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29-413: Loopback can take the form of communication channels with only one communication endpoint . Any message transmitted by such a channel is immediately and only received by that same channel. In telecommunications, loopback devices perform transmission tests of access lines from the serving switching center , which usually does not require the assistance of personnel at the served terminal . Loop around

58-473: A POSIX compatibility layer and are not otherwise inherently Unix systems. Many ancient UNIX systems no longer meet this definition. Broadly, any Unix-like system that behaves in a manner roughly consistent with the UNIX specification, including having a " program which manages your login and command line sessions "; more specifically, this can refer to systems such as Linux or Minix that behave similarly to

87-470: A UNIX system but have no genetic or trademark connection to the AT&;T code base. Most free/open-source implementations of the UNIX design, whether genetic UNIX or not, fall into the restricted definition of this third category due to the expense of obtaining Open Group certification, which costs thousands of dollars. Around 2001 Linux was given the opportunity to get a certification including free help from

116-403: A communication endpoint is a publish–subscribe topic or a group in group communication systems . This article related to telecommunications is a stub . You can help Misplaced Pages by expanding it . This computer networking article is a stub . You can help Misplaced Pages by expanding it . Unix-like A Unix-like (sometimes referred to as UN*X or *nix ) operating system

145-453: A computing system, or be routed by any routing device. Packets received on an interface with a loopback destination address must be dropped. Such packets are sometimes referred to as Martian packets . As with other bogus packets, they may be malicious and any problems they might cause can be avoided by applying bogon filtering . Some computer network equipment use the term "loopback" for a virtual interface used for management purposes. Unlike

174-587: A device's transmit pin connected to its receive pin will result in the device receiving exactly what it transmits. Moving this looping connection to the remote end of a cable adds the cable to this test. Moving it to the far end of a modem link extends the test further. This is a common troubleshooting technique and is often combined with a specialized test device that sends specific patterns and counts any errors that come back (see Bit Error Rate Test ). Some devices include built-in loopback capability. A simple serial interface loopback test, called paperclip test ,

203-453: A distinction is made between analog loopback , where the analog signal is looped back directly, and digital loopback , where the signal is processed in the digital domain before being re-converted to an analog signal and returned to the source. In telecommunications , loopback, or a loop, is a hardware or software method which feeds a received signal or data back to the sender. It is used as an aid in debugging physical connection problems. As

232-596: A historical connection to the AT&;T codebase. Most commercial UNIX systems fall into this category. So do the BSD systems, which are descendants of work done at the University of California, Berkeley in the late 1970s and early 1980s. Some of these systems have no original AT&T code but can still trace their ancestry to AT&T designs. These systems‍—‌largely commercial in nature‍—‌have been determined by

261-873: A misuse of their trademark. Their guidelines require "UNIX" to be presented in uppercase or otherwise distinguished from the surrounding text, strongly encourage using it as a branding adjective for a generic word such as "system", and discourage its use in hyphenated phrases. Other parties frequently treat "Unix" as a genericized trademark . Some add a wildcard character to the name to make an abbreviation like "Un*x" or "*nix", since Unix-like systems often have Unix-like names such as AIX , A/UX , HP-UX , IRIX , Linux , Minix , Ultrix , Xenix , and XNU . These patterns do not literally match many system names, but are still generally recognized to refer to any UNIX system, descendant, or work-alike, even those with completely dissimilar names such as Darwin / macOS , illumos / Solaris or FreeBSD . In 2007, Wayne R. Gray sued to dispute

290-431: A proper loopback interface, this type of loopback device is not used to talk with itself. Such an interface is assigned an address that can be accessed from management equipment over a network but is not assigned to any of the physical interfaces on the device. Such a loopback device is also used for management datagrams, such as alarms, originating from the equipment. The property that makes this virtual interface special

319-406: A test, many data communication devices can be configured to send specific patterns (such as all ones ) on an interface and can detect the reception of this signal on the same port. This is called a loopback test and can be performed within a modem or transceiver by connecting its output to its own input. A circuit between two points in different locations may be tested by applying a test signal on

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348-486: A variety of proprietary systems were developed based on it, including AIX , HP-UX , IRIX , SunOS , Tru64 , Ultrix , and Xenix . These largely displaced the proprietary clones. Growing incompatibility among these systems led to the creation of interoperability standards, including POSIX and the Single UNIX Specification . Various free, low-cost, and unrestricted substitutes for UNIX emerged in

377-429: Is a method of testing between stations that are not necessarily adjacent, wherein two lines are used, with the test being done at one station and the two lines are interconnected at the distant station. A patch cable may also function as loopback, when applied manually or automatically, remotely or locally, facilitating a loop-back test. Where a system (such as a modem ) involves round-trip analog-to-digital processing,

406-550: Is made up of many small, interchangeable components that can be added or removed as needed. This makes it easy to customize the operating system to suit the needs of different users or environments. The Open Group owns the UNIX trademark and administers the Single UNIX Specification, with the "UNIX" name being used as a certification mark . They do not approve of the construction "Unix-like", and consider it

435-405: Is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification . A Unix-like application is one that behaves like the corresponding Unix command or shell . Although there are general philosophies for Unix design, there is no technical standard defining the term, and opinions can differ about

464-415: Is sometimes used to identify serial ports of a computer and verify operation. It utilizes a terminal emulator application to send characters, with flow control set to off, to the serial port and receive the same back. For this purpose, a paperclip is used to short pin 2 to pin 3 (the receive and transmit pins) on a standard RS-232 interface using D-subminiature DE-9 or DB-25 connectors. Implementations of

493-560: Is that applications that use it will send or receive traffic using the address assigned to the virtual interface as opposed to the address on the physical interface through which the traffic passes. Loopback interfaces of this sort are often used in the operation of routing protocols , because they have the useful property that, unlike real physical interfaces, they will not go down when a physical port fails. The audio systems Open Sound System (OSS), Advanced Linux Sound Architecture (ALSA) and PulseAudio have loopback modules for recording

522-418: Is their ability to support multiple users and processes simultaneously. This allows users to run multiple programs at the same time and to share resources such as memory and disk space. This is in contrast to many older operating systems, which were designed to only support a single user or process at a time. Another important feature of Unix-like systems is their modularity . This means that the operating system

551-455: The Internet protocol suite include a virtual network interface through which network applications can communicate when executing on the same machine. It is implemented entirely within the operating system's networking software and passes no packets to any network interface controller . Any traffic that a computer program sends to a loopback IP address is simply and immediately passed back up

580-530: The Open Group to meet the Single UNIX Specification and are allowed to carry the UNIX name. Most such systems are commercial derivatives of the System V code base in one form or another, although Apple macOS 10.5 and later is a BSD variant that has been certified, and EulerOS and Inspur K-UX are Linux distributions that have been certified. A few other systems (such as IBM z/OS) earned the trademark through

609-431: The circuit in one location, and having the network device at the other location send a signal back through the circuit. If this device receives its own signal back, this proves that the circuit is functioning. A hardware loop is a simple device that physically connects the receiver channel to the transmitter channel. In the case of a network termination connector such as X.21 , this is typically done by simply connecting

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638-514: The 1980s and 1990s, including 4.4BSD , Linux , and Minix . Some of these have in turn been the basis for commercial "Unix-like" systems, such as BSD/OS and macOS . Several versions of (Mac) OS X/macOS running on Intel-based Mac computers have been certified under the Single UNIX Specification . The BSD variants are descendants of UNIX developed by the University of California at Berkeley, with UNIX source code from Bell Labs . However,

667-542: The BSD code base has evolved since then, replacing all the AT&T code. Since the BSD variants are not certified as compliant with the Single UNIX Specification, they are referred to as "UNIX-like" rather than "UNIX". Dennis Ritchie , one of the original creators of Unix, expressed his opinion that Unix-like systems such as Linux are de facto Unix systems. Eric S. Raymond and Rob Landley have suggested that there are three kinds of Unix-like systems: Those systems with

696-413: The audio output of applications for testing purposes. Unlike physical loopbacks, this does not involve double analog/digital conversion and no disruption is caused by hardware malfunctions. Communication endpoint A communication endpoint is a type of communication network node . It is an interface exposed by a communicating party or by a communication channel . An example of the latter type of

725-536: The degree to which a particular operating system or application is Unix-like. Some well-known examples of Unix-like operating systems include Linux and BSD . These systems are often used on servers as well as on personal computers and other devices. Many popular applications, such as the Apache web server and the Bash shell, are also designed to be used on Unix-like systems. One of the key features of Unix-like systems

754-523: The hostnames localhost or loopback . One notable exception to the use of the 127.0.0.0 / 8 network addresses is their use in Multiprotocol Label Switching (MPLS) traceroute error detection, in which their property of not being routable provides a convenient means to avoid delivery of faulty packets to end users. Any IP datagram with a source or destination address set to a loopback address must not appear outside of

783-592: The network software stack as if it had been received from another device. Unix-like systems usually name this loopback interface lo or lo0 . Various Internet Engineering Task Force (IETF) standards reserve the IPv4 address block 127.0.0.0 / 8 , in CIDR notation and the IPv6 address ::1 / 128 for this purpose. The most common IPv4 address used is 127.0.0.1 . Commonly these loopback addresses are mapped to

812-478: The pins together in the connector. Media such as optical fiber or coaxial cable , which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium. A modem can be configured to loop incoming signals from either the remote modem or the local terminal . This is referred to as loopback or software loop. A serial communications transceiver can use loopback for testing its functionality. For example,

841-508: The status of UNIX as a trademark, but lost his case, and lost again on appeal, with the court upholding the trademark and its ownership. "Unix-like" systems started to appear in the late 1970s and early 1980s. Many proprietary versions, such as Idris (1978), UNOS (1982), Coherent (1983), and UniFlex (1985), aimed to provide businesses with the functionality available to academic users of UNIX. When AT&T allowed relatively inexpensive commercial binary sublicensing of UNIX in 1979,

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