1. Building a Network with OSI
Chapter 3

2. Contents Explain the major functions of network hardware
Describe the functions of network software
Define each of these functions as part of the OSI seven-layer model

3. Overview

4. The Big Job
Whenever you have a big job to do, it helps to break it down into discrete chunks or functions
For example, moving your family from one city to another could be broken down into:
What to move
How to pack
How to load the van
How to unload the van
The job of moving data from one place to another is also a big job

5. OSI Seven-Layer Model
The big job of moving data from one place to another is broken down into functions defined by the OSI Seven-Layer Model

6. Case Study Example
Let’s look at the process of networking from a conceptual viewpoint
Assume we are just trying to move a file from one PC to another in a small office
One of the workers has just completed a new employee handbook
She needs to transfer the Word document to the other worker for review

7. Case Study Example
The file could be copied to a diskette, USB, or CD/DVD and handed over to the other worker – called Sneakernet – but that’s not necessary today
The document may be transferred using the network
The next section examines the various hardware required

8. Let’s Get Physical

9. Cables
Most networks use a cable like that shown as a physical channel to move the bits of data
Unshielded Twisted Pair (UTP) cable
Uses 4 wires: 2 for sending and 2 for receiving data

10. 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

11. Network Interface Card
Network Interface Cards (NICs – pronounced “Nicks”) are installed in PCs
Network cables are attached to the NICs

12. 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

13. Network Cabling System

14. Network Interface Cards
Since all the networked systems are connected to the same hub, 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
MAC address printed on surface of chip – it’s burned inside the chip.

15. MAC Addresses MAC addresses are 48 bits long
Usually represented using hexadecimal characters (12 hex digits = 48 bits)
Here’s a typical MAC address:
D49
Identifies the manufacturer
Unique serial number determined by the manufacturer
No two MAC addresses are ever the same!

16. WINIPCFG Used on Windows 98/Me systems to view network configuration
MAC address

17. ipconfig /all
Used on Windows NT/2000/XP systems to view network configuration
Determine your own MAC address using either WINIPCFG or ipconfig/all.
MAC address

18. Bits A MAC address is a series of ones and zeros called bits
Data is sent using pulses of electricity, light, or radio waves

19. Frames Data is sent across the network in frames
Frames are discrete chunks of data
There are a number of different frame types used in different networks. All NICs on the same network must use the same frame type or they will not be able to communicate with other NICs.

20. Fields Frames are made up of fields that contain information
Frames typically contain the recipient’s MAC address, the sender’s MAC address, the data itself, and a cyclic redundancy check (CRC) for error checking
CRCs (Cyclic Redundancy Check) are used to check the frame for accuracy
Frames are sometimes corrupted while in transit from one device to another
The data in the frame is divided by a number called the key, producing a four byte CRC value that is placed in the frame at the sending station
The receiving station does the same calculation on the received frame
If the value matches the CRC, the frame is good

21. Data What is inside the data part of the frame?
It could be part of a file, a print job, a web page, anything
NICs do not care what the data, or payload, is

22. 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

23. Processing Framesq
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

24. Getting Data on the Cable
Only one system may speak at a time since the cable is shared
Processes are used to keep two NICs from talking at the same time

25. Getting To Know You
How does the sending NIC know the MAC address of the NIC to which it is sending data?
Most of the time the two devices have talked before, so the destination MAC address is already known
If the MAC address is not known, then a broadcast message is sent over the network
The destination device will respond by sending its MAC address
A broadcast MAC address is FF-FF-FF-FF-FF-FF
How does the destination device know that it is the intended destination device? Remember, the MAC address is still unknown. On most networks today, another address called an IP address is used to identify the destination device.

26. Moving Frames – Step 1
The sending system’s network software hands some data to the NIC
The NIC begins building a frame

27. Moving Frames – Step 2
After the NIC creates the frame, it adds the CRC and data to it
It also puts its MAC address and the destination’s MAC address in the frame

28. Moving Frames – Step 3
When no other NIC is using the cable, it sends the frame through the cable

29. Moving Frames – Step 4 The frame propagates down the wire to the hub
The hub creates a copy of the frame to send to every other system
The receiving device processes the frame

30. Moving Frames – Step 5
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

31. Beyond the Single Wire – Network Software

32. Beyond the Single Wire
What if one system is using a modem to dial into the network?
What if one of the systems is a Macintosh?

33. Routers
A single network with single hub can only support up to 1,024 computers before the network becomes too slow
Routers are used to chop large networks up into smaller ones

34. 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 addressing method than MAC addresses is needed
TCP/IP are the most popular universal addressing protocols

35. TCP/IP Transmission Control Protocol (TCP) Internet Protocol (IP)
Gives each device on the network a unique numeric identifier
IP addresses consist of four 8-bit numbers
Each 8-bit number ranges from 0 to 255
No two systems on the same network share the same IP address

36. DHCP IP addresses must be manually configured on each device
Or they may be automatically configured using Dynamic Host Configuration Protocol (DHCP)
Each system in a network has two addresses
MAC address burned into a chip on the NIC
IP address configured through software

37. Sample Network with Addresses

38. IP packet with frame added
Packets
The network software creates a packet that contains the sending and receiving IP addresses along with the data
The packet is enclosed within a frame that contains the sending and receiving MAC addresses
IP packet in a frame
The diagram above does not include all the fields in an IP packet – it is a highly simplified IP packet.
IP packet
IP packet with frame added

39. Connecting to the Internet
To connect the local network to the Internet a router is needed
The local hub is connected to the router
The router is connected to the Internet through a phone line
The phone line uses a different kind of frame, so the router strips the frame and creates a new one

40. Connecting to the Internet
Router removing network frame and adding one for telephone line
Adding a router to the network

41. Moving through the Internet
The router strips off the MAC addresses and uses the type of addressing the phone company uses instead
The frame uses the IP address to guide it to the receiving system
The receiving router strips off the telephone 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

42. Assembly and Disassembly
Most data is much larger than a single frame
Network protocols chop up the data into smaller packets and gives 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
Transmission Control Protocol (TCP) is one such protocol

43. Sessions
If the receiving system also has a printer to share on the network, then it may receive packets for print jobs as well as packets for files
The sending system must also contact the receiving system to make sure it is ready to handle a print job as well
The software that handles these processes is called session software

44. Multiple Sessions Single session Multiple sessions
How many sessions does a PC have running at a time? If your network uses TCP/IP, use the following command to list each session line by line:
Netstat –a

45. Standardized Formats Macintoshes and PCs use very different formats
Standardized formats have been created that allow very different operating systems to exchange data

46. VPNs
Virtual Private Networks (VPNs) enable a user away from the office to connect to the corporate network via the Internet

47. Encryption Many networks encrypt data to prevent unauthorized access
Both the sending and receiving system must know the encryption method used

48. Network Applications
Users use network applications to exchange data on a network
My Network Places in Windows for files
Internet Explorer or Netscape Navigator for web pages
Outlook Express for

49. Copy a File Across a Network
The next few slides illustrate a typical process that takes place to copy a file from one machine to another over the network

50. My Network Places
The receiving station finds the remote file using My Network Places

51. Moving the Word Document
Drag and drop the Word document from My Network Places to the Desktop

52. Disassembly
The sending system chops the packet into segments and assigns sequence numbers

53. Addressing
Each packet is assigned addresses

54. Assembling the Frame
The NIC adds a frame around each packet containing the MAC addresses

55. Grabbing the Frame
When the network cable is not busy, the frame is sent down the wire
Every NIC looks at the frame. Only the receiving device grabs it

56. Checking the CRC
The receiving station checks the CRC to make sure the frame doesn’t have an error
It then strips off the frame header and passes it up to the next layer

57. Missing Packets
If one of the packets is missing, the receiving device asks for it to be resent

58. It’s Amazing!
Once the document has been reassembled, it is sent to the proper application
It’s amazing but almost all of this process is not seen by the user

59. The OSI Seven-Layer Model

60. Early Networking
In the early days of networking each company designed the networking process from the ground up
These proprietary systems could not talk to each other
For networking to grow a model needed to be created to standardize the process

61. OSI Model
The International Organization for Standardization (ISO) proposed the Open System Interconnection (OSI) Model
The OSI Model is a seven-layer model that describes the networking process
ISO is derived from the Greek word isos, which means equal.

62. OSI’s Seven Layers Layer 7 Application Layer 6 Presentation Layer 5
Session
Layer 4
Transport
Layer 3
Network
Layer 2
Data Link
Layer 1
Physical
Exam Tip
Memorize both the layer number and the name of each OSI layer.

63. Modular Design Each protocol is designed to deal with a specific layer
Each protocol needs to know how to interface with the layer immediately above and below it
These layers are not laws of physics – anybody that designs a network can do it any way they want. While many protocols fit neatly into one of the seven layers, others do not.

64. Layer 7 - Application
Defines a set of tools that programs may use to access the network
HTTP
FTP
Microsoft APIs

65. Layer 6 - Presentation
Presents data from the sending system in a form that applications on the receiving system can understand
ASCII and Unicode text formats
Encryption
Compression
Translates between different machine languages

66. Layer 5 - Session
Manages the connections between machines on the network
Connections may be for file transfers, for print jobs, for , or many other possibilities
Allows machines to keep track of who they are talking to

67. Layer 4 - Transport
Breaks up data it receives from the upper layers into smaller pieces for transport
On the receiving side the packets are reassembled from lower layers
Provides for error checking
Layer 4 is a pivotal layer in the process
Lower layers are concerned with moving data from point A to point B
Upper layers deal with the data

68. Layer 3 - Network
Helps to get packets from network to network by adding unique identifiers (like IP addresses) to the packets

69. Layer 2 – Data Link
Defines the rules for accessing and using the Physical layer
Specifies the rules for identifying devices
Determines which machine may use the network at a given time
Checks for errors
The Data Link layer is divided into two sub-layers:
Media Access Control (MAC)
Logical Link Control (LLC)
The LLC layer sits above the MAC layer and under the Network layer.
The MAC sub-layer controls access to the Physical layer. It encapsulates frames, adds source and destination MAC addresses and error checking, and decapsulates data at the receiving station.
The LLC sub-layer provides an interface with Network layer protocols. It provides for flow control and retransmission of corrupt packets.

70. Layer 1 - Physical
Defines the physical form taken by data when it travels across a cable
Defines how ones and zeros are turned into actual electrical signals on a wire, light pulses on a fiber optic cable, or radio waves in a wireless network

71. NICs and Layers
Network Interface Cards work at both layer 2 and layer 1
When it is said that a NIC is a layer 2 device, keep in mind that it also functions as a layer 1 device