Unit 6: The Link Layer
The final layer of the TCP/IP protocol stack that you will learn in this course is known as the link layer. This unit will explain how you can address machines on a network from that layer, use IP addresses to determine physical addresses, and identify the different mechanisms in the link layer that can correct packet collisions when data is transferred over the wire.
This unit guides you through the principles of the link layer. Then the textbook will direct your focus to computer networks with a discussion of how multiple hosts share one transmission medium. The chapter ends with a detailed discussion of the two types of computer networks that are important today from a deployment perspective: Ethernet and WiFi.
Completing this unit should take you approximately 17 hours.
Upon successful completion of this unit, you will be able to:
- demonstrate an understanding of the link layer;
- resolve IP addresses with physical addressing in the link layer;
- reduce packet collisions in a network using CSMA; and
- define CSMA and describe its use in the link layer.
6.1: Principles
Read the introduction to chapter 6 and the beginning of section 6.1, "Principles," on pages 211-212. Stop at section 6.1.1 "Framing," which you will read below.
The datalink layer uses the service provided by each of the different technologies found in the physical layer to send and receive bits between directly connected devices. The datalink layer receives packets from the network layer. What are the two main services provided by the datalink layer? Why are these services necessary?
6.1.1: Framing
Read section 6.1.1, "Framing," on pages 212-213. As you read, consider the following questions: What is the framing problem? What service does the datalink layer provide to resolve this problem?
6.1.2: Error Detection
Read section 6.1.2, "Error Detection," on pages 213-214. Datalink mechanisms also help solve problems related to detecting transmission error.
6.2: Medium Access Control
Read the beginning of section 6.2, "Medium Access Control," on pages 214-215. Stop at section 6.2.1 "Static Allocation Methods," which you will read below.
A computer network environment faces additional problems beyond the framing and error detection issues that are resolved in the datalink layer. What are these problems? How does the Medium Access Control algorithm(s) function in any of the computer network topologies?
6.2.1: Static Allocation Methods
Read section 6.2.1, "Static Allocation Methods," on pages 215-216. As you read, consider the following questions: What is static allocation? What are some of the static allocation methods utilized in the datalink layer to share resources in a computer network? Describe each method and how each handles the available resources.
6.2.2: ALOHA
Read this article, which provides a more detailed examination of the ALOHAnet structure.
Read section 6.2.2, "ALOHA," on pages 216-217. The University of Hawaii addressed the problem of sharing resources with its network of remote island campuses in a unique way. This description of the ALOHAnet explores their solution.
6.2.3: Carrier Sense Multiple Access (CSMA)
Read section 6.2.3, "Carrier Sense Multiple Access," on pages 217-218. The ALOHA solution has been enhanced by CSMA. As you read, be able to describe CSMA and how it works to share computer network resources.
6.2.4: CSMA With Collision Detection
Read section 6.2.4, "Carrier Sense Multiple Access with Collision Detection," on pages 218-222. In a wired environment, both ALOHA and CSMA performance are better able to detect collision. Can you explain why that is?
6.2.5: CSMA with Collision Avoidance (CSMA/CA)
Read section 6.2.5, "Carrier Sense Multiple Access with Collision Avoidance," on pages 222-225. CSMA/CA is found in the Wi-Fi environment. How are collisions avoided?
6.2.6: Deterministic Medium Access Control Algorithms
Read section 6.2.6, "Deterministic Medium Access Control algorithms," on pages 226-228. Some applications are sensitive to the time delay of transmissions. Deterministic Medium Access Control algorithms have been used to resolve this problem. Explore this section to see some of the deterministic algorithms.
6.3: Technologies
Read the beginning of section 6.3, "Datalink Layer Technologies," on page 229. Stop at section 6.3.1 "The Point-to-Point Protocol," which you will read below.
It would take a library full of volumes to discuss all of the technologies used on the Internet. Your textbook has condensed this information into a discussion of six major categories. The following subunits address the datalink services of these technologies.
6.3.1: Point-to-Point Protocol
Read section 6.3.1, "The Point-to-Point Protocol," on pages 228-230.
6.3.2: Ethernet
Read the beginning of section 6.3.2, "Ethernet," on pages 230-234. Stop at "Ethernet switches," which you will read below. MAC addresses are the datalink services found in an Ethernet environment.
6.3.2.1: Ethernet Switches
Read "Ethernet Switches" on pages 234-237. Stop at "The Spanning Tree Protocol (802.1d)," which you will read below. As you read, consider the following questions: What is an Ethernet switch? How does it function in the datalink layer? How does it utilize Medium Access Control mechanisms?
6.3.2.2: The Spanning Tree Protocol (802.1d)
Read "The Spanning Tree Protocol (802.1d)" on pages 237-240. Stop at the "Virtual LANs" section, which you will read below. The spanning tree protocol is a distributed standard that is used by switches to reduce the network topology to a spanning tree by eliminating all cycles. Explore the examples in your textbook to see how this technology processes frames in the datalink layer.
6.3.2.3: Virtual LANs
Read "Virtual LANs" on pages 240-241. Stop at section 6.3.3 "802.11 wireless networks," which you will read below. As you read, consider the following question: How do Ethernet switches create virtual LANs?
6.3.3: 802.11 Wireless Networks
Read section 6.3.3, "802.11 Wireless Networks," on pages 241-246. Wi-Fi is a very popular wireless networking technology. There are hundreds of millions of Wi-Fi devices, resulting in multiple wireless networking standards that use different frequency ranges and different physical layers. Explore the wireless revolution into the datalink layer in this section.
6.4: Summary and a Review of Hub, Switch, and Network Router
Read section 6.4, "Summary," on page 246. In chapter 6, you explored how Local Area Networks pose a problem for transporting frames since several devices share the same transmission channel. A variety of Medium Access Control algorithms have been necessary to regulate the access to the transmission channel by reducing collisions: ALOHA, CSMA, CSMA/CD, and CSMA/CA. Review the key technologies discussed in this section.
Watch this video for more details about networking devices such as switch,hub and routers. A network switch is a multiport network bridge that uses hardware addresses to process and forward data at the data link layer (layer 2) of the OSI model. A router is a networking device that forwards data packets between computer networks. An Ethernet hub, active hub, network hub, repeater hub, multiport repeater, or simply "hub", is a network hardware device for connecting multiple Ethernet devices together and making them act as a single network segment.
6.5: Practice Exercises
Read section 6.5, "Exercises," through the end of chapter 6, on pages 246-247. These exercises expand the key principles in this chapter. If you are a computer professional, you will enjoy the challenges and higher-level discussions in this section. If you are a novice, explore the presentations and spend more time on the topics that are meaningful to you.
Follow the steps detailed in this document in order to control your computer from a mobile device.
Unit 6 Assessment
Take this assessment to see how well you understood this unit.
- This assessment does not count towards your grade. It is just for practice!
- You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.