Understanding Wake on LAN: Does it Work When the Computer is Off?
When it comes to remote power management of computers, Wake on LAN (WoL) is a common feature that allows a computer to be awakened by a specially crafted network packet. However, the effectiveness of Wake on LAN can depend on the power state of the computer at the time of the packet.
When Does Wake on LAN Work?
If the computer is off: Wake on LAN does not work if the computer is completely powered down. In this state, the computer is in what is often called an "off" or "ACPI G3" state. In this mode, all power supplies and hardware components are powered off, and the computer is not capable of receiving any network packets, including Wake on LAN magic packets.
Do Computers Need to Be in Sleep or Hibernate Mode?
Requirement for Sleep or Hibernate Mode: For Wake on LAN to work, the computer must be in one of the supported sleep states: S3 (sleep mode) or S4 (hibernation mode). These states maintain a low-power state but still retain some power to the network adapter, allowing it to receive and process the Wake on LAN magic packet.
Once the magic packet is received, the computer will proceed according to its power state. If it's in S3 mode, it may not have all its power components active, but it will still be partially powered up to process the packet. For S4 mode (hibernation), the computer will need to restore full power and boot up from hibernation to fully operate.
Does Wake on LAN Work in Soft-Off or S4 State?
Soft-Off State (ACPI G2/S5 and Above): In some cases, computers have a "soft-off" or "ACPI G2/S5" state where the LAN adapter receives power continuously from the standby power bus, even when the rest of the computer is powered off. This can allow the network adapter to monitor for specific "wake" packets and send a power-on signal to the power supply. Once this happens, the computer behaves as if it were being powered on by the power button from a fully powered down state, bypassing the S3 or S4 sleep states.
However, it's important to note that this process is highly dependent on the specific hardware configuration and BIOS/UEFI settings. Not all systems support this behavior, and even if they do, the process might require the computer to fully power up and re-activate its components before it can fully resume its functions.
OS and Adapter Support for Wake on LAN
OS and Adapter Support: The ability to support Wake on LAN also heavily depends on the operating system and the network adapter. The network adapter must be "armed" for specific Wake on LAN scenarios, such as receiving a "magic packet." The operating system must also support Wake on LAN from a specific power state. For instance, Windows supports Wake on LAN from a variety of power states, but Unix-based systems might have different levels of support.
Conclusion
Wake on LAN does not work when the computer is completely powered down or in the "off" state (ACPI G3). However, when the computer is in a sleep state like S3 or S4, Wake on LAN can function effectively. Additionally, in some advanced systems, the LAN adapter might be able to receive a magic packet even in a soft-off state, providing a more seamless boot-up process.
When configuring Wake on LAN, ensure that your network adapter and operating system are properly set up to support this feature and that the computer is in a compatible sleep state. This setup will maximize the functionality and reliability of Wake on LAN in your network management workflow.
Frequently Asked Questions
Why doesn’t Wake on LAN work when the computer is completely powered down?
Wake on LAN requires at least a minimal amount of power to the network adapter to detect and process the magic packet. When the computer is fully powered down, all power supplies and hardware components are shut off, leaving no power for the adapter to function and receive the packet.
Can Wake on LAN work if the computer is in hibernate mode?
Yes, Wake on LAN can work in hibernate mode (S4 sleep state) provided that the operating system and network adapter are properly configured to support this feature. In this state, the computer will restore full power and boot from hibernation to fully operate when the magic packet is received.
What is a soft-off state, and how does it affect Wake on LAN?
A soft-off state, such as ACPI G2/S5, is a power state where the LAN adapter continues to receive power from the standby power bus, even when the rest of the computer is powered off. This allows the network adapter to monitor for specific "wake" packets and send a power-on signal to the power supply. Once this happens, the computer behaves as if it were being powered on by the power button from a fully powered down state.