The OSI Model

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Chapter 3
All Material Taken from Faster Smarter Network
Certification by Melissa Craft

• The OSI Model

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The OSI Model is often referred to as a Protocol
Stack
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The Layers

• The Upper layers (Application, Presentation, and
  Session) facilitate moving data between
  applications.
• The Middle Layers (Transport, Network) handle
  movement between network segments.
• The Lower Levels (Data-link, Physical) deal with
  physical and logical topology and enable movement
  across a single network segment.

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Encapsulation

• The sending computer passes data down in the
  stack (encapsulation).
• Each layer breaks the message down into smaller
  packets.
• When the message reaches the physical layer, it
  is converted into signals and sent across the
  media.

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Encapsulation (cont)

• The message is received by the destination
  computer and the message is sent upwards through
  the stack
• Each layer reassembles the message into larger
  packets
• Data is fully assembled at the application layer.

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The Physical Layer

• Transmits raw data in streams of bits across
  physical media
• Uses electrical impulses, light signals, or radio
  signals
• Connections are either multipoint or
  point-to-point
• Repeaters and hubs operate at this level

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The Data-Link Layer

• Supplies hardware addresses, identifies errors,
  and provides flow control
• Assembles bits from physical layer into frames
• Ethernet, token Ring, and ATM are Data-Link
  protocols
• Switches and Bridges operate at this level.

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The Data-Link Layer (cont.)

• Media Access Control (MAC)
• Contention-who gets to transmit?
• Polling-are you ready to transmit? (rarely used)
• Token passing-A special frame called the Token is
  passed around the ring. A device must have the
  token in order to transmit.

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The Data-Link Layer (cont.)

• MAC addresses
• Assigned to every NIC by the manufacturer
• Addresses are 6 bytes in length
• First 3 bytes identify manufacturer
• Hexadecimal digits (numbers 0-9, letters A-F)
• Address appears in Data-Link frame header

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The Data-Link Layer (cont.)

• Logical Link Control
• Uses control packets called Protocol Data Units
  (PDUs) which contain header information for
  network layer
• Can be used in connectionless or
  connection-oriented services
• Provides flow control and frame sequencing

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The Network Layer

• Provides internetworking services (different
  network segments)
• Determines best route to send data if multiple
  routes exist
• Error handling, congestion control, packet
  sequencing
• Datagrams are converted into frames when passed
  onto the data-link layer
• Routers operate at this level

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The Network Layer (cont.)

• Cell switching
• Circuit Switching
• Message Switching
• Packet Switching
• (See Pages 79-82)

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The Transport Layer

• Provides for transparent transfer between sender
  receiver
• Ensures that data is completely transferred
• Uses port numbers
• Multiplexing is made possible here
• Can be connectionless or connection oriented
• Uses ACK (acknowledgement) or NAK (negative
  acknowledgement) packets to verify data delivery
  if a connection-oriented mode is used

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The Session Layer

• Establishes connection between applications and
  manages the connection until termination
• Remote procedure calls (RPCs) function here. This
  is when a remote computer executes a program and
  it appears that it is running on the remote
  computer (Citrix and Telnet)
• Ensure proper termination and reallocation of
  resource afterwards
• Sets agreement services used duration of
  services
• Verification of IDs passwords occur here

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The Session Layer (cont.)

• Data is transmitted one of 3 ways in a session
• Simplex-data can travel only in one direction
  (am/fm radio or television)
• Half-duplex-data can travel both ways, but only
  one way at a time (walkie-talkies CB radios)
• Full-duplex-simultaneous communication in both
  directions (telephone)

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The Presentation Layer

• Provides way for data to be presented at the
  application layer
• Encryption/decryption
• Data compression/expansion
• Also know as syntax layer

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The Application Layer

• Supports end-user processes
• Typically the only layer involving end-user
  interaction
• Identifies sending receiving devices, quality
  of service (QoS) authentication privacy
• File transfers, E-mail, Web browsing, chat,
  network printing all happen here.

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Network Equipment and OSI Layers

• NICS operate at Layers 1 and 2.
• Hubs and repeaters operate at Layer 1.
• Bridges and Switches operate at layer 2.
• Routers operate at Layer 3
• Brouters operate at both layers 2 and 3.
• Gateways operate at Layers 5, 6, and 7.