Guide to Networking Essentials_

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					Guide to Networking Essentials, Fifth Edition   5-1




Chapter 5
Making Networks Work

At a Glance

Instructor’s Manual Table of Contents
   Overview

   Objectives

   Teaching Tips

   Quick Quizzes

   Class Discussion Topics

   Additional Projects

   Additional Resources

   Key Terms

   Technical Notes for Hands-On Projects
Guide to Networking Essentials, Fifth Edition                                           5-2


Lecture Notes

Overview
        Chapter 5 describes the role of the OSI reference model and the IEEE 802 networking
        model. Students learn about the importance of layered network designs and understand
        how each of the layers of the OSI model works. They learn about the encapsulation
        process as data moves across layers and how frames function and are created. Finally,
        students learn about the IEEE 802 networking model and related standards, including its
        relationship with the OSI reference model.


Objectives
       Explain the OSI reference model layers and their relationship to hardware and software
       Describe the function and creation of a data frame
       Explain the IEEE 802 networking model and related standards


Teaching Tips
Understanding the OSI and 802 Networking Models
    1. Explain the role of the OSI model (proposed by the ISO). Explain why the attempt to
       develop a working set of protocols and technologies based on the OSI model, and to put
       those efforts into common use, never materialized.


                 Explain that ISO is not an acronym; it comes from the Greek prefix iso, which
                 means “equal” or “the same.” The ISO, based in Geneva, Switzerland, is a
Teaching         network of national standard institutes from 140 countries. The expanded name
Tip              differs from language to language. For example, in France the organization is the
                 Organisation Internationale de Normalisation. The term ISO gives the network of
                 institutes a common name.



                 Note that the set of protocols that was developed to conform to the OSI model is
Teaching
                 called ISO. You can view the fruits of those labors at:
Tip
                 www.protocols.com/pbook/iso.htm.


    2. Explain that the IEEE 802 networking model provides detailed implementation
       specifications for a number of networking technologies.
Guide to Networking Essentials, Fifth Edition                                            5-3

Role of a Reference Model

    1. Explain the importance of having a networking reference model.

OSI Reference Model

    1. Provide a brief history of how the OSI reference model came to be.

    2. Explain that the model’s foundation is that networking can be separated into a series of
       related tasks. Explain that layering follows a “divide and conquer” approach.

    3. Understanding Layers. Explain the importance of having a layered networking
       reference model. Use an analogy of creating and delivering a letter through the U.S.
       mail to help explain the layered communication process.

Structure of the OSI Reference Model

    1. Use Figure 5-1 to describe the structure of the OSI reference model. Introduce the term
       protocol suite. Explain that protocols plus drivers equal network access.


                 Here are two good mnemonics to remember the seven layers of the OSI reference
Teaching         model. From the bottom up, starting with the Physical layer, the acronym is
Tip              “Programmers Do Not Throw Sausage Pizza Away.” From the top down, starting
                 with the Application layer, it’s “All People Seem To Need Data Processing.”


    2. Use Figure 5-2 to describe the role of OSI layers in an operating system context.

    3. Explain that rigidly defined boundaries called “interfaces” separate layers in the OSI
       reference model.

    4. Explain that in general, the purpose of any layer in the model is to provide services to
       the next higher layer, but also to shield that higher layer from the details of how its
       services are carried out.

    5. Use Figure 5-3 to introduce the concept of “peer layers”. Explain that communication
       between peer layers is “virtual”. Introduce the terms PDU, encapsulation, and
       decapsulation.

    6. Simulation 5-1 shows how data generated from an application travels down through the
       OSI model layers, with encapsulations added where necessary, and how the process is
       reversed on the receiving computer.


Teaching         For another animation of the encapsulation process, visit:
Tip              www.humboldt.edu/%7Eaeb3/telecom/Encapsulation.html.
Guide to Networking Essentials, Fifth Edition                                             5-4

    7. Describe the role of each of the seven OSI layers (in reverse order). For each layer,
       describe the tasks performed by the layer, provide examples of implementations of the
       layer, and describe possible problems that can occur at that layer:
           a. Application layer. Note that its PDU is called “data”.
           b. Presentation layer. Note that its PDU is called “data”. Don’t forget to introduce
               the term redirector.
           c. Session layer. Note that its PDU is called “data”.
           d. Transport layer. Note that its PDU is called “segment” (see Figure 5-4). Don’t
               forget to introduce the term flow control.
           e. Network layer. Note that its PDU is called “packet” (see Figure 5-5). Don’t
               forget to introduce the terms routing and access control.
           f. Data link layer. Note that its PDU is called “frame” (see Figure 5-6). Don’t
               forget to introduce the term Cyclic Redundancy Check (CRC).
           g. Physical layer. Don’t forget to introduce the term encoding.


Teaching         For an animation of the encoding process visit:
Tip              http://netbook.cs.purdue.edu/anmtions/anim03_1.htm.



Teaching         For a humorous analogy to the OSI model, read “James Bond Meets The 7 Layer
Tip              OSI Model” at: www.lewistech.com/rlewis/Resources/JamesBondOSI2.aspx.


Summary of the OSI Layers

    1. Use Table 5-1 to provide a summary of the role of each of the OSI layers.


                 Stress that although not all networking protocols adhere to the OSI model, a
                 network administrator’s clear understanding of the functions at each layer is
Teaching         essential in troubleshooting networks and network equipment and in
Tip              understanding how network devices operate. Many network devices are
                 described in terms of the OSI model. For example, you might hear the term
                 “Layer 3 switch”.


    2. Stress that no protocol suite developed after the OSI reference model was introduced
       has been free of its influence.
Guide to Networking Essentials, Fifth Edition                                             5-5

Quick Quiz 1
    1. Any computer that can access a network must have a protocol stack, also known as a(n)
       ____________________ because it consists of a collection of related software elements
       and services that correspond to the layers of the OSI model, instead of a single massive
       program.
       Answer: protocol suite

    2. The Presentation layer (Layer ____________________) handles data-formatting
       information for network communications.
       Answer: 6

    3. The Transport layer handles ____________________, which ensures that the recipient
       of transmitted data isn’t overwhelmed with more data than it can handle.
       Answer: flow control

    4. ____________________ is handled at the Network layer during the routing process; the
       router consults a list of rules before forwarding an incoming packet to determine
       whether a packet meeting certain criteria (such as source and destination address)
       should be permitted to reach the intended destination.
       Answer: Access control


Function of Data Frames in Network Communications
    1. A frame is the basic unit for network traffic as it travels across the medium.


                 Note that you often hear the term “packet” used instead of frame, but because
Teaching
                 this discussion focuses on the unit of data sent on to the medium, the appropriate
Tip
                 term is “frame.”


    2. Explain why networks split data into small pieces.

Examining the Structure of a Data Frame

    1. Use Figure 5-7 to describe the structure of a data frame.

Creating a Data Frame

    1. Use Figure 5-8 to describe how header/trailer information is added or removed as data
       passes from layer to layer.
Guide to Networking Essentials, Fifth Edition                                             5-6


Teaching         Explain that most protocols, such as TCP/IP, add header information only at the
Tip              Transport, Network, and Data Link layers.


Understanding Types of Data Frames

    1. Describe the difference between unicast, broadcast, and multicast frames.


                 Explain that the types of frames discussed in this section, unicast, broadcast, and
                 multicast, can also refer to the Network layer PDU (packets). Packets have their
                 own header information containing source and destination addresses that can also
Teaching
                 be unicast, broadcast, or multicast. The difference is that the packet header
Tip
                 contains logical addresses, such as TCP/IP addresses, assigned to the computer
                 manually, whereas the frame header contains the computer’s physical address
                 (MAC address) burned into the NIC.



Quick Quiz 2
    1. What is a frame?
       Answer: A frame is the basic unit for network traffic as it travels across the medium.

    2. What does a frame header contain?
       Answer: The frame header usually contains the address of the sender (source) and the
       address of the receiver (destination), information indicating the frame’s size or content,
       an alert signal to indicate data transmission, and clocking information to synchronize
       the transmission.

    3. In a(n) ____________________ frame, the frame’s destination address is a value of all
       binary 1s.
       Answer: broadcast

    4. ____________________ frames are created for any computers on a network that
       “listen” to a shared network address.
       Answer: Multicast


Understanding the IEEE 802 Networking Specifications
    1. Explain how the IEEE 802 project came to be. Explain that it concentrates on standards
       that describe a network’s physical elements.
Guide to Networking Essentials, Fifth Edition                                          5-7


Teaching         For more information on the IEEE and its standards, visit: www.ieee.org and
Tip              www.ieee802.org/.


    2. Explain that although IEEE 802 standards predate the OSI reference model, both were
       developed in collaboration and are compatible with one another.

IEEE 802 Specifications

    1. Use Table 5-2 to describe the role of the IEEE 802 standards categories. Explain that
       these categories, which encompass a large body of standards, are the focus of ongoing
       development and extension efforts at the IEEE through its working groups.


Teaching         For more information on the IEEE 802 family of standards, visit:
Tip              http://en.wikipedia.org/wiki/IEEE_802.


IEEE 802 Extensions to the OSI Reference Model

    1. Remind students that that the two lowest layers of the OSI model define how computers
       attach to specific network media, and specify how more than one computer can access
       the network without causing interference with other computers on the network.

    2. Use Figure 5-9 to explain that Project 802 took this work further to create the
       specifications (primarily 802.1 through 802.5) that define the most successful LAN
       technologies, including Ethernet and token ring, which together dominate the LAN
       world.

    3. Describe the role of the LLC (defined by 802.2) and MAC sublayers. Don’t forget to
       introduce the term Service Access Point (SAPs).

    4. Use Figure 5-10 to show how the IEEE 802 specifications map to the LLC and MAC
       sublayers.
Guide to Networking Essentials, Fifth Edition                                          5-8

Quick Quiz 3
    1. The IEEE ____________________ standard covers all forms of Ethernet media and
       interfaces, from 10 Mbps to 10 Gbps (10 Gigabit Ethernet).
       Answer: 802.3

    2. The ____________________ (LLC) sublayer (defined by 802.2) controls data-link
       communication and defines the use of logical interface points, called Service Access
       Points (SAPs), that other computers can use to transfer information from the LLC
       sublayer to the upper OSI layers.
       Answer: Logical Link Control

    3. What is the role of the MAC sublayer of the Data Link layer?
       Answer: The Media Access Control (MAC) sublayer manages access to the physical
       medium and, therefore, communicates with the Physical layer. It communicates directly
       with a computer’s NIC and is responsible for physical addressing. The physical address
       burned into every NIC is called a MAC address because it operates at this sublayer of
       the 802.2 specification.

    4. The ____________________ (IEEE) defined a set of LAN standards to ensure that
       network interfaces and cabling from multiple manufacturers would be compatible as
       long as they adhered to the same IEEE specification.
       Answer: Institute of Electrical and Electronics Engineers


Class Discussion Topics
    1. Ask students to form groups and review all the tasks and problems associated with the
       data link layer of the OSI reference model (see pages 170-172). Then, ask them to
       identify which of those tasks and problems belong to the LLC sublayer and which to the
       MAC sublayer. They should discuss and compare their lists in class.

    2. Have students heard about any other networking protocols besides the ones mentioned
       in this chapter? If so, at which layers do they think those protocols operate? Tip:
       Students should be able to list a few other protocols/technologies, such as PPP, SLIP,
       ATM, ARP, SMTP, SNMP, POP, etc.


Additional Projects
    1. The TCP/IP protocol stack differs from the OSI, but the layers of the former can be
       mapped to layers from the OSI reference model. Ask students to do some research to
       find out about this mapping. They should hand in a report indicating the mapping of
       these layers, including one or more graphics and a list of protocols that work at each
       layer.
Guide to Networking Essentials, Fifth Edition                                          5-9

    2. Ask students to use Ethereal (www.ethereal.com) to identify the fields of an Ethernet
       (Ethernet II or IEEE 802.3) frame and compare them with the ones studied in this
       chapter. Do they have a one-to-one relationship?


Additional Resources
    1. How OSI Works:
       http://computer.howstuffworks.com/osi.htm

    2. Open System Interconnection Reference Model:
       www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm#xtocid5

    3. The 7 Layers of the OSI Model:
       www.webopedia.com/quick_ref/OSI_Layers.asp

    4. OSI Reference Model Illustrated:
       http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci523729,00.html

    5. Data Link Layer:
       http://en.wikipedia.org/wiki/Data_link_layer

    6. Ethernet frame types and the EtherType field:
       http://en.wikipedia.org/wiki/Ethernet#Ethernet_frame_types_and_the_EtherType_field

    7. IEEE 802 LAN/MAN Standards Committee:
       www.ieee802.org/

    8. IEEE 802:
       http://en.wikipedia.org/wiki/IEEE_802


Key Terms
     802.2 — The IEEE specification in Project 802 for the Logical Link Control (LLC)
      sublayer of the OSI model’s Data Link layer.
     802.3 — The IEEE specification in Project 802 for Carrier Sense Multiple
      Access/Collision Detection (CSMA/CD) networks (more commonly called “Ethernet”).
      Ethernet users can attempt to access the medium any time it’s perceived as “quiet,” but
      they must back off and try to transmit again if they detect any collisions after
      transmission begins.
     802.5 — The IEEE specification in Project 802 for token ring LANs, which map a
      circulating ring structure onto a physical star and circulate a token to control access to
      the medium.
     802.11 — The IEEE specification in Project 802 for wireless networks.
     802.15 — The IEEE specification that covers emerging standards for wireless personal
      area networks (PANs).
Guide to Networking Essentials, Fifth Edition                                        5-10

     802.16 — The IEEE specification that covers wireless metropolitan area networks
      (MANs).
     access control — In the context of the Network layer and routing, the process whereby
      a router consults a list of rules before forwarding an incoming packet. The rules
      determine whether a packet meeting certain criteria (such as source and destination
      address) should be permitted to reach the intended destination.
     Application layer — Layer 7 in the OSI reference model provides interfaces that
      enable applications to request and receive network services. See also OSI reference
      model.
     broadcast frames — Data frames with destination addresses that specify that all
      computers on a network must read and process these frames.
     Cyclical Redundancy Check (CRC) — A mathematical recipe that generates a
      specific value, called a checksum, based on a frame’s contents. The CRC is calculated
      before frame transmission and then included with the frame; on receipt, the CRC is
      recalculated and compared to the sent value. If the two agree, it’s assumed that the data
      frame was delivered intact; if they disagree, the frame must be retransmitted.
     Data Link layer — Layer 2 in the OSI reference model is responsible for managing
      access to the networking medium and ensuring error-free delivery of data frames from
      sender to receiver. See also OSI reference model.
     data section — The frame component that’s the actual data being sent across a
      network. The size of this section can vary from less than 50 bytes to 16 KB, depending
      on the network type.
     decapsulation — The process of stripping the header from a PDU as it makes its way
      up the communication layers before being passed to the next higher layer. See also
      protocol data unit (PDU).
     encapsulation — The process of adding header information to a PDU as it makes its
      way down the communication layers before being passed to the next lower layer. See
      also protocol data unit (PDU).
     encoding — The representation of 0s and 1s as a physical signal, such as electrical
      voltage or a light pulse.
     flow control — A process designed to regulate information transfer between a sender
      and a receiver. Flow control is often necessary when there’s a speed differential
      between sender and receiver.
     frame — The basic unit for network traffic as it travels across the medium. Data is
      broken into these smaller, more manageable pieces for faster, more efficient delivery.
     frame header — Information added to the beginning of data being sent, which
      contains, among other things, addressing and sequencing information.
     frame trailer — Information added to the end of the data being sent in a frame; it
      generally contains error-checking information, such as the CRC.
     International Organization for Standardization (ISO) — The international
      standards-setting body based in Geneva, Switzerland, which sets worldwide technology
      standards.
     layers — The functional subdivisions of the OSI reference model. See also OSI
      reference model.
     Logical Link Control (LLC) — The upper sublayer of the IEEE Project 802 model for
      the Data Link layer of the OSI model. It handles error-free delivery and controls the
      flow of frames between sender and receiver across a network.
Guide to Networking Essentials, Fifth Edition                                           5-11

     Media Access Control (MAC) — The lower sublayer of the IEEE Project 802 model
      for the Data Link layer of the OSI model. It handles access to network media and
      mapping between logical and physical network addresses for NICs.
     multicast frames — Frames that use a special destination address so that any computer
      listening for this address can read and process the frame’s data.
     Network layer — Layer 3 of the OSI reference model handles addressing and routing
      PDUs across internetworks in which sender and receiver must traverse multiple
      networks. See also protocol data unit (PDU) and OSI reference model.
     Open Systems Interconnection (OSI) — The family of ISO standards developed in
      the 1970s to facilitate functionality of networking services among dissimilar computers
      on a global scale. The OSI initiative was unsuccessful, owing to a fatal combination of
      an all-inclusive standards-setting effort and a failure to develop standard protocol
      interfaces to help developers implement its manifold requirements.
     OSI reference model — ISO Standard 7498 defines a frame of reference for
      understanding networks by dividing the process of network communication into seven
      layers. Each layer is defined in terms of the services and data it handles on behalf of the
      layer directly above it and the services and data it needs from the layer directly below it.
      The OSI reference model remains the OSI initiative’s most enduring legacy.
     Physical layer — Layer 1, the bottom layer of the OSI reference model, transmits and
      receives signals and specifies the physical details of cables, adapter cards, connectors,
      and hardware behavior. See also OSI reference model.
     Presentation layer — At Layer 6 of the OSI reference model, data can be encrypted
      and/or compressed to facilitate delivery. Platform-specific application formats are
      translated into generic data formats for transmission or from generic data formats into
      platform-specific application formats for delivery to the Application layer. See also OSI
      reference model.
     Project 802 — The IEEE effort that produced the collection of 802 networking
      specifications and standards.
     protocol data unit (PDU) — A unit of information passed as a self-contained data
      structure from one layer to another on its way up or down the network protocol stack.
     protocol suite — A family of related protocols in which higher-layer protocols provide
      application services and request handling facilities, and lower-layer protocols manage
      the intricacies of Layers 1 to 4 in the OSI reference model.
     redirector — A software component that intercepts requests for service from a
      computer and redirects requests that can’t be handled locally across the network to a
      networked resource that can handle the request.
     routing — A Network-layer service that determines how to deliver an outgoing packet
      of data from sender to receiver. Routing entails several methods for managing delivery,
      and requires error and status reporting so that senders can determine whether packets
      are reaching the receivers.
     Service Access Points (SAPs) — Logical interface points used to transfer information
      from the LLC sublayer to the upper OSI layers. See also Logical Link Control (LLC).
     Session layer — Layer 5 of the OSI reference model is responsible for setting up,
      maintaining, and ending ongoing sequences of communications (called sessions) across
      a network. See also OSI reference model.
     Transport layer — Layer 4 of the OSI reference model is responsible for fragmenting
      large PDUs from the Session layer for delivery across the network, inserting integrity
      controls, and managing delivery mechanisms to allow for error-free reassembly on the
Guide to Networking Essentials, Fifth Edition                                            5-12

      receiving end of a network transmission. See also OSI reference model and protocol
      data unit (PDU).
     unicast frame — A data frame addressed to a single recipient.


Technical Notes for Hands-On Projects
Hands-On Project 5-1: In this project, students use the ipconfig command-line utility.

Hands-On Project 5-2: This project requires a Web browser and Internet access.

				
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