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.