Teaching Tip Before moving on, point out the Cross Check on Page 9, which asks the students to check their knowledge about information in Chapter 1 on Cisco. It asks, What does Cisco say about CompTIA Network+ certification? Where would you go to get more details?
Note on page 10: ISO may look like a mispelled acronym, but it’s actually a word, derived from the Greek word isos , which means equal.
Tip on Page 11: Be sure to memorize both the name and the number of each OSI layer. Network techs use terms such as “Layer 4” and “Transport layer” synonymously. Students have long used mnemonics for memorizing such lists. One of my favorites for the OSI seven-layer model is Please Do Not Throw Sausage Pizza Away. Yum! Note on Page 11: Keep in mind that these layers are not laws of physics — anybody who wants to design a network can do it any way he or she wants. While many protocols fit nearly into one of the seven layers, others do not.
Note on bottom of Page 11: This section is a conceptual overview of the hardware and software functions of a network. Your network may have different hardware or software, but it will share the same functions!
Take time to point out the Try This! On page 15, giving each student time to determine the MAC address of a NIC using IPCONFIG/ALL.
Note on Page 15: A number of different frame types are 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.
Tech Tip (Page 16): CRC in Depth Most CRCs are only 4 bytes long, yet the average frame carries around 1500 bytes of data. How can 4 bytes tell you if all 1500 bytes in the data are correct? That’s the magic of CRCs. Without going into the grinding details, think of the CRC as just the remainder of a division problem. (Remember learning remainders from division back in elementary school?) The NIC sending the frame does a little math to make the CRC. Using binary arithmetic, it works a division problem on the data using a divisor called a key. This key is the same on all the NICs in your network —it’s built in at the factory. The result of this division is the CRC. When the frame gets to the receiving NIC, it divides the data by the same key. If the receiving NIC’s answer is the same as the CRC, it knows the data is good.
How does the destination device know that it is the intended destination device? On most networks today, an address called an IP address is used to identify the destination device. More on IP addresses later in this chapter, and again, later in the book (Chapters 7 through 11).
The sending system hands data to the NIC The NIC makes the frame
The NIC adds the CRC and data to the frame The NIC puts its MAC address and the destination’s MAC address in the frame
NIC sends the frame when no other NIC is using the cable
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
Tip on Page 20: The CompTIA Network+ exam tests you on the details of the OSI seven-layer model, so know that the Data Link layer is the only layer that has any sublayers.
Tech Tip (Page 21): NIC and Layers [a shortened version] Network cards operate at both Layer 2 and Layer 1 of the OSI seven-layer model. If cornered to answer one or the other, however, go with the more common answer, Layer 2.
Tip on Page 21: MAC addresses are also known as physical addresses.
Note on Page 22: TCP/IP is the most famous network protocol, but there are others.
Tip on Page 23: Head to Chapter 7, “TCP/IP Basics,” and Chapter 8, “The Wonderful World of Routing,” to get much deeper into routers.
Note on Page 23: This is a highly simplified IP packet. I am not including lots of little parts of the IP packet in this diagram because they are not important to what you need to understand right now —but don’t worry, you’ll see them later in the book!
Allow time for the students to do the activity in the Try This! on Page 28. No need to explain the results of running the netstat command, other than to ensure that they appreciate the large number of sessions that are open on each computer.
Tech Tip (Page 28): Acrobat as Open Standard Adobe released the PDF standard to ISO in 2007 and PDF became the ISO 32000 open standard. Adobe Acrobat remains the premier application for reading and editing PDF documents, so most folks call PDF documents Acrobat files.
Chapter 2 Networking
Building a Network with the OSI Model Chapter 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
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 majorfunctions of the real item 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 Standardization) proposed the OSI seven-layer model
Layer 7 - Application The Seven Layer 6 - PresentationLayers in Action Layer 5 - Session Layer 4 - Transport Layer 3 - Network The OSI Model Layer 2 - Data Link Layer 1 - Physical
Welcome to MHTechEd!A conceptual viewpointof 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
She could…• Copy the file to a flash drive and walk it over to the other person (sneakernet)• Transfer the file using the network
Let’s Get Physical…and examine the network hardwarecomponents required for this transfer
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
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
Figure 2.7 The network so far, with the Physical layerhardware
Network Interface Cards• Network Interface Cards (NICs) are installed in PCs• 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 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
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 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
MAC Addresses• MAC addresses are 48-bits long• Usually represented using hexadecimal characters (12 hex digits = 48 bits) – A typical MAC address: 004005-607D49Organizationally unique Device ID identifier (OUI) No two MAC addresses are ever the same!
ipconfig /all MAC addressFigure 2.12 Output from IPCONFIG/ALL
Moving DataFigure 2.13 Data moving along a wire
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
Frame as a canister Figure 2.18 Frame as a canister
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
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
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
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
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
Beyond the Single Wire – Network Software and Layers 3 – 7
SubnetsFigure 2.26 Large LAN complete (left), and broken into two subnets (right)
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)
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
• 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
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 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 addresses 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
Connectingto 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 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
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
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
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
Figure 2.41 Different data formats were often unreadable between systems
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
• 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