Course Overview Principles of Operating Systems

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Course OverviewPrinciples of Operating Systems

• Introduction
• Computer System Structures
• Operating System Structures
• Processes
• Process Synchronization
• Deadlocks
• CPU Scheduling

• Memory Management
• Virtual Memory
• File Management
• Security
• Networks
• Distributed Systems
• Case Studies
• Conclusions

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Chapter Overview Networking

• Motivation
• Objectives
• Topology
• Network Types
• Communication
• Design Strategies

• Examples
• Client/Server Model
• Middleware
• Important Concepts and Terms
• Chapter Summary

3
Motivation

• resource sharing
• users can access all resources available on the
  machines connected to the network
• computation speedup
• computation-intensive tasks can be partitioned
  into subtasks and distributed over several
  computers on the network
• specialized or more powerful computers can be
  used instead of or in addition to the local
  machine of the user
• reliability
• the overall system can continue to operate even
  if some of its components fail
• communication
• fast exchange of information
• remote procedure calls

4
Objectives

• be aware of benefits and problems of computers
  connected via networks
• know relevant network topologies and network
  types
• understand the issues involved in communication
  of computers via networks
• apply networking concepts to the client/server
  model

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Terminology

• site
• indicates the location of computer systems
• host
• specific system at a site
• frequently implies the execution of a program or
  the availability of services
• local resources
• all the resources available directly within a
  specific computer system system
• local sometimes also refers to a particular site,
  not one single computer system

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Terminology cont.

• remote resources
• mainly for exchange of information
• sometimes specialized resource types
• network operating system
• users are aware of the individual machines in the
  network
• resources are accessible via login or explicit
  transfer of data
• distributed operating system
• users are unaware of the underlying machines and
  networks
• remote resources are accessible in the same way
  as local resources

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Computer Network

• set of computers linked to each other through
  some type of network
• physical link
• virtual link
• indirect via other computers
• logical link
• more abstract level, independent of the physical
  realization

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Computer Network Diagram
logical link
Network
physical link
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Network Topology

• describes the interconnection structure of the
  network
• fully connected
• partially connected
• hierarchical network
• tree structure
• star network
• ring network
• bus network
• unless otherwise noted, the topology refers to
  the physical interconnections

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Comparison Criteria

• important properties of the configuration
• site refers to one or several computers with a
  single access point to the network
• link is a connection between two sites
• basic cost
• setup of the communication between two sites
• communication cost
• costs of transferring a message from one site to
  another
• time required for the transfer
• reliability
• effects of a failure in a link or site

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Fully Connected

• each site is directly linked with all the others
• basic cost
• high requires a direct link between every two
  sites
• quadratic w.r.t. the number of sites
• communication cost
• low no intermediate steps
• reliability
• very high if messages can be rerouted

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Fully Connected Diagram
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Partially Connected

• direct links exist between some pairs of sites
• basic cost
• lower than for the fully connected network
• communication cost
• higher than for the fully connected network
• reliability
• lower than for the fully connected network

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Partially Connected Diagram
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Hierarchical Network

• the sites are organized as a tree
• often used for corporate networks
• headquarters -gt main offices -gt regional offices
• basic cost
• much lower than fully, usually lower than
  partially conn.
• communication cost
• higher than for the fully connected network
• acceptable if communication patterns match the
  hierarchical structure
• reliability
• medium the failure of a site or link partitions
  the network into disjoint subtrees

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Hierarchical Network Diagram
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Star Network

• all sites in the network are linked to a central
  hub
• the hub may be a special device for communication
  only, or a regular site
• basic cost
• linear with the number of sites
• communication cost
• very low if there isnt too much traffic
• central hub may become a bottleneck
• reliability
• depends on the central hub if it fails, the
  network is completely partitioned
• the failure of a site or link affects only that
  site

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Star Network Diagram
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Ring Network

• each site is connected to its two neighbors
• links can be uni- or bi-directional
• basic cost
• linear with the number of sites
• communication cost
• linear with the number of sites
• n-1 transfers for unidirectional, n/2 transfers
  for bi-directional links in the worst case
• reliability
• low the failure of one site or link partitions
  the network

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Ring Network Diagram
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Directional Ring Network Diagram
unidirectional
bi-directional
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Bus Network

• single shared link (bus)
• multiple simultaneous access must be coordinated
• all sites are directly connected to the bus
• basic cost
• linear with the number of sites
• communication cost
• low as long as traffic is not too high
• network contention can become a problem for high
  traffic situations
• reliability
• the bus is the critical component

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Bus Network Diagram
linear bus
ring bus
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Hybrid Networks

• combination of different network topologies
• different topologies usually use different
  protocols
• requires routers or bridges to translate between
  different protocols and routing mechanisms
• more or less unavoidable in practice
• Ethernet LAN
• hierarchical WAN

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Hybrid Network Diagram
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Network Types

• geographical distribution of networks
• local area networks (LANs)
• single or adjacent buildings
• wide area networks (WANs)
• sites distributed over a large geographical area
• has a major impact in the design of networks
• speed, reliability, protocol, security

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Local Area Networks

• connect sites within a small geographical area
• make resources available to all sites
• applications, files, special devices
• high-speed communication links
• Mega-Bit to Giga-Bit per second transfer rate
• wire, optical fiber, infrared
• requires expensive links
• gateways provide access to other networks
• other LANs
• WANs
• Internet

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LAN Examples

• Ethernet
• usually wire (shielded or twisted-pair cable)
• TCP/IP protocol
• variations
• 10BaseT
• 100BaseT
• FDDI
• optical fiber
• token ring protocol
• 100 MBit/s

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Wide Area Networks

• connect geographically distributed sites
• exchange of information
• access to special purpose computers
• not useful for access to peripheral devices
• links must cross long distances
• speed, reliability, security problems
• may involve other organizations

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WAN Technology

• link technology
• telephone lines
• microwave links
• satellite connections
• specialized communication processors
• interface to LANs (protocol, transfer rate)
• transmission of information
• routing
• billing, usage statistics

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WAN Examples

• Arpanet
• Internet
• company networks

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Internet as WAN Example

• LANs are connected to regional networks
• regional network are connected with routers and
  high-speed links
• special purpose links
• telephone connections
• connections from LANs to the Internet
• telephone modems (up to 50 KBit/s)
• ISDN (up to 128 KBit/s)
• T1 connections (1.544 MBit/s)
• can be combined into multiple links

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Communication

• internal workings of networks
• naming
• routing
• packet strategies
• connection strategies
• contention

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Naming

• the entities to exchange information are
  processes
• processes must specify the recipient
• sometimes also the sender
• names are more appropriate for human users
• must be unique, at least within the horizon of
  the user
• computers use numbers internally
• hierarchical numbering scheme for unique
  addresses
• mapping of names to numbers (addresses)
• single file with all names and addresses
• impractical for large systems
• distributed scheme
• more complicated to implement

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Domain Name Service

• naming scheme used in the Internet
• specifies the naming structure of sites and hosts
  
• conversion from names to addresses
• logical names
• consist of several fields separated by periods
• www.cis.njit.edu refers to a host named www in
  the CIS department at NJIT

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Name Resolution

• the name is parsed in reverse order
• edu indicates that the requested host is in an
  educational institution, and the name server for
  the edu domain is contacted for the address of
  njit.edu refers
• the edu name server returns the address of the
  host acting as name server for njit.edu refers
• the njit.edu returns the address of the
  cis.njit.edu name server, or directly the address
  of the requested host
• this results in an Internet address (IP-address)
  of the form 128.132.55.116
• caching improves access speed and reliability

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Routing

• in WANs there are usually multiple potential
  connection from host A to host B
• a routing table contains information about
  possible communication paths
• alternative routes
• speed, costs
• routing schemes
• fixed
• virtual
• dynamic

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Fixed Routing

• the path from host A to host B is specified in
  advance
• it does not change unless severe obstacles are
  encountered
• e.g. hardware failure
• used for heavily used connections
• minimization of communication costs or transfer
  time

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Virtual Routing

• the path from A to B is fixed for the duration of
  one session
• different paths for different sessions are
  possible
• more flexible than fixed routing
• reasonably easy to administrate
• parts of a message take the same path

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Dynamic Routing

• the path for a message from A to B is chosen at
  the time of sending the message
• different messages from the same session may take
  different paths
• composition of messages at the receiver can
  become complicated
• messages may arrive out of order
• takes into account the traffic conditions
• link failures, load changes

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Router

• responsible for routing messages
• can be a computer, or a special purpose device
• has routing tables for the networks it is
  connected to
• possibly cached copies of the name files from the
  name server
• examines the destination IP address and decides
  where to send the message

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Packet Strategies

• variable-length messages are commonly implemented
  through fixed-length packets
• datagrams, frames are alternative names for
  packets
• connections are established to transfer packets
  constituting a message
• to increase reliability, acknowledgment packets
  can be sent

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Connection Strategies

• establishing communication between processes that
  want to exchange information
• circuit switching
• a connection is established for the whole
  duration of the session
• similar to the telephone system
• inefficient resource utilization
• message switching
• packet switching

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Circuit Switching

• a connection is established for the whole
  duration of the session
• similar to the telephone system
• inefficient resource utilization
• the communication line is reserved even if there
  is no activity
• little overhead for individual messages

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Message Switching

• a connection is established for the transfer of
  one particular message
• similar to letters sent through the postal
  service
• better resource utilization
• many messages can use the same link
• no waste due to inactivity of individual
  processes
• more overhead for messages
• each message must contain administrative
  information
• receiver, sender, etc.

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Packet Switching

• individual packets from one message are sent
  separately
• very good resource utilization
• better load balancing
• efficient treatment of packets since all are of
  the same size
• overhead for individual packets
• receiver, sender
• packets must be reassembled into messages
• may not be suitable for time-sensitive information

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Contention

• several hosts may want to use a link
  simultaneously
• mainly relevant for bus topologies
• without coordination, data will be scrambled and
  becomes useless
• solutions
• collision detection
• token passing
• message slots

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Collision Detection

• a host can only use a link if it is free
• if two hosts start transmitting at the same time,
  a collision will occur
• the collision must be detected
• the hosts will try again with some random delay
• many collisions in high-traffic situations
• limited number of hosts on a network segment
• used in the Ethernet protocol
• CSMA/CD carrier sense with multiple
  access/collision detection

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Token Passing

• a unique message (token) circulates in the
  network
• normally a ring topology
• a host may transmit only if it has the token
• must wait until the token arrives
• forwards the token after the transmission is
  finished
• precautions must be taken for a lost token
• constant performance, independent of traffic load
• worse than CSMA/CD in low traffic, better for
  high traffic
• used in IBM Token Ring networks

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Message Slots

• a number of fixed-length message slots circulate
  in the system
• normally a ring topology
• variable-size messages may have to be partitioned
• a host must wait until an empty slot arrives
• used int the Cambridge Digital Communication Ring
• experimental system
• little experience with performance

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Network Design

• complex problem involving various levels of
  abstraction
• names, IP addresses, messages, packets
• coordination between different protocols
• one approach is to partition the problem into
  several layers
• ISO/OSI protocol layers
• ISO/OSI network model

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OSI Model

• Open Systems Interconnection (OSI) reference
  model
• developed by the International Organization for
  Standardization (ISO)
• is commonly used as abstract model
• TCP/IP is much more widely used in practice
• less complex
• more mature

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OSI Layers
interaction with the user file transfer, remote
login, email, distributed data bases
Application
conversion of different formats and data
representations characters (ASCII, ISO),
transmission modes
Presentation
communication protocols between processes remote
login, file and mail transfer (at the process
level)
Session
transfer of data between processes message
handling, error recovery, flow control
Transport
independence from network technologies network
routing, addressing, connection management
Network
reliable transfer of information on physical
links packet handling, error detection and
recovery on lower level
Data Link
transmission of a bit stream over the physical
medium mechanical and electrical network interface
Physical
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Network Example

• exchange of information between hosts on
  different Ethernet networks
• TCP/IP
• TCP transmission control protocol
• IP Internet protocol
• very widely used
• available for practically all computer systems
• simpler than the ISO/OSI model
• four layers instead of seven
• Ethernet
• very popular LAN

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TCP/IP Example
Host A
Host B
logical connection
Router
IP
Network 1
Network 2
NAP 1
NAP 2
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Client/Server Computing

• applications are separated into tasks
• client tasks
• user interface, presentation, some processing
• server tasks
• data management, storage, computation-intensive
  processing
• cooperation between client and server
• clients request services from servers
• servers return results to clients
• network environment
• frequently LAN, sometimes WAN

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Client/Server Diagram
Servers
Clients
Network (LAN, WAN)
Stallings 98
58
Client/Server Architecture
Client
Server
Presentation Services
Request
Application Logic (Client Side)
Application Logic (Server Side)
Response
Communication
Communication
Protocol
Operating System
Operating System
Hardware Platform
Hardware Platform
Stallings 98
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Client/Server Example Data Base
Client
Presentation Services
Server
Application Logic (Client Side)
Application Logic (Server Side)
Request
Data Base Logic
Data Base Logic DBMS
Response
Communication
Communication
Protocol
Operating System
Operating System
Hardware Platform
Hardware Platform
Stallings 98
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Client/Server Usage

• a data base is a good example for client/server
• centralized maintenance
• possibly large storage space requirements
• computation-intensive operations
• sorting, searching, joins
• powerful server with large hard disk(s)
• client provides user interface, smaller
  computations
• potential problem balance between client
  server
• searching should not be done by the client
• network traffic to transfer the whole data base
• computation power required

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Client/Server Advantages

• lower cost than mainframes
• better utilization of PCs, workstations
• distribution or centralization of critical
  services
• possible, as needed
• resource sharing
• load distribution

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Client/Server Problems

• client administration
• different platforms (hardware, OS, language)
• software distribution
• reliable
• verifiable
• security
• network
• information
• integration of various systems
• legacy systems
• multi-vendor, multi-platform environments

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Middleware

• tools and methods that provide a uniform access
  mechanism to systems across all platforms
• standardized interfaces and protocols
• examples
• Common Object Request Broker Architecture (CORBA)
• Common Object Model (COM), Object Linking and
  Embedding (OLE)
• Java

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Middleware Diagram
Application
APIs
Middleware (distributed system services)
Platform Interfaces
Platform
Stallings 98
65
Middleware Example
Data Base Server
Servers
Clients
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Tiered Architectures

• distinction between various types of networked
  computer system
• drive towards a global shared information space
  (WWW)
• dynamic, executable content
• platform-independent implementation
• integration of existing systems

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Tiered Architectures cont.

• utilization of resources
• processing power of underutilized computers
• access to networked resources
• total cost of ownership (TCO)
• basic costs for infrastructure
• cost per computer system

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Single-Tiered Architecture

• typical mainframe with directly connected
  terminals
• all resources are available through the mainframe
• advantages and problems
• easy to manage
• high cost
• low flexibility
• connectivity
• usually star network with mainframe as central hub

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Single-Tier Diagram
mainframe
terminals
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Two-Tiered Architecture

• typical client/server model
• server provides various services
• clients have limited processing power
• advantages and problems
• lower cost
• better use of desktop processing power
• higher flexibility
• access to services on various servers
• more difficult to manage and program
• connectivity
• flexible, usually LAN
• separation of business logic from client
  processing

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Two-Tier Diagram
servers
PCs, workstations
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Three-Tiered Architecture

• integration of existing systems (legacy
  systems)
• frequently via middleware
• reduction of management and program development
  problems through platform-independent methods
• CORBA, Java
• advantages and problems
• reasonable to manage and program
• platform-independent applications
• intermediate components
• high flexibility
• integration of legacy systems
• LAN-based connectivity

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Three-Tier Diagram
mainframes
servers
PCs, workstations
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Important Concepts and Terms

• application layer
• bus
• computer system
• CSMA/CD
• distributed operating system
• Ethernet
• fully connected
• hierarchical network
• host
• ISO/OSI model
• Internet
• local
• machine
• middleware

• network operating system
• node
• operating system
• partially connected
• physical layer
• processes, tasks
• remote
• resource
• ring
• service
• site
• star
• TCP/IP
• tier
• topology

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Chapter Summary

• most computer systems are connected to networks
• local area network (LAN)
• wide area network (WAN), especially Internet
• networking enables communication, collaboration,
  and access to shared resources
• networks require infrastructure
• interconnections (links)
• protocols
• a client/server model is frequently used in
  networks
• middleware integrates various platforms and
  applications on networks