Building a Network with the Building a Network with the OSI ModelOSI Model

Chapter 2Chapter 2

ObjectivesObjectivesObjectivesObjectives• Describe models such as the OSI seven-layerDescribe models such as the OSI seven layer

model• Explain the major functions of networkExplain the major functions of network

hardware with OSI Layers 1-2• Describe the functions of network software

with OSI Layers 3-7

OverviewOverview

The CompTIA Network+ ChallengeThe CompTIA Network+ ChallengeThe CompTIA Network+ ChallengeThe CompTIA Network+ Challenge

• Understand every aspect of networking• Understand every aspect of networking– Use the Open Systems Interconnect (OSI)

modelmodel– Conceptualize the parts of a network

• The OSI Seven-Layer Model providesThe OSI Seven-Layer Model provides– A powerful tool for diagnosing

problemsproblems– A common language to describe

networksnetworks

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

Working with ModelsWorking with Models

Biography of a Model• What does “model” mean to you?

– Computer models that predict weatherPlastic model airplane– Plastic model airplane

– Fashion model

Figure 2.2 Types of Models

A model has all the major f ti f th l itfunctions of the real item

Figure 2.3 Simple model airplane

The OSI sevenThe OSI seven--layer modellayer model• What functions define all networks?

The OSI sevenThe OSI seven--layer modellayer model

• What details can be omitted?• ISO (International Organization for• ISO (International Organization for Standardization) proposed the OSI seven‐layer modelseven layer model

Layer 7 - Application

Layer 6 - Presentation

S

The Seven The Seven Layers in ActionLayers in Action Layer 5 - Session

Layer 4 - Transport

Layers in ActionLayers in Action

The OSI Model Layer 3 - Network

Layer 2 - Data Link

Layer 1 - PhysicalLayer 1 - Physical

Welcome to Welcome to MHTechEdMHTechEd!!A conceptual viewpoint of networkingof networking

– One of the workers has just completed a newjust completed a new employee handbook

– She needs to transfer the Word document to the other worker for 

Figure 2.4 Janelle and Tiffany, hard at work

review

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 k• Transfer the file using the network

Let’s Get Physical…Let’s Get Physical…d i h k h dd i h k h dand examine the network hardware and examine the network hardware 

components required for this transfercomponents required for this transfer

CablingCablingCablingCabling• Most networks use a cable• Most networks use a cable, 

like this one, as a physical channel to move the bits of data

Unshielded Twisted PairUnshielded Twisted Pair (UTP) cable

Figure 2.5 UTP cabling

HubsHubsHubsHubs• Each computer system has 

a cable leading to a device gcalled a hub– Usually located in a closet

• The hub sends the data received from one system to all the other systemsto all the other systems attached to it

Figure 2.6 Typical hub

Figure 2.7 The network so far, with the Physical layerhardware

Network Interface CardsNetwork Interface Cards• Network Interface Cards (NICs) are installed in PCs

• Network cables attach to the NICs

Figure 2.8 Typical NIC

NIC to Hub ConnectionsNIC to Hub Connections• Cables run from the NIC in the PC to a jackNIC in the PC to a jack on the wall

• Cables run through the gwalls to the closet where they connect to a hub

Fig re 2 9 NIC ith cable connectFigure 2.9 NIC with cable connect-ing the PC to the wall jack

Network Cabling SystemNetwork Cabling System

Figure 2.10 The MHTechEd network

The NIC MAC address printed on surface of chip – and burned

• Each system must have a unique identifier

pinside the chip.

identifier• Media Access Control (MAC) address– A unique address burned 

into a ROM chip on the network card

– Each MAC address is 12Each MAC address is 12 hex characters or 48 bits in length

Figure 2.11 MAC address

MAC AddressesMAC 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:

004005‐607D49

Organizationally unique identifier (OUI)

Device ID

No two MAC addresses are ever the same!

ipconfig /all

MAC address

Figure 2.12 Output from IPCONFIG/ALL

Moving DataMoving DataMoving DataMoving Data

Figure 2.13 Data moving along a wireg g g

Moving DataMoving Datagg

Figure 2-14: Oscilloscope of data

Moving DataMoving Datagg

Figure 2.15 Data as ones and zeroes

Frames

Figure 2-16 Inside the NIC

Inside a frameInside a frame• Frames are made up of fields that contain informationinformation

• Frames contain the recipient’s MAC address, the sender’s MAC address, the data itself, and a cyclic , , yredundancy check (CRC) for error checking

Figure 2.17 Generic frame

Frame as a canisterFrame as a canister

Fig re 2 18 Frame as a canisterFigure 2.18 Frame as a canister

Frame SizeFrame SizeFrame SizeFrame Size• Different networks use different sizes of framesframes

• 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 orderg p p

Processing FramesProcessing FramesProcessing FramesProcessing Frames• All devices on the network see the frame, b t l th d i th t it i dd d tbut only the device that it is addressed to will process it– Every frame is received by every NICEvery 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 LineGetting Data on the Linegg• Since the cable is shared, only one system may speak at a time

• Processes are used to keep two NICsProcesses are used to keep two NICs from talking at the same time

Incoming Frame!Incoming Frame!

Figure 2.19 Incoming frame!

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

• If the MAC address is not known, a broadcastIf 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 receivedAfter the frame is receivedAfter the frame is receivedAfter 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 operatingsends the data portion to the network operating system for processing

– If the value does not match, the frame has errors and must be resentmust be resent

The TwoThe TwoThe Two The Two Aspects of NICsAspects of NICsAspects of NICsAspects 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

Beyond the Single Wire Beyond the Single Wire ––y gy gNetwork SoftwareNetwork Softwareand Layers 3 and Layers 3 –– 77

SubnetsSubnets

Figure 2 26 Large LAN complete (left)Figure 2.26 Large LAN complete (left), and broken into two subnets (right)

Network ProtocolsNetwork Protocols• Network protocols define rules for how systems

dd d h t h d t i t h kare addressed, how to chop data up into chunks, how to deal with routers, and so on

• As a network grows a more universal address-• As a network grows, a more universal address-ing method than MAC addresses is needed

• TCP/IP: – Transmission Control Protocol (TCP)– Internet Protocol (IP)

IP IP –– Playing on Layer 3, the Playing on Layer 3, the Network LayerNetwork Layer• IP address: a unique numeric identifier• An IP address is a logical address while a g

MAC address is a physical address• IP uses a dotted-decimal notation• Each 8-bit number ranges from 0 to 255

– Example: 192.168.4.232N t t th t k h• No two systems on the same network share 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 address 

• An IP router (most• An IP router (most common) forwards IP packets

• Works at Layer 3 the• Works at Layer 3, the Network layer

Fig re 2 27 T pical small ro terFigure 2.27 Typical small router

Figure 2.28 MHTechEd addressing

Figure 2.29 Router added to the OSI model for the network

Frames (packets) within FramesFrames (packets) within FramesFrames (packets) within FramesFrames (packets) within Frames• Network software creates a packet thatNetwork software creates a packet that

contains the sending and receiving IP addresses along with the data

Fig re 2 30 IP packetFigure 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 MACcontains the sending and receiving MAC addresses

IP packet in a frame

Figure 2.32 IP packet in a frame

Connecting to the Internet Connecting to the Internet gg• 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 linecable or phone line• The cable or phone line uses a different kind of

frame so the router strips the frame and createsframe, so the router strips the frame and creates a new one

Figure 2.33 Adding a router to the network

C tiC tiConnecting Connecting to the Internetto 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 typeThe 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 f d dd th MAC dd f thcompany frame and adds the MAC address for the

receiving system• The NIC strips off the MAC header and hands the• The NIC strips off the MAC header and hands the

frame off to the NOS

Assembly and Disassembly Assembly and Disassembly –– Layer 4, Layer 4, y yy y yythe the Transport LayerTransport 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 numberp , 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 orderg

Figure 2.37 OSI updated

Talking on a NetworkTalking on a Network –– Layer 5Layer 5Talking on a Network Talking on a Network Layer 5,Layer 5,the the Session LayerSession 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 Standardized Formats –– or Why Layer 6, or Why Layer 6, Presentation, Has No FriendsPresentation, Has No Friends

• Presentation layer tasks solved an old problem• Macintoshes and PCs use very different formats

St d di d f t h b t d th t• Standardized formats have been created that allow very different operating systems to exchange dataexchange data

Figure 2.41 Different data formats were goften unreadable between systems

Figure 2 42 Everyone recognized PDF files!Figure 2.42 Everyone recognized PDF files!

Figure 2.43 OSI updated

Network Applications Network Applications –– Layer 7, Layer 7, pppp y ,y ,the the Application LayerApplication Layer

• Users use Application layer network applications pp y ppto exchange data on a network– Network in Windows Vista (My Network Places in

earlier Windows)earlier Windows)– Web browser like Internet Explorer or Netscape

Navigator– Outlook Express for e-mail– 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 DocumentHow Tiffany Gets Her DocumentHow Tiffany Gets Her DocumentHow Tiffany Gets Her Document

• The next few slides illustrate a typical process• 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 troubleshootingThe OSI seven layer model is a troubleshooting tool

• Example: Jane can’t print to the networked printer– Layer 1 and 2: NIC shows activity?

L 3 D t h IP– Layer 3: Does computer have a proper IP address?

– Move up through the layers to discover problemMove up through the layers to discover problem area