Unit 6: The Link Layer
The final layer of the TCP/IP protocol stack that we discuss is 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 mechanisms in the link layer that can correct packet collisions when data is transferred over the wire. This unit discusses the link layer by focusing on how multiple hosts share one transmission medium. The unit 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);
- illustrate a variety of L2 networks using bridges, L2 switches, and repeaters by building or troubleshooting the networks;
- 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.
- explain how physical addressing resolves IP addresses at the link layer;
6.1: Principles of the Link Layer
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?
What is the framing problem? What service does the datalink layer provide to resolve this problem?
Datalink mechanisms also help solve problems related to detecting transmission errors.
6.2: Medium Access Control
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 do the Medium Access Control algorithms function in each computer network topology?
What is static allocation? What are some of the static allocation methods utilized in the datalink layer to share resources in a computer network? Make sure you can describe each method and how they handle the available resources.
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.
ALOHAnet originated at the University of Hawaii in the early 1970s. This network revolutionized wireless networking with its innovative ALOHA random access method using UHF frequencies to enable communications between remote units and a central station without dedicated scheduling. The system began as Pure ALOHA and was later refined with Slotted ALOHA, which had enhanced efficiency using synchronized transmission timings that mitigated data collisions. This formed the foundational technology for Ethernet, Wi-Fi, and mobile technology using a system of random time intervals.
How did Reservation ALOHA (R-ALOHA) reduce network delays and support higher traffic levels? What mechanism allows stations to maintain control over specific slots after successful transmissions? Consider how the two-channel star configuration of ALOHAnet optimized network communication for interactive computing. This design choice has influenced contemporary network communication protocols and the efficient use of network resources.
CSMA enhanced the ALOHA solution. Make sure you can describe CSMA and how it works to share computer network resources.
In a wired environment, both ALOHA and CSMA performance are better able to detect collisions. Can you explain why that is?
CSMA/CA is found in the Wi-Fi environment. How are collisions avoided?
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: Link Layer Technologies
It would take a library full of volumes to discuss all of the technologies used on the Internet. We have condensed this information into a discussion of six major categories. The following resources address the datalink services of these technologies.
The Point-to-Point Protocol (PPP) evolved from the Serial Line IP (SLIP) to facilitate reliable internet connectivity over direct links. PPP includes framing, option negotiation, and session authentication that ensures a secure structure for data transmission. Important features include a flag sequence framing and protocol identification, enabling versatile authentication methods through the Extensible Authentication Protocol (EAP). Why did PPP replace SLIP, and how does its frame structure support data integrity?
Read this section about MAC addresses, which are the datalink services found in an Ethernet environment.
Read this article to learn about the difference between the two current implementations of the Ethernet protocol. Pay attention to the differences in the frame format between the two, and make sure you can determine which type of frame is being received for a captured frame. As you read, focus on the historical development of Ethernet, its role in local area networks (LANs), and the significance of IEEE 802.3 standards. How has Ethernet evolved? How can you differentiate it from the IEEE 802.3 standards?
What is an Ethernet switch? How does it function in the datalink layer? How does it utilize Medium Access Control mechanisms?
The spanning tree protocol is a distributed standard switch used to reduce the network topology to a spanning tree by eliminating all cycles. Explore these examples to see how this technology processes frames in the datalink layer.
Read this section for a deeper look at how VLANs are implemented in networks and how switches react when they receive frames with a particular VLAN tag. How do switches direct frames based on VLAN tag, dropping or forwarding depending on VLAN members on a particular VLAN?
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. This section explores the wireless revolution in the datalink layer.
The Address Resolution Protocol (ARP) is a vital network function that translates IP addresses to MAC addresses. ARP comes into play when a network device, such as a host or router, needs to determine the MAC address associated with a known IP address in the same local network to facilitate direct communication. In this video, you will see how a device broadcasts an ARP request to retrieve the MAC address of another device. This process ensures that data is transmitted to the correct recipient. As you watch, consider the enhanced efficiency ARP provides to network operations. Be prepared to discuss the potential impact on data transmission if ARP were unavailable and the role ARP plays in directing data to the correct device in a network.
6.4: Summary
Read this summary of how Local Area Networks pose a problem for transporting frames when several devices share the same transmission channel. Various 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 these key technologies here.
Watch this video for more details about networking devices such as switches, hubs, 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
These exercises expand on the key principles from this unit. 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.
This is an optional assignment. Follow the steps in this document to practice controlling your computer from a mobile device.
Unit 6 Assessment
- 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.