1. Name the seven layers of the OSI model.

Answer: Application (Layer 7), presentation (Layer 6), session (Layer 5), transport (Layer 4), network (Layer 3), data link (Layer 2), and physical (Layer 1). Some mnemonics to help you recall the names of the layers are: All People Seem To Need Data Processing (Layers 7 to 1), Please Do Not Take Sausage Pizzas Away (Layers 1 to 7), and the ever-popular Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7).

2. What is the main purpose(s) of Layer 7?

Answer: Layer 7 (the application layer) provides standardized services to applications. The definition for this layer is typically ambiguous because it varies. The key is that it does not define a user interface, but instead it is a sort of toolbox used by application developers. For example, a web browser is an application that uses HTTP, as defined as a TCP/IP application layer protocol, to transfer the contents of web page between a server and client.

3. What is the main purpose(s) of Layer 6?

Answer: Layer 6 (the presentation layer) defines data formats, compression, and possibly encryption.

4. What is the main purpose(s) of Layer 5?

Answer: Layer 5 (the session layer) controls the conversation between two endpoints. Although the term used is session, the term conversation more accurately describes what is accomplished. The session layer ensures that not only communication, but also useful sets of communication between endpoints is accomplished.

5. What is the main purpose(s) of Layer 4?

Answer: Layer 4 (the transport layer) provides end-to-end error recovery, if requested.

6. What is the main purpose(s) of Layer 3?

Answer: Layer 3 (the network layer) defines logical addressing and routing as a means of delivering data across an entire network. IP and IPX are two examples of Layer 3-equivalent protocols.

7. What is the main purpose(s) of Layer 2?

Answer: The data link layer defines addressing specific to a particular medium as part of the means of providing delivery of data across that medium. It also includes the protocols used to determine what device(s) accesses the media at any point in time.

8. What is the main purpose(s) of Layer 1?

Answer: Layer 1 (physical layer) is responsible for encoding energy signals onto the medium and interpreting a received energy signal. Layer 1 also defines the connector and cabling details.

9. Describe the process of data encapsulation as data is processed from creation until it exits a physical interface to a network. Use the OSI model as an example.

Answer: Data encapsulation represents the process of a layer adding a header (and possibly a trailer) to the data as it is processed by progressively lower layers in the protocol specification. In the context of OSI, each layer could add a header so that— other than the true application data—there would be six other headers (Layers 2 to 7) and a trailer for Layer 2, with this L2PDU being encoded by the physical layer onto the network media.

10. Name three benefits to layering networking protocol specifications.

Answer: Some examples of benefits to layering networking protocol specifications include reduced complexity, standardized interfaces, modular engineering, interoperable technology, accelerated evolution, and simplified teaching and learning. Questions such as this on the exam require some subjective interpretation of the wording on your part.

11. What header or trailer does a router discard as a side effect of routing?

Answer: A router discards the data-link header and trailer as a side effect of routing. This is because the network layer, where routing is defined, is interested in delivering the network layer (Layer 3) PDU from end to end. Routing uses intermediate data links

(Layer 2) to transport the data to the next routers and eventually to the true destination. The data-link header and trailer are useful only to deliver the data to the next router or host, so the header and trailer are discarded by each router.

12. What OSI layer typically encapsulates using both a header and a trailer?

Answer: The data link layer typically encapsulates using both a header and a trailer. The trailer typically includes a frame check sequence (FCS), which is used to perform error detection.

13. What terms are used to describe the contents of the data encapsulated by the data link, network, and transport layers, respectively?

Answer: Frame, packet, and segment, respectively.

14. Explain the meaning of the term L5PDU.

Answer: PDU stands for protocol data unit. A PDU is the entity that includes the headers and trailers created by a particular networking layer, plus any encapsulated data. For instance, an L5PDU includes Layer 5 headers and the encapsulated data.

15. Explain how Layer x on one computer communicates with Layer x on another computer.

Answer: Each layer of a networking model works with the same layer on another computer with which it wants to communicate. The protocol defined by each layer uses a header that is transmitted between the computers to communicate what each computer wants to do.

16. List the terms behind the acronym TCP/IP.

Answer: Transmission Control Protocol and Internet Protocol.

17. List the terms behind the acronym OSI. Answer: Open Systems Interconnection.

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