Chapter 3 Operating Systems and Networks

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Chapter 3 Operating Systems and Networks

• The Evolution of Operating Systems
• Operating System Architecture
• Coordinating the Machines Activities
• Handling Competition Among Processes
• Networks
• Network Protocols
• Security

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3.1 The Evolution of Operating Systems

• Single-Processor Systems
• Batch processing the execution of jobs by
  collecting them in a single batch, then executing
  them without further interaction with the user.
• Interactive processing the execution of programs
  that carried on a dialogue with the user through
  remote terminals or workstations.
• Queue a storage organization in which objects
  are ordered in first-in, first-out (FIFO)
  fashion.
• Real-time processing the coordination between
  the machine and its environment.

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• Time-sharing the technique of dividing time into
  intervals, or time slices, and then restricting
  the execution of a job to only one time slice at
  a time. multitasking
• Multiprocessor Systems
• Load balancing making sure that the processors
  are used efficiently
• Scaling breaking tasks into a number of subtasks
  compatible with the number of processors in the
  machine.

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3.2 Operating System Architecture

• Shell defines the interface between the
  operating system and its users.
• Modern shells perform this task by means of a
  Graphical User Interface (GUI)
• Window Manager allocates blocks of space on the
  screen, called window, and keeps track of which
  application is associated with each window.

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Kernel

• File manager coordinates the use of the
  machines mass storage facilities.
• Device drivers communicate with the controllers
  to carry out operations on the machines
  peripheral devices.
• Memory manager coordinates the use of main memory

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Virtual Memory

• The illusion of additional memory space by
  rotating programs and data back and forth between
  main memory and mass storage.
• Divide the required space into units called pages
  and store the contents of these pages in mass
  storage.
• Scheduler determines which activities are to be
  considered for execution
• Dispatcher controls the allocation of time
  slices to these activities.

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Boot strapping

• A CPU is designed so that its program counter
  starts with a particular predetermined address
  each time the CPU is turned on.
• It is at this location that the CPU expects to
  find the first instruction to be executed. This
  portion of memory is normally constructed in such
  a way that its content is permanent. ROM
• Bootstrap the small program that is permanently
  stored at the above location.

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3.3 Coordinating the Machine Activities
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Process

• The activity of executing a program.
• A process encompasses the current status of the
  activity, called the process state.
• This state includes the current position in the
  program being executed as well as the values in
  the other CPU registers and the associated memory
  cells.

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Process Administration

• Process table To keep track of all the
  processes, the scheduler maintains a block of
  information in main memory.
• Memory area assigned to the process, the priority
  of the process, and whether the process is ready
  or waiting
• Ready a state in which its progress can
  continue.
• Waiting its progress is currently delayed until
  some external event occurs
• Process switch The procedure of changing from
  one process to another

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Interrupt

• When the CPU receives an interrupt signal, it
  completes its current machine cycle, saves its
  position in the current process, and begins
  executing a program, called the interrupt
  handler, that is stored at a predetermined
  location in main memory.

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The Client/Server Model

• Client makes requests of other units
• Server satisfies the requests made by clients
• COBRA provides a standard for the network-wide
  communication between software units known as
  objects.

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3.4 Handling Competition Among Processes

• Semaphores test-and-set
• Critical region a sequence of instructions that
  can be executed by only one process at a time.
  Mutual exclusion
• To enter the critical region, a process must find
  the semaphore clear and then set the semaphore
  before entering the critical region then upon
  exiting the critical region, the process must
  clear the semaphore.

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Deadlock

• The condition in which two or more processes are
  blocked from progressing because each is waiting
  fro access to resources allocated to another.
• 3 necessary conditions
• There is competition for nonshareable resources.
• The resources are requested on a partial basis
  that is, having received some resources, a
  process will return later to request more.
• Once a resource has been allocated, it cannot be
  forcibly retrieved.

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• The operating system has made the nonshareable
  resource appear shareable by creating the
  illusion of more than one printer. The technique
  of holding data for output at a later but more
  convenient time is called spooling.

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FIGURE 3.9 A deadlock resulting from
competition for
nonshareable railroad intersections
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3.5 Networks

• LANs local area networks
• WANs wide area networks
• 4 popular configurations Ring, Bus, Star,
  Irregular

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The Internet

• Network of networks
• Each network in the Internet is connected to
  another network by a machine called a router.
• A collection of networks clusters known as
  domains, each of which normally consists of those
  networks operated by a single organization.

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Internet addressing

• 32 bits consisting of two parts a pattern
  identifying the domain in which the machine
  resides (network identifier) and a pattern
  identifying the particular machine within the
  domain (host address).
• The network identifier is assigned by the
  InterNIC (Internet Network Information Center)

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Figure 3.11 The distinction between a bridge
and a router
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Figure 3.12 A typical approach to connecting to
the Internet
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• An individual normally obtains access to the
  Internet through membership in an organization
  with a domain.
• Several companies, called Internet access
  providers, offer Internet access to individuals
  on a commercial basis.
• Hypertext contains words, phrases, or images
  that are linked to other documents.

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3.6 Network Protocols

• Protocols The rules that govern the
  communication between different components within
  a computer system.
• How messages are addressed, how the right to
  transmit messages is delegated among the
  machines, how the duties of packaging messages
  for transmission and unpacking received messages
  are to be handled.

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Figure 3.14 A simple Web page expressed in HTML
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Figure 3.15 Communication over a ring network
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Controlling Transmission Privileges

• Ring a unique bit pattern, called a token, is
  passed around the ring. Possession of this token
  gives a machine the authority to transmit its own
  message without the token, a machine is only
  allowed to forward messages.

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Ethernet

• CSMA/CD Carrier Sense, Multiple Access with
  Collision Detection
• Each message transmitted by any machine be
  broadcast to all the machines on the bus. Each
  machine monitors all the messages but keeps only
  those addressed to itself. To transmit a message,
  a machine waits until the bus is silent, at which
  time it begins transmitting while continuing to
  monitor the bus.
• If another machine also begins transmitting, both
  machines detect the clash and pause for a brief
  random period of time before trying to transmit
  again.

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The Layered Approach to Internet Software

• The transport layer divides long messages into
  segments of a size compatible with the underlying
  network layer. It then adds sequence numbers to
  these segments so that the original message can
  be reconstructed at the messages destination.

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Figure 3.16 Communication over a bus network
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Network Layer

• The network layer oversees the intermediate steps
  made by packets as they find their way across the
  Internet. It does this by appending an
  intermediate destination address to each packet.

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Figure 3.17 Package-shipping example
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Link Layer

• To deal with the communication details particular
  to the individual network (Ring, bus) in which
  the machine resides.
• To assist with this readdressing process, the
  network layer maintains a routing table that
  contains the final destination addresses that it
  has dealt with recently and the intermediate
  addresses to which it forwarded each of those
  packets.

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Figure 3.18 The Internet software layers
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Figure 3.19 Following a message through the
Internet (continued)
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Figure 3.19 Following a message through the
Internet (continued)
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Figure 3.19 Following a message through the
Internet
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Figure 3.20 Choosing between TCP and UDP
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• Network layer hands its transport layer only
  those packets that addressed to the local
  machine.
• Only the link and network layers are involved in
  the forwarding of packets destined to other
  machines.
• Once the message is complete, the transport layer
  hands it to its application layer.

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TCP/IP protocol suite

• A collection of protocols defining the four-level
  hierarchy used by the Internet.
• TCP defines a version of the transport layer.
• Before sending data, a transport layer based on
  TCP sends a message to the transport layer at the
  destination telling it that data are about to be
  sent and which application layer software is to
  receive the data.
• It then waits for this message to be acknowledged
  before starting to send message segments.

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• TCP transport layers at the origin and
  destination work together by means of
  acknowledgments and segment retransmissions to
  confirm that all segments of a message are
  successfully transferred to the destination.

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• IP is the Internets standard for the network
  layer.
• Each time an IP network layer prepares a packet
  to be handed to the link layer, it appends a
  value called a hop count (64), or time to live,
  to that packet.
• This value is the limit to the number of times
  the packet should be forwarded as it tries to
  find its way through the Internet.
• With this information, the network layer can
  protect the Internet from packets circling
  endlessly within the system.

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3.7 Security

• One popular approach to encryption of messages
  sent over the Internet is called public-key
  encryption.
• Public-key encryption involves the use of two
  values called keys. One key, known as the public
  key, is used to encode messages and is known to
  by all people authorized to generate messages
  the other, known as the private key, is required
  to decode messages and is know by only the person
  who is to receive messages.

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Single-Key Cryptography

• Encoding The process of creating a coded
  message.
• Decoding The process of unscrambling a coded
  message using a key.
• When the same key is used for both encoding and
  decoding, the code is called single-key
  cryptography.
• The more encoded messages you have, the easier it
  is to break the code. One way is to identifier
  the most frequently used characteristics and
  character sequences in those message.

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• Virus a program segment that attaches itself to
  other programs in the computer system.
• Worm an autonomous program that transfers itself
  through the network, taking up residence in
  machines and forwarding copies of itself through
  the network.

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ADSL (Asymmetric Digital Subscribe line)

• ADSL offers a 1.536Mbps downlink channel (T1
  minus the 193rd bit), but only a 16Kbps uplink
  channel.
• The old 4KHz analog telephone channel is also on
  there.
• The uplink has enough bandwidth for the user to
  order movies, and the downlink has enough
  bandwidth to send them encoded in MPEG-1.