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Access control

In physical security and information security, access control (AC) is the selective restriction of access to a place or other resource, while access management describes the process. The act of accessing may mean consuming, entering, or using. Permission to access a resource is called authorization.

Access control on digital platforms is also termed admission control. The protection of external databases is essential to preserve digital security.

Access control is considered to be a significant aspect of privacy that should be further studied. Access control policy is part of an organization’s security policy. In order to verify the access control policy, organizations use an access control model.

Physical security

Geographical access control may be enforced by personnel (e.g. border guard, bouncer, ticket checker), or with a device such as a turnstile. There may be fences to avoid circumventing this access control. An alternative of access control in the strict sense (physically controlling access itself) is a system of checking authorized presence, see e.g. Ticket controller (transportation). A variant is exit control, e.g. of a shop (checkout) or a country.

The term access control refers to the practice of restricting entrance to a property, a building, or a room to authorized persons. Physical access control can be achieved by a human (a guard, bouncer, or receptionist), through mechanical means such as locks and keys, or through technological means such as access control systems like the mantrap. Within these environments, physical key management may also be employed as a means of further managing and monitoring access to mechanically keyed areas or access to certain small assets.

Physical access control is a matter of who, where, and when. An access control system determines who is allowed to enter or exit, where they are allowed to exit or enter, and when they are allowed to enter or exit. Historically, this was partially accomplished through keys and locks. When a door is locked, only someone with a key can enter through the door, depending on how the lock is configured. Mechanical locks and keys do not allow restriction of the key holder to specific times or dates. Mechanical locks and keys do not provide records of the key used on any specific door, and the keys can be easily copied or transferred to an unauthorized person. When a mechanical key is lost or the key holder is no longer authorized to use the protected area, the locks must be re-keyed.

Electronic access control

Electronic access control (EAC) uses computers to solve the limitations of mechanical locks and keys. A wide range of credentials can be used to replace mechanical keys. The electronic access control system grants access based on the credential presented. When access is granted, the door is unlocked for a predetermined time and the transaction is recorded. When access is refused, the door remains locked and the attempted access is recorded. The system will also monitor the door and alarm if the door is forced open or held open too long after being unlocked.

When a credential is presented to a reader, the reader sends the credential's information, usually a number, to a control panel, a highly reliable processor. The control panel compares the credential's number to an access control list, grants or denies the presented request, and sends a transaction log to a database. When access is denied based on the access control list, the door remains locked. If there is a match between the credential and the access control list, the control panel operates a relay that in turn unlocks the door. The control panel also ignores a door open signal to prevent an alarm. Often the reader provides feedback, such as a flashing red LED for an access denied and a flashing green LED for an access granted.

The above description illustrates a single factor transaction. Credentials can be passed around, thus subverting the access control list. For example, Alice has access rights to the server room, but Bob does not. Alice either gives Bob her credential, or Bob takes it; he now has access to the server room. To prevent this, two-factor authentication can be used. In a two factor transaction, the presented credential and a second factor are needed for access to be granted; another factor can be a PIN, a second credential, operator intervention, or a biometric input.

There are three types (factors) of authenticating information:
 * something the user knows, e.g. a password, pass-phrase or PIN
 * something the user has, such as smart card or a key fob
 * something the user is, such as the users fingerprint, verified by biometric measurement

Passwords are a common means of verifying a user's identity before access is given to information systems. In addition, a fourth factor of authentication is now recognized: someone you know, whereby another person who knows you can provide a human element of authentication in situations where systems have been set up to allow for such scenarios. For example, a user may have their password, but have forgotten their smart card. In such a scenario, if the user is known to designated cohorts, the cohorts may provide their smart card and password, in combination with the extant factor of the user in question, and thus provide two factors for the user with the missing credential, giving three factors overall to allow access.

Credential
A credential is a physical/tangible object, a piece of knowledge, or a facet of a person's physical being that enables an individual access to a given physical facility or computer-based information system. Typically, credentials can be something a person knows (such as a number or PIN), something they have (such as an access badge), something they are (such as a biometric feature), something they do (measurable behavioural patterns), or some combination of these items. This is known as multi-factor authentication. The typical credential is an access card or key-fob, and newer software can also turn users' smartphones into access devices.

There are many card technologies including magnetic stripe, bar code, Wiegand, 125 kHz proximity, 26-bit card-swipe, contact smart cards, and contactless smart cards. Also available are key-fobs, which are more compact than ID cards, and attach to a key ring. Biometric technologies include fingerprint, facial recognition, iris recognition, retinal scan, voice, and hand geometry. The built-in biometric technologies found on newer smartphones can also be used as credentials in conjunction with access software running on mobile devices. In addition to older more traditional card access technologies, newer technologies such as near-field communication (NFC), Bluetooth low energy or Ultra-wideband (UWB) can also communicate user credentials to readers for system or building access.

Access control system components
Components of an access control system include:
 * An access control panel (also known as a controller)
 * An access-controlled entry, such as a door, turnstile, parking gate, elevator, or other physical barrier
 * A reader installed near the entry. (In cases where the exit is also controlled, a second reader is used on the opposite side of the entry.)
 * Locking hardware, such as electric door strikes and electromagnetic locks
 * A magnetic door switch for monitoring door position
 * Request-to-exit (RTE) devices for allowing egress. When a RTE button is pushed, or the motion detector detects motion at the door, the door alarm is temporarily ignored while the door is opened. Exiting a door without having to electrically unlock the door is called mechanical free egress. This is an important safety feature. In cases where the lock must be electrically unlocked on exit, the request-to-exit device also unlocks the door.

Access control topology

Access control decisions are made by comparing the credentials to an access control list. This look-up can be done by a host or server, by an access control panel, or by a reader. The development of access control systems has observed a steady push of the look-up out from a central host to the edge of the system, or the reader. The predominant topology circa 2009 is hub and spoke with a control panel as the hub, and the readers as the spokes. The look-up and control functions are by the control panel. The spokes communicate through a serial connection; usually RS-485. Some manufactures are pushing the decision making to the edge by placing a controller at the door. The controllers are IP enabled, and connect to a host and database using standard networks

Types of readers
Access control readers may be classified by the functions they are able to perform:
 * Basic (non-intelligent) readers: simply read card number or PIN, and forward it to a control panel. In case of biometric identification, such readers output the ID number of a user. Typically, Wiegand protocol is used for transmitting data to the control panel, but other options such as RS-232, RS-485 and Clock/Data are not uncommon. This is the most popular type of access control readers. Examples of such readers are RF Tiny by RFLOGICS, ProxPoint by HID, and P300 by Farpointe Data.
 * Semi-intelligent readers: have all inputs and outputs necessary to control door hardware (lock, door contact, exit button), but do not make any access decisions. When a user presents a card or enters a PIN, the reader sends information to the main controller, and waits for its response. If the connection to the main controller is interrupted, such readers stop working, or function in a degraded mode. Usually semi-intelligent readers are connected to a control panel via an RS-485 bus. Examples of such readers are InfoProx Lite IPL200 by CEM Systems, and AP-510 by Apollo.
 * Intelligent readers: have all inputs and outputs necessary to control door hardware; they also have memory and processing power necessary to make access decisions independently. Like semi-intelligent readers, they are connected to a control panel via an RS-485 bus. The control panel sends configuration updates, and retrieves events from the readers. Examples of such readers could be InfoProx IPO200 by CEM Systems, and AP-500 by Apollo. There is also a new generation of intelligent readers referred to as "IP readers". Systems with IP readers usually do not have traditional control panels, and readers communicate directly to a PC that acts as a host.

Some readers may have additional features such as an LCD and function buttons for data collection purposes (i.e. clock-in/clock-out events for attendance reports), camera/speaker/microphone for intercom, and smart card read/write support.

Access control system topologies
1. Serial controllers. Controllers are connected to a host PC via a serial RS-485 communication line (or via 20mA current loop in some older systems). External RS-232/485 converters or internal RS-485 cards have to be installed, as standard PCs do not have RS-485 communication ports.

Advantages: Disadvantages: 2. Serial main and sub-controllers. All door hardware is connected to sub-controllers (a.k.a. door controllers or door interfaces). Sub-controllers usually do not make access decisions, and instead forward all requests to the main controllers. Main controllers usually support from 16 to 32 sub-controllers.
 * RS-485 standard allows long cable runs, up to 4000 feet (1200 m)
 * Relatively short response time. The maximum number of devices on an RS-485 line is limited to 32, which means that the host can frequently request status updates from each device, and display events almost in real time.
 * High reliability and security as the communication line is not shared with any other systems.
 * RS-485 does not allow Star-type wiring unless splitters are used
 * RS-485 is not well suited for transferring large amounts of data (i.e. configuration and users). The highest possible throughput is 115.2 kbit/sec, but in most system it is downgraded to 56.2 kbit/sec, or less, to increase reliability.
 * RS-485 does not allow the host PC to communicate with several controllers connected to the same port simultaneously. Therefore, in large systems, transfers of configuration, and users to controllers may take a very long time, interfering with normal operations.
 * Controllers cannot initiate communication in case of an alarm. The host PC acts as a master on the RS-485 communication line, and controllers have to wait until they are polled.
 * Special serial switches are required, in order to build a redundant host PC setup.
 * Separate RS-485 lines have to be installed, instead of using an already existing network infrastructure.
 * Cable that meets RS-485 standards is significantly more expensive than regular Category 5 UTP network cable.
 * Operation of the system is highly dependent on the host PC. In the case that the host PC fails, events from controllers are not retrieved, and functions that require interaction between controllers (i.e. anti-passback) stop working.

Advantages: Disadvantages: 3. Serial main controllers & intelligent readers. All door hardware is connected directly to intelligent or semi-intelligent readers. Readers usually do not make access decisions, and forward all requests to the main controller. Only if the connection to the main controller is unavailable, will the readers use their internal database to make access decisions and record events. Semi-intelligent reader that have no database and cannot function without the main controller should be used only in areas that do not require high security. Main controllers usually support from 16 to 64 readers. All advantages and disadvantages are the same as the ones listed in the second paragraph. 4. Serial controllers with terminal servers. In spite of the rapid development and increasing use of computer networks, access control manufacturers remained conservative, and did not rush to introduce network-enabled products. When pressed for solutions with network connectivity, many chose the option requiring less efforts: addition of a terminal server, a device that converts serial data for transmission via LAN or WAN.
 * Work load on the host PC is significantly reduced, because it only needs to communicate with a few main controllers.
 * The overall cost of the system is lower, as sub-controllers are usually simple and inexpensive devices.
 * All other advantages listed in the first paragraph apply.
 * Operation of the system is highly dependent on main controllers. In case one of the main controllers fails, events from its sub-controllers are not retrieved, and functions that require interaction between sub-controllers (i.e. anti-passback) stop working.
 * Some models of sub-controllers (usually lower cost) do not have the memory or processing power to make access decisions independently. If the main controller fails, sub-controllers change to degraded mode in which doors are either completely locked or unlocked, and no events are recorded. Such sub-controllers should be avoided, or used only in areas that do not require high security.
 * Main controllers tend to be expensive, therefore such a topology is not very well suited for systems with multiple remote locations that have only a few doors.
 * All other RS-485-related disadvantages listed in the first paragraph apply.

Advantages: Disadvantages: All the RS-485-related advantages and disadvantages also apply. 5. Network-enabled main controllers. The topology is nearly the same as described in the second and third paragraphs. The same advantages and disadvantages apply, but the on-board network interface offers a couple of valuable improvements. Transmission of configuration and user data to the main controllers is faster, and may be done in parallel. This makes the system more responsive, and does not interrupt normal operations. No special hardware is required in order to achieve redundant host PC setup: in the case that the primary host PC fails, the secondary host PC may start polling network controllers. The disadvantages introduced by terminal servers (listed in the fourth paragraph) are also eliminated. 6. IP controllers. Controllers are connected to a host PC via Ethernet LAN or WAN.
 * Allows utilizing the existing network infrastructure for connecting separate segments of the system.
 * Provides a convenient solution in cases when the installation of an RS-485 line would be difficult or impossible.
 * Increases complexity of the system.
 * Creates additional work for installers: usually terminal servers have to be configured independently, and not through the interface of the access control software.
 * Serial communication link between the controller and the terminal server acts as a bottleneck: even though the data between the host PC and the terminal server travels at the 10/100/1000 Mbit/sec network speed, it must slow down to the serial speed of 112.5 kbit/sec or less. There are also additional delays introduced in the process of conversion between serial and network data.

Advantages: Disadvantages: 7. IP readers. Readers are connected to a host PC via Ethernet LAN or WAN.
 * An existing network infrastructure is fully utilized, and there is no need to install new communication lines.
 * There are no limitations regarding the number of controllers (as the 32 per line in cases of RS-485).
 * Special RS-485 installation, termination, grounding and troubleshooting knowledge is not required.
 * Communication with the controllers may be done at the full network speed, which is important if transferring a lot of data (databases with thousands of users, possibly including biometric records).
 * In case of an alarm, controllers may initiate connection to the host PC. This ability is important in large systems, because it serves to reduce network traffic caused by unnecessary polling.
 * Simplifies installation of systems consisting of multiple sites that are separated by large distances. A basic Internet link is sufficient to establish connections to the remote locations.
 * Wide selection of standard network equipment is available to provide connectivity in various situations (fiber, wireless, VPN, dual path, PoE)
 * The system becomes susceptible to network related problems, such as delays in case of heavy traffic and network equipment failures.
 * Access controllers and workstations may become accessible to hackers if the network of the organization is not well protected. This threat may be eliminated by physically separating the access control network from the network of the organization. Most IP controllers utilize either Linux platform or proprietary operating systems, which makes them more difficult to hack. Industry standard data encryption is also used.
 * Maximum distance from a hub or a switch to the controller (if using a copper cable) is 100 meters (330 ft).
 * Operation of the system is dependent on the host PC. In case the host PC fails, events from controllers are not retrieved and functions that require interaction between controllers (i.e. anti-passback) stop working. Some controllers, however, have a peer-to-peer communication option in order to reduce dependency on the host PC.

Advantages:
 * Most IP readers are PoE capable. This feature makes it very easy to provide battery backed power to the entire system, including the locks and various types of detectors (if used).
 * IP readers eliminate the need for controller enclosures.
 * There is no wasted capacity when using IP readers (e.g. a 4-door controller would have 25% of unused capacity if it was controlling only 3 doors).
 * IP reader systems scale easily: there is no need to install new main or sub-controllers.
 * Failure of one IP reader does not affect any other readers in the system.

Disadvantages: The advantages and disadvantages of IP controllers apply to the IP readers as well.
 * In order to be used in high-security areas, IP readers require special input/output modules to eliminate the possibility of intrusion by accessing lock and/or exit button wiring. Not all IP reader manufacturers have such modules available.
 * Being more sophisticated than basic readers, IP readers are also more expensive and sensitive, therefore they should not be installed outdoors in areas with harsh weather conditions, or high probability of vandalism, unless specifically designed for exterior installation. A few manufacturers make such models.

Security risks

The most common security risk of intrusion through an access control system is by simply following a legitimate user through a door, and this is referred to as tailgating. Often the legitimate user will hold the door for the intruder. This risk can be minimized through security awareness training of the user population or more active means such as turnstiles. In very high-security applications this risk is minimized by using a sally port, sometimes called a security vestibule or mantrap, where operator intervention is required presumably to assure valid identification.

The second most common risk is from levering a door open. This is relatively difficult on properly secured doors with strikes or high holding force magnetic locks. Fully implemented access control systems include forced door monitoring alarms. These vary in effectiveness, usually failing from high false positive alarms, poor database configuration, or lack of active intrusion monitoring. Most newer access control systems incorporate some type of door prop alarm to inform system administrators of a door left open longer than a specified length of time.

The third most common security risk is natural disasters. In order to mitigate risk from natural disasters, the structure of the building, down to the quality of the network and computer equipment vital. From an organizational perspective, the leadership will need to adopt and implement an All Hazards Plan, or Incident Response Plan. The highlights of any incident plan determined by the National Incident Management System must include Pre-incident planning, during incident actions, disaster recovery, and after-action review.

Similar to levering is crashing through cheap partition walls. In shared tenant spaces, the divisional wall is a vulnerability. A vulnerability along the same lines is the breaking of sidelights.

Spoofing locking hardware is fairly simple and more elegant than levering. A strong magnet can operate the solenoid controlling bolts in electric locking hardware. Motor locks, more prevalent in Europe than in the US, are also susceptible to this attack using a doughnut-shaped magnet. It is also possible to manipulate the power to the lock either by removing or adding current, although most Access Control systems incorporate battery back-up systems and the locks are almost always located on the secure side of the door.

Access cards themselves have proven vulnerable to sophisticated attacks. Enterprising hackers have built portable readers that capture the card number from a user's proximity card. The hacker simply walks by the user, reads the card, and then presents the number to a reader securing the door. This is possible because card numbers are sent in the clear, no encryption being used. To counter this, dual authentication methods, such as a card plus a PIN should always be used.

Many access control credentials unique serial numbers are programmed in sequential order during manufacturing. Known as a sequential attack, if an intruder has a credential once used in the system they can simply increment or decrement the serial number until they find a credential that is currently authorized in the system. Ordering credentials with random unique serial numbers is recommended to counter this threat.

Finally, most electric locking hardware still has mechanical keys as a fail-over. Mechanical key locks are vulnerable to bumping.

The need-to-know principle
The need to know principle can be enforced with user access controls and authorization procedures and its objective is to ensure that only authorized individuals gain access to information or systems necessary to undertake their duties.

Computer security
In computer security, general access control includes authentication, authorization, and audit. A more narrow definition of access control would cover only access approval, whereby the system makes a decision to grant or reject an access request from an already authenticated subject, based on what the subject is authorized to access. Authentication and access control are often combined into a single operation, so that access is approved based on successful authentication, or based on an anonymous access token. Authentication methods and tokens include passwords, biometric analysis, physical keys, electronic keys and devices, hidden paths, social barriers, and monitoring by humans and automated systems.

In any access-control model, the entities that can perform actions on the system are called subjects, and the entities representing resources to which access may need to be controlled are called objects (see also Access Control Matrix). Subjects and objects should both be considered as software entities, rather than as human users: any human users can only have an effect on the system via the software entities that they control.

Although some systems equate subjects with user IDs, so that all processes started by a user by default have the same authority, this level of control is not fine-grained enough to satisfy the principle of least privilege, and arguably is responsible for the prevalence of malware in such systems (see computer insecurity).

In some models, for example the object-capability model, any software entity can potentially act as both subject and object.

, access-control models tend to fall into one of two classes: those based on capabilities and those based on access control lists (ACLs).
 * In a capability-based model, holding an unforgeable reference or capability to an object provides access to the object (roughly analogous to how possession of one's house key grants one access to one's house); access is conveyed to another party by transmitting such a capability over a secure channel
 * In an ACL-based model, a subject's access to an object depends on whether its identity appears on a list associated with the object (roughly analogous to how a bouncer at a private party would check an ID to see if a name appears on the guest list); access is conveyed by editing the list. (Different ACL systems have a variety of different conventions regarding who or what is responsible for editing the list and how it is edited.)

Both capability-based and ACL-based models have mechanisms to allow access rights to be granted to all members of a group of subjects (often the group is itself modeled as a subject).

Access control systems provide the essential services of authorization, identification and authentication (I&A), access approval, and accountability where:

 * authorization specifies what a subject can do
 * identification and authentication ensure that only legitimate subjects can log on to a system
 * access approval grants access during operations, by association of users with the resources that they are allowed to access, based on the authorization policy
 * accountability identifies what a subject (or all subjects associated with a user) did

Access control models
Access to accounts can be enforced through many types of controls. An access control paradigm whereby access rights are granted to users through the use of policies which evaluate attributes (user attributes, resource attributes and environment conditions) In DAC, the data owner determines who can access specific resources. For example, a system administrator may create a hierarchy of files to be accessed based on certain permissions. Compared to other approaches like RBAC or ABAC, the main difference is that in GBAC access rights are defined using an organizational query language instead of total enumeration. Using this network administrators can more effectively manage activity and access based on individual needs. A lattice is used to define the levels of security that an object may have and that a subject may have access to. The subject is only allowed to access an object if the security level of the subject is greater than or equal to that of the object. In MAC, users do not have much freedom to determine who has access to their files. For example, security clearance of users and classification of data (as confidential, secret or top secret) are used as security labels to define the level of trust. OrBAC model allows the policy designer to define a security policy independently of the implementation RBAC allows access based on the job title. RBAC largely eliminates discretion when providing access to objects. For example, a human resources specialist should not have permissions to create network accounts; this should be a role reserved for network administrators. RAC method, also referred to as Rule-Based Role-Based Access Control (RB-RBAC), is largely context based. Example of this would be allowing students to use labs only during a certain time of day; it is the combination of students' RBAC-based information system access control with the time-based lab access rules. Information is accessed based on the responsibilities assigned to an actor or a business role
 * 1) Attribute-based Access Control (ABAC)
 * 1) Discretionary Access Control (DAC)
 * 1) Graph-based Access Control (GBAC)
 * 1) History-Based Access Control (HBAC) Access is granted or declined based on the real-time evaluation of a history of activities of the inquiring party, e.g. behavior, time between requests, content of requests. For example, the access to a certain service or data source can be granted or declined on the personal behavior, e.g. the request interval exceeds one query per second.
 * 2) History-of-Presence Based Access Control (HPBAC) Access control to resources is defined in terms of presence policies that need to be satisfied by presence records stored by the requestor. Policies are usually written in terms of frequency, spread and regularity. An example policy would be "The requestor has made k separate visitations, all within last week, and no two consecutive visitations are apart by more than T hours."
 * 3) Identity-Based Access Control (IBAC)
 * 1) Lattice-Based Access Control (LBAC)
 * 1) Mandatory Access Control (MAC)
 * 1) Organization-Based Access Control (OrBAC)
 * 1) Role-Based Access Control (RBAC)
 * 1) Rule-Based Access Control (RAC)
 * 1) Responsibility Based Access Control

Telecommunication
In telecommunication, the term access control is defined in U.S. Federal Standard 1037C with the following meanings:
 * 1)  A service feature or technique used to permit or deny use of the components of a communication system.
 * 2)  A technique used to define or restrict the rights of individuals or application programs to obtain data from, or place data onto, a storage device.
 * 3)  The definition or restriction of the rights of individuals or application programs to obtain data from, or place data into, a storage device.
 * 4)  The process of limiting access to the resources of an AIS (Automated Information System) to authorized users, programs, processes, or other systems.
 * 5)  That function performed by the resource controller that allocates system resources to satisfy user requests.

This definition depends on several other technical terms from Federal Standard 1037C.

Attribute accessors
Special public member methods – accessors (aka getters) and mutator methods (often called setters) are used to control changes to class variables in order to prevent unauthorized access and data corruption.

Public policy
In public policy, access control to restrict access to systems ("authorization") or to track or monitor behavior within systems ("accountability") is an implementation feature of using trusted systems for security or social control.

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Access time

Access time is the time delay or latency between a request to an electronic system, and the access being completed or the requested data returned

In a computer and software systems, it is the time interval between the instant at which an instruction control unit initiates a call to retrieve data or a request to store data, and the instant at which delivery of the data is completed or the storage is started. Note that in distributed software systems, access time or latency) should be measured through '99th percentile''.

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

title: Access time


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Baud

In telecommunication and electronics, baud (symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel.

It is the unit for symbol rate or modulation rate in symbols per second or pulses per second. It is the number of distinct symbol changes (signalling events) made to the transmission medium per second in a digitally modulated signal or a bd rate line code.

Baud is related to gross bit rate, which can be expressed in bits per second (bit/s). If there are precisely two symbols in the system (typically 0 and 1), then baud and bits per second are equivalent.

Naming
The baud unit is named after Émile Baudot, the inventor of the Baudot code for telegraphy, and is represented according to the rules for SI units. That is, the first letter of its symbol is uppercase (Bd), but when the unit is spelled out, it should be written in lowercase (baud) except when it begins a sentence or is capitalized for another reason, such as in title case. It was defined by the CCITT (now the ITU) in November 1926. The earlier standard had been the number of words per minute, which was a less robust measure since word length can vary.

Definitions
The symbol duration time, also known as the unit interval, can be directly measured as the time between transitions by looking at an eye diagram of the signal on an oscilloscope. The symbol duration time Ts can be calculated as:

 * $$ T_\text{s} =  {1 \over f_\text{s}}, $$

where fs is the symbol rate. There is also a chance of miscommunication which leads to ambiguity.

 * Example: Communication at the baud rate 1000 Bd means communication by means of sending 1000 symbols per second. In the case of a modem, this corresponds to 1000 tones per second; similarly, in the case of a line code, this corresponds to 1000 pulses per second. The symbol duration time is $1⁄1000$ second (that is, 1 millisecond).

The baud is scaled using standard metric prefixes, so that for example
 * 1 kBd (kilobaud) = 1000 Bd
 * 1 MBd (megabaud) = 1000 kBd
 * 1 GBd (gigabaud) = 1000 MBd

Relationship to gross bit rate
The symbol rate is related to gross bit rate expressed in bit/s. The term baud has sometimes incorrectly been used to mean bit rate, since these rates are the same in old modems as well as in the simplest digital communication links using only one bit per symbol, such that binary digit "0" is represented by one symbol, and binary digit "1" by another symbol. In more advanced modems and data transmission techniques, a symbol may have more than two states, so it may represent more than one bit. A bit (binary digit) always represents one of two states.

If $N$ bits are conveyed per symbol, and the gross bit rate is $R$, inclusive of channel coding overhead, the symbol rate $f_{s}$ can be calculated as

 * $$ f_\text{s}  =   {R \over N}. $$

By taking information per pulse N in bit/pulse to be the base-2-logarithm of the number of distinct messages M that could be sent, Hartley constructed a measure of the gross bit rate R as

 * $$ R  =   f_\text{s} N\quad  $$  where $$\quad N = \left \lceil \log_2(M) \right \rceil.$$

Here, the $$\left \lceil x \right \rceil$$ denotes the ceiling function of $$x$$. Where $$x$$ is taken to be any real number greater than zero, then the ceiling function rounds up to the nearest natural number (e.g. $$\left \lceil 2.11 \right \rceil = 3$$).

In that case, $M = 2^{N}$ different symbols are used. In a modem, these may be time-limited sinewave tones with unique combinations of amplitude, phase and/or frequency. For example, in a 64QAM modem, $M = 64$, and so the bit rate is $N = log_{2}(64) = 6$ times the baud rate. In a line code, these may be M different voltage levels.

The ratio is not necessarily an integer; in 4B3T coding, the bit rate is $4⁄3$ of the baud rate. (A typical basic rate interface with a 160 kbit/s raw data rate operates at 120 kBd.)

Codes with many symbols, and thus a bit rate higher than the symbol rate, are most useful on channels such as telephone lines with a limited bandwidth but a high signal-to-noise ratio within that bandwidth. In other applications, the bit rate is less than the symbol rate. Eight-to-fourteen modulation as used on audio CDs has bit rate $8⁄17$ of the baud rate.

title: Baud


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Acoustic coupler

In telecommunications, an acoustic coupler is an interface device for coupling electrical signals by acoustical means—usually into and out of a telephone.

The link is achieved through converting electric signals from the phone line to sound and reconverting sound to electric signals needed for the end terminal, such as a teletypewriter, and back, rather than through direct electrical connection.

History and applications
Prior to its breakup in 1984, Bell System's legal monopoly over telephony in the United States allowed the company to impose strict rules on how consumers could access their network. Customers were prohibited from connecting equipment not made or sold by Bell to the network. The same set-up was operative in nearly all countries, where the telephone companies were nationally owned. In many households, telephones were hard-wired to wall terminals before connectors like RJ11 and BS 6312 became standardized.

The situation was similar in other countries. In Australia, until 1975 the PMG, a Government monopoly, owned all telephone wiring and equipment in user premises and prohibited attachment of third party devices, and while most handsets were connected by 600 series connectors, these were rare in Australia so imported equipment could not be directly connected in any case, despite the general electrical compatibility.

It was not until a landmark U.S. court ruling regarding the Hush-A-Phone in 1956 that the use of a phone attachment (by a third party vendor) was allowed for the first time; though AT&T's right to regulate any device connected to the telephone system was upheld by the courts, they were instructed to cease interference towards Hush-A-Phone users. A second court decision in 1968 regarding the Carterfone further allowed any device not harmful to the system to be connected directly to the AT&T network. This decision enabled the proliferation of later innovations like answering machines, fax machines, and modems.

When inventors began developing devices to send non-voice signals over a telephone line, the need for a workaround for the Bell restrictions was apparent. As early as 1937, telefax machines used by newspapers were using some kind of couplers, possibly acoustic but more likely magnetic for single-directional communication. Multiplexed bidirectional telephone coupling was not needed by these early fax machines.

Robert Weitbrecht created a workaround for the Bell restrictions in 1963. He developed a coupling device that converted sound from the ear piece of the telephone handset to electrical signals, and converted the electrical pulses coming from the teletypewriter to sound that goes into the mouth piece of the telephone handset. His acoustic coupler is known as the Weitbrecht Modem.

The Weitbrecht Modem inspired other engineers to develop other modems to work with 8-bit ASCII terminals at a faster rate. Such modems or couplers were developed around 1966 by John van Geen at the Stanford Research Institute (now SRI International), that mimicked handset operations. An early commercial model was built by Livermore Data Systems in 1968. One would dial the computer system (which would have telephone company datasets) on one's phone, and when the connection was established, place the handset into the acoustic modem. Since the handsets were all supplied by the telephone company, most had the same shape, simplifying the physical interface. A microphone and a speaker inside the modem box would pick up and transmit the signaling tones, and circuitry would convert those audio frequency-shift keying encoded binary signals for an RS232 output socket. With luck one could get 300 baud (~bits/second) transmission rates, but 150 baud was more typical. That speed was sufficient for typewriter-based terminals, as the IBM 2741, running at 134.5 baud, or a teleprinter, running at 110 baud.

The practical upper limit for acoustic-coupled modems was 1200 baud, first made available in 1973 by Vadic and 1977 by AT&T. 1200 baud endpoints became widespread in 1985 with the advent of the Hayes Smartmodem 1200A, though it used an RJ11 jack and was not an acoustic coupler. Such devices facilitated the creation of dial-up bulletin board systems, a forerunner of modern internet chat rooms, message boards, and e-mail.

Design
Usually, a standard telephone handset was placed into a cradle that had been engineered to fit closely (by the use of rubber seals) around the microphone and earpiece of the handset. A modem would modulate a loudspeaker in the cup attached to the handset's microphone, and sound from the loudspeaker in the telephone handset's earpiece would be picked up by a microphone in the cup attached to the earpiece. In this way signals could be passed in both directions.

Acoustic couplers were sensitive to external noise and depended on the widespread standardization of the dimensions of telephone handsets. Once they were made legal and the AT command set became standard for modems, direct electrical connections to telephone networks rapidly became the preferred method of attaching modems, and the use of acoustic couplers dwindled. Acoustic couplers were still used until at least the late 1990s by people travelling in areas of the world where electrical connection to the telephone network is illegal or impractical. Many models of TDDs (Telecommunications Device for the Deaf) still have a built-in acoustic coupler, which allow more universal use with pay phones and for 911 calls by deaf people.

Popular culture
An acoustic coupler (a Novation CAT 300 baud model) is prominently shown early in the 1983 film WarGames, when character David Lightman (portrayed by actor Matthew Broderick) places a telephone handset into the cradle of a film prop acoustic modem to accentuate the act of using telephone lines for interconnection to the developing computer networks of the period—in this case, a military command computer. The earliest major motion picture depicting an acoustic coupler was probably the 1968 Steve McQueen film Bullitt.

title: Acoustic coupler


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Active laser medium

The active laser medium (also called a gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source.

Examples of active laser media include:
 * Certain crystals, typically doped with rare-earth ions (e.g. neodymium, ytterbium, or erbium) or transition metal ions (titanium or chromium); most often yttrium aluminium garnet (Y3Al5O12), yttrium orthovanadate (YVO4), or sapphire (Al2O3); and not often caesium cadmium bromide (CsCdBr3) (solid-state lasers)
 * Glasses, e.g. silicate or phosphate glasses, doped with laser-active ions;
 * Gases, e.g. mixtures of helium and neon (HeNe), nitrogen, argon, krypton, carbon monoxide, carbon dioxide, or metal vapors; (gas lasers)
 * Semiconductors, e.g. gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or gallium nitride (GaN).
 * Liquids, in the form of dye solutions as used in dye lasers.

In order to fire a laser, the active gain medium must be changed into a state in which population inversion occurs. The preparation of this state requires an external energy source and is known as laser pumping. Pumping may be achieved with electrical currents (e.g. semiconductors, or gases via high-voltage discharges) or with light, generated by discharge lamps or by other lasers (semiconductor lasers). More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams.

Example of a model of gain medium

The simplest model of optical gain in real systems includes just two, energetically well separated, groups of sub-levels. Within each sub-level group, fast transitions ensure that thermal equilibrium is reached quickly. Stimulated emissions between upper and lower groups, essential for gain, require the upper levels to be more populated than the corresponding lower ones. This situation is called population-inversion. It is more readily achieved if unstimulated transition rates between the two groups are slow, i.e. the upper levels are metastable. Population inversions are more easily produced when only the lowest sublevels are occupied, requiring either low temperatures or well energetically split groups.

In the case of amplification of optical signals, the lasing frequency is called signal frequency. If the externally provided energy required for the signal's amplification is optical, it would necessarily be at the same or higher pump frequency.

Cross-sections
The simple medium can be characterized with effective cross-sections of absorption and emission at frequencies $$~\omega_{\rm p}~$$ and $$~\omega_{\rm s}$$.
 * Have $$~N~$$ be concentration of active centers in the solid-state lasers.
 * Have $$~N_1~$$ be concentration of active centers in the ground state.
 * Have $$~N_2~$$ be concentration of excited centers.
 * Have $$~N_1+N_2=N$$.

The relative concentrations can be defined as $$~n_1=N_1/N~$$ and $$~n_2=N_2/N$$.

The rate of transitions of an active center from the ground state to the excited state can be expressed like this: $$~ W_{\rm u}=\frac{I_{\rm p}\sigma_{\rm ap}}{ \hbar \omega_{\rm p} }+\frac{I_{\rm s}\sigma_{\rm as}}{ \hbar \omega_{\rm s} } ~$$.

While the rate of transitions back to the ground state can be expressed like: $$~W_{\rm d}=\frac{ I_{\rm p} \sigma_{\rm ep}}{ \hbar \omega_{\rm p} }+\frac{I_{\rm s}\sigma_{\rm es}}{ \hbar \omega_{\rm s} } +\frac{1}{\tau}~$$, where $$~\sigma_{\rm as} ~$$ and $$~\sigma_{\rm ap} ~$$ are effective cross-sections of absorption at the frequencies of the signal and the pump, $$~\sigma_{\rm es} ~$$ and $$~\sigma_{\rm ep} ~$$ are the same for stimulated emission, and $$~\frac{1}{\tau}~$$ is rate of the spontaneous decay of the upper level.

Then, the kinetic equation for relative populations can be written as follows:

$$~ \frac {{\rm d}n_2} {{\rm d}t} = W_{\rm u} n_1 - W_{\rm d} n_2 $$,

$$~ \frac{{\rm d}n_1}{{\rm d}t}=-W_{\rm u} n_1 + W_{\rm d} n_2 ~$$ However, these equations keep $$~ n_1+n_2=1 ~$$.

The absorption $$~ A ~$$ at the pump frequency and the gain $$~  G ~$$ at the signal frequency can be written as follows:

$$~ A = N_1\sigma_{\rm pa} -N_2\sigma_{\rm pe} ~$$ and $$~ G = N_2\sigma_{\rm se} -N_1\sigma_{\rm sa} ~$$.

Steady-state solution
In many cases the gain medium works in a continuous-wave or quasi-continuous regime, causing the time derivatives of populations to be negligible.

The steady-state solution can be written:

$$~ n_2=\frac{W_{\rm u}}{W_{\rm u}+W_{\rm d}} ~$$, $$~ n_1=\frac{W_{\rm d}}{W_{\rm u}+W_{\rm d}}.$$

The dynamic saturation intensities can be defined:

$$~ I_{\rm po}=\frac{\hbar \omega_{\rm p}}{(\sigma_{\rm ap}+\sigma_{\rm ep})\tau} ~$$, $$~ I_{\rm so}=\frac{\hbar \omega_{\rm s}}{(\sigma_{\rm as}+\sigma_{\rm es})\tau} ~$$.

The absorption at strong signal: $$~ A_0=\frac{ND}{\sigma_{\rm as}+\sigma_{\rm es}}~$$.

The gain at strong pump: $$~ G_0=\frac{ND}{\sigma_{\rm ap}+\sigma_{\rm ep}}~$$, where $$~ D= \sigma_{\rm pa} \sigma_{\rm se} - \sigma_{\rm pe} \sigma_{\rm sa} ~$$ is determinant of cross-section.

Gain never exceeds value $$~G_0~$$, and absorption never exceeds value $$~A_0 U~$$.

At given intensities $$~I_{\rm p}~$$, $$~I_{\rm s}~$$ of pump and signal, the gain and absorption can be expressed as follows:

$$~A=A_0\frac{U+s}{1+p+s}~$$, $$~G=G_0\frac{p-V}{1+p+s}~$$,

where $$~p=I_{\rm p}/I_{\rm po}~$$, $$~s=I_{\rm s}/I_{\rm so}~$$, $$~U=\frac{(\sigma_{\rm as}+\sigma_{\rm es})\sigma_{\rm ap}}{D}~$$, $$~V=\frac{(\sigma_{\rm ap}+\sigma_{\rm ep})\sigma_{\rm as}}{D}~$$.

Identities
The following identities take place: $$U-V=1 ~ $$, $$~ A/A_0 +G/G_0=1~.\ $$

The state of gain medium can be characterized with a single parameter, such as population of the upper level, gain or absorption.

Efficiency of the gain medium
The efficiency of a gain medium can be defined as $$~ E =\frac{I_{\rm s} G}{I_{\rm p}A}~$$.

Within the same model, the efficiency can be expressed as follows: $$~E =\frac{\omega_{\rm s}}{\omega_{\rm p}} \frac{1-V/p}{1+U/s}~$$.

For efficient operation, both intensities—pump and signal—should exceed their saturation intensities: $$~\frac{p}{V}\gg 1~$$, and $$~\frac{s}{U}\gg 1~$$.

The estimates above are valid for a medium uniformly filled with pump and signal light. Spatial hole burning may slightly reduce the efficiency because some regions are pumped well, but the pump is not efficiently withdrawn by the signal in the nodes of the interference of counter-propagating waves.

title: Active laser medium


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Adaptive communications

Adaptive communications can mean any communications system, or portion thereof, that automatically uses feedback information obtained from the system itself or from the signals carried by the system to modify dynamically one or more of the system operational parameters to improve system performance or to resist degradation. The modification of a system parameter may be discrete, as in hard-switched diversity reception, or may be continuous, as in a predetection combining algorithm.

title: Adaptive communications


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Adaptive predictive coding

Adaptive predictive coding (APC) is a narrowband analog-to-digital conversion that uses a one-level or multilevel sampling system in which the value of the signal at each sampling instant is predicted according to a linear function of the past values of the quantized signals.

APC is related to linear predictive coding (LPC) in that both use adaptive predictors. However, APC uses fewer prediction coefficients, thus requiring a higher sampling rate than LPC.

title: Adaptive predictive coding


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Adder–subtractor

In digital circuits, an adder–subtractor is a circuit that is capable of adding or subtracting numbers (in particular, binary). Below is a circuit that adds or subtracts depending on a control signal. It is also possible to construct a circuit that performs both addition and subtraction at the same time.

Construction

Having an n-bit adder for A and B, then S = A + B. Then, assume the numbers are in two's complement. Then to perform B − A, two's complement theory says to invert each bit of A with a NOT gate then add one. This yields S = B + $\overline{A}$ + 1, which is easy to do with a slightly modified adder.

By preceding each A input bit on the adder with a 2-to-1 multiplexer where: that has control input D that is also connected to the initial carry, then the modified adder performs This works because when D = 1 the A input to the adder is really $\overline{A}$ and the carry in is 1. Adding B to $\overline{A}$ and 1 yields the desired subtraction of B − A.
 * Input 0 (I0) is A
 * Input 1 (I1) is $\overline{A}$
 * addition when D = 0, or
 * subtraction when D = 1.

A way you can mark number A as positive or negative without using a multiplexer on each bit is to use an XOR gate to precede each bit instead. This produces the same truth table for the bit arriving at the adder as the multiplexer solution does since the XOR gate output will be what the input bit is when D = 0 and the inverted input bit when D = 1.
 * The first input to the XOR gate is the actual input bit
 * The second input for each XOR gate is the control input D

Role in the arithmetic logic unit
Adders are a part of the core of an arithmetic logic unit (ALU). The control unit decides which operations an ALU should perform (based on the op code being executed) and sets the ALU operation. The D input to the adder–subtractor above would be one such control line from the control unit.

The adder–subtractor above could easily be extended to include more functions. For example, a 2-to-1 multiplexer could be introduced on each Bi that would switch between zero and Bi; this could be used (in conjunction with D = 1) to yield the two's complement of A since −A = $\overline{A}$ + 1.

A further step would be to change the 2-to-1 multiplex on A to a 4-to-1 with the third input being zero, then replicating this on Bi thus yielding the following output functions:
 * 0 (with both the Ai and Bi inputs set to zero and D = 0)
 * 1 (with both the Ai and Bi inputs set to zero and D = 1)
 * A (with the Bi input set to zero)
 * B (with the Ai input set to zero)
 * A + 1 (with the Bi input set to zero and D = 1)
 * B + 1 (with the Ai input set to zero and D = 1)
 * A + B
 * A − B
 * B − A
 * $\overline{A}$ (with Ai set to invert; Bi set to zero; and D = 0)
 * −A (with Ai set to invert; Bi set to zero; and D = 1)
 * $\overline{B}$ (with Bi set to invert; Ai set to zero; and D = 0)
 * −B (with Bi set to invert; Ai set to zero; and D = 1)

By adding more logic in front of the adder, a single adder can be converted into much more than just an adder—an ALU.

title: Adder–subtractor


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Address (disambiguation)

An address is a collection of information used to give the location of a building or a plot of land.

Address or The Address may also refer to:

 * Address (film), an unreleased Indian film
 * The Address (film), a 2014 film by Ken Burns

Buildings

 * Address Boulevard, a hotel in Dubai
 * Address Downtown, a hotel in Dubai

Computing

 * , an HTML element
 * An (often-virtual) location in an address space which corresponds to a logical or physical entity

Other uses

 * Public speaking, the process of speaking to a group of people in a structured, deliberate manner
 * Style (manner of address), a legal, official, or recognized title

title: Address (disambiguation)


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Adjacent-channel interference

Adjacent-channel interference (ACI) is interference caused by extraneous power from a signal in an adjacent channel. ACI may be caused by inadequate filtering (such as incomplete filtering of unwanted modulation products in FM systems), improper tuning or poor frequency control (in the reference channel, the interfering channel or both).

ACI is distinguished from crosstalk.

Origin
The adjacent-channel interference which receiver A experiences from a transmitter B is the sum of the power that B emits into A's channel—known as the "unwanted emission", and represented by the ACLR (Adjacent Channel Leakage Ratio)—and the power that A picks up from B's channel, which is represented by the ACS (Adjacent Channel Selectivity). B emitting power into A's channel is called adjacent-channel leakage (unwanted emissions). It occurs for two reasons. First, because RF filters require a roll-off, and do not eliminate a signal completely. Second, due to intermodulation in B's amplifiers, which cause the transmitted spectrum to spread beyond what was intended. Therefore, B emits some power in the adjacent channel which is picked up by A. A receives some emissions from B's channel due to the roll off of A's selectivity filters. Selectivity filters are designed to "select" a channel. Similarly, B's signal suffers intermodulation distortion passing through A's RF input amplifiers, leaking more power into adjacent frequencies.

Avoidance procedure
Broadcast regulators frequently manage the broadcast spectrum in order to minimize adjacent-channel interference. For example, in North America, FM radio stations in a single region cannot be licensed on adjacent frequencies — that is, if a station is licensed on 99.5 MHz in a city, the first-adjacent frequencies of 99.3 MHz and 99.7 MHz cannot be used anywhere within a certain distance of that station's transmitter, and the second-adjacent frequencies of 99.1 MHz and 99.9 MHz are restricted to specialized usages such as low-power stations. Similar restrictions formerly applied to third-adjacent frequencies as well (i.e. 98.9 MHz and 100.1 MHz in the example above), but these are no longer observed.

title: Adjacent-channel interference


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Advanced Data Communication Control Procedures

In telecommunication, Advanced Data Communication Control Procedures (or Protocol) (ADCCP) is a bit-oriented data link layer protocol developed by the American National Standards Institute. It is functionally equivalent to the ISO High-Level Data Link Control (HDLC) protocol.

Although the ISO and ANSI standards writers coordinated their work, so the differences between the standards are mainly editorial, there is one meaningful difference: ADCCP's definition of the basic subset required to implement balanced asynchronous mode includes the RSET frame, while HDLC makes it optional.

One major difference between the two is the unnumbered (U) format. When extended (7-bit) sequence numbers are used, I and S frames have two-byte control fields. Like early versions of HDLC, ADCCP specifies a 2-byte control field format with the P/F flag duplicated. Later HDLC specifications, in particular ISO/IEC 13239, changed that to specify that U frames have 1-byte control fields in all cases.


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Aerial insert

In telecommunications an aerial insert is a segment of cabling that rises from ground to a point above ground, followed by an overhead run, e.g. on poles, followed by a drop back into the ground. An aerial insert is used in places where it is not possible or practical to place a cable underground. Aerial inserts might be encountered in crossing deep ditches, canals, rivers, or subway lines.

Aerial inserts can be found for the same reason also in underground power transmission lines. However for this purpose, the aerial insert is in most cases realized as classical overhead line. It is also possible that a powerline consists of multiple underground and overhead sections of different length.

title: Aerial insert


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Aeronautical Emergency Communications System Plan

In telecommunication, the Aeronautical Emergency Communications System Plan (AECS) provides for the operation of aeronautical communications stations, on a voluntary, organized basis, to provide the President and the Federal Government, as well as multiple heads of state and local governments, or their designated representatives, and the aeronautical industry with an expeditious means of communications during an emergency.


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AIOD leads

In land-line telephony, AIOD leads are Terminal equipment leads used solely to transmit automatic identified outward dialing (AIOD) data from a PBX to the public switched telephone network or to switched service networks (e.g., EPSCS), so that a vendor can provide a detailed monthly bill identifying long distance calling usage by individual PBX stations, tie trunks, or the attendant console. It resembles common channel signalling in that the AIOD leads provide data for all trunks, but is used only for billing, thus resembling automatic number identification.

title: AIOD leads


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Airborne radio relay

Airborne radio relay is a technique employing aircraft fitted with radio relay stations for the purpose of increasing the range, flexibility, or physical security of communications systems. The aircraft may be manned or unmanned aerial vehicles.

Use in Vietnam
One of the first uses of airborne radio relay was by the United States Army's 1st Cavalry Division in the Battle of Ia Drang during the Vietnam War, which employed the technique to improve communications with commanders at headquarters. The action of war had shifted to the borders of Laos and Cambodia, where the hilly terrain made the monetary and human cost of seizing and holding high ground, and airlifting and installing radio relay equipment prohibitive. In 1968, the Department of the Army provided four specially-equipped relay aircraft to the Division, which proved invaluable throughout the country, in particular, during the 1st Cavalry Division's relief of Khe Sanh in 1968.

The use of airborne radio relay was a great success, although two problems arose during the Vietnam War. The first was the limitations of the aircraft used as relays. The 1st Cavalry Division had originally used C-7 Caribous as the relay aircraft, but when these planes were turned over to the Air Force, the equipment was installed in single-engine Otter aircraft, which were too underpowered to carry the heavy equipment required for relay. Eventually, the 1st Signal Brigade was provided with six specially-equipped U-21 aircraft for use in relay operations. The second problem was that of radio frequency interference: the limited frequency spectrum in use for combat radios meant that relay aircraft often interfered with the communication of ground units when their frequencies were overridden by the airborne units. The Army eventually assigned certain frequencies for airborne relay only, although this further limited the frequencies available to ground units.

title: Airborne radio relay


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Alarm sensor

In telecommunication, the term alarm sensor has the following meanings:

1. In communications systems, a device that can sense an abnormal condition within the system and provide a signal indicating the presence or nature of the abnormality to either a local or remote alarm indicator, and (b) may detect events ranging from a simple contact opening or closure to a time-phased automatic shutdown and restart cycle.

2. In a physical security system, an approved device used to indicate a change in the physical environment of a facility or a part thereof.

3. In electronic security systems, a physical device or change/presence of any electronic signal/logic which causes trigger to electronic circuit to perform application specific operation. In electronic alarm systems the use of this trigger event done by such devices is to turn on the alarm or siren producing sound and/or perform a security calling through telephone lines.

Note: Alarm sensors may also be redundant or chained, such as when one alarm sensor is used to protect the housing, cabling, or power protected by another alarm sensor.

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

title: Alarm sensor


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A-law algorithm

An A-law algorithm is a standard companding algorithm, used in European 8-bit PCM digital communications systems to optimize, i.e. modify, the dynamic range of an analog signal for digitizing. It is one of the two companding algorithms in the G.711 standard from ITU-T, the other being the similar μ-law, used in North America and Japan.

For a given input $$x$$, the equation for A-law encoding is as follows: $$ F(x) = \sgn(x) \begin{cases} \dfrac{A |x|}{1 + \ln(A)}, & |x| < \dfrac{1}{A}, \\[1ex] \dfrac{1+ \ln(A |x|)}{1 + \ln(A)}, & \dfrac{1}{A} \leq |x| \leq 1, \end{cases} $$

where $$A$$ is the compression parameter. In Europe, $$A = 87.6$$.

A-law expansion is given by the inverse function: $$ F^{-1}(y) = \sgn(y) \begin{cases} \dfrac{|y| (1 + \ln(A))}{A}, & |y| < \dfrac{1}{1 + \ln(A)}, \\   \dfrac{e^{-1 + |y| (1 + \ln(A))}}{A}, & \dfrac{1}{1 + \ln(A)} \leq |y| < 1. \end{cases} $$

The reason for this encoding is that the wide dynamic range of speech does not lend itself well to efficient linear digital encoding. A-law encoding effectively reduces the dynamic range of the signal, thereby increasing the coding efficiency and resulting in a signal-to-distortion ratio that is superior to that obtained by linear encoding for a given number of bits.

Comparison to μ-law
The μ-law algorithm provides a slightly larger dynamic range than the A-law at the cost of worse proportional distortion for small signals. By convention, A-law is used for an international connection if at least one country uses it.

title: A-law algorithm