Exploring the Fundamentals of Networking: Data Link Layer and Beyond

Posted on Nov 14, 2024 in Computers

1. What is the Data Link Layer?

The data link layer provides a means to exchange data using common local media. It performs two basic services:

  • Allows the upper layers to access the media using techniques such as frames.
  • Controls how data is placed in the media and received from the media using techniques such as access control methods and error detection.

2. Structure of a Frame

  • Header: Contains control information (Addressing)
  • Payload: The packet from the network layer.
  • Trailer: Contains control information at the end of the PDU.

3. Physical Topologies

  • Point-to-Point (P2P): The medium access method used by the data link protocol is determined by the P2P logical topology, not the physical topology. This means that the logical point-to-point connection between two nodes may not necessarily be between two physical nodes at each end of a single physical link.
  • Multi-Access: Allows several nodes to communicate using the same shared media. Data from a single node can be placed on the medium at any time. All nodes see all the frames that are on the medium, but only the node to which the frame is addressed processes the frame’s contents.
  • Ring: Each node receives a frame in turn. If the frame is not addressed to the node, the node passes the frame to the next node. This enables a media access control technique using a ring called token passing.

4. Major Protocols

  • Ethernet
  • Point-to-Point Protocol (PPP)
  • High-Level Data Link Control (HDLC)
  • Frame Relay
  • Asynchronous Transfer Mode (ATM)

5. The Ethernet Protocol

Ethernet is a family of networking technologies, as defined in the 802.2 and 802.3 standards. The standards define the protocols of Ethernet Layer 2 and Layer 1 technologies. Ethernet is the most widely used LAN technology and supports data bandwidths of 10, 100, 1000, or 10,000 Mbps.

6. Physical Layer Standards

  • Physical and electrical properties of the media.
  • Mechanical properties.
  • Representation of bits by signals.
  • Definition of control information signals.

7. Fundamental Principles of the Physical Layer

  • Encoding: Converts a stream of data bits into a predefined code. They also provide control codes for identifying the beginning and end of a frame.
  • Signaling: Is the method of representing bits 0 and 1. The bit time is the time it takes for each signal to occupy the specific media.

8. Manchester Encoding

Bit values are represented as voltage transitions. This transition can be used to ensure that the receiving node’s bit timing is synchronized with the transmitting node.

9. Useful Transfer Capacity

A measure of usable data transferred during a period of time. It is the measure of most interest to network users. This capacity represents the performance without the overhead of traffic used to establish sessions, acknowledgments, and encapsulations.

10. Types of Wireless Media

  • IEEE 802.11 (Wi-Fi): A LAN technology with a non-deterministic system using a CSMA/CA media access process.
  • IEEE 802.15 (WPAN: Wireless Personal Area Network): Also known as Bluetooth. Uses pairing between devices within a range of 1 to 100 meters.
  • IEEE 802.16 (WiMAX): Uses point-to-multipoint connections to provide wireless broadband access.

11. Collision Handling in Ethernet

Why does a host on an Ethernet segment experiencing a collision use a random delay before attempting to retransmit a frame?

They use a random delay to help prevent the stations from experiencing another collision during retransmission.

12. Switches vs. Hubs

What are the two characteristics that make switches preferable to hubs in Ethernet networks?

  • Increased communication performance.
  • Reduction of collisions to a minimum.

13. Upper Link Sublayer Functions

What are the three functions of the upper link sublayer in the OSI model?

  • Establishes a connection with the upper layer.
  • Protects the network layer protocols from changes in the physical media.
  • Identifies the network layer protocol.

14. Purpose of ARP

What is the main purpose of ARP?

To resolve IPv4 addresses to MAC addresses.

15. Ethernet in the OSI Model

In which layers of the OSI model does Ethernet operate?

Physical layer and data link layer.

16. Router Configuration Access Methods

  • Console: A direct connection using a cable that connects to a PC’s serial port and a special connector on the router.
  • Telnet and SSH: These connection types are established remotely (not physically at the router’s location). A password is required.
  • Auxiliary: This connection is similar to the console, but a network cable is used. It extends from the auxiliary port of the router to the PC’s Ethernet card. This mode is used when the console connection has problems with certain parameters.

17. Cisco Router Configuration File Types

  • Startup Configuration File (startup-config): Used during system startup to configure the device. It is stored in NVRAM (non-volatile RAM), so the file remains intact even when the device is powered off.
    Startup-config files are loaded into RAM when the device starts. Once in RAM, the configuration is considered the running configuration (running-config).
  • Running Configuration (running-config): Once in RAM, this configuration is used to operate the network device. It is changed when the administrator modifies the device configuration, with changes affecting the Cisco device immediately.
    Changes can be saved to the startup-config in NVRAM. If not saved, changes made in RAM are lost when the router restarts.

18. Router Implementation Modes

  • User Mode: Has limited capabilities but allows for basic operations. In this mode, you cannot change any settings on the router.
  • Privileged Mode: In this mode, you can change the router configuration. The router will grant permission to perform any operation, provided you have the necessary passwords.

19. Using the CLI Help

  • Context-Sensitive Help: Provides a list of commands and their arguments within the context of the current mode. To access this help, type a “?” at any input prompt. Allows you to check the name of a command or see how a command can be used.
    Can also be used to verify commands that start with a specific sequence of characters.
  • Checking Command Syntax: When you enter a command and press Enter, the command interpreter parses it from left to right. If it cannot understand something, it displays an error message. The message can be due to three different causes: Ambiguous (missing characters for the IOS to understand), Incomplete (missing arguments), or Invalid (incorrectly entered command).
  • Hotkeys and Shortcuts: The command-line interface has hotkeys and shortcuts for easier configuration and troubleshooting.

20. Configuring Router Interfaces

Scenario: A physically connected network fails to function. Upon investigation, you realize that the main router’s connections are down. Configure FastEthernet 0/0 with IP address 192.168.0.10 and subnet mask 255.255.255.0, and Serial 0/1/0 with IP address 10.0.1.1 and subnet mask 255.0.0.0.

Router> enable (enter privileged mode)
Router# configure terminal (enter configuration mode)
Router(config)# interface fastEthernet 0/0 (enter interface configuration mode)
Router(config-if)# ip address 192.168.0.10 255.255.255.0 (configure interface settings)
Router(config-if)# no shutdown (enable the interface)
Router(config-if)# exit (exit to the previous mode)
Router(config)# interface serial 0/1/0 (enter interface configuration mode)
Router(config-if)# ip address 10.0.1.1 255.0.0.0 (configure interface settings)
Router(config-if)# no shutdown (enable the interface)
Router(config-if)# exit (exit to the previous mode)
Router(config)# exit (exit to the previous mode)
Router# show ip interface brief (display interface status - FastEthernet 0/0 and Serial 0/1/0 should be up)