• 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 Wi-Fi.

    Completing this unit should take you approximately 7 hours.

    • Upon successful completion of this unit, you will be able to:

      • explain how physical addressing resolves IP addresses at the link layer;
      • illustrate the methods for error control and flow control at the data link layer (DLL), including error detection codes and the sliding window protocol;
      • illustrate different framing techniques used at the DLL, such as length count, bit stuffing, and flag delineation;
      • describe the difference between data link technologies, such as the Point-to-Point (PPP), Ethernet V2, and 802.3 protocols;
      • describe how packet collisions in a network are controlled using carrier-sense multiple access with collision detection (CSMA/CD);
      • build and troubleshoot a variety of L2 networks using bridges, L2 switches, and repeaters;
      • use Virtual LANs (VLANs) to create multiple LANs in the same physical environment;
      • illustrate the Spanning Tree Protocol (STP), why it is needed, and how it breaks loops in an L2 network;
      • describe different allocation methods used by the data link layer; and
      • use the 802.11 protocol to build and use wireless networks.

    • 6.1: Principles

      • Principles Page

        Read this section. 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

        • Framing Page

          As you read this section, 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

        • Error Detection Page

          Read this material. Datalink mechanisms also help solve problems related to detecting transmission error.

    • 6.2: Medium Access Control

      • Medium Access Control Page

        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

        • Static Allocation Methods Page

          As you read this section, 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

        • ALOHAnet Page

          Read this article, which provides a more detailed examination of the ALOHAnet structure.

        • ALOHA Page

          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)

        • Carrier Sense Multiple Access Page

          The ALOHA solution has been enhanced by CSMA. As you read this section, be able to describe CSMA and how it works to share computer network resources.

      • 6.2.4: CSMA With Collision Detection

      • 6.2.5: CSMA with Collision Avoidance (CSMA/CA)

      • 6.2.6: Deterministic Medium Access Control Algorithms

        • Deterministic Medium Access Control algorithms Page
          Read this section. 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

      • Datalink Layer Technologies Page

        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

      • 6.3.2: Ethernet

        • Ethernet Page

          Read this section. MAC addresses are the datalink services found in an Ethernet environment.

        • Ethernet vs 802.3 URL

          Read this article to learn about the difference between the two current implementation of the Ethernet protocol. Place special attention to the frame format clearly differentiating between the two formats and being able to determine which type of frame is being receive for a captured frame.

        • 6.3.2.1: Ethernet Switches

          • Ethernet Switches Page

            As you read this section, 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)

          • The Spanning Tree Protocol (802.1d) Page

            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

          • Virtual LANs Page

            As you read this section, consider the following question: How do Ethernet switches create virtual LANs?

          • Virtual Networks Page

            Read this material for a deeper understanding of how VLANs are implemented in networks and how switches react when they receive frames with a particular VLAN tag. Explore how switches direct frames based on VLAN tag, dropping or forwarding depending on VLAN members on a particular VLAN. 

          • 802.11 Wireless Networks Page

            Read this section. 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.

          • MAC to IP Address Resolution Page

            Read this article, which describes how the physical data link layer address (the MAC address) is mapped to a corresponding IP address. Pay special attention to the packet structure that allows a node to obtain the MAC address of a particular node given the IP address of the intended recipient. Make sure that you understand why this is needed.

    • 6.4: Summary and a Review of Hub, Switch, and Network Router

      • Summary Page

        Read this summary of Chapter 6. In this chapter, 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.

      • Repeaters, Hubs, Bridge Switches, and Routers Page

        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

      • Exercises Page

        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.

      • Local Area Network Service Scan and Wake on LAN Page

        Follow the steps detailed in this document in order to control your computer from a mobile device.

    • Unit 6 Assessment

      • Unit 6 Assessment Quiz
        Receive a grade

        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.