text:
Aurora (disambiguation)

An aurora is a natural light display in the sky on Earth seen predominantly in the high latitudes.

Aurora may also refer to:

Common uses

 * Aurora (mythology), the Roman goddess of dawn
 * Aurora (given name), a feminine given name (and list of people with the name)
 * Aurora (heraldry), the heraldic display of an aurora

Brazil

 * Aurora, Ceará
 * Aurora, Santa Catarina

Philippines

 * Aurora (province), a province in the Philippines
 * Aurora, Isabela
 * Aurora, Zamboanga del Sur

Romania

 * Aurora, Cujmir, Mehedinți County
 * Cap Aurora, Mangalia, Constanța County

United States

 * Aurora, Arkansas
 * Aurora, Colorado
 * Aurora, Illinois
 * Aurora, Indiana
 * Aurora, Iowa
 * Aurora, Kansas
 * Aurora, Kentucky
 * Aurora, New Orleans, Louisiana
 * Aurora, Maine
 * Aurora, Minnesota
 * Aurora Township, Steele County, Minnesota
 * Aurora, Missouri
 * Aurora, Nebraska
 * Aurora, Nevada
 * Aurora, Cayuga County, New York
 * Aurora, Erie County, New York
 * Aurora, North Carolina
 * Aurora, Ohio
 * Aurora, Oregon
 * Aurora County, South Dakota
 * Aurora, South Dakota
 * Aurora, Texas
 * Aurora, Utah
 * Aurora, West Virginia
 * Aurora, Florence County, Wisconsin, a town
 * Aurora (community), Florence County, Wisconsin, an unincorporated community
 * Aurora, Kenosha County, Wisconsin
 * Aurora, Taylor County, Wisconsin
 * Aurora, Washington County, Wisconsin
 * Aurora, Waushara County, Wisconsin

Other places

 * Aurora, Victoria, Australia
 * Aurora, Ontario, Canada
 * Aurora (Turin), Italy
 * Aurora Cave, New Zealand
 * Aurora, Western Cape, South Africa
 * Aurora, Suriname
 * Aurora Island or Maéwo, Vanuatu
 * Aurora Islands, phantom isles (once considered near the Falklands)

Education

 * Aurora University, Illinois, United States
 * Aurora College, Northwest Territories, Canada
 * Aurora College (Invercargill), New Zealand
 * Community College of Aurora, Colorado, United States

Structures

 * Aurora (Spencer, Virginia), a historic home near Spencer, Patrick County, Virginia, United States
 * Aurora Apartment Hotel, a historic high-rise building located in San Antonio, Texas, United States
 * Aurora Bridge, Seattle, Washington, United States
 * Aurora, Dichteren, a windmill in Gelderland, Netherlands
 * Aurora Melbourne Central, a skyscraper in Melbourne, Australia
 * Aurora Place, a skyscraper in Sydney, Australia
 * Aurora Stadium, Launceston, Australia
 * Aurora Tower, a skyscraper in Brisbane, Australia
 * Aurora Wheel, a Ferris wheel in Kuwana, Mie Prefecture, Japan

Visual arts

 * Aurora (Artemisia Gentileschi), a c1625-1627 painting by Artemisia Gentileschi
 * Aurora (Reni), a 1614 Baroque ceiling fresco by Guido Reni
 * Aurora (sculpture), a 1992 sculpture by Mark di Suvero

Books

 * Prix Aurora Awards, a Canadian literature award
 * Aurora, a novel by Michel Leiris
 * Aurora, a novel by David Koepp
 * Aurora (novel), a 2015 novel by Kim Stanley Robinson
 * Aurora, an airship in the novel Airborn
 * Aurora-Verlag, a New York-based German language publisher co-founded in 1944 by Lion Feuchtwanger
 * Aurora (comics), a Marvel comics character

Periodicals

 * Aurora (literary journal), a literary journal published from 1821 to 1837
 * Aurora (newspaper), an 1899–1957 Swedish publication
 * Aurora de Chile, the first periodical in Chilean history
 * Philadelphia Aurora, a newspaper published from 1794 to 1824

Film and television

 * Aurora (1984 film), an Italian drama
 * Aurora (2006 film), a Ukrainian film
 * Aurora (2010 film), a Romanian crime story
 * Aurora (2014 film), a Chilean film
 * Aurora (2018 Filipino film), a Philippine film
 * Aurora (2018 Kyrgyz film), a Kyrgyzstani film
 * Aurora (TV series), a Spanish-language telenovela
 * "Aurora" (Stargate Atlantis), an episode of Stargate Atlantis
 * Aurora Community Channel, an Australian TV channel
 * Aurora (Sleeping Beauty), the title character from the animated film Sleeping Beauty
 * Princess Aurora, the Sleeping Beauty character in Once Upon a Time

Gaming

 * Aurora Engine, a BioWare game engine
 * Sega Aurora, a gaming hardware platform
 * Aurora, a gaming PC brand by Alienware
 * Aurora, the main protagonist from the role-playing video game Child of Light
 * The Aurora, a crashed spaceship in the survival adventure game Subnautica

Works

 * Aurora (opera), a 1908 opera by Ettore Panizza
 * Aurora (tone poem), an orchestral tone poem by William Lloyd Webber

Bands and artists

 * Aurora (singer) (born 1996), Norwegian singer-songwriter
 * Aurora (Christian band), an American/British girl group
 * Aurora (electronic music group), a British group
 * Aurora (punk band), a Hungarian group
 * Aurora, a German dark wave band later known as Aurora Sutra

Albums

 * Aurora (Angela Chang album), 2004
 * Aurora (Antiskeptic album), 2003
 * Aurora (Asia album), 1986
 * Aurora (Aurora album), 2000
 * Aurora (Bada album), 2004
 * Aurora (Breaking Benjamin album), 2020
 * Aurora (Daisy Jones & the Six album), 2023
 * Aurora (Esmerine album), 2005
 * Aurora (Jean-Luc Ponty album), 1976
 * Aurora (Nico Touches the Walls album), 2009
 * Aurora (Susumu Hirasawa album), 1994
 * Aurora (Ben Frost album), 2014
 * Aurora (Bea Miller album), 2018
 * Aurora, a 2009 album by Avishai Cohen
 * Aurora, a 1995 album by Crash Vegas
 * Aurora, a 2013 album by Joey Moe
 * Aurora, a 2021 album by Humbe
 * Aurora, a 2023 album by Nikolija

Songs

 * "Aurora" (Foo Fighters song), 1999
 * "Aurora" (Lights Action song), 2008
 * "Aurora", a song by 36 Crazyfists from Rest Inside the Flames
 * "Aurora", a song by the Andrews Sisters
 * "Aurora", a song by Björk from Vespertine
 * "L'aurora", a song by Eros Ramazzotti from Dove c'è musica
 * "Aurora", a song by Gloria Trevi from De Película
 * "Aurora", a song by Hans Zimmer
 * "Aurora", a song by Lapush from Someplace Closer to Here
 * "Aurora", a song by Susumu Hirasawa from Aurora
 * "Aurora 2", a Susumu Hirasawa remix of "Aurora" from Solar Ray
 * "Aurora", a song by Vanessa Mae from Storm
 * "Aurora", a song by Veruca Salt from Tank Girl

Organizations

 * Aurora (pen manufacturer), an Italian manufacturer
 * Aurora Cannabis, a Canadian licensed producer of medical cannabis
 * Aurora Energy (disambiguation), various electricity companies
 * Aurora Film Corporation, an Indian film production and distribution company
 * Aurora Flight Sciences (a.k.a. Aurora), flight research subsidiary of Boeing
 * Aurora Gaming, a Serbian esports organization
 * Aurora Health Care, a health care system in Wisconsin, United States
 * Aurora Innovation, an American self-driving car company
 * Aurora (university network), a network of European universities
 * Aurora Plastics Corporation, a defunct American toy and hobby company
 * Aurora Productions, Hollywood, an American film production house
 * Aurora Publishing (Hungary), a German-Hungarian publishing company
 * Aurora Publishing (United States), the American subsidiary of Japanese publisher Ohzora Publishing

Astronomy

 * 94 Aurora, an asteroid
 * Aurora 7, the second American spacecraft to orbit the Earth
 * Aurora Flight Sciences (a.k.a. Aurora), flight research subsidiary of Boeing
 * Aurora on Mars, an atmospheric phenomenon on Mars
 * Aurora programme, a human spaceflight programme of the European Space Agency
 * Aurora, an albedo feature on Mercury

Biology

 * Aurora (genus), a synonym of a genus of sponges
 * Aurora, a synonym of the moth genus Peoria (moth)
 * Aurora kinase, an enzyme
 * Aurora (grape) or Aurore, a hybrid wine grape variety

Computing

 * Apache Aurora, a Mesos framework for both long-running services and cron jobs
 * Aurora (typeface), a serif font
 * Mobile OS Aurora, a Russian operating system spun off from Sailfish
 * Aurora OS, formerly named Eeebuntu OS, a discontinued Ubuntu-based OS for netbooks
 * Aurora (protocol), a communications protocol from Xilinx
 * Aurora Generator Test, a 2007 experiment by Idaho National Laboratory
 * Aurora HDR, high-dynamic-range software for photography
 * Operation Aurora, a 2009 cyber attack
 * Aurora, a tablet in the NOVO7 series
 * Aurora, a version of the Firefox browser
 * AURORA, an entrant in the NIST hash function competition

High performance computing

 * Aurora (supercomputer), a Exascale-class supercomputer at the U.S.-based Argonne National Laboratory under development in the 2010s and 2020s
 * Aurora, a supercomputer produced by Eurotech
 * NEC SX-Aurora TSUBASA, a vector processing architecture from NEC
 * Amazon Aurora, a high-performance database developed by Amazon

Aerospace

 * Aurora (aircraft), a rumored American reconnaissance aircraft
 * Aurora (airline), a Russian airline
 * Aurora D8, an airliner concept currently under development
 * Lockheed CP-140 Aurora, a maritime patrol aircraft used by the Royal Canadian Air Force
 * Aurora Space Station, a concept for a commercial space station
 * Aurora Flight Sciences (a.k.a. Aurora), flight research subsidiary of Boeing

Automobiles

 * Aurora (1957 automobile), an American automobile
 * Aurora Solar Car
 * Oldsmobile Aurora, a full-size sedan made from 1995 to 2003
 * Zenvo Aurora, a Danish sports car

Rail

 * Aurora station (disambiguation), stations named Aurora
 * Aurora (train), a domestic Trans Europ Express in Italy
 * Aurora Winter Train, seasonal passenger service on the Alaska Railroad
 * Aurora, a South Devon Railway Comet class locomotive
 * Brand name to be applied to the British Rail Class 810

Road

 * Aurora Boulevard, a major thoroughfare in Quezon City and San Juan City, Metro Manila
 * Aurora Avenue, Seattle, Washington

Watercraft

 * List of ships named Aurora

Sports

 * Aurora Formula One, a motor racing championship
 * Aurora F.C., a Guatemalan football club
 * Aurora FC (Canada), a Canadian soccer club
 * Club Aurora, a Bolivian football club
 * FBC Aurora, a Peruvian football club
 * Aurora Seriate Calcio, a defunct Italian football club
 * Aurora Pro Patria 1919, an Italian football club
 * S.P. Aurora, a defunct Sammarinese football club

Other uses

 * Jagjit Singh Aurora (1916–2005), Indian soldier
 * Aurora Prize for Awakening Humanity, an international humanitarian award
 * Aurora Solar Thermal Power Project, a proposed solar-powered electricity generator in South Australia

title: Aurora (disambiguation)


text:
Authenticator

An authenticator is a means used to confirm a user's identity, that is, to perform digital authentication. A person authenticates to a computer system or application by demonstrating that he or she has possession and control of an authenticator. In the simplest case, the authenticator is a common password.

Using the terminology of the NIST Digital Identity Guidelines, the party to be authenticated is called the claimant while the party verifying the identity of the claimant is called the verifier. When the claimant successfully demonstrates possession and control of one or more authenticators to the verifier through an established authentication protocol, the verifier is able to infer the claimant's identity.

Classification
Authenticators may be characterized in terms of secrets, factors, and physical forms.

Authenticator secrets
Every authenticator is associated with at least one secret that the claimant uses to demonstrate possession and control of the authenticator. Since an attacker could use this secret to impersonate the user, an authenticator secret must be protected from theft or loss.

The type of secret is an important characteristic of the authenticator. There are three basic types of authenticator secret: a memorized secret and two types of cryptographic keys, either a symmetric key or a private key.

Memorized secret
A memorized secret is intended to be memorized by the user. A well-known example of a memorized secret is the common password, also called a passcode, a passphrase, or a personal identification number (PIN).

An authenticator secret known to both the claimant and the verifier is called a shared secret. For example, a memorized secret may or may not be shared. A symmetric key is shared by definition. A private key is not shared.

An important type of secret that is both memorized and shared is the password. In the special case of a password, the authenticator is the secret.

Cryptographic key
A cryptographic authenticator is one that uses a cryptographic key. Depending on the key material, a cryptographic authenticator may use symmetric-key cryptography or public-key cryptography. Both avoid memorized secrets, and in the case of public-key cryptography, there are no shared secrets as well, which is an important distinction.

Examples of cryptographic authenticators include OATH authenticators and FIDO authenticators. The name OATH is an acronym from the words "Open AuTHentication" while FIDO stands for Fast IDentity Online. Both are the results of an industry-wide collaboration to develop an open reference architecture using open standards to promote the adoption of strong authentication.

By way of counterexample, a password authenticator is not a cryptographic authenticator. See the section for details.

Symmetric key
A symmetric key is a shared secret used to perform symmetric-key cryptography. The claimant stores their copy of the shared key in a dedicated hardware-based authenticator or a software-based authenticator implemented on a smartphone. The verifier holds a copy of the symmetric key.

Public-private key pair
A public-private key pair is used to perform public-key cryptography. The public key is known to (and trusted by) the verifier while the corresponding private key is bound securely to the authenticator. In the case of a dedicated hardware-based authenticator, the private key never leaves the confines of the authenticator.

Authenticator factors and forms
An authenticator is something unique or distinctive to a user (something that one has), is activated by either a PIN (something that one knows), or is a biometric ("something that is unique to oneself"). An authenticator that provides only one of these factors is called a single-factor authenticator whereas a multi-factor authenticator incorporates two or more factors. A multi-factor authenticator is one way to achieve multi-factor authentication. A combination of two or more single-factor authenticators is not a multi-factor authentication, yet may be suitable in certain conditions.

Authenticators may take a variety of physical forms (except for a memorized secret, which is intangible). One can, for example, hold an authenticator in one's hand or wear one on the face, wrist, or finger.

It is convenient to describe an authenticator in terms of its hardware and software components. An authenticator is hardware-based or software-based depending on whether the secret is stored in hardware or software, respectively.

An important type of hardware-based authenticator is called a security key, also called a security token (not to be confused with access tokens, session tokens, or other types of security tokens). A security key stores its secret in hardware, which prevents the secret from being exported. A security key is also resistant to malware since the secret is at no time accessible to software running on the host machine.

A software-based authenticator (sometimes called a software token) may be implemented on a general-purpose electronic device such as a laptop, a tablet computer, or a smartphone. For example, a software-based authenticator implemented as a mobile app on the claimant's smartphone is a type of phone-based authenticator. To prevent access to the secret, a software-based authenticator may use a processor's trusted execution environment or a Trusted Platform Module (TPM) on the client device.

A platform authenticator is built into a particular client device platform, that is, it is implemented on device. In contrast, a roaming authenticator is a cross-platform authenticator that is implemented off device. A roaming authenticator connects to a device platform via a transport protocol such as USB.

Examples
The following sections describe narrow classes of authenticators. For a more comprehensive classification, see the NIST Digital Identity Guidelines.

Single-factor authenticators
To use an authenticator, the claimant must explicitly indicate their intent to authenticate. For example, each of the following gestures is sufficient to establish intent:

 * The claimant types a password into a password field, or
 * The claimant places their finger on a fingerprint reader, or
 * The claimant presses a button to indicate approval

The latter is called a test of user presence (TUP). To activate a single-factor authenticator (something that one has), the claimant may be required to perform a TUP, which avoids unintended operation of the authenticator.

A password is a secret that is intended to be memorized by the claimant and shared with the verifier. Password authentication is the process whereby the claimant demonstrates knowledge of the password by transmitting it over the network to the verifier. If the transmitted password agrees with the previously shared secret, user authentication is successful.

OATH OTP
One-time passwords (OTPs) have been used since the 1980s. In 2004, an Open Authentication Reference Architecture for the secure generation of OTPs was announced at the annual RSA Conference. The Initiative for Open Authentication (OATH) launched a year later. Two IETF standards grew out of this work, the HMAC-based One-time Password (HOTP) algorithm and the Time-based One-time Password (TOTP) algorithm specified by RFC 4226 and RFC 6238, respectively. By OATH OTP, we mean either HOTP or TOTP. OATH certifies conformance with the HOTP and TOTP standards.

A traditional password (something that one knows) is often combined with a one-time password (something that one has) to provide two-factor authentication. Both the password and the OTP are transmitted over the network to the verifier. If the password agrees with the previously shared secret, and the verifier can confirm the value of the OTP, user authentication is successful.

One-time passwords are generated on demand by a dedicated OATH OTP authenticator that encapsulates a secret that was previously shared with the verifier. Using the authenticator, the claimant generates an OTP using a cryptographic method. The verifier also generates an OTP using the same cryptographic method. If the two OTP values match, the verifier can conclude that the claimant possesses the shared secret.

A well-known example of an OATH authenticator is Google Authenticator, a phone-based authenticator that implements both HOTP and TOTP.

Mobile Push
A mobile push authenticator is essentially a native app running on the claimant's mobile phone. The app uses public-key cryptography to respond to push notifications. In other words, a mobile push authenticator is a single-factor cryptographic software authenticator. A mobile push authenticator (something that one has) is usually combined with a password (something that one knows) to provide two-factor authentication. Unlike one-time passwords, mobile push does not require a shared secret beyond the password.

After the claimant authenticates with a password, the verifier makes an out-of-band authentication request to a trusted third party that manages a public-key infrastructure on behalf of the verifier. The trusted third party sends a push notification to the claimant's mobile phone. The claimant demonstrates possession and control of the authenticator by pressing a button in the user interface, after which the authenticator responds with a digitally signed assertion. The trusted third party verifies the signature on the assertion and returns an authentication response to the verifier.

The proprietary mobile push authentication protocol runs on an out-of-band secondary channel, which provides flexible deployment options. Since the protocol requires an open network path to the claimant's mobile phone, if no such path is available (due to network issues, e.g.), the authentication process can not proceed.

FIDO U2F
A FIDO Universal 2nd Factor (U2F) authenticator (something that one has) is a single-factor cryptographic authenticator that is intended to be used in conjunction with an ordinary web password. Since the authenticator relies on public-key cryptography, U2F does not require an additional shared secret beyond the password.

To access a U2F authenticator, the claimant is required to perform a test of user presence (TUP), which helps prevent unauthorized access to the authenticator's functionality. In practice, a TUP consists of a simple button push.

A U2F authenticator interoperates with a conforming web user agent that implements the U2F JavaScript API. A U2F authenticator necessarily implements the CTAP1/U2F protocol, one of the two protocols specified in the FIDO Client to Authenticator Protocol.

Unlike mobile push authentication, the U2F authentication protocol runs entirely on the front channel. Two round trips are required. The first round trip is ordinary password authentication. After the claimant authenticates with a password, the verifier sends a challenge to a conforming browser, which communicates with the U2F authenticator via a custom JavaScript API. After the claimant performs the TUP, the authenticator signs the challenge and returns the signed assertion to the verifier via the browser.

Multi-factor authenticators
To use a multi-factor authenticator, the claimant performs full user verification. The multi-factor authenticator (something that one has) is activated by a PIN (something that one knows), or a biometric (something that is unique to oneself"; e.g. fingerprint, face or voice recognition), or some other verification technique. ,

ATM card
To withdraw cash from an automated teller machine (ATM), a bank customer inserts an ATM card into a cash machine and types a Personal Identification Number (PIN). The input PIN is compared to the PIN stored on the card's chip. If the two match, the ATM withdrawal can proceed.

Note that an ATM withdrawal involves a memorized secret (i.e., a PIN) but the true value of the secret is not known to the ATM in advance. The machine blindly passes the input PIN to the card, which compares the customer's input to the secret PIN stored on the card's chip. If the two match, the card reports success to the ATM and the transaction continues.

An ATM card is an example of a multi-factor authenticator. The card itself is something that one has while the PIN stored on the card's chip is presumably something that one knows. Presenting the card to the ATM and demonstrating knowledge of the PIN is a kind of multi-factor authentication.

Secure Shell
Secure Shell (SSH) is a client-server protocol that uses public-key cryptography to create a secure channel over the network. In contrast to a traditional password, an SSH key is a cryptographic authenticator. The primary authenticator secret is the SSH private key, which is used by the client to digitally sign a message. The corresponding public key is used by the server to verify the message signature, which confirms that the claimant has possession and control of the private key.

To avoid theft, the SSH private key (something that one has) may be encrypted using a passphrase (something that one knows). To initiate a two-factor authentication process, the claimant supplies the passphrase to the client system.

Like a password, the SSH passphrase is a memorized secret but that is where the similarity ends. Whereas a password is a shared secret that is transmitted over the network, the SSH passphrase is not shared, and moreover, use of the passphrase is strictly confined to the client system. Authentication via SSH is an example of passwordless authentication since it avoids the transmission of a shared secret over the network. In fact, SSH authentication does not require a shared secret at all.

FIDO2
The FIDO U2F protocol standard became the starting point for the FIDO2 Project, a joint effort between the World Wide Web Consortium (W3C) and the FIDO Alliance. Project deliverables include the W3C Web Authentication (WebAuthn) standard and the FIDO Client to Authenticator Protocol (CTAP). Together WebAuthn and CTAP provide a strong authentication solution for the web.

A FIDO2 authenticator, also called a WebAuthn authenticator, uses public-key cryptography to interoperate with a WebAuthn client, that is, a conforming web user agent that implements the WebAuthn JavaScript API. The authenticator may be a platform authenticator, a roaming authenticator, or some combination of the two. For example, a FIDO2 authenticator that implements the CTAP2 protocol is a roaming authenticator that communicates with a WebAuthn client via one or more of the following transport options: USB, near-field communication (NFC), or Bluetooth Low Energy (BLE). Concrete examples of FIDO2 platform authenticators include Windows Hello and the Android operating system.

A FIDO2 authenticator may be used in either single-factor mode or multi-factor mode. In single-factor mode, the authenticator is activated by a simple test of user presence (e.g., a button push). In multi-factor mode, the authenticator (something that one has) is activated by either a PIN (something that one knows) or a biometric ("something that is unique to oneself").

Security code
First and foremost, strong authentication begins with multi-factor authentication. The best thing one can do to protect a personal online account is to enable multi-factor authentication. There are two ways to achieve multi-factor authentication:

 * 1) Use a multi-factor authenticator
 * 2) Use a combination of two or more single-factor authenticators

In practice, a common approach is to combine a password authenticator (something that one knows) with some other authenticator (something that one has) such as a cryptographic authenticator.

Generally speaking, a cryptographic authenticator is preferred over an authenticator that does not use cryptographic methods. All else being equal, a cryptographic authenticator that uses public-key cryptography is better than one that uses symmetric-key cryptography since the latter requires shared keys (which may be stolen or misused).

Again all else being equal, a hardware-based authenticator is better than a software-based authenticator since the authenticator secret is presumably better protected in hardware. This preference is reflected in the NIST requirements outlined in the next section.

NIST authenticator assurance levels
NIST defines three levels of assurance with respect to authenticators. The highest authenticator assurance level (AAL3) requires multi-factor authentication using either a multi-factor authenticator or an appropriate combination of single-factor authenticators. At AAL3, at least one of the authenticators must be a cryptographic hardware-based authenticator. Given these basic requirements, possible authenticator combinations used at AAL3 include:

 * 1) A multi-factor cryptographic hardware-based authenticator
 * 2) A single-factor cryptographic hardware-based authenticator used in conjunction with some other authenticator (such as a password authenticator)

See the NIST Digital Identity Guidelines for further discussion of authenticator assurance levels.

Restricted authenticators
Like authenticator assurance levels, the notion of a restricted authenticator is a NIST concept. The term refers to an authenticator with a demonstrated inability to resist attacks, which puts the reliability of the authenticator in doubt. Federal agencies mitigate the use a restricted authenticator by offering subscribers an alternative authenticator that is not restricted and by developing a migration plan in the event that a restricted authenticator is prohibited from use at some point in the future.

Currently, the use of the public switched telephone network is restricted by NIST. In particular, the out-of-band transmission of one-time passwords (OTPs) via recorded voice messages or SMS messages is restricted. Moreover, if an agency chooses to use voice- or SMS-based OTPs, that agency must verify that the OTP is being transmitted to a phone and not an IP address since Voice over IP (VoIP) accounts are not routinely protected with multi-factor authentication.

Comparison
It is convenient to use passwords as a basis for comparison since it is widely understood how to use a password. On computer systems, passwords have been used since at least the early 1960s. More generally, passwords have been used since ancient times.

In 2012, Bonneau et al. evaluated two decades of proposals to replace passwords by systematically comparing web passwords to 35 competing authentication schemes in terms of their usability, deployability, and security. (The cited technical report is an extended version of the peer-reviewed paper by the same name. ) They found that most schemes do better than passwords on security while every scheme does worse than passwords on deployability. In terms of usability, some schemes do better and some schemes do worse than passwords.

Google used the evaluation framework of Bonneau et al. to compare security keys to passwords and one-time passwords. They concluded that security keys are more usable and deployable than one-time passwords, and more secure than both passwords and one-time passwords.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Authenticator


text:
Automatic callback

In telecommunication, an automatic callback is a computer telephony calling feature that permits a user, when encountering a busy condition or other condition where the called individual is unavailable, to instruct the system to retain the called number and to establish the call when there is an available line or when the called number is no longer busy. Automatic callback may be implemented in the terminal, in the telephone exchange, or shared between them. Automatic callback is not the same as camp-on.

Using callback on popular business telephone systems
Comdial Digitech, DSU, Impact Place an intercom call and press CAMP. Your phone will disconnect from the attempted call. When the phone you rang is available, your phone will ring with five ring bursts. Press intercom to ring the other phone. To use with calls made in the voice-announce mode, press intercom before the camp button. To cancel, press intercom and dial "#6".

Comdial ExecuTech System 2000 Make an intercom call. At the busy signal, dial "*6". Hang up. When the desired extension becomes idle, the calling telephone receives five tone bursts. To answer callback rings, lift the handset. The called telephone will ring. To cancel auto call back before it rings, press "ITCM", dial "#6" and hang up.

Comdial Digital Impression When you reach a station that is busy or does not answer, press CAMP. When the phone you wish to reach becomes idle, your phone will ring with five short tones. Press ITCM to cause the other phone to ring. To cancel the callback, press ITCM and dial "#6". If the extension you call in voice announce mode is not answered, press ITCM before pressing CAMP.

Database Systems Corp. PACER Phone System Custom callback is integrated into the CRM application that signals the phone system to redial a number on a particular date and time. Call is automatically assigned to the original agent or assigned to a hunt group associated with a particular campaign.

Executone Encore CX Press CALLBK when you hear the busy tone. Answer the callback by lifting the handset or pressing MON.

Inter-Tel Eclipse IDS Integrated Operator Terminal Press the Call Back key at the busy signal. Press the RLS key. When your line is free and the extension you called is idle, your extension will ring. When the calls rings back to you, press the RLS key.

Inter-Tel Eclipse2 Associate Display and Basic Digital Phone Press "6" at the busy signal and hang up. Your phone will ring when the extension if available. Press "6" again to cancel before you get your callback.

Isoetec Digital Systems Display/Data Phone Press "Cb." Soft key at the busy signal. Replace the handset or press "HF". Wait for the double tone. When the extension is no longer busy, it will automatically call you back.

Isoetec IDS M Series Telephones When you hear the busy signal, press the "CALL BACK" key. Hang up. When you are signaled, lift the handset or press the "HF" key. Press the blinking "CALL BACK" key.

PCS Digital Telephone Press "cbck" at the busy signal. When a station is available, pick up the handset. To cancel, press "del".

Vodavi StarPlus Phone System Press the pre-programmed CALL BACK button. Hang up. When the busy station becomes available, you will be signaled.

title: Automatic callback


text:
Automatic call distributor

An automated call distribution system, commonly known as automatic call distributor or automatic call dispatcher (ACD), is a telephony device that answers and distributes incoming calls to a specific group of terminals or agents within an organization. ACDs direct calls based on parameters that may include the caller's telephone number, the number they dialed, the time of day or a response to an automated voice prompt. Advanced ACD systems may use digital technologies such as computer telephony integration (CTI), computer-supported telecommunications applications (CSTA) or IVR as input to determine the route to a person or voice announcement that will serve the caller. Experts claim that "the invention of ACD technology made the concept of a call centre possible."

Background
A Private Branch Exchange (PBX) is a telephone exchange device that acts as a switchboard to route phone calls within an organisation. This technology developed into Automated Call Distribution systems using computer technology to automatically connect incoming calls to recipients based on programmable logic.

Although ACDs appeared in the 1950s, one of the first large and separate ACDs was a modified 5XB switch used by the New York Telephone Company in the early 1970s to distribute calls among hundreds of 4-1-1 information operators. Robert Hirvela developed and received a patent for technology that was used to create the Rockwell Galaxy Automatic Call Distributor, which was used by Continental Airlines for more than 20 years. Since then, ACDs have integrated incoming call management and voice messaging software into its capabilities.

Application
ACD systems route incoming calls to people according to defined rules that may include, for example, the time of day, the day of the week, the geographic location of the caller and the availability of people to respond. The rules should aim to route the call to a person qualified to address the caller's needs. Routing can use caller ID, automatic number identification, interactive voice response or dialed number identification services to determine how calls are handled. ACD systems are often found in offices that handle large volumes of incoming phone calls from callers who have no need to talk to a specific person, but require assistance from any of multiple persons (e.g., customer service representatives or emergency services dispatch centers) at the earliest opportunity.

There are several contact routing strategies that can be set up within an algorithm based on a company's needs. Skills-based routing is determined by an operator's knowledge to handle a caller's inquiry. Virtual contact centers can also be used to aggregate the skill sets of agents to help multiple vendors, where all real-time and statistical information can be shared amongst the contact center sites. An additional function for these external routing applications is to enable Computer telephony integration (CTI), which improves efficiency for call center agents by matching incoming phone calls with relevant data via screen pop.

Distribution methods
Methods for distributing incoming calls from a queue include
 * Linear Call Distribution – Calls are distributed in order, starting at the beginning each time
 * Circular/Rotary Call Distribution – Calls are distributed in order, starting with the next in order
 * Uniform Call Distribution – Calls are distributed uniformly, starting with the person who has handled the fewest calls
 * Simultaneous Call Distribution – Calls are presented to all available extensions simultaneously
 * Weighted Call Distribution – Calls are distributed according to a configurable weighting, such as differing skill sets within customer service representatives

title: Automatic call distributor


text:
Automatic data processing

Automatic data processing (ADP) may refer to:
 * Automatic Data Processing, a computing services company.
 * Data processing using mechanical or electronic equipment, Electronic data processing

title: Automatic data processing


text:
Automatic link establishment

Automatic Link Establishment, commonly known as ALE, is the worldwide de facto standard for digitally initiating and sustaining HF radio communications. ALE is a feature in an HF communications radio transceiver system that enables the radio station to make contact, or initiate a circuit, between itself and another HF radio station or network of stations. The purpose is to provide a reliable rapid method of calling and connecting during constantly changing HF ionospheric propagation, reception interference, and shared spectrum use of busy or congested HF channels.

Mechanism
A standalone ALE radio combines an HF SSB radio transceiver with an internal microprocessor and MFSK modem. It is programmed with a unique ALE address, similar to a phone number (or on newer generations, a username). When not actively in contact with another station, the HF SSB transceiver constantly scans through a list of HF frequencies called channels, listening for any ALE signals transmitted by other radio stations. It decodes calls and soundings sent by other stations and uses the bit error rate to store a quality score for that frequency and sender-address.

To reach a specific station, the caller enters the ALE Address. On many ALE radios this is similar to dialing a phone number. The ALE controller selects the best available idle channel for that destination address. After confirming the channel is indeed idle, it then sends a brief selective calling signal identifying the intended recipient. When the distant scanning station detects ALE activity, it stops scanning and stays on that channel until it can confirm whether or not the call is for it. The two stations' ALE controllers automatically handshake to confirm that a link of sufficient quality has been established, then notify the operators that the link is up. If the callee fails to respond or the handshaking fails, the originating ALE node usually selects another frequency either at random or by making a guess of varying sophistication.

Upon successful linking, the receiving station generally emits an audible alarm and shows a visual alert to the operator, thus indicating the incoming call. It also indicates the callsign or other identifying information of the linked station, similar to Caller ID. The operator then un-mutes the radio and answers the call then can talk in a regular conversation or negotiates a data link using voice or the ALE built-in short text message format. Alternatively, digital data can be exchanged via a built-in or external modem (such as a STANAG 5066 or MIL-STD-188-110B serial tone modem) depending on needs and availability. The ALE built-in text messaging facility can be used to transfer short text messages as an "orderwire" to allow operators to coordinate external equipment such as phone patches or non-embedded digital links, or for short tactical messages.

Common applications
An ALE radio system enables connection for voice conversation, alerting, data exchange, texting, instant messaging, email, file transfer, image, geo-position tracking, or telemetry. With a radio operator initiating a call, the process normally takes a few minutes for the ALE to pick an HF frequency that is optimum for both sides of the communication link. It signals the operators audibly and visually on both ends, so they can begin communicating with each other immediately. In this respect, the longstanding need in HF radio for repetitive calling on pre-determined time schedules or tedious monitoring static is eliminated. It is useful as a tool for finding optimum channels to communicate between stations in real-time. In modern HF communications, ALE has largely replaced HF prediction charts, propagation beacons, chirp sounders, propagation prediction software, and traditional radio operator educated guesswork. ALE is most commonly used for hooking up operators for voice contacts on SSB (single-sideband modulation), HF internet connectivity for email, SMS phone texting or text messaging, real-time chat via HF text, Geo Position Reporting, and file transfer. High Frequency Internet Protocol or HFIP may be used with ALE for internet access via HF.

Techniques
The essence of ALE techniques is the use of automatic channel selection, scanning receivers, selective calling, handshaking, and robust burst modems. An ALE node decodes all received ALE signals heard on the channel(s) it monitors. It uses the fact that all ALE messages use forward error correction (FEC) redundancy. By noting how much error-correction occurred in each received and decoded message, an ALE node can detect the "quality" of the path between the sending station and itself. This information is coupled with the ALE address of the sending node and the channel the message was received on, and stored in the node's Link Quality Analysis (LQA) memory. When a call is initiated, the LQA lookup table is searched for matches involving the target ALE address and the best historic channel is used to call the target station. This reduces the likelihood that the call has to be repeated on alternate frequencies. Once the target station has heard the call and responded, a bell or other signalling device will notify both operators that a link has been established. At this point, the operators may coordinate further communication via orderwire text messages, voice, or other means. If further digital communication is desired, it may take place via external data modems or via optional modems built into the ALE terminal.

This unusual usage of FEC redundancy is the primary innovation that differentiates ALE from previous selective calling systems which either decoded a call or failed to decode due to noise or interference. A binary outcome of "Good enough" or not gave no way of automatically choosing between two channels, both of which are currently good enough for minimum communications. The redundancy-based scoring inherent in ALE thus allows for selecting the "best" available channel and (in more advanced ALE nodes) using all decoded traffic over some time window to sort channels into a list of decreasing probability-to-contact, significantly reducing co-channel interference to other users as well as dramatically decreasing the time needed to successfully link with the target node.

Techniques used in the ALE standard include automatic signaling, automatic station identification (sounding), polling, message store-and-forward, linking protection and anti-spoofing to prevent hostile denial of service by ending the channel scanning process. Optional ALE functions include polling and the exchange of orderwire commands and messages. The orderwire message, known as AMD (Automatic Message Display), is the most commonly used text transfer method of ALE, and the only universal method that all ALE controllers have in common for displaying text. It is common for vendors to offer extensions to AMD for various non-standard features, although dependency on these extensions undermines interoperability. As in all interoperability scenarios, care should be taken to determine if this is acceptable before using such extensions.

History and precedents
ALE evolved from older HF radio selective calling technology. It combined existing channel-scanning selective calling concepts with microprocessors (enabling FEC decoding and quality scoring decisions), burst transmissions (minimizing co-channel interference), and transponding (allowing unattended operation and incoming-call signalling). Early ALE systems were developed in the late 1970s and early 1980s by several radio manufacturers. The first ALE-family controller units were external rack mounted controllers connected to control military radios, and were rarely interoperable across vendors.

Various methods and proprietary digital signaling protocols were used by different manufacturers in first generation ALE, leading to incompatibility. Later, a cooperative effort among manufacturers and the US government resulted in a second generation of ALE that included the features of first generation systems, while improving performance. The second generation 2G ALE system standard in 1986, MIL-STD-188-141A, was adopted in FED-STD-1045 for US federal entities. In the 1980s, military and other entities of the US government began installing early ALE units, using ALE controller products built primarily by US companies. The primary application during the first 10 years of ALE use was government and military radio systems, and the limited customer base combined with the necessity to adhere to MILSPEC standards kept prices extremely high. Over time, demand for ALE capabilities spread and by the late 1990s, most new government HF radios purchased were designed to meet at least the minimum ALE interoperability standard, making them eligible for use with standard ALE node gear. Radios implementing at least minimum ALE node functionality as an option internal to the radio became more common and significantly more affordable. As the standards were adopted by other governments worldwide, more manufacturers produced competitively priced HF radios to meet this demand. The need to interoperate with government organizations prompted many non-government organizations (NGOs) to at least partially adopt ALE standards for communication. As non-military experience spread and prices came down, other civilian entities started using 2G ALE. By the year 2000, there were enough civilian and government organizations worldwide using ALE that it became a de facto HF interoperability standard for situations where a priori channel and address coordination is possible.

In the late 1990s, a third generation 3G ALE with significantly improved capability and performance was included in MIL-STD-188-141B, retaining backward compatibility with 2G ALE, and was adopted in NATO STANAG 4538. Civilian and non-government adoption rates are much lower than 2G ALE due to the extreme cost as compared to surplus or entry-level 2G gear as well as the significantly increased system and planning complexity necessary to realize the benefits inherent in the 3G specification. For many militaries, whose needs for maximized intra-organizational capability and capacity always strain existing systems, the additional cost and complexity of 3G are less problematic.

Reliability
ALE enables rapid unscheduled communication and message passing without requiring complex message centers, multiple radios and antennas, or highly trained operators. With the removal of these potential sources of failure, the tactical communication process becomes much more robust and reliable. The effects extend beyond mere force multiplication of existing communications methods; units such as helicopters, when outfitted with ALE radios, can now reliably communicate in situations where the crew are too busy to operate a traditional non-line of sight radio. This ability to enable tactical communication in conditions where dedicated trained operators and hardware are inappropriate is often considered to be the true improvement offered by ALE.

ALE is a critical path toward increased interoperability between organizations. By enabling a station to participate nearly simultaneously in many different HF networks, ALE allows for convenient cross-organization message passing and monitoring without requiring dedicated separate equipment and operators for each partner organization. This dramatically reduces staffing and equipment considerations, while enabling small mobile or portable stations to participate in multiple networks and subnetworks. The result is increased resilience, decreased fragility, increased ability to communicate information effectively, and the ability to rapidly add to or replace communication points as the situation demands.

When combined with Near Vertical Incidence Skywave (NVIS) techniques and sufficient channels spread across the spectrum, an ALE node can provide greater than 95% success linking on the first call, nearly on par with SATCOM systems. This is significantly more reliable than cellphone infrastructure during disasters or wars yet is mostly immune to such considerations itself.

Standards and protocols
Global standards for ALE are based on the original US MIL-STD 188-141A and FED-1045, known as 2nd Generation (2G) ALE. 2G ALE uses non-synchronised scanning of channels, and it takes several seconds to half a minute to repeatedly scan through an entire list of channels looking for calls. Thus it requires sufficient duration of transmission time for calls to connect or link with another station that is unsynchronised with its calling signal. The vast majority of ALE systems in use in the world at the present time are 2G ALE.

2G technical characteristics
The more common 2G ALE signal waveform is designed to be compatible with standard 3 kHz SSB narrowband voice channel transceivers. The modulation method is 8ary Frequency Shift Keying or 8FSK, also sometimes called Multi Frequency Shift Keying MFSK, with eight orthogonal tones between 750 and 2500 Hz. Each tone is 8 ms long, resulting in a transmitted over-the-air symbol rate of 125 baud or 125 symbols per second, with a raw data rate of 375 bits per second. The ALE data is formatted in 24-bit frames, which consist of a 3-bit preamble followed by three ASCII characters, each seven bits long. The received signal is usually decoded using digital signal processing techniques that are capable of recovering the 8FSK signal at a negative decibel signal-to-noise ratio (i.e., the signal may be recovered even when it is below the noise level). The over-the-air layers of the protocol involve the use of forward error correction, redundancy, and handshaking transponding similar to those used in ARQ techniques.

3G technical characteristics
Newer standards of ALE, called 3rd Generation or 3G ALE, use accurate time synchronization (via a defined time-synch protocol as well as the option of GPS-locked clocks) to achieve faster and more dependable linking. Through synchronization, the calling time to achieve a link may be reduced to less than 10 seconds. The 3G ALE modem signal also provides better robustness and can work in channel conditions that are less favorable than 2G ALE. Dwell groups, limited callsigns, and shorter burst transmissions enable more rapid intervals of scanning. All stations in the same group scan and receive each channel at precisely the same time window. Although 3G ALE is more reliable and has significantly enhanced channel-time efficiency, the existence of a large installed base of 2G ALE radio systems and the wide availability of moderately priced (often military surplus) equipment, has made 2G the baseline standard for global interoperability.

Basis for HF interoperability communications
Interoperability is a critical issue for the disparate entities which use radiocommunications to fulfill the needs of organizations. Largely due to the ubiquity of 2G ALE, it became the primary method for providing interoperability on HF between governmental and non-governmental disaster relief and emergency communications entities, and amateur radio volunteers. With digital techniques increasingly employed in communications equipment, a universal digital calling standard was needed, and ALE filled the gap. Nearly every major HF radio manufacturer in the world builds ALE radios to the 2G standard to meet the high demand that new installations of HF radio systems conform to this standard protocol. Disparate entities that historically used incompatible radio methods were then able to call and converse with each other using the common 2G ALE platform. Some manufacturers and organizations have used the AMD feature of ALE to expand the performance and connectivity. In some cases, this has been successful, and in other cases, the use of proprietary preamble or embedded commands has led to interoperability problems.

Tactical communication and resource management
ALE serves as a convenient method of beyond line of sight communication. Originally developed to support military requirements, ALE is useful to many organizations who find themselves managing widely located units. United States Immigration and Customs Enforcement and United States Coast Guard are two members of the Customs Over the Horizon Enforcement Network (COTHEN), a MIL-STD 188-141A ALE network. All U.S. armed forces operate multiple similar networks. Similarly, shortwave utility listeners have documented frequency and callsign lists for many nations' military and guard units, as well as networks operated by oil exploration and production companies and public utilities in many countries.

Emergency / disaster relief or extraordinary situation response communications
ALE radio communication systems for both HF regional area networks and HF interoperability communications are in service among emergency and disaster relief agencies as well as military and guard forces. Extraordinary response agencies and organizations use ALE to respond to situations in the world where conventional communications may have been temporarily overloaded or damaged. In many cases, it is in place as alternative back-channel for organizations that may have to respond to situations or scenarios involving the loss of conventional communications. Earthquakes, storms, volcanic eruptions, and power or communication infrastructure failures are typical situations in which organizations may deem ALE necessary to operations. ALE networks are common among organizations engaged in extraordinary situation response such as: natural and man-made disasters, transportation, power, or telecommunication network failures, war, peacekeeping, or stability operations. Organizations known to use ALE for Emergency management, disaster relief, ordinary communication or extraordinary situation response include: Red Cross, FEMA, Disaster Medical Assistance Teams, NATO, Federal Bureau of Investigation, United Nations, AT&T, Civil Air Patrol, SHARES, State of California Emergency Management Agency (CalEMA), other US States' Offices of Emergency Services or Emergency Management Agencies, and Amateur Radio Emergency Service (ARES).

International HF telecommunications for disaster relief
The International Telecommunication Union (ITU), in response to the need for interoperation in international disaster response spurred largely by humanitarian relief, included ALE in its Telecommunications for Disaster Relief recommendations. The increasing need for instant connectivity for logistical and tactical disaster relief response communications, such as the 2004 Indian Ocean earthquake tsunami led to ITU actions of encouragement to countries around the world toward loosening restrictions on such communications and equipment border transit during catastrophic disasters. The IARU Global Amateur Radio Emergency Communications Conferences (GAREC) and IARU Global Simulated Emergency Tests have included ALE.

Use in amateur radio
Amateur radio operators began sporadic ALE operation on a limited basis in the early to mid-1990s, with commercial ALE radios and ALE controllers. In 2000, the first widely available software ALE controller for the Personal Computer, PCALE, became available, and hams started to set up stations based on it. In 2001, the first organized and coordinated global ALE nets for International Amateur Radio began. In August 2005, ham radio operators supporting communications for emergency Red Cross shelters used ALE for Disaster Relief operations during the Hurricane Katrina disaster. After the event, hams developed more permanent ALE emergency/disaster relief networks, including internet connectivity, with a focus on interoperation between organizations. The amateur radio HFLink Automatic Link Establishment system uses an open net protocol to enable all amateur radio operators and amateur radio nets worldwide to participate in ALE and share the same ALE channels legally and interoperably. Amateur radio operators may use it to call each other for voice or data communications.

Amateur radio interoperability adaptations
Amateur radio operators commonly provide local, regional, national, and international emergency / disaster relief communications. The need for interoperability on HF led to the adoption of Automatic Link Establishment ALE open networks by hams. Amateur radio adapted 2G ALE techniques, by using the common denominators of the 2G ALE protocol, with a limited subset of features found in the majority of all ALE radios and controllers. Each amateur radio ALE station uses the operator's call sign as the address, also known as the ALE Address, in the ALE radio controller. The lowest common denominator technique enables any manufacturer's ALE radios or software to be used for HF interoperability communications and networking. Known as Ham-Friendly ALE, the amateur radio ALE standard is used to establish radio communications, through a combination of active ALE on internationally recognized automatic data frequencies, and passive ALE scanning on voice channels. In this technique, active ALE frequencies include pseudorandom periodic polite station identification, while passive ALE frequencies are silently scanned for selective calling. ALE systems include Listen Before Transmit as a standard function, and in most cases this feature provides better busy channel detection of voice and data signals than the human ear. Ham-Friendly ALE technique is also known as 2.5G ALE, because it maintains 2G ALE compatibility while employing some of the adaptive channel management features of 3G ALE, but without the accurate GPS time synchronization of 3G ALE.

Disaster relief HF network
Hot standby ALE nets are in constant operation 24/7/365 for International Emergency and Disaster Relief communications. The Ham Radio Global ALE High Frequency Network, which began service in June 2007, is the world's largest intentionally open ALE network. It is a free open network staffed by volunteers, and used by amateur radio operators supporting disaster relief organizations.

International coordination
International amateur radio ALE High Frequency channels are frequency coordinated with all Regions of the International Amateur Radio Union (IARU entity of ITU), for international, regional, national, and local use in the Amateur Radio Service. All Amateur Radio ALE channels use "USB" Upper Sideband standard. Different rules, regulations, and bandplans of the region and local country of operation apply to use of various channels. Some channels may not be available in every country. Primary or global channels are in common with most countries and regions.

International channels
''This listing is current as of February 2020. See HFLINK for more information about Amateur Radio Service ALE Automatic Link Establishment. '' Frequency table notes: Automatic Link Establishment ALE channel frequencies in the Amateur Radio Service are internationally coordinated with selective calling Selcall channels for interoperability purposes. Net is the ALE net address or Selcall net name.

title: Automatic link establishment


text:
Automatic message exchange

Automatic message exchange (AME): In an adaptive high-frequency (HF) radio network, an automated process allowing the transfer of a message from message injection to addressee reception, without human intervention. Through the use of machine-addressable transport guidance information, i.e., the message header, the message is automatically routed through an on-line direct connection through single or multiple transmission media.

Source: from Federal Standard 1037C

title: Automatic message exchange


text:
Automatic redial

In telecommunication, an automatic redial is a service feature that allows the user to dial, by depressing a single key or a few keys, the most recent telephone number dialed at that instrument.

Note: Automatic redial is often associated with the telephone instrument, but may be provided by a PBX, or by the central office. Synonym last number redial. Contrast with automatic calling unit.

Often one must subscribe to a caller ID for use of this function on a landline.

title: Automatic redial


text:
Automatic sounding

In telecommunication, automatic sounding is the testing of selected channels for quality by providing a very brief identifying transmission that may be used by other stations to evaluate connectivity, and availability, and to identify known working channels for immediate or later use for communications or calling. They are often used to maintain connectivity in digital communications high frequency radio networks.

Automatic soundings are primarily intended to increase the efficiency of the automatic link establishment (ALE) function, thereby increasing system throughput.

In ALE, the sounding information consists of a heavily error-corrected short message identifying the sender. Recipients decode it and use the bit error rate to calculate and store a (channel, node, quality) tuple. As ionospheric conditions and mobile-node locations change, these quality tuples will shift. The stored data can be used to maximize the chance that the best channel to link with a given partner will be chosen first.

title: Automatic sounding


text:
Automatic switching system

In data communications, an automatic switching system is a switching system in which all the operations required to execute the three phases of Information transfer transactions are automatically executed in response to signals from a user end-instrument.

In an automatic switching system, the information-transfer transaction is performed without human intervention, except for initiation of the access phase and the disengagement phase by a user.

In telephony, it refers to a telephone exchange in which all the operations required to set up, supervise, and release connections required for telephone calls are automatically performed in response to signals from a calling device. This distinction lost importance as manual switching declined during the 20th century.

title: Automatic switching system


text:
Auxiliary power

Auxiliary power is electric power that is provided by an alternate source and that serves as backup for the primary power source at the station main bus or prescribed sub-bus.

An offline unit provides electrical isolation between the primary power source and the critical technical load whereas an online unit does not.

A Class A power source is a primary power source, i.e., a source that assures an essentially continuous supply of power.

Types of auxiliary power services include Class B, a standby power plant to cover extended outages of the order of days; Class C, a 10-to-60-second quick-start unit to cover short-term outages of the order of hours; and Class D, an uninterruptible non-break unit using stored energy to provide continuous power within specified voltage and frequency tolerances.

Uses/Implementations
Many uses and implementations of auxiliary power are experimented with to increase its efficiency. One such experimentation was to find a better way to operate a diesel engine with fuel cell based auxiliary power units. The method of which is to separate hydrogen-rich gas from the diesel fuel to generate electricity separately in an auxiliary power unit. With this process, an effective reduction in emissions can be achieved by lowering the consumed volume of gas per hour. However, upon  power demands reaching 60% a sharp decrease in performance occurs, which can be solved by using a diesel or kerosine fuel with a maximum CO concentration of 1.5%.

There are a variety of other implementations of auxiliary power units in energy systems. This explains for how a significant portion of emissions come from commercial vehicles. Diesel engines operating within densely populated areas, running within an ineffective range in order to power their auxiliary systems, such as refrigeration, are contributors to a large portion of the emissions from automobiles. Using a model with a diesel powered four-stroke engine on a truck with 100% load capacity driving a combination of typical urban and city road cycles, emissions and auxiliary power demand were recorded. Then by using the calculated auxiliary power demand, a source was developed to support the demand for the auxiliary systems in the form of a PEM fuel cell. The end product of the PEM fuel cell was able to support the auxiliary systems of the truck using a maximum of 5 kW of power. This input was able to sustain the cooling chamber, cabin air-conditioning, radio unit, etc. The introduction of this fuel cell also contributed to a 9% reduction in diesel fuel consumption and 9.6% reduction in emissions.

Legal Requirements for Industries
The United States Environmental Protection Agency has set out rules and guidelines for how auxiliary and supplemental power sources (ASPS) that provide secondary power to wastewater treatment plants in case of a blackout. ASPS should be able to supply enough power to run the plant effectively, and be available for start-up in a short period of time in case of emergency. Types of ASPS necessary for adequate power generation include: internal combustion engines, microturbines, solar cells, fuel cells, and wind turbines. ASPS technology is required to be reliable enough to start up quickly, and run for extended periods of time, (i.e. 48 hours or more) with sufficient fuel.

Efficiency
As previously affirmed, auxiliary power units are commonly used to improve the efficiency of electrical system. The use of auxiliary power units for range extended electric automobiles has been shown to improve the control of energy flow and distribution throughout the system, improving its overall efficiency.

For closed systems with extreme power consumption such as tankers and other vessels at sea, the use and quality of auxiliary power systems have a great impact on the efficiency of the overall system. The different uses of auxiliary power for an array of ships and ship activities and how these different power schemes change the overall efficiency and/emissions of the ship's system. Studies have indicated that while ships travel between ports within the same bay, total ship exhaust emissions are due to primarily their auxiliary boiler and auxiliary engine power systems, due to the time and speed required to transit the port waters with the large berth of the vessel. Findings also lead to the conclusion that the power output capabilities of auxiliary engines at a certain point do not increase with the size of the vessel, or the vessel's installed main engine power. There are a great many factors such as machinery variables, power schemes, and size and power of vessels, that there are too many factors to take into account in order to portray an accurate representation of the ratio between main power and auxiliary power output. More surveys and studies should be done in order to achieve this more accurate result.

title: Auxiliary power


text:
Availability

In reliability engineering, the term availability has the following meanings:

 * The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e. a random, time.
 * The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment.

Normally high availability systems might be specified as 99.98%, 99.999% or 99.9996%.

Representation
The simplest representation of availability (A) is a ratio of the expected value of the uptime of a system to the aggregate of the expected values of up and down time (that results in the "total amount of time" C of the observation window)

 * $$A = \frac{E[\mathrm{uptime}]}{E[\mathrm{uptime}]+E[\mathrm{downtime}]} = \frac{E[\mathrm{uptime}]}{C}$$

Another equation for availability (A) is a ratio of the Mean Time To Failure (MTTF) and Mean Time Between Failure (MTBF), or

 * $$A = \frac{MTTF}{MTTF + MTTR} = \frac{MTTF}{MTBF}$$

If we define the status function $$X(t)$$ as

 * $$X(t)=

\begin{cases} 1, & \text{sys functions at time } t\\ 0, & \text{maintenance} \end{cases} $$

therefore, the availability A(t) at time t > 0 is represented by

 * $$ A(t)=\Pr[X(t)=1]=E[X(t)]. \, $$

Average availability must be defined on an interval of the real line. If we consider an arbitrary constant $$c>0$$, then average availability is represented as

 * $$ A_c = \frac{1}{c} \int_0^c A(t)\,dt.

$$

Limiting (or steady-state) availability is represented by
 * $$ A = \lim_{c \rightarrow \infty} A_c. $$

Limiting average availability is also defined on an interval $$[0,c]$$ as,

 * $$ A_\infty =\lim_{c \rightarrow \infty} A_c = \lim_{c \rightarrow \infty}\frac{1}{c} \int_0^c A(t)\,dt,\quad c > 0.

$$

Availability is the probability that an item will be in an operable and committable state at the start of a mission when the mission is called for at a random time, and is generally defined as uptime divided by total time (uptime plus downtime).

Methods and techniques to model availability
Reliability Block Diagrams or Fault Tree Analysis are developed to calculate availability of a system or a functional failure condition within a system including many factors like:

 * Reliability models
 * Maintainability models
 * Maintenance concepts
 * Redundancy
 * Common cause failure
 * Diagnostics
 * Level of repair
 * Repair status
 * Dormant failures
 * Test coverage
 * Active operational times / missions / sub system states
 * Logistical aspects like; spare part (stocking) levels at different depots, transport times, repair times at different repair lines, manpower availability and more.
 * Uncertainty in parameters

Furthermore, these methods are capable to identify the most critical items and failure modes or events that impact availability.

Definitions within systems engineering
Availability, inherent (Ai) The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment. It excludes logistics time, waiting or administrative downtime, and preventive maintenance downtime. It includes corrective maintenance downtime. Inherent availability is generally derived from analysis of an engineering design: It is based on quantities under control of the designer.
 * 1) The impact of a repairable-element (refurbishing/remanufacture isn't repair, but rather replacement) on the availability of the system, in which it operates, equals mean time between failures MTBF/(MTBF+ mean time to repair MTTR).
 * 2) The impact of a one-off/non-repairable element (could be refurbished/remanufactured) on the availability of the system, in which it operates, equals the mean time to failure (MTTF)/(MTTF + the mean time to repair MTTR).

Availability, achieved (Aa) The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment (i.e., that personnel, tools, spares, etc. are instantaneously available). It excludes logistics time and waiting or administrative downtime. It includes active preventive and corrective maintenance downtime.

Availability, operational (Ao) The probability that an item will operate satisfactorily at a given point in time when used in an actual or realistic operating and support environment. It includes logistics time, ready time, and waiting or administrative downtime, and both preventive and corrective maintenance downtime. This value is equal to the mean time between failure (MTBF) divided by the mean time between failure plus the mean downtime (MDT). This measure extends the definition of availability to elements controlled by the logisticians and mission planners such as quantity and proximity of spares, tools and manpower to the hardware item.

Refer to Systems engineering for more details

Basic example
If we are using equipment which has a mean time to failure (MTTF) of 81.5 years and mean time to repair (MTTR) of 1 hour:

 * MTTF in hours = $81.5 × 365 × 24 = 713940$ (This is a reliability parameter and often has a high level of uncertainty!)

 * Inherent availability (Ai) $= 713940 / (713940+1) = 713940 / 713941 = 99.999860%$

—Ả≥〈〉〈〉〈〉
 * Inherent unavailability $= 1 / 713940 = 0.000140%$

Outage due to equipment in hours per year = 1/rate = 1/MTTF = 0.01235 hours per year.

Literature
Availability is well established in the literature of stochastic modeling and optimal maintenance. Barlow and Proschan [1975] define availability of a repairable system as "the probability that the system is operating at a specified time t." Blanchard [1998] gives a qualitative definition of availability as "a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called for at an unknown random point in time." This definition comes from the MIL-STD-721. Lie, Hwang, and Tillman [1977] developed a complete survey along with a systematic classification of availability.

Availability measures are classified by either the time interval of interest or the mechanisms for the system downtime. If the time interval of interest is the primary concern, we consider instantaneous, limiting, average, and limiting average availability. The aforementioned definitions are developed in Barlow and Proschan [1975], Lie, Hwang, and Tillman [1977], and Nachlas [1998]. The second primary classification for availability is contingent on the various mechanisms for downtime such as the inherent availability, achieved availability, and operational availability. (Blanchard [1998], Lie, Hwang, and Tillman [1977]). Mi [1998] gives some comparison results of availability considering inherent availability.

Availability considered in maintenance modeling can be found in Barlow and Proschan [1975] for replacement models, Fawzi and Hawkes [1991] for an R-out-of-N system with spares and repairs, Fawzi and Hawkes [1990] for a series system with replacement and repair, Iyer [1992] for imperfect repair models, Murdock [1995] for age replacement preventive maintenance models, Nachlas [1998, 1989] for preventive maintenance models, and Wang and Pham [1996] for imperfect maintenance models. A very comprehensive recent book is by Trivedi and Bobbio [2017].

Applications
Availability factor is used extensively in power plant engineering. For example, the North American Electric Reliability Corporation implemented the Generating Availability Data System in 1982.

title: Availability


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Backbone

Backbone may refer to:

 * Vertebral column, core part of the axial skeleton in vertebrate animals

Film

 * Backbone (1923 film), a 1923 lost silent film starring Alfred Lunt
 * Backbone (1975 film), a 1975 Yugoslavian drama directed by Vlatko Gilić

Albums

 * Backbones (album), a 2004 Wishbone Ash compilation album
 * Backbone (Backbone album), 1998
 * Backbone (Boney James album), 1993
 * Backbone (Roam album), 2016
 * Backbone (Anthony Callea album), 2016
 * Backbone (Status Quo album), 2019

Songs

 * "At the End of the Day / Backbone", 1993 song by Baby Animals
 * "Backbone", 2005 song by Gojira from the album From Mars to Sirius
 * "Backbone" (Daughtry song), 2018 song by American rock band Daughtry

Other music

 * Backbone, a rock band led by former Grateful Dead drummer Bill Kreutzmann

Other arts, entertainment, and media

 * Backbone (solitaire), a solitaire game
 * Backbone (magazine), a Canadian business magazine
 * Backbone One (gaming device), a hardware and software platform
 * Tails Noir, a 2021 indie video game known as Backbone until 2023
 * Backbone Entertainment, a video game development company

Places

 * Backbone, Virginia, US
 * European backbone, or "Blue Banana", a geographic corridor of urbanisation in Western Europe
 * Backbone State Park, oldest state park in Iowa, US

Science and technology

 * Backbone chain, in polymer chemistry, the framework of the molecule
 * Backbone network, the top level of a hierarchical computer network
 * Internet backbone, principal data routes between interconnected networks and core routers in the Internet
 * Backbone.js, a JavaScript library used to build applications

Other uses

 * Backbone (British radio communications network)

title: Backbone


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Back-to-back connection

A back-to-back connection is the direct connection of the output of one device to the input of a similar or related device.

Telecommunications
In telecommunications, a back-to-back connection can be formed by connecting a transmitter directly to a receiver without a transmission line in between. This is used for equipment measurements and testing purposes. The back-to-back connection eliminates the effects of the transmission medium.

In some cases, the output of a receiving device is instead connected to the input of a transmitting device.

Power transmission
A back-to-back connection for electric power transmission is a high-voltage direct-current (HVDC) system with both ends in the same switchyard. This is used to couple asynchronously operated power grids or for connecting power grids of different frequencies where no DC transmission line is necessary.

Electronics
In electronics, a back-to-back connection is the connection of two identical or similar components in series with the opposite polarity. This is used to convert polarised components to non-polar use. Common examples include:
 * electrolytic capacitors
 * zener diodes

title: Back-to-back connection


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Backward channel

In a data transmission circuit a backward channel is the channel that passes data in a direction opposite to that of its associated forward channel. The backward channel is usually used for transmission of request, supervisory, acknowledgement, or error-control signals. The direction of flow of these signals is opposite to that in which user information is being transferred. The backward-channel bandwidth is usually less than that of the primary channel, that is, the forward (user information) channel. For example, ADSL's upstream channel, considered a backward channel for some types of analysis, typically has a bandwidth less than one-fourth of the downstream channel.

In data transmission, it is a secondary channel in which the direction of transmission is constrained to be opposite to that of the primary, i.e., the forward (user-information) channel. The direction of transmission in the backward channel is restricted by the control interchange circuit that controls the direction of transmission in the primary channel.

title: Backward channel


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Balanced line

In telecommunications and professional audio, a balanced line or balanced signal pair is an electrical circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths, to ground, and to other circuits. The primary advantage of the balanced line format is good rejection of common-mode noise and interference when fed to a differential device such as a transformer or differential amplifier.

As prevalent in sound recording and reproduction, balanced lines are referred to as balanced audio.

A common form of balanced line is twin-lead, used for radio frequency communications. Also common is twisted pair, used for traditional telephone, professional audio, or for data communications. They are to be contrasted to unbalanced lines, such as coaxial cable, which is designed to have its return conductor connected to ground, or circuits whose return conductor actually is ground (see earth-return telegraph). Balanced and unbalanced circuits can be interfaced using a device called a balun.

Circuits driving balanced lines must themselves be balanced to maintain the benefits of balance. This may be achieved by transformer coupling (repeating coils) or by merely balancing the impedance in each conductor.

Lines carrying symmetric signals (those with equal amplitudes but opposite polarities on each leg) are often incorrectly referred to as "balanced", but this is actually differential signalling. Balanced lines and differential signalling are often used together, but they are not the same thing. Differential signalling does not make a line balanced, nor does noise rejection in balanced cables require differential signalling.

Explanation
Transmission of a signal over a balanced line reduces the influence of noise or interference due to external stray electric fields. Any external signal sources tend to induce only a common mode signal on the line, and the balanced impedances to ground minimizes differential pickup due to stray electric fields. The conductors are sometimes twisted together to ensure that each conductor is equally exposed to any external magnetic fields that could induce unwanted noise.

Some balanced lines also have electrostatic shielding to reduce the amount of noise introduced. The cable is often wrapped in foil, copper wire, or a copper braid. This shield provides immunity to RF interference but does not provide immunity to magnetic fields.

Some balanced lines use 4-conductor star quad cable to provide immunity to magnetic fields. The geometry of the cable ensures that magnetic fields will cause equal interference of both legs of the balanced circuit. This balanced interference is a common-mode signal that can easily be removed by a transformer or balanced differential receiver.

A balanced line allows a differential receiver to reduce the noise on a connection by rejecting common-mode interference. The lines have the same impedance to ground, so the interfering fields or currents induce the same voltage in both wires. Since the receiver responds only to the difference between the wires, it is not influenced by the induced noise voltage. If a balanced line is used in an unbalanced circuit, with different impedances from each conductor to ground, currents induced in the separate conductors will cause different voltage drops to ground, thus creating a voltage differential, making the line more susceptible to noise. Examples of twisted pairs include category 5 cable.

Compared to unbalanced lines, balanced lines reduce the amount of noise per distance, allowing a longer cable run to be practical. This is because electromagnetic interference will affect both signals the same way. Similarities between the two signals are automatically removed at the end of the transmission path when one signal is subtracted from the other.

Telephone systems
The first application for balanced lines was for telephone lines. Interference that was of little consequence on a telegraph system (which is in essence digital) could be very disturbing for a telephone user. The initial format was to take two single-wire unbalanced telegraph lines and use them as a pair. This proved insufficient, however, with the growth of electric power transmission which tended to use the same routes. A telephone line running alongside a power line for many miles will inevitably have more interference induced in one leg than the other since one of them will be nearer to the power line. This issue was addressed by swapping the positions of the two legs every few hundred yards with a cross-over, thus ensuring that both legs had equal interference induced and allowing common-mode rejection to do its work. As the telephone system grew, it became preferable to use cable rather than open wires to save space, and also to avoid poor performance during bad weather. The cable construction used for balanced telephone cables was twisted pair; however, this did not become widespread until repeater amplifiers became available. For an unamplified telephone line, a twisted pair cable could only manage a maximum distance of 30 km. Open wires, on the other hand, with their lower capacitance, had been used for enormous distances—the longest was the 1500 km from New York to Chicago built in 1893. Loading coils were used to improve the distance achievable with cable but the problem was not finally overcome until amplifiers started to be installed in 1912. Twisted pair balanced lines are still widely used for local loops, the lines that connect each subscriber's premises to their respective exchange.

Telephone trunk lines, and especially frequency division multiplexing carrier systems, are usually 4-wire circuits rather than 2-wire circuits (or at least they were before fibre-optic became widespread) and require a different kind of cable. This format requires the conductors to be arranged in two pairs, one pair for the sending (go) signal and the other for the return signal. The greatest source of interference on this kind of transmission is usually the crosstalk between the go and return circuits themselves. The most common cable format is star quad, where the diagonally opposite conductors form the pairs. This geometry gives maximum common mode rejection between the two pairs. An alternative format is DM (Dieselhorst-Martin) quad which consists of two twisted pairs with the twisting at different pitches.

Audio systems
An example of balanced lines is the connection of microphones to a mixer in professional systems. Classically, both dynamic and condenser microphones used transformers to provide a differential-mode signal. While transformers are still used in the large majority of modern dynamic microphones, more recent condenser microphones are more likely to use electronic drive circuitry. Each leg, irrespective of any signal, should have an identical impedance to ground. Pair cable (or a pair-derivative such as star quad) is used to maintain the balanced impedances and close twisting of the cores ensures that any interference is common to both conductors. Providing that the receiving end (usually a mixing console) does not disturb the line balance, and is able to ignore common-mode (noise) signals, and can extract differential ones, then the system will have excellent immunity to induced interference.

Typical professional audio sources, such as microphones, have three-pin XLR connectors. One connects to the shield or chassis ground, while the other two are for the signal conductors. The signal wires can carry two copies of the same signal with opposite polarity (differential signalling) but need not do so. They are often termed "hot" and "cold," and the AES14-1992(r2004) Standard [and EIA Standard RS-297-A] suggest that the pin that carries the positive signal that results from a positive air pressure on a transducer will be deemed 'hot'. Pin 2 has been designated as the 'hot' pin, and that designation serves useful for keeping a consistent polarity in the rest of the system. Since these conductors travel the same path from source to destination, the assumption is that any interference is induced upon both conductors equally. The appliance receiving the signals compares the difference between the two signals (often with disregard to electrical ground) allowing the appliance to ignore any induced electrical noise. Any induced noise would be present in equal amounts and in identical polarity on each of the balanced signal conductors, so the two signals’ difference from each other would be unchanged. The successful rejection of induced noise from the desired signal depends in part on the balanced signal conductors receiving the same amount and type of interference. This typically leads to twisted, braided, or co-jacketed cables for use in balanced signal transmission.

Balanced and differential
Many explanations of balanced lines assume symmetric signals (i.e. signals equal in magnitude but of opposite polarity) but this can lead to confusion of the two concepts—signal symmetry and balanced lines are quite independent of each other. Essential in a balanced line is identical impedances in the two conductors in the driver, line and receiver (impedance balancing). These conditions ensure that external noise affects each leg of the line equally and thus appears as a common mode signal that is rejected by the receiver. There are balanced drive circuits that have excellent common-mode impedance balancing between the legs but do not provide symmetric signals. Symmetric differential signals concern headroom and are not necessary for interference rejection.

Baluns
Interfacing balanced and unbalanced lines requires a balun. For example, baluns can be used to send line level audio or E-carrier level 1 signals over coaxial cable (which is unbalanced) through 300 ft of balanced category 5 cable by using a pair of baluns at each end of the CAT5 run. As the signal travels through the balanced line, noise is induced and added to the signal. As the CAT5 line is carefully impedance balanced, the noise induces equal (common-mode) voltages in both conductors. At the receiving end, the balun responds only to the difference in voltage between the two conductors, thus rejecting the noise picked up along the way and leaving the original signal intact.

A once common application of a radio frequency balun was found at the antenna terminals of a television receiver. Typically a 300-ohm balanced twin lead antenna input could only be connected to a coaxial cable from a cable TV system through a balun.

Characteristic impedance
The characteristic impedance $$Z_0$$ of a transmission line is an important parameter at higher frequencies of operation. For a parallel 2-wire transmission line,

 * $$Z_0 = \frac{1}{\pi}\sqrt{\frac{\mu}{\epsilon}} \ln\left(\frac{l}{R} + \sqrt{\left(\frac{l}{R}\right)^2-1}~\right),$$

where $$l$$ is half the distance between the wire centres, $$R$$ is the wire radius and $$\mu$$, $$\epsilon$$ are respectively the permeability and permittivity of the surrounding medium. A commonly used approximation that is valid when the wire separation is much larger than the wire radius and in the absence of magnetic materials is

 * $$Z_0 = \frac{120 ~ \Omega}{\sqrt{\epsilon_r}}\ln\left(\frac{2l}{R}\right),

$$

where $$\epsilon_r$$ is the relative permittivity of the surrounding medium.

Electric power lines
In electric power transmission, the three conductors used for three-phase power transmission are referred to as a balanced line since the instantaneous sum of the three line voltages is nominally zero. However, balance in this field is referring to the symmetry of the source and load: it has nothing to do with the impedance balance of the line itself, the sense of the meaning in telecommunications.

For the transmission of single-phase electric power as used for railway electrification, two conductors are used to carry in-phase and out-of-phase voltages such that the line is balanced.

Bipolar HVDC lines at which each pole is operated with the same voltage toward ground are also balanced lines.

Balanced transmission standards

 * Ethernet over twisted pair
 * RS-422
 * RS-485
 * Low-voltage differential signalling (LVDS)

title: Balanced line


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Balance return loss

In telecommunications, balance return loss is one of two things:
 * A measure of the degree of balance between two impedances connected to two conjugate sides of a hybrid set, coil, network, or junction.
 * A measure of the effectiveness with which a balancing network simulates the impedance of a two-wire circuit at a hybrid coil.

title: Balance return loss


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Balancing network

In a hybrid set, hybrid coil, or resistance hybrid, balancing network is a circuit used to match, i.e., to balance, the impedance of a uniform transmission line, (e.g., a twisted metallic pair, coaxial cable, etc.) over a selected range of frequencies. A balancing network is required to ensure isolation between the two ports of the four-wire side of the hybrid.

A balancing network can also be a device used between a balanced device or line and an unbalanced device or line for the purpose of transforming from balanced to unbalanced or from unbalanced to balanced.

Source: from Federal Standard 1037C and from MIL-STD-188

title: Balancing network


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Bandwidth compression

In telecommunication, the term bandwidth compression has the following meanings:
 * The reduction of the bandwidth needed to transmit a given amount of data in a given time.
 * The reduction of the time needed to transmit a given amount of data in a given bandwidth.

Bandwidth compression implies a reduction in normal bandwidth of an information-carrying signal without reducing the information content of the signal. This can be accomplished with lossless data compression techniques. For more information read the Increasing speeds section in the Modem article. Bandwidth Compression is a core feature of WAN Optimization appliances to improve bandwidth efficiency.

title: Bandwidth compression


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Barrage jamming

Barrage jamming is an electronic warfare technique that attempts to blind ("jam") radar systems by filling the display with noise, rendering the broadcaster's blip invisible on the display, and often those in the nearby area as well. "Barrage" refers to systems that send signals in many bands of frequencies compared to the bandwidth of any single radar. This allows the jammer to jam multiple radars at once, and reduces or eliminates the need for adjustments to respond to any single radar.

Early radar systems typically operated on a single frequency, and could only change that frequency by changing internal electronics. Against these radars, it was possible to use conventional radio sets to send out signals on the same band, causing the radar display to be filled with noise whenever the antenna was pointed in the general direction of the jammer. However, given that each individual radar would be operating on different frequencies, this "spot jamming" technique required multiple radio sets in order to jam more than one radar at a time, and true wide-band barrage jamming was very difficult.

Early barrage jammers in World War II used photomultiplier tubes to amplify a wideband noise source, An improved approach appeared with the introduction of the carcinotron in the early 1950s, a vacuum tube that generates microwaves whose frequency can be adjusted across a very wide band simply by changing the input voltage. A single carcinotron could be swept through the entire bandwidth of any potential radar network, jamming all of the radars in such rapid sequence that it appeared to be constant noise on all frequencies at all times. A downside to this approach is that the signal only spends a brief period of time at any one radar's frequency; depending on the scanning rate, the radar may only be jammed during certain periods, but if the rate is increased to offset this, the amount of noise in any one pulse period is reduced. More complex jammers can scan only the bands it sees being used, improving its effectiveness.

Barrage jamming was extremely effective against 1950s radars, to the point where there was some belief that the carcinotron might render ground-based radars useless, especially in the early warning radar role. By the 1960s a number of techniques had been introduced to combat barrage jamming. Frequency agile radars, which change their frequency from pulse to pulse, force the jammer spread its signal across the entire bandwidth, ensuring the signal is diluted. Combining this with extremely powerful signals and highly focused antennas allowed new radars to overpower the jammers, "burning through" the jamming. Simple techniques, like turning off the receivers when the antenna was pointed close to the jammer, allowed the radar to continue tracking other targets. The use of phased array antennas and signal processing techniques that reduced sidelobes also improved performance.

Barrage jammers also have the disadvantage that they are very easy to detect using a wideband receiver. This can be used to track the jammer using a variety of techniques. A well developed instance of this was deployed by the RAF in their RX12874 network, which could track jammer-carrying aircraft with accuracy equal to a radar. More generally, a barrage jammer's signal is so easy to receive that it makes an excellent early warning signal on its own.

title: Barrage jamming