All users within the network cannot access the Internet. I will begin troubleshooting at the OSI model's physical layer. What approach should I take to resolve this issue?

When troubleshooting a network issue where all users cannot access the Internet, starting at the physical layer of the OSI model is a systematic approach. Here are the steps you would take:

1. **Check the Status of Hardware Devices:** Make sure routers, switches, modems, and other network devices are powered on and showing normal operation indicators. Inspect any visible damage or unusual indicators like red lights or no lights, which might suggest a hardware failure.

2. **Examine Cables and Connections:** Ensure all cables are plugged in correctly and securely at both ends. Look for signs of damage, such as frayed or cut cables that could lead to connectivity problems.

3. **Verify Network Cards and Interfaces:** Check that the network interfaces on users' computers and on network devices are functioning correctly. Look for any signs of physical damage or check the device manager in the operating system to ensure that the network card is recognized and enabled.

4. **Test with a Different Device:** Try plugging in a known working device or cable to see if the problem persists. This will help you determine if the problem is with a specific cable, port, or device.

5. **Check for Power Issues:** Ensure that all devices are receiving power and that there are no issues with the power sources or surge protectors. A power failure to a key network device could cause network-wide connectivity issues.

6. **Physical Layer Tools:** Use network testing tools like a cable tester to check for breaks or faults within the cable infrastructure.

7. **Environmental Check:** Verify if any environmental factors could affect the physical layer, such as construction work, water damage, electromagnetic interference, etc.

Once you have systematically checked the physical connections and devices, if the problem is not resolved, you'd proceed to the next layer of the OSI model, which is the data link layer, where you would check switches and bridge connections, MAC addresses, and VLAN configurations. However, based on the nature of the issue – all users being unable to access the Internet – it might also be valuable to verify network-wide services such as DNS, routing, and Internet service status.

Extra: The OSI model is a conceptual framework used to understand and design the flow of data in a network. It is organized into seven layers, each with distinct functions:

1. **Physical Layer:** Concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.

2. **Data Link Layer:** Provides node-to-node data transfer—a link between two directly connected nodes. It detects and possibly corrects errors that may occur in the physical layer.

3. **Network Layer:** Deals with structuring and managing a multi-node network, including addressing, routing, and traffic control.

4. **Transport Layer:** Ensures complete data transfer.

5. **Session Layer:** Establishes, manages, and terminates connections between applications.

6. **Presentation Layer:** Translates data between a networking service and an application; includes character encoding, data compression, and encryption/decryption.

7. **Application Layer:** Provides network services to end-user applications such as web browsing, email, and file transfers.

By beginning at the physical layer, you are starting with the foundational elements that support network connectivity and moving up through the layers only after ensuring that the lower layers are functioning correctly. This methodical approach can lead to a more efficient troubleshooting process and often resolves many common network issues quickly without the need for more advanced diagnostics.