; Building a Network with the OSI Model
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Building a Network with the OSI Model


  • pg 1
									Building a Network with the
        OSI Model

         Chapter 2
• Describe models such as the OSI seven layer
• Explain the major functions of network
  hardware with OSI Layers 1-2
• Describe the functions of network software
  with OSI Layers 3-7
 The CompTIA Network+ Challenge
• Understand every aspect of networking
  – Use the Open Systems Interconnect (OSI)
  – Conceptualize the parts of a network
• The OSI Seven-Layer Model provides
  – A powerful tool for diagnosing
  – A common language to describe
Figure 2.1 Using the OSI terminology – Layer 3 –
in a typical setup screen
Working with Models
Biography of a Model
• What does “model” mean to you?
  – Computer models that predict weather
  – Plastic model airplane
  – Fashion model

                Figure 2.2 Types of Models
A model has all the major
functions of the real item
f   ti     f th     l it

   Figure 2.3 Simple model airplane
The OSI seven-layer model
• What functions define all networks?
• What details can be omitted?
  ISO (International Organization for 
• ISO (International Organization for
  Standardization) proposed the OSI 
  seven layer model
  seven‐layer model
                   Layer 7 - Application

  The Seven        Layer 6 - Presentation

Layers in Action               Session
                     Layer 5 - S

                    Layer 4 - Transport

                     Layer 3 - Network
  The OSI Model
                    Layer 2 - Data Link

                     Layer 1 - Physical
     Welcome to MHTechEd!
A conceptual viewpoint 
of networking
of networking
  – One of the workers has 
    just completed a new
    just completed a new 
    employee handbook
  – She needs to transfer 
    the Word document to      Figure 2.4 Janelle and Tiffany,
    the other worker for      hard at work
She could…
  Copy the file to a flash drive and walk it 
• Copy the file to a flash drive and walk it
  over to the other person (sneakernet)
•T      f h fil        i   h
  Transfer the file using the networkk
Let’s Get Physical…
  d     i    h        kh d
and examine the network hardware 
components required for this transfer
  Most networks use a cable, 
• Most networks use a cable
  like this one, as a physical 
  channel to move the bits of 

  Unshielded Twisted Pair
       (UTP) cable

                                  Figure 2.5 UTP cabling
• Each computer system has 
  a cable leading to a device 
  called a hub
   – Usually located in a closet
• The hub sends the data 
  received from one system 
  to all the other systems 
  to all the other systems
  attached to it
                                   Figure 2.6 Typical hub
Figure 2.7 The network so far, with the Physical layer
    Network Interface Cards
• Network Interface Cards 
  (NICs) are installed in 
• Network cables attach 
  to the NICs

                             Figure 2.8 Typical NIC
   NIC to Hub Connections
• Cables run from the 
  NIC in the PC to a jack 
  NIC in the PC to a jack
  on the wall
• Cables run through the 
  walls to the closet 
  where they connect to 
  a hub

                             Figure 2 9        ith       connect-
                             Fig re 2.9 NIC with cable connect
                             ing the PC to the wall jack
Network Cabling System

    Figure 2.10 The MHTechEd network
   The NIC                               MAC address printed on
                                       surface of chip – and burned
                                             inside the chip.
• Each system must 
  have a unique 
• Media Access Control 
  (MAC) address
  – A unique address burned 
    into a ROM chip on the 
    network card
    Each MAC address is 12 
  – Each MAC address is 12
    hex characters or 48 bits in 

                                    Figure 2.11 MAC address
            MAC Addresses
• MAC addresses are 48‐bits long
  Usually represented using hexadecimal characters 
• Usually represented using hexadecimal characters
  (12 hex digits = 48 bits)
   – A typical MAC address:

Organizationally unique      Device ID
    identifier (OUI)

            No two MAC addresses are ever the same!
     ipconfig /all

                                       MAC address

Figure 2.12 Output from IPCONFIG/ALL
   Moving Data

  g                   g     g
Figure 2.13 Data moving along a wire
   Moving Data

Figure 2-14: Oscilloscope of data
    Moving Data

Figure 2.15 Data as ones and zeroes

Figure 2-16 Inside the NIC
             Inside a frame
• Frames are made up of fields that contain 
• Frames contain the recipient’s MAC address, the 
  sender’s MAC address, the data itself, and a cyclic 
                       ,               ,        y
  redundancy check (CRC) for error checking

              Figure 2.17 Generic frame
Frame as a canister

   Figure 2.18
   Fig re 2 18 Frame as a canister
              Frame Size
• Different networks use different sizes of 
• Many frames hold about 1500 bytes of data
• The sending software breaks up large 
  amounts of data into smaller chunks
• The receiving station must then put the 
                 g            p p
  chunks back together in the proper order
      Processing Frames
• All devices on the network see the frame, 
  but only the device that it is addressed to 
  b t l th d i th t it i dd              dt
  will process it
    Every frame is received by every NIC
  – Every frame is received by every NIC
  – The MAC address is used to decide if the frame 
    belongs to a given device
Getting Data on the Line
• Since the cable is shared, only one 
  system may speak at a time
• Processes are used to keep two NICs
  Processes are used to keep two NICs 
  from talking at the same time
Incoming Frame!

 Figure 2.19 Incoming frame!
     Getting To Know You
• Usually two devices have talked before, so 
  the destination MAC address is already 
  the destination MAC address is already
  If the MAC address is not known, a broadcast 
• If the MAC address is not known, a broadcast
  message is sent over the network
  – The destination device will respond by sending 
    its MAC address
  – A MAC broadcast address is FF‐FF‐FF‐FF‐FF‐FF
Figure 2.20 Building the frame
Figure 2.21 Adding the data and CRC to the frame
Figure 2.22 Sending the frame
Figure 2.23 Reading an incoming frame
After the frame is received
• The receiving station checks the CRC value in
  the frame
  – If the value matches what it should, then the NIC
    sends the data portion to the network operating
    system for processing
  – If the value does not match, the frame has errors and
    must be resent
   The Two
Aspects of NICs

   Figure 2.24 Layer 1 and Layer 2 are now properly
                 applied to the network
Figure 2.25 LLC and MAC, the two parts of the Data Link Layer
  y           g
Beyond the Single Wire –
   Network Software
    and Layers 3 – 7

       2.26                       (left)
Figure 2 26 Large LAN complete (left),
   and broken into two subnets (right)
      Network Protocols
• Network protocols define rules for how systems
       dd     d h     t h     d t     i t h k
  are addressed, how to chop data up into chunks,
  how to deal with routers, and so on
• As a network grows, a more universal address-
  ing method than MAC addresses is needed
  – Transmission Control Protocol (TCP)
  – Internet Protocol (IP)
IP – Playing on Layer 3, the Network Layer
  • IP address: a unique numeric identifier
  • An IP address is a logical address while a
    MAC address is a physical address
  • IP uses a dotted-decimal notation
  • Each 8-bit number ranges from 0 to 255
    – Example:
    No two systems on th same network share
  • N t        t      the       t   k h
    the same IP address
  Routers are used to 
• Routers are used to
  chop large networks up 
  into smaller ones
  Routers forward 
• Routers forward
  packets by logical 
  An IP router (most 
• An IP router (most
  common) forwards IP 
  Works at Layer 3, the 
• Works at Layer 3 the
  Network layer

                            Figure 2.27 Typical       router
                            Fig re 2 27 T pical small ro ter
Figure 2.28 MHTechEd addressing
Figure 2.29 Router added to the OSI model for the network
Frames (packets) within Frames
  • Network software creates a packet that
    contains the sending and receiving IP
    addresses along with the data

             Figure 2.30
             Fig re 2 30 IP packet
Figure 2.31 IP packet in a frame (as a canister)
• The packet is enclosed within a frame that
  contains the sending and receiving MAC
                      IP packet in a frame

           Figure 2.32 IP packet in a frame
    Connecting to the Internet
• A router connects a local network to the Internet
• The local hub is connected to the router
• The router is connected to the Internet through a
  cable or phone line
• The cable or phone line uses a different kind of
  frame so the router strips the frame and creates
  a new one
Figure 2.33 Adding a router to the network
 C        ti
to the Internet

              Figure 2.34 Router removing network frame
                 and adding one for the cable line
Figure 2.35 Router in action (notice addresses)
• The router replaces the MAC address with the type
  of address used by the cable or phone company
• The frame uses the IP address to guide it to the
  receiving system
• The receiving router strips off the cable or phone
  company f           d dd th           dd      f th
            frame and adds the MAC address for the
  receiving system
• The NIC strips off the MAC header and hands the
  frame off to the NOS
           y              y     y
   Assembly and Disassembly – Layer 4,
   the Transport Layer
  Most data is much larger than a single frame
• Most data is much larger than a single frame
• Network protocols chop up the data into smaller 
  packets, and give each one a sequence number
  p       ,    g                 q
• The sequence numbers are used by the receiving system 
  to put the packets back in order, and to assemble them
• This compares to the numbering of boxes by UPS
Figure 2.36 Labeling the boxes
  Transport layer is the assembler/disassembler
• Transport layer is the assembler/disassembler
• Transport layer also initializes requests for packets 
  that weren’t received in good order
Figure 2.37 OSI updated
     Talking on a Network – Layer 5
           the Session Layer
• One system may be talking to many other
  systems simultaneously
• The software that handles these processes is
  called session software, working at Layer 5
Figure 2.38 Handling multiple inputs
Figure 2.39 Each request becomes a session
Figure 2.40 OSI updated
  Standardized Formats – or Why Layer 6,
       Presentation, Has No Friends

• Presentation layer tasks solved an old problem
• Macintoshes and PCs use very different formats
  Standardized f       t h     b       t d th t
• St d di d formats have been created that
  allow very different operating systems to
  exchange data
Figure 2.41 Different data formats were
   often unreadable between systems
Figure 2 42 Everyone recognized PDF files!
Figure 2.43 OSI updated
    Network Applications – Layer 7,
             pp              y    ,
        the Application Layer
             pp           y            pp
• Users use Application layer network applications
  to exchange data on a network
   – Network in Windows Vista (My Network Places in
     earlier Windows)
   – Web browser like Internet Explorer or Netscape
   – Outlook Express for e-mail
• All operating systems have APIs at the Application
  layer for network-aware applications
Figure 2.44 Network applications at work
Figure 2.45 OSI updated
      How Tiffany Gets Her Document

• The next few slides illustrate a typical process
  that takes place to copy a file from one machine
  to another over a network
Figure 2.46 Network application showing computers
              on the MHTechEd network
Figure 2.47 Copying the Word document
Figure 2.48 Chopping the Word document
Figure 2.49 Creating and addressing packets
Figure 2.50 Creating frames
Figure 2.51 Tiffany’s system grabbing a frame
• The OSI seven layer model is a troubleshooting
• Example: Jane can’t print to the networked
  – Layer 1 and 2: NIC shows activity?
  –L      3 Does computer h
    Layer 3: D           t have a proper IP
  – Move up through the layers to discover problem

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