Operating Systems

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Operating Systems
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Internet Addresses
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Universal Identifiers

• Universal Communication Service - Communication
  system which allows any host to communicate with
  any other host
• Name what an object is
• Address where it is
• Route how to get there

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IP Address

• Unique 32-bit address

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IP Classes
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IP Classes
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Layers
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TCP/IP vs OSI
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Open System Interconnect

• The layers of the OSI model provide a framework
  for understanding networking.
• The OSI model is not a protocol, but rather a
  reference model (abstract structure) that
  describes the functions and interactions of
  various data communication protocols.

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OSI Stack

• Layers 5 through 7, the "higher" layers, are
  concerned with communication between
  applications.
• Layers 1 through 4, the "lower" layers, are
  concerned with transmitting raw data between
  computers.

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Physical Layer

• The Physical Layer deals with transmitting and
  receiving bits across a physical medium.
• Includes the following types of hardware devices
  that send and receive signals over each type of
  physical medium
• Network interface cards (NICs)
• Fiber optic transceivers
• Radio transceivers
• Modems

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Physical Layer (cont.)

• Concerned only with transmitting and receiving
  physical signals that represent data bits.
• Demo
• Most error detection and all error correction are
  the responsibility of higher layers.

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Data Link Layer

• The Data Link Layer deals with frames over a
  single physical link. A Data Link Layer address
  is the unique address built into a NIC.
• It uses the signaling services of the Physical
  Layer below it.
• To Layer 3 above, it provides the service of
  addressing a message to a device located across a
  single physical transmission path.

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Data Link Layer (cont.)

• Network Layer handles data in units called
  "packets.
• Network Layer passes a packet to the Data Link
  Layer, which encapsulates the packet by placing a
  header and trailer around it (frame).
• Frame header includes the unique NIC addresses of
  the sending and receiving nodes.
• Frame trailer contains error-checking information

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Data Link Layer (cont.)

• Network Layer handles data in units called
  "packets.
• Network Layer passes a packet to the Data Link
  Layer, which encapsulates the packet by placing a
  header and trailer around it (frame).
• Frame header includes the unique NIC addresses of
  the sending and receiving nodes.
• Frame trailer contains error-checking information

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Data Link Layer (cont.)Transmitting Node

• Accepts data packets of arbitrary length from the
  Network Layer.
• Accepts the address of an adjacent node to which
  it is to transmit the data.
• Adds sequence information to the frame in case
  they get out of sequence during error recovery.
• Adds error detection and correction codes to the
  frames.
• Does not send frames to the Physical Layer at a
  faster rate than the receiving Data Link Layer
  process can handle them.

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Data Link Layer (cont.)Receiving Node

• Receives bits passed up from the Physical Layer
  and interprets groups of bits as frames.
• Checks each frame for errors and takes corrective
  measures if necessary.
• Handles problems, such as missing frames, through
  handshaking with its peer process.
• Puts the frames back in the correct sequence to
  reconstruct the packet.
• Passes each decapsulated packet up to the Network
  Layer.
• Demo

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Network Layer

• The Network Layer deals with packets.
• Responsible for transmitting data packets between
  source and destination nodes that may not be
  connected by the same physical link
• Message may pass through an "intermediate node."
  Network Layer's job is to use intermediate nodes
  to transmit a packet to its destination.
• Demo

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Transport Layer

• The Transport Layer carries messages between
  processes (remember - multi-tasking).
• Transport Layer addresses are process addresses
  called "port numbers."
• The lowest layer in which peer software processes
  at either end of a connection carry on a
  conversation.
• Demo

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Transport Layer (cont.)Services

• Addressing
• deliver data to a specific process within a node.
• Connection management
• responsible for establishing and releasing
  connections between processes.
• During data transfer, processes on both hosts
  communicate to verify that data is being received
  without errors or loss.
• When communication is complete, the peer
  processes terminate the connection to free up
  resources for other processes.
• Flow Control and Buffering
• responsible for ensuring that the receiving node
  has enough memory buffers to store the incoming
  data
• data is not transmitted faster than the receiving
  node can accept it.

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Session Layer

• Where a conversation starts and stops
• The rules of the conversation are agreed upon.
• Facilitates a step-by-step interaction or session
  between two entities.
• Example - an interactive user session begins with
  a user logging on to the computer and ends with
  the user logging off.

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Session Layer (cont.)Electronic Conversation

• Parties first agree to talk to one another.
• They (usually) do not talk simultaneously.
• Divide the conversation into parts.
• End the conversation in an orderly fashion.

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Presentation Layer

• The process concerned with how data is
  presented on the computer and represented inside
  the computer.
• The "representation" layer.
• deals with the format of stored computer
  information.
• Three main services
• Data representation
• Data security
• Data compression

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Presentation Layer (cont.)Services

• Data Representation - resolves differences
  between different types of encoding systems.
• Byte ordering within integers
• Character coding (EBCDIC and ASCII)
• Format of floating point numbers
• Boolean
• Data Security - encrypts and decrypts data
• Authentication
• data encryption
• Data Compression - reduces the number of bytes
  that must be transmitted by translating the data
  into a more efficient form that requires less
  storage.

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Application Layer

• Includes some programs that interact directly
  with users, and some that provide services to
  user applications.
• E-mail
• USENET newsgroups
• File transfer and access
• Virtual terminals
• Web browsers and servers