instances of (possibly

different) OSes on a single machine

• Distributed OS: Large scale networked systems/machines

1: OS & Network Overview 13

Network Overview

1: OS & Network Overview 14

OSI Reference model

1: OS & Network Overview 15

OSI vs TCP/IP

1: OS & Network Overview 16

IP Related Protocols

1: OS & Network Overview 17

IPv4 System Model

1: OS & Network Overview 18

IP Datagram Format

1: OS & Network Overview 19

Fragmentation and Re-Assembly

1: OS & Network Overview 20

Fragmentation Example

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Getting a Datagram from Src to Dest

1: OS & Network Overview 22

Getting a Datagram from Src to Dst

1: OS & Network Overview 23

Getting a Datagram from Src to Dst

1: OS & Network Overview 24

Getting a Datagram from Src to Dst

1: OS & Network Overview 25

Network Commands

1: OS & Network Overview 26

User Datagram Protocol (UDP)

1: OS & Network Overview 27

Simple Packet Multiplexer (UDP)

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User Datagram Protocol (UDP)

• Applications may prefer using UDP:

• don’t need reliable delivery ;

• do not need connection oriented service or cannot afford the connection setup overhead ;

• have their own special needs, such as streaming of real-time audio or video ;

• tftp, RIP, DNS, SNMP, RTP, etc.

1: OS & Network Overview 29

TCP Overview

1: OS & Network Overview 30

TCP Segment Format

1: OS & Network Overview 31

TCP Segment Format (cont’d)

1: OS & Network Overview 32

TCP Connection Management

1: OS & Network Overview 33

3-Way Handshaking

1: OS & Network Overview 34

TCP: Initial Sequence Number

1: OS & Network Overview 35

Data Transfer

1: OS & Network Overview 36

TCP Connections: Close

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Application Layer

1: OS & Network Overview 38

Questions