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24 février 2011 4 24 /02 /février /2011 05:57

In computing, the Advanced Configuration and Power Interface (ACPI) specification provides an open standard for unified operating system-centric device configuration and power management     (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

ACPI, first released in December 1996, defines platform-independent interfaces for hardware discovery, configuration, power management and monitoring. The specification is central to Operating System-directed configuration and Power Management(OSPM)      Dell Studio XPS 1640 Battery ;

a term used to describe a system implementing ACPI, which therefore removes device management responsibilities from legacy firmware interfaces. The standard was originally developed by Intel, Microsoft, and Toshiba - later joined by HP and Phoenix. - and last (as of 2010) published as "Revision 4.0a", on April 5, 2010   Dell Vostro 1710 Battery .


ACPI aims to consolidate and improve upon existing power and configuration standards for hardware devices. It provides a transition from existing standards to entirely ACPI-compliant hardware, with some ACPI operating systems already removing support for legacy hardware       Dell KM958 battery .

With the intention of replacing Advanced Power Management, the MultiProcessor Specification and the Plug and Play BIOS Specification, the standard brings power management into operating system control (OSPM), as opposed to the previous BIOS central system, which relied on platform-specific firmware to determine power management and configuration policy  Dell Studio 1555 battery .

The ACPI specification contains numerous related components for hardware and software programming, as well as a unified standard for device/power interaction and bus configuration. As a document that unifies many previous standards it covers many areas, for system and device builders as well as system programmers         Sony VGP-BPS13 battery .

Some software developers have trouble implementing ACPI and express concerns about the requirements that bytecode from an external source must be run by the system with full privileges. Linus Torvalds, creator of the Linux kernel, once described it as "a complete design disaster in every way", in relation to his view that "modern PCs are horrible"    Sony VGP-BPS13/B battery .

Microsoft Windows 98 was the first operating system with full support for ACPI , with Windows 2000, Windows XP, Windows Vista, Windows 7, eComStation, FreeBSD, NetBSD,OpenBSD, HP-UX, OpenVMS, Linux and PC versions of SunOS all having at least some support for ACPI Sony VGP-BPS13/S battery .

OSPM responsibilities

ACPI requires that once an OSPM-compatible operating system has activated ACPI on a computer, it then takes over and has exclusive control of all aspects of power management and device configuration   Sony VGP-BPS13A/B battery .

The OSPM implementation must expose an ACPI-compatible environment to device drivers, which exposes certain system, device and processor states.

Global states

The ACPI specification defines the following seven states (so-called global states) for an ACPI-compliant computer-system  Sony VGP-BPS13B/B battery :

  • G0 (S0): Working
  • G1Sleeping subdivides into the four states S1 through S4:
    • S1: All processor caches are flushed, and the CPU(s) stop executing instructions. Power to the CPU(s) and RAM is maintained; devices that do not indicate they must remain on may be powered down  Sony VGP-BPL9 battery .
    • S2: CPU powered off
    • S3: Commonly referred to as StandbySleep, or Suspend to RAM. RAM remains powered
    • S4Hibernation or Suspend to Disk. All content of main memory is saved to non-volatile memory such as a hard drive, and is powered down  Sony VGP-BPS13B/B battery .
  • G2 (S5), Soft Off: G2 is almost the same as G3 Mechanical Off, but some components remain powered so the computer can "wake" from input from the keyboard, clock, modem, LAN, or USBdevice  Sony VGP-BPL15 battery .
  • G3Mechanical Off: The computer's power consumption approaches close to zero, to the point that the power cord can be removed and the system is safe for dis-assembly (typically, only the real-time clock is running off its own small battery)  Dell Inspiron E1505 battery .

Furthermore, the specification defines a Legacy state: the state on an operating system which does not support ACPI. In this state, the hardware and power are not managed via ACPI, effectively disabling ACPI  Dell Latitude E6400 battery .

Device states

The device states D0-D3 are device-dependent:

  • D0 Fully On is the operating state.
  • D1 and D2 are intermediate power-states whose definition varies by device  HP Pavilion dv6000 Battery .
  • D3 Off has the device powered off and unresponsive to its bus.

Processor states

The CPU power states C0-C3 are defined as follows  SONY VAIO VGN-FZ Battery :

  • C0 is the operating state.
  • C1 (often known as Halt) is a state where the processor is not executing instructions, but can return to an executing state essentially instantaneously       SONY VAIO VGN-FZ18 Battery .
  • All ACPI-conformant processors must support this power state. Some processors, such as the Pentium 4, also support an Enhanced C1 state (C1E or Enhanced Halt State) for lower power consumption  SONY VAIO VGN-FZ21E Battery .
  • C2 (often known as Stop-Clock) is a state where the processor maintains all software-visible state, but may take longer to wake up. This processor state is optional  SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .
  • C3 (often known as Sleep) is a state where the processor does not need to keep its cache coherent, but maintains other state. Some processors have variations on the C3 state (Deep Sleep, Deeper Sleep, etc.) that differ in how long it takes to wake the processor. This processor state is optional  SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

Performance states

While a device or processor operates (D0 and C0, respectively), it can be in one of several power-performance states. These states are implementation-dependent, but P0 is always the highest-performance state, with P1 to Pn being successively lower-performance states, up to an implementation-specific limit of n no greater than 16  SONY VAIO VGN-FW21E Battery .

P-states have become known as SpeedStep in Intel processors, as PowerNow! or Cool'n'Quiet in AMD processors, and as PowerSaver in VIA processors.

Hardware Interface

ACPI-compliant systems interact with hardware through either a "Function Fixed Hardware (FFH) Interface" or a platform-independent hardware programming model which relies on platform-specific ACPI Machine Language (AML) provided by the original equipment manufacturer (OEM)    SONY VAIO VGN-NR11Z/S Battery .

Function Fixed Hardware interfaces are platform-specific features, provided by platform manufacturers for the purposes of performance and failure recovery. Standard Intel-based PCs have a fixed function interface defined by Intel SONY VAIO VGN-NR11Z/T Battery ,

which provides a set of core functionality that reduces an ACPI-compliant system's need for full driver stacks for providing basic functionality during boot time or in the case of major system failure  Sony VAIO VGN-FZ21E Battery .

Firmware interface

ACPI defines a large number of tables that provide the interface between an ACPI-compliant operating system and system firmware. These allow description of system hardware in a platform-independent manner, and are presented as either fixed formatted data structures or in AML   Sony VAIO VGN-FW21E Battery .

The main AML table is the DSDT (differentiated system description table).

The Root System Description Pointer is located in a platform-dependent manner, and describes the rest of the tables  Sony VAIO VGN-NR11S/S Battery .

ACPI Component Architecture (ACPICA)

The ACPI Component Architecture (ACPICA) provides an open-source OS-independent reference implementation of the ACPI specification.


The first revision of the ACPI specification was released in December 1996 supporting 16 and 32-bit addressing spaces  Sony VAIO VGN-NR11Z/S Battery .

It wasn't until August 2000 that ACPI received 64-bit address support as well as support for multiprocessor workstations and servers with revision 2.0. In September 2004, revision 3.0 gave the ACPI specification support for SATA connectors, PCI Express bus, >256 multiprocessor support, ambient light sensors and user-presence devices  Sony VAIO VGN-NR11M/S Battery ,

as well as extending the Thermal model beyond the previous processor centric support. The latest of the major publications is that of revision 4.0. Released in June 2009, the 4.0 specification added many new features to the design; most notable are USB 3.0 support, logical processor idling support, and x2APIC support  Sony VAIO VGN-NR11Z/T Battery .

Power management keys are three keys on computer keyboards which control the computer's power management status. They are the Power key, theSleep key, and the Wake key:

  • The Power key is used both to power on and to power off the computer       SONY VAIO VGN-FZ180E Battery .
  • The Sleep key is used to put the computer to sleep, when it uses significantly lower power than when it is active, but can be woken quickly.
  • The Wake key wakes the system from standby   SONY VAIO VGN-FZ220E Battery .

The power saving modes and the transition between them is usually implemented using ACPI.

On computers running Windows Vista, the Windows key can also act as a power management key, and turns on the computer when pressed  SONY VAIO VGN-FZ340E Battery .

Keyboard layout

The placement of the three power management keys on a computer keyboard varies depending on the keyboard's model.

Ideally, these keys should be placed far enough from the rest of the keys       SONY VAIO VGN-FZ430E Battery ,

since the consequences of accidentally pressing one of them while using the computer (with the exception of the Wake key, which does nothing in this situation) can range from being annoying to causing data loss. For instance, if the user has edited a file, but not saved it yet        SONY VAIO VGN-FZ460E Battery ,

on some applications shutting down the system (which happens when a finger slip hits the Power key) can easily lose all the changes (although generally the more used operating systems ask the user to save the file before shut down). While the results of wrongly pressing the Sleep key aren't as extreme (unless the computer has problems returning from sleep)  SONY VAIO VGN-FZ480E Battery,

it can cause problems in real-time games or any application which depends on a network connection.

One common placement for these keys is in the right of the top row of the keyboard, replacing the Print Screen/SysRq, Scroll Lock, and Pause/Break keys, which are moved one row down      SONY VAIO VGN-FZ4000 Battery .

Hereby the 2x3 block of edit and navigation keys is made to occupy the empty space just above the arrow keys. This annoys some people, because it confuses their muscle memory.

Another common placement for these keys is just below the Delete, End, and Page down keys, in the empty space just above the Arrow keys      SONY VAIO VGN-FZ31E Battery .

When used with normal-sized keys, this can cause a high probability of accidental keypresses, especially when the typist is used to a keyboard without any keys on that position, or with the alternate arrangement described above. This leads some people to physically remove the keycaps       SONY VAIO VGN-FZ31B Battery .

Some keyboard models avoid this problem by using keys with a lower height and requiring a higher force to be pressed.

Many keyboards have found a solution to this problem, in that a specific key has to be held down while the power management key is pressed  SONY VAIO VGN-FZ31J Battery .

Often, this is found in the top-right of the keyboard, to the right of the lock button lights, and can be described as "opposite the escape button". With this button, the power, sleep, and wake keys don't do anything when pressed alone, and the function key does nothing, either    SONY VAIO VGN-FZ31M Battery .

Most of the original ADB Apple Macintosh keyboards, as well as the first USB keyboard, had a key to turn the machine on in lieu of a physical power button on the computer itself. The symbol on it was usually a triangle pointing left (?)  SONY VAIO VGN-FZ31Z Battery .

On most keyboards it was located at the extreme upper-right corner, above the function keys and above the right most key, or in the same vertical position and in the center. The power button was replaced with a disc eject button when the Apple Pro Keyboard was introduced, and Apple removed the ability of Macs to be switched on from the keyboard SONY VAIO VGN-FZ38M Battery .

Pressing the power button when running Mac OS brought up the shutdown dialog box, on keyboards with an eject button the combination Control-Eject is used instead (for most x-Power keyboard combinations, x-Eject is used as the replacement combination)  SONY VGP-BPS8 Battery .

Wake-on-LAN (WOL) is an Ethernet computer networking standard that allows a computer to be turned on or woken up by a network message. The message is usually sent by a program executed on another computer on the same local area network SONY VGP-BPS13/S Battery .

Equivalent terms include Wake On WANRemote Wake-upPower On By LANPower Up By LANResume by LANResume on LANWake Up On LAN SONY VGP-BPS13A/B Battery .

In case the computer being woken is communicating via Wi-Fi, a supplementary standard called Wake on Wireless LAN (WoWLAN) must be employed .

The WOL and WoWLAN standards are often supplemented by vendors to provide protocol-transparent on-demand services, for example in the Apple Bonjour wake-on-demand feature  SONY VGP-BPS13B/B Battery .

Technical details

System requirements

Wake-on-LAN support is implemented on the motherboard (BIOS) of a computer and the network interface (firmware), and is consequently not dependent on the operating system (and NIC drivers) running on the hardware   SONY VGP-BPS13A/S Battery .

Some operating systems can control Wake-on-LAN behaviour via hardware drivers. If the network interface is a plug-in card rather than being integrated into the motherboard, the card may need to be connected to the motherboard by a cable. Motherboards with an embedded Ethernet controller which supports Wake-on-LAN do not need a cable   SONY VGP-BPS13AS Battery .

The power supply must meetATX 2.01 specifications.

How it works

Wake-on-LAN is implemented using a special network message called a magic packet. The magic packet contains the MAC address of the destination computer       Dell Inspiron 1320n Battery .

The listening computer waits for a magic packet addressed to it and then initiates system wake-up.

The magic packet is sent on the data link or layer 2 in the OSI model and broadcast to all NICs using the network broadcast address; the IP-address (layer 3 in the OSI model) is not used  Dell Inspiron 1464 Battery .

This is why Wake-on-LAN is platform-independent. Any application, on any platform, can wake up computers running on any other platform.

It is a common misconception that because Wake-on-LAN is built upon broadcast technology it can only be used within the current network subnet. Whilst this is generally the case there are some exceptions  Dell Inspiron 1564 Battery .

In order for Wake-on-LAN to work, parts of the network interface need to stay on. This consumes standby power, much less than normal operating power. If Wake-on-LAN is not needed, disabling it may reduce power consumption slightly while the computer is switched off but still plugged in  Dell Inspiron 1764 Battery .

Magic Packet

The magic packet is a broadcast frame containing anywhere within its payload 6 bytes of all 255 (FF FF FF FF FF FF in hexadecimal), followed by sixteen repetitions of the target computer's 48-bitMAC address  Dell Studio 1450 Battery .

Since the magic packet is only scanned for the string above, and not actually parsed by a full protocol stack, it may be sent as any network- and transport-layer protocol. It is typically sent as a UDPdatagram to port 7 or 9, but actually it can be sent on any port  Dell Studio 1457 Battery .

Limitations of Wake-on-LAN Magic Packets

A standard magic packet has the following basic limitations:

  • Requires destination computer MAC address (also may require a SecureOn password)
  • Does not provide a delivery confirmation  Dell Latitude D610 Battery
  • May not work outside of the local (or the local segment of the) network
  • Require hardware support of Wake-On-LAN on destination computer       Toshiba NB100 Battery

The Wake-on-LAN implementation is designed to be very simple and to be quickly processed by the circuitry present on the network interface card (NIC) with minimal power requirement. Because Wake-on-LAN operates below the protocol layer the MAC address is required and makes IP addresses and DNS names meaningless    Toshiba Satellite M65 battery .

Subnet Directed Broadcasts

A principal limitation of standard broadcast Wake-On-LAN is that broadcast packets are generally not routed. This prevents the technique being used in larger networks or over the internet. Subnet Directed Broadcasts (SDB) may be used to overcome this limitation     Toshiba Satellite M60 battery .

SDB may require changes to intermediate router configuration. Subnet directed broadcasts are treated as normal network packets until processed by the final (local) router. This router converts the packet into a true broadcast packet. This technique allows a broadcast to be initiated on a remote network but requires all intervening routers to forward the SDB Dell Latitude D830 Battery .

When preparing a network to forward SDB packets, care must be taken to filter such that only desired (e.g. WoL) SDB packets are permitted—otherwise the network becomes unprotected against DDoS attacks such as the Smurf Attack  Dell Latitude D620 Battery .

Sending the magic packet

There are many ways to send the magic packet. Software is available for all modern platforms, including Windows, Apple and Linux, plus many smart phones. Also there are web sites on the Internet that allow a magic packet to be sent online without charge  Dell Inspiron Mini 10 Battery .

Example source code for a developer to add Wake-on-LAN to a program is readily available in many computer languages.

Some home routers are able to send magic packets to LAN, for example routers with DD-WRT firmware have Wake On Lan client  Sony VGN-FW11S Battery .

Troubleshooting magic packets

Wake-on-LAN can be a frustrating technology to implement. This is because it requires appropriate BIOS, network card and, sometimes, operating system and router support to function reliably. In some cases hardware may wake from one low power state but not from others    Sony VGN-FW11M Battery .

This means that due to hardware issues the computer may be waking up from the "fully off state" (S5) but doesn't wake from sleep or hibernation.

There are software tools to help with Wake-on-LAN troubleshooting       Sony VGN-FW139E/H battery .

These monitoring tools allow confirmation that the magic packet has arrived at a particular PC. This allows networking issues to be isolated from other hardware issues. In some cases they also confirm that the packet was destined for a specific PC or allow magic packets to be 'promiscuously sniffed' from the network  Dell Latitude E5400 Battery .

In Windows Vista and higher, you can also determine how the OS was powered up. You can use powercfg /lastwake in the CMD prompt and it will list the "Wake Source". The Wake-on-LAN event should also be logged in the System event log Dell Latitude E4200 Battery .

Security Issues

Magic packets are sent via the data link or OSI-2 layer, which can be used or abused by anyone on the same LAN, unless the L2 LAN equipment is capable of (and configured for) filtering such traffic to match site-wide security requirements    Dell Inspiron 300M Battery .

Firewalls may be used to prevent clients within the public WAN from accessing the broadcast addresses of inside LAN segments.

Certain NICs support a security feature called "SecureOn". It allows users to store within the NIC a hexadecimal password of 6 bytesDell Vostro A840 Battery .

Clients have to append this password to the magic packet. TheNIC wakes the system only if the MAC address and password are correct. This security measure significantly decreases the risk of successful brute force attacks, by increasing the search space by 48 bits (6 bytes), up to 296 combinations if the MAC address is entirely unknown Dell Studio 1737 battery .

Still, only a few NIC and router manufacturers support such security features.

Abuse of the Wake-on-LAN feature only allows computers to be switched on; it does not in itself bypass password and other forms of security   Dell Inspiron E1505 battery .

TLS Encryption

Some PCs include technology built into the chipset to improve security for Wake-on-LAN. For example, Intel AMT (a component of Intel vPro technology), includes Transport Layer Security (TLS), an industry-standard protocol that strengthens encryption  Dell RM791 battery .

AMT uses TLS encryption to secure an out-of-band communication tunnel to an AMT-based PC for remote management commands such as Wake-on-LAN. AMT secures the communication tunnelAdvanced Encryption Standard (AES) 128-bit encryption and RSA keys with modulus lengths of 2,048 bits  Dell XPS M1530 battery .

Because the encrypted communication is out-of-band, the PC’s hardware and firmware receive the magic packet before network traffic reaches the software stack for the operating system (OS). Since the encrypted communication occurs “below” the OS level, it is less vulnerable to attacks by viruses, worms, and other threats that typically target the OS level  Dell XPS M2010 battery .

IT shops using Wake-on-LAN through the Intel AMT implementation can wake an AMT PC over network environments that require TLS-based security, such as IEEE 802.1x, Cisco Self Defending Network (SDN), and Microsoft Network Access Protection (NAP) environments   Acer Aspire One battery .

The Intel implementation also works for wireless networks.

Hardware implications

Older motherboards must have a WAKEUP-LINK header onboard connected to the network card via a special 3-pin cable; however, systems supporting the PCI 2.2 standard and with a PCI 2.2 compliant network adapter card do not usually require a Wake-on-LAN cable as the required standby power is relayed through the PCI bus      Toshiba Satellite P10 Battery .

PCI version 2.2 supports PME (Power Management Events). PCI cards send and receive PME signals via the PCI socket directly, without the need for a Wake-on-LAN cable     SONY VGN-FZ210CE Battery .

Wake-on-LAN usually needs to be enabled in the Power Management section of a PC motherboard's BIOS setup utility, although on some systems, such as Apple computers, it is enabled by default. It may also be necessary to configure the computer to reserve power for the network card when the system is shut down     Dell Precision M70 Battery .

In addition, in order to get Wake-on-LAN to work it is sometimes required to enable this feature on the interface card. Details of how to do this depend upon the operating system and the device driver       Toshiba Satellite L305 Battery.

Laptops powered by the Intel Centrino® Processor Technology or newer (with explicit BIOS support) allow waking up the machine using wireless Wake on Wireless LAN (WoWLAN)      Toshiba Satellite T4900 Battery .

In most modern PCs, ACPI is notified of the "waking up" and take control of the Power up. In ACPI, OSPM must record the "wake source" or the device that is causing the power-up. The device being the "Soft" power switch, the NIC (via Wake-on-LAN), the cover being opened, a temperature change, etc     Toshiba PA3399U-2BRS battery .

Other machine states and LAN wakeup signals

In the early days of Wake-on-LAN the situation was relatively simple: a machine was connected to power but switched off, and it was arranged that a special packet be sent to switch the machine on     Toshiba Satellite A200 Battery .

Since then many options have been added and standards agreed upon. A machine can be in 7 power states from S0 (fully on) through S5 (powered down but plugged in) and disconnected from power (G3, Mechanical Off), with names such as "sleep", "standby", and "hibernate"     Toshiba Satellite 1200 Battery .

In some reduced-power modes the system state is stored in RAM and the machine can wake up very quickly; in others the state is saved to disk and the motherboard powered down, taking at least several seconds to awake. The machine can be woken from a reduced-power state by a variety of signals     Toshiba Satellite M300 Battery .

In a particular example, the Gigabyte 8KNXP motherboard with built-in Intel PRO/1000 CT network adapter, there are 3 motherboard BIOS settings and 6 network adapter settings which affect wakeup. The problem is often to prevent the machine waking up immediately after going to a reduced power state     SONY VGP-BPS13A/Q Battery .

The machine's BIOS must be set to allow Wake-on-LAN. To allow wakeup from powered-down state S5, wakeup on PME (Power Management Event) is also required. The Intel adapter allows "Wake on Directed Packet", "Wake on Magic Packet", "Wake on Magic Packet from power off state", and "Wake on Link"       SONY VGP-BPS13A/Q Battery .

Wake on Directed Packet is particularly useful as the machine will automatically come out of standby or hibernation when it is referenced, without the user or application needing to explicitly send a magic packet       SONY VGP-BPS13B/Q Battery .

Unfortunately in many networks waking on directed packet (any packet with the adapter's MAC address or IP address) or on link is likely to cause wakeup immediately after going to a low-power state. Details for any particular motherboard and network adapter are to be found in the relevant manuals    SONY VGP-BPS13B/Q Battery ;

there is no general method. Knowledge of signals on the network may also be needed to prevent spurious wakening.

Unattended operation

For a machine which is normally unattended precautions need to be taken to make the Wake-on-LAN function as reliable as possibleSONY VGP-BPS13/Q Battery .

For a machine procured to work in this way, Wake-on-LAN functionality is an important part of the purchase procedure.

Some machines do not support Wake-on-LAN after they have been disconnected from power (e.g., when power is restored after a power failure)       SONY VGP-BPS13/Q Battery .

Use of an uninterruptible power supply (UPS) will give protection against a short period without power, although the battery will discharge during a prolonged power cut.

Awakening without the operation presence

If a machine that is not designed to support Wake-on-LAN is left powered down after power failure   SONY VGP-BPS21A/B Battery ,

it may be possible to set the BIOS to start it up automatically on restoration of power, so that it is never left in an unresponsive state. A typical BIOS setting is AC back function which may be onoff, or memoryOn is the correct setting in this case; memory, which restores the machine to the state it was in when power was lost, may leave a machine which was hibernating in an unwakeable stateSONY VGP-BPS21/S Battery .

Other problems can affect the ability to start or control the machine remotely: hardware failure of the machine or network, failure of the BIOS settings battery (the machine will halt when started before the network connection is made, displaying an error message and requiring a keypress)       SONY VGP-BPS21B Battery ,

loss of control of the machine due to software problems (machine hang, termination of remote control or networking software, etc.), and virus infection or hard disk corruption. Therefore, the use of a reliable server-class machine with RAID drives, redundant power supplies, etc.      SONY VGP-BPS21A Battery ,

will help to maximize availability. Additionally, a device which can switch the machine off and on again, controlled perhaps by a remote signal, can force a reboot which will clear problems due to misbehaving software      SONY VGP-BPS21 Battery .

For a machine not in constant use, energy can be conserved by putting the machine into low-power RAM standby after a short timeout period. If a connection delay of a minute or two is acceptable, the machine can timeout into hibernation, powered off with its state saved to disk      Sony VGP-BPS21A/B Battery .

Wake on Internet

The computer being woken does not know whether the wakeup signal comes from another machine on the same network or from anywhere else       Sony VGP-BPS21/S Battery .

If the magic packet can be made to reach a computer, it can originate anywhere (e.g., from the Internet). This can be achieved by a Virtual Private Network (VPN), which makes the remote computer appear to be a member of the Local Area Network (LAN)  Sony VGP-BPS21A Battery .

In the absence of a VPN, a computer connected to a router can be woken if a magic packet sent over the Internet is routed to it. This requires any firewall to be set up to allow entry of the Wake-on-LAN signal to a specified port. The port can be forwarded to the computer to be woken up       Sony VGP-BPS21 Battery ;

or some routers permit the packet to be broadcast to the entire LAN . However, some routers do not support this as they will not forward broadcast packets.

Wake-on-Ring (WOR), sometimes referred to as Wake-on-Modem (WOM)   Sony Vaio PCG-5G2L Battery ,

is a specification that allows supported computers and devices to "wake up" or turn on from a sleeping, hibernating or "soft off" state (e.g. ACPI state G1 or G2), and begin operation.

The basic premise is that a special signal is sent over phone lines to the computer through its dial-up modem, telling it to fully power-on and begin operation    Sony Vaio PCG-5G3L Battery .

Common uses were archive databases andBBSes, although hobbyist use was significant.

Fax machines use a similar system, in which they are mostly idle until receiving an incoming fax signal, which spurs operation.

This style of remote operation has mostly been supplanted by Wake-on-LAN, which is newer but works in much the same way  Sony Vaio PCG-5J1L Battery .

Ring Indicator, abbreviated as RI, is a control signal that exists inside an RS-232 serial communications cable that goes between a computer and a modem. The signal is sent from the modem to the computer, typically over pin 9 of a nine-pin (DE9) serial port, or pin 22 over a 25-pin (DB25) port        Sony Vaio PCG-5K2L Battery .

Its purpose is to indicate to the computer that the phone line is ringing.

As implemented in modern PC hardware, a hardware interrupt is generated that can be captured by the processor any time the RI signal changes state       Sony Vaio PCG-5J2L Battery .

This functionality was originally present in the8250 UART - the chip responsible for providing the serial port - which was present in the original IBM PC XT. Virtually all serial hardware for newer PC's, including the popular 16550 UART, has maintained this functionality     Sony Vaio PCG-5K1L Battery .

Having support for this hardware interrupt means that a program or operating system can be informed of a change in state of the RI pin, without requiring the software to constantly "poll" the state of the pin     Sony Vaio PCG-6S2L Battery .

Unlike nearly all of the status pins in the typical PC's 9-pin RS-232 port, RI is a one-way signal from the modem to the computer (or more correctly, the DCE to the DTE) that does not correspond to another signal that carries similar information the opposite waySony Vaio PCG-6S3L Battery .

Conceptually, TXD is paired with RXD, DTR is paired with DSR, and RTS paired with CTS, as each of these signals usually carry the same type of information. The only remaining signal pins are DCD and RI, however both of these signals go in the same direction (tothe computer or DTE) rather than in opposite directions (as the other three mentioned pairs) Sony Vaio PCG-6V1L Battery .

As used on modems

On an external modem with LED lights, the status of the Ring Indicator pin is often coupled to the "AA" (auto answer) light. Normally, the AA light is either off if the modem is not in auto-answer mode, or on if the auto answer mode is enabled (see ATS0 in the Hayes command set)  Sony Vaio PCG-6W1L Battery .

However, when a ring is detected and the RI signal is asserted, the state of the AA light is simultaneously flipped - so it temporarily turns on if auto-answer mode is off, or off if it's on. This doesn't change the auto answer status - rather, it is simply meant as a visual indication of ringing and the assertion of the RI signal Sony Vaio PCG-6W2L Battery .

In practice, most telecommunications programs or server hosts do not actually use the RI signal to detect ringing. Rather, they typically wait for the "RING" message sent to the host over the data pins.

The asserted RI signal lasts nearly exactly as long as the ringing on the telephone line - for example  Sony Vaio PCG-6W3L Battery ,

2.0 seconds per ring burst in North America. The RI signal follows the ringing pattern closely - in fact, it is possible for a computer program to detect distinctive ring patterns from the phone company by monitoring the transitions of the RI signal. If the phone line rings, for example, two short burts and then a long burst, the RI signal will imitate this pattern  Sony Vaio PCG-7111L Battery .

In practice, distinctive ring detection is a feature built in to the command sets of many modern modems. Distinctive ring allows a single phone line to be "shared purpose" whereby it has multiple phone numbers, distinguished only by the ringing pattern, and the modem only answers calls whose ring pattern identify the call as a data call  Sony Vaio PCG-7112L Battery .

As used with other hardware

Even when not used with modems, the signal still exists on a standard PC serial port. Although the signal is unused by most hardware, it is notably used by some older uninterruptible power supplies(UPS's) to signal a power failure state to the computer  Sony Vaio PCG-7113L Battery .

Although newer UPS devices typically send serial data over the "receive data" pins (such is called a "smart" UPS), using the Ring Indicator pin is particularly cheap and simple to implement, because it is essentially a simple signal that can be sent to the computer in the form of either a constant positive or constant negative voltage  Sony Vaio PCG-7133L Battery .

A UPS that uses the Ring Indicator pin to signal a power failure is referred to as a "dumb" UPS[3]. A "mark" (negative voltage) on the Ring Indicator pin indicates a power-fail state. The serial extension cables included with such UPSes are frequently contain conductors only for the Ring Indicator and ground pins   Sony Vaio PCG-7Z2L Battery .


The Ring Indicator pin can also be configured in the BIOS on some computer motherboards to wake the computer from sleep mode. This functionality is known as wake-on-ring, and is conceptually similar to the wake-on-LAN feature found on many Ethernet controllers   Sony Vaio PCG-8Y1L Battery .

A home automation controller from a popular company known as X10 Wireless Technology is a notable example of this type of use. The controller, which connects to the computer's serial port, can be programmed to assert the RI line on the serial port upon detecting a home automation event of the user's choice  Sony Vaio PCG-8Y2L Battery ,

waking up the computer to query the controller for the event ID and then perform any response programmed for the event.

Despite RS232 being considered somewhat of a "legacy" standard, the wake-on-ring functionality is still considered important enough that it is implemented into USB-to-serial converters  Sony Vaio PCG-8Z1L Battery .

When implemented, the USB-to-serial converter remains on standby while the computer is in sleep mode, and sends an ACPI "wake" event over the USB port when the appropriate Ring Indicator transition is detected.

Motherboard BIOSes may refer to this feature with other names or phrases, such as "Power on by external modems" Sony Vaio PCG-8Z2L Battery .

Floating RI interference in serial cables

Some poor quality serial cables can cause computer problems with the Ring Indicator signal  Sony VAIO PCG-5G2L Battery .

This happens if there is inductive interference between the RI line and the RXD/TXD lines when the cable is used with equipment that does not provide any signal on the RI line, and often occurs when using serial extension cords with equipment (which usually carry all 9 or more pins) that only uses a small subset of the serial lines  Sony VAIO PCG-5G3L Battery .

In this scenario, the voltage on the RI line freely "floats ", and can pick up interference from neighboring signals. If the RI line picks up interference from the RXD/TXD lines (which change from high to low rapidly with every bit transmitted, often thousands of times per second)   Sony VAIO PCG-5J1L Battery ,

it can cause the computer to erroneously detect enormous quantities of transitions on the RI line. Each of these transitions generates CPU interrupts and kernel events and can disrupt application software and waste a significant amount of CPU time. If the RI line is unused but carried through a cable next to other wiring         Sony VAIO PCG-5K2L Battery ,

it is important to tie it to either a high or low signal instead of leaving it disconnected and floating. Simply tying it to the output of some other pin on the computer that outputs a steady RS232-level voltage (such as DTR or RTS) will eliminate this problem, regardless of which signal is actually being output on the pin chosen        Sony VAIO PCG-5J2L Battery .

Connecting the pin to ground should be avoided, because the resulting lack of voltage differential (0 volts) is considered an undefined signal in the RS232 standard  Sony VAIO PCG-5K1L Battery .

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