SIMNET 10-001 _CCENT_ OSI Reference Model

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SIMNET 10-001 _CCENT_ OSI Reference Model Powered By Docstoc
					SIMNET 10-001 (CCENT)
OSI REFERENCE MODEL
Chris Seedyk
Network Engineer, MPRI
The Layered Approach
   In the early days of networking, communication
    between hosts as proprietary
   In the 1970s, the Open Systems Interconnection
    reference model (OSI) was created
     Helpsvendors create interoperable network devices
     Breaks the operation of networks down into layers
       Each   layer accomplishes a specific function
The Layered Approach
   Reference model – conceptual blueprint of how
    communications should take place
     Addresses process required for communications
     Divided processes into layer (logical groupings)

   When a communication system is designed in this
    method, it is referred to as layered architecture
   This allows designers and implementers to view a
    complete and consistent picture of system
    operations
The Layered Approach
   Example: starting a company
     You  sit down and think of tasks to be completed, who
      will complete them, and in what order
     You group these tasks into departments: ordering,
      processing, and shipping
       Each  department has its own tasks to complete the job
       Each department completes their own portion of the job and
        trusts the other departments to complete their
     When  the company is formed, each department has a
      blueprint of operations; they then develop methods to
      accomplish their goals
The Layered Approach
   Example: starting a company (con’d)
     1. Ordering receives a customer order; the fees are
      collected and the approved order is passed to
      processing
     2. Processing receives the order and extracts the
      required order items; these items are gathered and
      packages; the collection is sent to receiving
     3. Shipping receives the packaged items and extracts
      the customer information; the order is verified and sent
      to the customer
The Layered Approach
   This same layered approach can be applied to a
    variety of development processes
     Software development
     Business process development

     Network/communication systems design (OSI model)

   They key is that each part (layer) of the process uses its
    methods (protocols) to accomplish their job and hand the
    process over to another part (layer)
   This process is known as binding. The processes that are
    related are grouped together in a particular layer
The Layered Approach
   Advantages of Reference Models for Networking
     Divides the communication process into smaller
      components; aiding design, development and
      troubleshooting
     Allows multi-vendor develop through a standard
      component design
     Encourages industry standardization
     Allows various types of hardware and software to
      communicate
     Prevents changes in one layer from directly affecting
      another layer
The OSI Reference Model
   Creates a precedent and guide for an open
    network protocol set
   The OSI model has seven layer
     Application  (layer 7)
     Presentation (layer 6)
     Session (layer 5)
     Transport (layer 4)
     Network (layer 3)
     Data link (layer 2)
     Physical (layer 1)
The OSI Reference Model
   Just as with the starting a company example, each
    layer of the OSI model:
     Accomplishes a specific task
     Passes information to the next layer

   The OSI model layers are divided into two groups
    of layers
     Upper  layers: Application, Presentation, and Session
     Lower layers: Transport, Network, Data Link, and
      Physical
The OSI Reference Model
   Different devices operate at different layers of the
    OSI model
     Hubs/repeaters:   Physical layer (layer 1)
     Switches: Data link layer (layer 2)

     Routers: Network layer (layer 3)

     Some devices, such as network hosts and application
      servers, operate at all 7 layers of the OSI model
The OSI Reference Model
   Remember, each layer of the OSI model performs a
    specific function:
     Application: file, print, and application services
     Presentation: data encryption and translation services
     Session: dialog control
     Transport: end-to-end connection
     Network: routing
     Data Link: framing
     Physical: physical topology

   Let’s take a detailed look at the OSI Layer
The OSI Reference Model
   Application Layer (layer 7)
     Where   users actually communicate to the computer
     Acts as an interface between actual application
      programs
       Programs   may not necessary run at layer 7
     Provide  ways for applications to send information
      through protocol stacks
     Identifies and establishes the availability of the
      intended communication partner
The OSI Reference Model
   Presentation layer (layer 6)
     Performs data translation and code formatting
     Passes formatted information to the application layer
     Two primary functions
       Translate   machine/computer data into human-readable
        form
       Coding and conversion functions
     OSI model specifies standards on how data should be
      converted
     Also performs data compression, data decompression,
      and encryption.
The OSI Reference Model
   Session layer (layer 5)
     Sets-up,   manages, and tears downs sessions
       Sessions   between Presentation layer entities
     Coordinates communications between systems
     Organizes communication into three modes
       Simplex
       Half-duplex
       Full   duplex
     Keeps     application data separate between application
The OSI Reference Model
   Transport layer (layer 4)
     Segments  and reassembled data into a stream
     Provides end-to-end transport services

     Establishes logical connections between sending and
      destination hots
     Provides mechanisms for multiplexing upper-layer
      applications, establishing sessions between hosts, and
      tearing down virtual hosts
The OSI Reference Model
   Transport layer (con’d)
     Flow   Control
       Provides   buffering to prevent overflow and data-loss
     Connection-oriented      Communication
       Periodic   connection checks to ensure reliable connection
     Windowing
       Used to control the amount of outstanding, unacknowledged
        data segments
     Acknowledgements
       Receiving  notification that data was received
       Positive Acknowledgement with Retransmission
The OSI Reference Model
   Network layer (layer 3)
     Manages   addressing
     Determines the best way to move data

     Transports between devices that aren’t locally attached

     Uses two kinds of packets
       Data: used to transport user data
       Route update: update neighboring routers about changes

     Routers(and sometimes layer 3 switches) are the most
      prominent layer 3 devices
The OSI Reference Model
   Network layer (con’d)
     The   routing process
      A   packet is received on an interfaces and the IP address is
        checked
       If the packet is destined for another network, the router
        checks the routing table
       If a path is found, the packet is forwarded; if not, the
        packet is dropped
The OSI Reference Model
   Network layer (con’d)
     Several important points about routers; consider
      committing these to memory
       Routers, by default, do not forward broadcast or multicast
       Routers use logical addresses to determine the next hop
       Routers can used access lists to control access the networks
       Routers can perform bridging functions if needed
       Routers (and all layer 3 devices) provide connections
        between VLANs
       Routers can provide quality of service (QoS) for specific
        types of network traffic
The OSI Reference Model
   Data Link layer (layer 2)
     Providesfor physical transmission of data
     Handles error notification and flow control

     Formats messages from network layer into frames
       Frames    are later broken down into bits for the physical layer
     Has   two sub-layers
       LogicalLink Control (LLC) – identifying and encapsulating
        network protocols
       Media Access Control (MAC) – defines how packets are
        placed on the network (logical topology)
The OSI Reference Model
   Data Link layer (con’d)
     Switches (most) and bridges operate at layer 2
     Specialized hardware on a switch creates an
      application-specific integrated circuit (ASIC)
       This   is what allows switches to create collision domains
     Switches  store records of the location of specific MAC
      addresses for switching
     Filters can be used to only forward frame to specific
      network segments; this is called transparent bridging
The OSI Reference Model
   Physical layer (layer 1)
     Has   two primary functions:
       Sends bits
       Receives bits

     Communications   directly with various types of media
     Where physical signals and translated into bits and
      bytes and placed on a medium
     Specifies the electrical, mechanical, procedural, and
      functional requirements for activating, maintaining, and
      deactivating and physical link
The OSI Reference Model
   Physical layer (layer 1)
     Hubs    and repeaters operate at layer 1
       Hub   = multiport repeater
     Any  signal received on a hub is broadcast to all ports
      of a hub
     All devices on a hub:
       Are in the same collision domain
       Are in the same broadcast domain
       Share the same bandwidth
     Only one host at a time can transmit on a medium using
      a hub
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SIMNET 10-001 (CCENT)

				
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