The document describes the OSI 7-layer model and how it can be used to understand network communication. It explains each of the 7 layers, including the physical, data link, network, transport, session, presentation and application layers. Examples are provided to illustrate how data moves through each layer of the OSI model when transferring a file between two computers on a network.

1. Building a Network with the
OSI Model
Chapter 2

2. Objectives
• Describe models such as the OSI seven-layer
model
• Explain the major functions of network
hardware with OSI Layers 1-2
• Describe the functions of network software
with OSI Layers 3-7

3. Overview

4. The CompTIA Network+ Challenge
• Understand every aspect of networking
– Use the Open Systems Interconnect (OSI)
model
– Conceptualize the parts of a network

5. • The OSI Seven-Layer Model provides
– A powerful tool for diagnosing
problems
– A common language to describe
networks

6. Figure 2.1 Using the OSI terminology – Layer 3 –
in a typical setup screen

7. Working with Models

8. 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

9. A model has all the major
functions of the real item
Figure 2.3 Simple model airplane

10. The OSI seven-layer model
• What functions define all networks?
• What details can be omitted?
• ISO (International Organization for
Standardization) proposed the OSI
seven-layer model

11. Layer 7 - Application
The Seven Layer 6 - Presentation
Layers in Action Layer 5 - Session
Layer 4 - Transport
Layer 3 - Network
The OSI Model
Layer 2 - Data Link
Layer 1 - Physical

12. Welcome to MHTechEd!
A conceptual viewpoint
of networking
– One of the workers has
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
review

13. She could…
• Copy the file to a flash drive and walk it
over to the other person (sneakernet)
• Transfer the file using the network

14. Let’s Get Physical…
and examine the network hardware
components required for this transfer

15. Cabling
• Most networks use a cable,
like this one, as a physical
channel to move the bits of
data
Unshielded Twisted Pair
(UTP) cable
Figure 2.5 UTP cabling

16. Hubs
• 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
attached to it
Figure 2.6 Typical hub

17. Figure 2.7 The network so far, with the Physical layer
hardware

18. Network Interface Cards
• Network Interface Cards
(NICs) are installed in
PCs
• Network cables attach
to the NICs
Figure 2.8 Typical NIC

19. NIC to Hub Connections
• Cables run from the 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 NIC with cable
connect-ing the PC to the wall jack

20. Network Cabling System
Figure 2.10 The MHTechEd network

21. The NIC MAC address printed on
surface of chip – and burned
inside the chip.
• Each system must
have a unique
identifier
• Media Access Control
(MAC) address
– A unique address burned
into a ROM chip on the
network card
– Each MAC address is 12
hex characters or 48 bits in
length
Figure 2.11 MAC address

22. MAC Addresses
• MAC addresses are 48-bits long
• Usually represented using hexadecimal characters
(12 hex digits = 48 bits)
– A typical MAC address:
004005-607D49
Organizationally unique Device ID
identifier (OUI)
No two MAC addresses are ever the same!

23. ipconfig /all
MAC address
Figure 2.12 Output from IPCONFIG/ALL

24. Moving Data
Figure 2.13 Data moving along a wire

25. Moving Data
Figure 2-14: Oscilloscope of data

26. Moving Data
Figure 2.15 Data as ones and zeroes

27. Frames
Figure 2-16 Inside the NIC

28. Inside a frame
• Frames are made up of fields that contain
information
• Frames contain the recipient’s MAC address, the
sender’s MAC address, the data itself, and a cyclic
redundancy check (CRC) for error checking
Figure 2.17 Generic frame

29. Frame as a canister
Figure 2.18 Frame as a canister

30. Frame Size
• Different networks use different sizes of
frames
• 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
chunks back together in the proper order

31. Processing Frames
• All devices on the network see the frame,
but only the device that it is addressed to
will process it
– Every frame is received by every NIC
– The MAC address is used to decide if the frame
belongs to a given device

32. 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
from talking at the same time

33. Incoming Frame!
Figure 2.19 Incoming frame!

34. Getting To Know You
• Usually two devices have talked before, so
the destination MAC address is already
known
• 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

35. Figure 2.20 Building the frame

36. Figure 2.21 Adding the data and CRC to the frame

37. Figure 2.22 Sending the frame

38. Figure 2.23 Reading an incoming frame

39. 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

40. The Two Aspects
of NICs
Figure 2.24 Layer 1 and Layer 2 are now properly
applied to the network

41. Figure 2.25 LLC and MAC, the two parts of the Data Link Layer

42. Beyond the Single Wire –
Network Software
and Layers 3 – 7

43. Subnets
Figure 2.26 Large LAN complete (left),
and broken into two subnets (right)

44. Network Protocols
• Network protocols define rules for how systems
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
• TCP/IP:
– Transmission Control Protocol (TCP)
– Internet Protocol (IP)

45. 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: 192.168.4.232
• No two systems on the same network share
the same IP address

46. • Routers are used to
chop large networks up
into smaller ones
• Routers forward packets
by logical address
• An IP router (most
common) forwards IP
packets
• Works at Layer 3, the
Network layer
Figure 2.27 Typical small router

47. Figure 2.28 MHTechEd addressing

48. Figure 2.29 Router added to the OSI model for the network

49. Frames (packets) within Frames
• Network software creates a packet that
contains the sending and receiving IP
addresses along with the data
Figure 2.30 IP packet

50. Figure 2.31 IP packet in a frame (as a canister)

51. • The packet is enclosed within a frame that
contains the sending and receiving MAC
addresses
IP packet in a frame
Figure 2.32 IP packet in a frame

52. 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

53. Figure 2.33 Adding a router to the network

54. Connecting
to the Internet
Figure 2.34 Router removing network frame
and adding one for the cable line

55. Figure 2.35 Router in action (notice addresses)

56. • 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 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

57. Assembly and Disassembly – Layer 4,
the Transport Layer
• 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
• 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

58. Figure 2.36 Labeling the boxes

59. • Transport layer is the assembler/disassembler
• Transport layer also initializes requests for packets
that weren’t received in good order

60. Figure 2.37 OSI updated

61. 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

62. Figure 2.38 Handling multiple inputs

63. Figure 2.39 Each request becomes a session

64. Figure 2.40 OSI updated

65. 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 formats have been created that
allow very different operating systems to
exchange data

66. Figure 2.41 Different data formats were
often unreadable between systems

67. Figure 2.42 Everyone recognized PDF files!

68. Figure 2.43 OSI updated

69. Network Applications – Layer 7,
the Application Layer
• 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
Navigator
– Outlook Express for e-mail
• All operating systems have APIs at the Application
layer for network-aware applications

70. Figure 2.44 Network applications at work

71. Figure 2.45 OSI updated

72. 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

73. Figure 2.46 Network application showing computers
on the MHTechEd network

74. Figure 2.47 Copying the Word document

75. Figure 2.48 Chopping the Word document

76. Figure 2.49 Creating and addressing packets

77. Figure 2.50 Creating frames

78. Figure 2.51 Tiffany’s system grabbing a frame

79. • The OSI seven-layer model is a troubleshooting
tool
• Example: Jane can’t print to the networked
printer
– Layer 1 and 2: NIC shows activity?
– Layer 3: Does computer have a proper IP
address?
– Move up through the layers to discover problem
area