Apply Your Knowledge

Exercises

4.1. Identifying OSI Layers

Developing a real-world project in a chapter focusing on the OSI model is almost an impossible task. For the Network+ exam, you will need to be able to identify the various layers of the OSI model and the network devices that correspond to each level. With that in mind, in lieu of a hands-on project, this chapter provides a practical exercise to reinforce the concepts discussed in this chapter. Your ability to correctly complete this exercise will show sufficient knowledge of the OSI model to get you through any questions on it in the exam.

Estimated time: 10 minutes

1.	Refer to the worksheet below and identify two functions for each layer of the OSI model.
2.	Check your responses against the information in Table 4.2.

Exam Questions

1:	At which layer of the OSI model does a switch operate? Physical Data-link Network Session
A1:	b. A switch uses the MAC addresses of connected devices to make forwarding decisions and therefore operates at the data-link layer of the OSI model. None of the other answers apply. For more information, see the section “The Layers at Which Devices Operate,”in this chapter.
2:	Which of the following devices operate at Layer 2 of the OSI model? (Choose the three best answers.) Switch Network card Hub Bridge
A2:	a, b, d. Switches, bridges, and NICs operate at the data-link layer of the OSI model, which is also known as Layer 2. A hub is defined as a physical-layer (that is, Layer 1) device. For more information, see the section “The Layers at Which Devices Operate,” in this chapter.
3:	Which layer of the OSI model is responsible for synchronizing the exchange of data between two devices at the application level? Transport Session Presentation Data-link
A3:	b. The synchronization of data between applications is performed at the session layer of the OSI model. None of the other answers apply. For more information, see the section “OSI Reference Model 101,” in this chapter.
4:	Which of the following transport-layer protocols offer guaranteed delivery? (Choose the two best answers.) SPX UDP IPX TCP
A4:	a, d. Both SPX and TCP are connection-oriented protocols that offer guaranteed delivery of data. UDP and IPX are both connectionless protocols. For more information, see the section “OSI Reference Model 101,” in this chapter.
5:	Which layer of the OSI model is responsible for route discovery? Session Data-link Network Transport
A5:	c. Route discovery is performed by protocols that operate at the network layer of the OSI model. None of the other answers are valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
6:	What are the two sublayers of the data-link layer? Logical link control Logical loop control Media access control Multiple access control
A6:	a, c. The data-link layer of the OSI model is divided into two distinct sublayers: the LLC sublayer and the MAC sublayer. None of the other answers are valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
7:	Which of the following are responsibilities of the transport layer? (Choose the two best answers.) Performs error detection and handling for the transmitted signals Synchronizes data exchange between two applications Performs error checking and verification Establishes, maintains, and breaks connections between devices
A7:	c, d. The network layer is responsible for, among other things, performing error checking and verification, and establishing, maintaining, and breaking connections between devices. None of the other answers are valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
8:	Which layer of the OSI model defines the method by which the network media are accessed on a logical level? Data-link Physical Session Presentation
A8:	a. Standards at the data-link layer define how the network is accessed on a logical level. You should not confuse the function of the data-link layer with that of the physical layer, which performs similar functions but at a physical rather than a logical level. For more information, see the section “OSI Reference Model 101,” in this chapter.
9:	At which layer of the OSI model does a hub operate? Application Network Physical Data-link
A9:	c. A hub operates at the physical layer of the OSI model. None of the other answers are valid. For more information, see the section “The Layers at Which Devices Operate,” in this chapter.
10:	Which of the following terms is not used to describe a logical grouping of bits? Datagram Segment Package Packet
A10:	c. The term package is not valid when referring to a logical grouping of bits. All the other answers are valid terms. For more information, see the section “OSI Reference Model 101,” in this chapter.
11:	Which layer of the OSI model defines the signal rates and voltages that are used? Data-link Physical Session Presentation
A11:	b. The physical layer of the OSI model defines the physical characteristics of the network, including voltages and signaling rates. None of the other answers are valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
12:	At which layer of the OSI model does a bridge operate? Physical Data-link Network Transport
A12:	b. Bridges use the MAC addresses of devices to make forwarding decisions. Therefore, they are classified as data-link-layer devices. None of the other answers are valid. For more information, see the section “The Layers at Which Devices Operate,” in this chapter.
13:	At which layer of the OSI model does an NIC operate? Physical Data-link Network Transport
A13:	b. NICs operate at the data-link layer of the OSI model. None of the other answers are valid. For more information, see the section “The Layers at Which Devices Operate,” in this chapter.
14:	At which layer of the OSI model do encryption and decryption take place? Physical Session Application Presentation
A14:	d. Encryption is a function that takes place at the presentation layer of the OSI model. None of the other answers are valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
15:	Which of the following are commonly used flow control strategies? (Choose the two best answers.) Buffering Segmentation Windowing Direct flow management
A15:	a, c. Windowing and buffering are commonly used flow control strategies. Segmentation is the term used to describe the division of packets to enable them to be transported across the network. Answer d is not valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
16:	You have been called in to troubleshoot a 10Base2 network that is experiencing problems. After performing some tests, you determine that the problem lies with one of the terminators on the network. At which layer of the OSI model does the problem exist? Physical Data-link Network Session
A16:	a. Cable and connectors are physical elements of the network, so they relate to the physical layer of the OSI model. None of the other answers are valid. For more information, see the section “The Layers at Which Devices Operate,” in this chapter.
17:	Which of the following OSI layers is responsible for establishing connections between two devices? Session Network Transport Data-link
A17:	c. The transport layer is responsible for establishing connections between two devices. None of the other answers are valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
18:	At which layer do the protocols that handle route discovery reside? Transport Network Session Application
A18:	b. Protocols at the network layer are responsible for route discovery. None of the other answers are valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
19:	At the transport layer, two types of protocols are used for sending data to a remote system. What terms are used to describe these protocols? (Choose the two best answers.) Connection oriented Connection reliant Connection dependent Connectionless
A19:	a, d. The two terms used to describe protocols at the transport layer are connection oriented and connectionless. The terms in Answers b and c are not valid. For more information, see the section “OSI Reference Model 101,” in this chapter.
20:	At which layer of the OSI model does a router operate? Application Session Network Transport
A20:	c. A router uses the logical network address to make decisions and is therefore a network-layer device. None of the other answers are valid. For more information, see the section “The Layers at Which Devices Operate,” in this chapter.

Suggested Readings and Resources

1. Habraken, Joe. Absolute Beginner's Guide to Networking, third edition. Que Publishing, 2001.

2. Hallberg, Bruce A. Networking: A Beginners Guide (Network Professional's Library). Osborne McGraw-Hill, 2001.

3. “TechEncyclopedia,” www.techencyclopedia.com.

4. “OSI Technical Information,” www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/osi_prot.htm.

5. “OSI Model Tutorial,” www.itp-journals.com/search/t04124.htm.

6. Computer networking tutorials and advice, compnetworking.about.com.