Advantech ARK-3400 User Manual

Contents

67 ARK-3400 User ManualAppendix C Application NotesC.2.3 How it worksWake-on-LAN is not restricted to LAN (Local area network) traffic.The general process of waking a computer up remotely over a network connectioncan be explained thusly:The target computer is shut down (Sleeping, Hibernating or Soft Off, i.e. ACPI stateG1 or G2), with power reserved for the network card. The network card listens for aspecific packet, called the "Magic Packet." The Magic Packet is broadcast on thebroadcast address for that particular subnet (or an entire LAN, though this requiresspecial hardware and/or configuration). When the listening computer receives thispacket, the network card checks the packet for the correct information. If the MagicPacket is valid, the network card turns on the computer to full power and boots theoperating system.The magic packet is sent on the data link or OSI-2 layer and broadcast to all NICs(within the network of the broadcast address). Therefore, it does not matter whetherthe remote host has a fixed or dynamic IP-address (OSI-3 layer).In order for Wake on LAN to work, parts of the network interface need to stay on. Thisincreases the standby power used by the computer. If Wake on LAN is not needed,turning it off may reduce power consumption while the computer is off but stillplugged in.C.2.4 Magic PacketThe Magic Packet is a broadcast frame containing anywhere within its payload 6bytes of ones (resulting in hexadecimal FF FF FF FF FF FF) followed by sixteen rep-etitions of the target computer's MAC address.Since the Magic Packet is only scanned for the string above, and not actually parsedby a full protocol stack, it may be sent as a broadcast packet of any network- andtransport-layer protocol. It is typically sent as a UDP datagram to port 0, 7 or 9, or, informer times, as an IPX packet.