Hypertext Transfer Protocol
The Hypertext Transfer Protocol is an application protocol for distributed, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, where hypertext documents include hyperlinks to other resources that the user can access, for example by a mouse click or by tapping the screen in a web browser. HTTP was developed to facilitate the World Wide Web. Development of HTTP was initiated by Tim Berners-Lee at CERN in 1989. Development of HTTP standards was coordinated by the Internet Engineering Task Force and the World Wide Web Consortium, culminating in the publication of a series of Requests for Comments; the first definition of HTTP/1.1, the version of HTTP in common use, occurred in RFC 2068 in 1997, although this was made obsolete by RFC 2616 in 1999 and again by the RFC 7230 family of RFCs in 2014. A version, the successor HTTP/2, was standardized in 2015, is now supported by major web servers and browsers over Transport Layer Security using Application-Layer Protocol Negotiation extension where TLS 1.2 or newer is required.
HTTP functions as a request–response protocol in the client–server computing model. A web browser, for example, may be the client and an application running on a computer hosting a website may be the server; the client submits an HTTP request message to the server. The server, which provides resources such as HTML files and other content, or performs other functions on behalf of the client, returns a response message to the client; the response contains completion status information about the request and may contain requested content in its message body. A web browser is an example of a user agent. Other types of user agent include the indexing software used by search providers, voice browsers, mobile apps, other software that accesses, consumes, or displays web content. HTTP is designed to permit intermediate network elements to improve or enable communications between clients and servers. High-traffic websites benefit from web cache servers that deliver content on behalf of upstream servers to improve response time.
Web browsers cache accessed web resources and reuse them, when possible, to reduce network traffic. HTTP proxy servers at private network boundaries can facilitate communication for clients without a globally routable address, by relaying messages with external servers. HTTP is an application layer protocol designed within the framework of the Internet protocol suite, its definition presumes an underlying and reliable transport layer protocol, Transmission Control Protocol is used. However, HTTP can be adapted to use unreliable protocols such as the User Datagram Protocol, for example in HTTPU and Simple Service Discovery Protocol. HTTP resources are identified and located on the network by Uniform Resource Locators, using the Uniform Resource Identifiers schemes http and https. URIs and hyperlinks in HTML documents form interlinked hypertext documents. HTTP/1.1 is a revision of the original HTTP. In HTTP/1.0 a separate connection to the same server is made for every resource request. HTTP/1.1 can reuse a connection multiple times to download images, stylesheets, etc after the page has been delivered.
HTTP/1.1 communications therefore experience less latency as the establishment of TCP connections presents considerable overhead. The term hypertext was coined by Ted Nelson in 1965 in the Xanadu Project, in turn inspired by Vannevar Bush's 1930s vision of the microfilm-based information retrieval and management "memex" system described in his 1945 essay "As We May Think". Tim Berners-Lee and his team at CERN are credited with inventing the original HTTP, along with HTML and the associated technology for a web server and a text-based web browser. Berners-Lee first proposed the "WorldWideWeb" project in 1989—now known as the World Wide Web; the first version of the protocol had only one method, namely GET, which would request a page from a server. The response from the server was always an HTML page; the first documented version of HTTP was HTTP V0.9. Dave Raggett led the HTTP Working Group in 1995 and wanted to expand the protocol with extended operations, extended negotiation, richer meta-information, tied with a security protocol which became more efficient by adding additional methods and header fields.
RFC 1945 introduced and recognized HTTP V1.0 in 1996. The HTTP WG planned to publish new standards in December 1995 and the support for pre-standard HTTP/1.1 based on the developing RFC 2068 was adopted by the major browser developers in early 1996. By March that year, pre-standard HTTP/1.1 was supported in Arena, Netscape 2.0, Netscape Navigator Gold 2.01, Mosaic 2.7, Lynx 2.5, in Internet Explorer 2.0. End-user adoption of the new browsers was rapid. In March 1996, one web hosting company reported that over 40% of browsers in use on the Internet were HTTP 1.1 compliant. That same web hosting company reported that by June 1996, 65% of all browsers accessing their servers were HTTP/1.1 compliant. The HTTP/1.1 standard as defined in RFC 2068 was released in January 1997. Improvements and updates to the HTTP/1.1 standard were released under RFC 2616 in June 1999. In 2007, the HTTPbis Working Group was formed, in part, to revise and clarify the HTTP/1.1 specification. In June 2014, the WG released an updated six-part specification obsoleting RFC 2616: RFC 7230, HTTP/1.1: Message Syntax and Routing RFC 7231, HTTP/1.1: Semantics and Content RFC 7232, HTTP/1.1: Conditional Requests RFC 7233, HTTP/1.1: Range Requests RFC 7234, HTTP/1.1: Caching RFC 7235, HTTP/1
A network switch is a computer networking device that connects devices on a computer network by using packet switching to receive and forward data to the destination device. A network switch is a multiport network bridge that uses hardware addresses to process and forward data at the data link layer of the OSI model; some switches can process data at the network layer by additionally incorporating routing functionality. Such switches are known as layer-3 switches or multilayer switches. Switches for Ethernet are the most common form of network switch; the first Ethernet switch was introduced by Kalpana in 1990. Switches exist for other types of networks including Fibre Channel, Asynchronous Transfer Mode, InfiniBand. Unlike less advanced repeater hubs, which broadcast the same data out of each of its ports and let the devices decide what data they need, a network switch forwards data only to the devices that need to receive it. A switch is a device in a computer network. Multiple data cables are plugged into a switch to enable communication between different networked devices.
Switches manage the flow of data across a network by transmitting a received network packet only to the one or more devices for which the packet is intended. Each networked device connected to a switch can be identified by its network address, allowing the switch to direct the flow of traffic maximizing the security and efficiency of the network. A switch is more intelligent than an Ethernet hub, which retransmits packets out of every port of the hub except the port on which the packet was received, unable to distinguish different recipients, achieving an overall lower network efficiency. An Ethernet switch operates at the data link layer of the OSI model to create a separate collision domain for each switch port; each device connected to a switch port can transfer data to any of the other ports at any time and the transmissions will not interfere. Because broadcasts are still being forwarded to all connected devices by the switch, the newly formed network segment continues to be a broadcast domain.
Switches may operate at higher layers of the OSI model, including the network layer and above. A device that operates at these higher layers is known as a multilayer switch. Segmentation involves the use of a switch to split a larger collision domain into smaller ones in order to reduce collision probability, to improve overall network throughput. In the extreme case, each device is located on a dedicated switch port. In contrast to an Ethernet hub, there is a separate collision domain on each of the switch ports; this allows computers to have dedicated bandwidth on point-to-point connections to the network and to run in full-duplex mode. Full-duplex mode has only one transmitter and one receiver per collision domain, making collisions impossible; the network switch plays an integral role in most modern Ethernet local area networks. Mid-to-large sized. Small office/home office applications use a single switch, or an all-purpose device such as a residential gateway to access small office/home broadband services such as DSL or cable Internet.
In most of these cases, the end-user device contains a router and components that interface to the particular physical broadband technology. User devices may include a telephone interface for Voice over IP. Switches are most used as the network connection point for hosts at the edge of a network. In the hierarchical internetworking model and similar network architectures, switches are used deeper in the network to provide connections between the switches at the edge. In switches intended for commercial use, built-in or modular interfaces make it possible to connect different types of networks, including Ethernet, Fibre Channel, RapidIO, ATM, ITU-T G.hn and 802.11. This connectivity can be at any of the layers mentioned. While the layer-2 functionality is adequate for bandwidth-shifting within one technology, interconnecting technologies such as Ethernet and token ring is performed more at layer 3 or via routing. Devices that interconnect at the layer 3 are traditionally called routers, so layer 3 switches can be regarded as primitive and specialized routers.
Where there is a need for a great deal of analysis of network performance and security, switches may be connected between WAN routers as places for analytic modules. Some vendors provide firewall, network intrusion detection, performance analysis modules that can plug into switch ports; some of these functions may be on combined modules. Through port mirroring, a switch can create a mirror image of data that can go to an external device such as intrusion detection systems and packet sniffers. A modern switch may implement power over Ethernet, which avoids the need for attached devices, such as a VoIP phone or wireless access point, to have a separate power supply. Since switches can have redundant power circuits connected to uninterruptible power supplies, the connected device can continue operating when regular office power fails. Modern commercial switches use Ethernet interfaces; the core function of an Ethernet switch is to provide a multiport layer 2 bridging function. Many switches perform operations at other layers.
A device capable of more than bridging is known as a multilayer switch. Switches may learn about topologies at many layers and forward at one or more layers. A layer 1 network device transfers data, but does not manage any of the traffic coming through it, an example is Ethernet hub. Any packet entering a port is repeated to the output of eve
Secure Shell is a cryptographic network protocol for operating network services securely over an unsecured network. Typical applications include remote command-line login and remote command execution, but any network service can be secured with SSH. SSH provides a secure channel over an unsecured network in a client–server architecture, connecting an SSH client application with an SSH server; the protocol specification distinguishes between two major versions, referred to as SSH-1 and SSH-2. The standard TCP port for SSH is 22. SSH is used to access Unix-like operating systems, but it can be used on Microsoft Windows. Windows 10 uses OpenSSH as its default SSH client. SSH was designed as a replacement for Telnet and for unsecured remote shell protocols such as the Berkeley rlogin and rexec protocols; those protocols send information, notably passwords, in plaintext, rendering them susceptible to interception and disclosure using packet analysis. The encryption used by SSH is intended to provide confidentiality and integrity of data over an unsecured network, such as the Internet, although files leaked by Edward Snowden indicate that the National Security Agency can sometimes decrypt SSH, allowing them to read the contents of SSH sessions.
SSH uses public-key cryptography to authenticate the remote computer and allow it to authenticate the user, if necessary. There are several ways to use SSH. Another is to use a manually generated public-private key pair to perform the authentication, allowing users or programs to log in without having to specify a password. In this scenario, anyone can produce a matching pair of different keys; the public key is placed on all computers that must allow access to the owner of the matching private key. While authentication is based on the private key, the key itself is never transferred through the network during authentication. SSH only verifies whether the same person offering the public key owns the matching private key. In all versions of SSH it is important to verify unknown public keys, i.e. associate the public keys with identities, before accepting them as valid. Accepting an attacker's public key without validation will authorize an unauthorized attacker as a valid user. On Unix-like systems, the list of authorized public keys is stored in the home directory of the user, allowed to log in remotely, in the file ~/.ssh/authorized_keys.
This file is respected by SSH only if it is not writable by anything apart from the root. When the public key is present on the remote end and the matching private key is present on the local end, typing in the password is no longer required. However, for additional security the private key itself can be locked with a passphrase; the private key can be looked for in standard places, its full path can be specified as a command line setting. The ssh-keygen utility produces the private keys, always in pairs. SSH supports password-based authentication, encrypted by automatically generated keys. In this case, the attacker could imitate the legitimate server side, ask for the password, obtain it. However, this is possible only if the two sides have never authenticated before, as SSH remembers the key that the server side used; the SSH client raises a warning before accepting the key of a new unknown server. Password authentication can be disabled. SSH is used to log into a remote machine and execute commands, but it supports tunneling, forwarding TCP ports and X11 connections.
SSH uses the client-server model. The standard TCP port 22 has been assigned for contacting SSH servers. An SSH client program is used for establishing connections to an SSH daemon accepting remote connections. Both are present on most modern operating systems, including macOS, most distributions of Linux, OpenBSD, FreeBSD, NetBSD, Solaris and OpenVMS. Notably, versions of Windows prior to 1709 do not include SSH by default. Proprietary and open source versions of various levels of complexity and completeness exist. File managers for UNIX-like systems can use the FISH protocol to provide a split-pane GUI with drag-and-drop; the open source Windows program WinSCP provides similar file management capability using PuTTY as a back-end. Both WinSCP and PuTTY are available packaged to run directly off a USB drive, without requiring installation on the client machine. Setting up an SSH server in Windows involves enabling a feature in Settings app. In Windows 10 version 1709, an official Win32 port of OpenSSH is available.
SSH is important in cloud computing to solve connectivity problems, avoiding the security issues of exposing a cloud-based virtual machine directly on the Internet. An SSH tunnel can provide a secure path over the Internet, through a firewall to a virtual machine. In 1995, Tatu Ylönen, a researcher at Helsinki University of Technology, designed the first version of the protocol prompted by a password-sniffing attack at his university network; the goal of SSH was to replace the earlier rlogin, TELNET, FTP and rsh protocols, which did not provide strong authentication nor guarantee confidentiality. Ylönen released his implementation as freeware in July 1995, an
In computer networking, Gigabit Ethernet is the various technologies for transmitting Ethernet frames at a rate of a gigabit per second, as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999 supplanting Fast Ethernet in wired local networks, as a result of being faster; the cables and equipment are similar to previous standards and have been common and economical since 2010. Half-duplex gigabit links connected through repeater hubs were part of the IEEE specification, but the specification is not updated anymore and full-duplex operation with switches is used exclusively. Ethernet was the result of the research done at Xerox PARC in the early 1970s. Ethernet evolved into a implemented physical and link layer protocol. Fast Ethernet increased speed from 10 to 100 megabits per second. Gigabit Ethernet was the next iteration; the initial standard for Gigabit Ethernet was produced by the IEEE in June 1998 as IEEE 802.3z, required optical fiber. 802.3z is referred to as 1000BASE-X, where -X refers to either -CX, -SX, -LX, or -ZX.
For the history behind the "X" see Fast Ethernet. IEEE 802.3ab, ratified in 1999, defines Gigabit Ethernet transmission over unshielded twisted pair category 5, 5e or 6 cabling, became known as 1000BASE-T. With the ratification of 802.3ab, Gigabit Ethernet became a desktop technology as organizations could use their existing copper cabling infrastructure. IEEE 802.3ah, ratified in 2004 added two more gigabit fiber standards, 1000BASE-LX10 and 1000BASE-BX10. This was part of a larger group of protocols known as Ethernet in the First Mile. Gigabit Ethernet was deployed in high-capacity backbone network links. In 2000, Apple's Power Mac G4 and PowerBook G4 were the first mass-produced personal computers featuring the 1000BASE-T connection, it became a built-in feature in many other computers. There are five physical layer standards for Gigabit Ethernet using optical fiber, twisted pair cable, or shielded balanced copper cable; the IEEE 802.3z standard includes 1000BASE-SX for transmission over multi-mode fiber, 1000BASE-LX for transmission over single-mode fiber, the nearly obsolete 1000BASE-CX for transmission over shielded balanced copper cabling.
These standards use 8b/10b encoding, which inflates the line rate by 25%, from 1000 Mbit/s to 1250 Mbit/s, to ensure a DC balanced signal. The symbols are sent using NRZ. Optical fiber transceivers are most implemented as user-swappable modules in SFP form or GBIC on older devices. IEEE 802.3ab, which defines the used 1000BASE-T interface type, uses a different encoding scheme in order to keep the symbol rate as low as possible, allowing transmission over twisted pair. IEEE 802.3ap defines Ethernet Operation over Electrical Backplanes at different speeds. Ethernet in the First Mile added 1000BASE-LX10 and -BX10. 1000BASE-X is used in industry to refer to Gigabit Ethernet transmission over fiber, where options include 1000BASE-SX, 1000BASE-LX, 1000BASE-LX10, 1000BASE-BX10 or the non-standard -EX and -ZX implementations. Included are copper variants using the same 8b/10b line code. 1000BASE-CX is an initial standard for Gigabit Ethernet connections with maximum distances of 25 meters using balanced shielded twisted pair and either DE-9 or 8P8C connector.
The short segment length is due to high signal transmission rate. Although it is still used for specific applications where cabling is done by IT professionals, for instance the IBM BladeCenter uses 1000BASE-CX for the Ethernet connections between the blade servers and the switch modules, 1000BASE-T has succeeded it for general copper wiring use. 1000BASE-KX is part of the IEEE 802.3ap standard for Ethernet Operation over Electrical Backplanes. This standard defines one to four lanes of backplane links, one RX and one TX differential pair per lane, at link bandwidth ranging from 100Mbit to 10Gbit per second; the 1000BASE-KX variant uses 1.25 GBd electrical signalling speed. 1000BASE-SX is an optical fiber Gigabit Ethernet standard for operation over multi-mode fiber using a 770 to 860 nanometer, near infrared light wavelength. The standard specifies a maximum length of 220 meters for 62.5 µm/160 MHz×km multi-mode fiber, 275 m for 62.5 µm/200 MHz×km, 500 m for 50 µm/400MHz×km, 550 m for 50 µm/500 MHz×km multi-mode fiber.
In practice, with good quality fiber and terminations, 1000BASE-SX will work over longer distances. This standard is popular for intra-building links in large office buildings, co-location facilities and carrier-neutral Internet exchanges. Optical power specifications of SX interface: Minimum output power = −9.5 dBm. Minimum receive sensitivity = −17 dBm. 1000BASE-LX is an optical fiber Gigabit Ethernet standard specified in IEEE 802.3 Clause 38 which uses a long wavelength laser, a maximum RMS spectral width of 4 nm. 1000BASE-LX is specified to work over a distance of up to 5 km over 10 µm single-mode fiber. 1000BASE-LX can run over all common types of multi-mode fiber with a maximum segment length of 550 m. For link distances greater than 300 m, the use of a special launch conditioning patch cord may be required; this launches the laser at a precise offset from the center of the fiber which causes it to spread across the diameter of the fiber core, reducing the effect known as differential mode delay which occurs when the laser couples onto only a small number of available modes in multi-mode f
Graphical user interface
The graphical user interface is a form of user interface that allows users to interact with electronic devices through graphical icons and visual indicators such as secondary notation, instead of text-based user interfaces, typed command labels or text navigation. GUIs were introduced in reaction to the perceived steep learning curve of command-line interfaces, which require commands to be typed on a computer keyboard; the actions in a GUI are performed through direct manipulation of the graphical elements. Beyond computers, GUIs are used in many handheld mobile devices such as MP3 players, portable media players, gaming devices and smaller household and industrial controls; the term GUI tends not to be applied to other lower-display resolution types of interfaces, such as video games, or not including flat screens, like volumetric displays because the term is restricted to the scope of two-dimensional display screens able to describe generic information, in the tradition of the computer science research at the Xerox Palo Alto Research Center.
Designing the visual composition and temporal behavior of a GUI is an important part of software application programming in the area of human–computer interaction. Its goal is to enhance the efficiency and ease of use for the underlying logical design of a stored program, a design discipline named usability. Methods of user-centered design are used to ensure that the visual language introduced in the design is well-tailored to the tasks; the visible graphical interface features of an application are sometimes referred to as chrome or GUI. Users interact with information by manipulating visual widgets that allow for interactions appropriate to the kind of data they hold; the widgets of a well-designed interface are selected to support the actions necessary to achieve the goals of users. A model–view–controller allows flexible structures in which the interface is independent from and indirectly linked to application functions, so the GUI can be customized easily; this allows users to select or design a different skin at will, eases the designer's work to change the interface as user needs evolve.
Good user interface design relates to users more, to system architecture less. Large widgets, such as windows provide a frame or container for the main presentation content such as a web page, email message or drawing. Smaller ones act as a user-input tool. A GUI may be designed for the requirements of a vertical market as application-specific graphical user interfaces. Examples include automated teller machines, point of sale touchscreens at restaurants, self-service checkouts used in a retail store, airline self-ticketing and check-in, information kiosks in a public space, like a train station or a museum, monitors or control screens in an embedded industrial application which employ a real-time operating system. By the 1980s, cell phones and handheld game systems employed application specific touchscreen GUIs. Newer automobiles use GUIs in their navigation systems and multimedia centers, or navigation multimedia center combinations. Sample graphical desktop environments A GUI uses a combination of technologies and devices to provide a platform that users can interact with, for the tasks of gathering and producing information.
A series of elements conforming a visual language have evolved to represent information stored in computers. This makes it easier for people with few computer skills to use computer software; the most common combination of such elements in GUIs is the windows, menus, pointer paradigm in personal computers. The WIMP style of interaction uses a virtual input device to represent the position of a pointing device, most a mouse, presents information organized in windows and represented with icons. Available commands are compiled together in menus, actions are performed making gestures with the pointing device. A window manager facilitates the interactions between windows and the windowing system; the windowing system handles hardware devices such as pointing devices, graphics hardware, positioning of the pointer. In personal computers, all these elements are modeled through a desktop metaphor to produce a simulation called a desktop environment in which the display represents a desktop, on which documents and folders of documents can be placed.
Window managers and other software combine to simulate the desktop environment with varying degrees of realism. Smaller mobile devices such as personal digital assistants and smartphones use the WIMP elements with different unifying metaphors, due to constraints in space and available input devices. Applications for which WIMP is not well suited may use newer interaction techniques, collectively termed post-WIMP user interfaces; as of 2011, some touchscreen-based operating systems such as Apple's iOS and Android use the class of GUIs named post-WIMP. These support styles of interaction using more than one finger in contact with a display, which allows actions such as pinching and rotating, which are unsupported by one pointer and mouse. Human interface devices, for the efficient interaction with a GUI include a computer keyboard used together with keyboard shortcuts, pointing devices for the cursor control: mouse, pointing stick, trackball, virtual keyboards, head-up displays. There are actions performed by programs that affect the GUI.
For example, there are components like inotify or D-Bus to facilitate communication between computer programs. Ivan Sutherland developed Sketchpad in 1963 held as the first graphical co
Simple Network Management Protocol
Simple Network Management Protocol is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behavior. Devices that support SNMP include cable modems, switches, workstations and more. SNMP is used in network management for network monitoring. SNMP exposes management data in the form of variables on the managed systems organized in a management information base which describe the system status and configuration; these variables can be remotely queried by managing applications. Three significant versions of SNMP have been deployed. SNMPv1 is the original version of the protocol. More recent versions, SNMPv2c and SNMPv3, feature improvements in performance and security. SNMP is a component of the Internet Protocol Suite as defined by the Internet Engineering Task Force, it consists of a set of standards for network management, including an application layer protocol, a database schema, a set of data objects.
In typical uses of SNMP, one or more administrative computers called managers have the task of monitoring or managing a group of hosts or devices on a computer network. Each managed system executes a software component called an agent which reports information via SNMP to the manager. An SNMP-managed network consists of three key components: Managed devices Agent – software which runs on managed devices Network management station – software which runs on the managerA managed device is a network node that implements an SNMP interface that allows unidirectional or bidirectional access to node-specific information. Managed devices exchange node-specific information with the NMSs. Sometimes called network elements, the managed devices can be any type of device, but not limited to, access servers, cable modems, hubs, IP telephones, IP video cameras, computer hosts, printers. An agent is a network-management software module. An agent has local knowledge of management information and translates that information to or from an SNMP-specific form.
A network management station executes applications that control managed devices. NMSs provide the bulk of the memory resources required for network management. One or more NMSs may exist on any managed network. SNMP agents expose management data on the managed systems as variables; the protocol permits active management tasks, such as configuration changes, through remote modification of these variables. The variables accessible via SNMP are organized in hierarchies. SNMP itself does not define which variables a managed system should offer. Rather, SNMP uses an extensible design; these hierarchies are described as a management information base. MIBs describe the structure of the management data of a device subsystem; each OID identifies a variable that can be read or set via SNMP. MIBs use the notation defined by Structure of Management Information Version 2.0, a subset of ASN.1. SNMP operates in the application layer of the Internet protocol suite. All SNMP messages are transported via User Datagram Protocol.
The SNMP agent receives requests on UDP port 161. The manager may send requests from any available source port to port 161 in the agent; the agent response is sent back to the source port on the manager. The manager receives notifications on port 162; the agent may generate notifications from any available port. When used with Transport Layer Security or Datagram Transport Layer Security, requests are received on port 10161 and notifications are sent to port 10162. SNMPv1 specifies five core protocol data units. Two other PDUs, GetBulkRequest and InformRequest were added in SNMPv2 and the Report PDU was added in SNMPv3. All SNMP PDUs are constructed as follows: The seven SNMP PDU types as identified by the PDU-type field are as follows: GetRequest A manager-to-agent request to retrieve the value of a variable or list of variables. Desired variables are specified in variable bindings. Retrieval of the specified variable values is to be done as an atomic operation by the agent. A Response with current values is returned.
SetRequest A manager-to-agent request to change the value of a variable or list of variables. Variable bindings are specified in the body of the request. Changes to all specified variables are to be made as an atomic operation by the agent. A Response with new values for the variables is returned. GetNextRequest A manager-to-agent request to discover available variables and their values. Returns a Response with variable binding for the lexicographically next variable in the MIB; the entire MIB of an agent can be walked by iterative application of GetNextRequest starting at OID 0. Rows of a table can be read by specifying column OIDs in the variable bindings of the request. GetBulkRequest A manager-to-agent request for multiple iterations of GetNextRequest. An optimized version of GetNextRequest. Returns a Response with multiple variable bindings walked from the variable binding or bindings in the request. PDU specific non-repeaters and max-repetitions fields are used to control response behavior.
GetBulkRequest was introduced in SNMPv2. Response Returns variable bindings and acknowledgement from agent to manager for GetRequest, SetRequest, GetNextRequest, GetBulkRequest and InformRequest. Error reporting is provided by error-index fields. Although it was used as a response to both gets and sets, this P
Integrated Services Digital Network
Integrated Services Digital Network is a set of communication standards for simultaneous digital transmission of voice, video and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data; the key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. The ISDN standards define several kinds of access interfaces, such as Basic Rate Interface, Primary Rate Interface, Narrowband ISDN, Broadband ISDN. ISDN is a circuit-switched telephone network system, which provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than an analog phone can provide, it offers circuit-switched connections, packet-switched connections, in increments of 64 kilobit/s.
In some countries, ISDN found major market application for Internet access, in which ISDN provides a maximum of 128 kbit/s bandwidth in both upstream and downstream directions. Channel bonding can achieve a greater data rate. ISDN is employed as data-link and physical layers in the context of the OSI model. In common use, ISDN is limited to usage to Q.931 and related protocols, which are a set of signaling protocols establishing and breaking circuit-switched connections, for advanced calling features for the user. They were introduced in 1986. In a videoconference, ISDN provides simultaneous voice and text transmission between individual desktop videoconferencing systems and group videoconferencing systems. Integrated services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice and fax, over a single line. Multiple devices can be attached to the line, used as needed; that means an ISDN line can take care of what were expected to be most people's complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.
It refers to integrated switching and transmission in that telephone switching and carrier wave transmission are integrated rather than separate as in earlier technology. The entry level interface to ISDN is the Basic Rate Interface, a 128 kbit/s service delivered over a pair of standard telephone copper wires; the 144 kbit/s overall payload rate is divided into two 64 kbit/s bearer channels and one 16 kbit/s signaling channel. This is sometimes referred to as 2B+D; the interface specifies the following network interfaces: The U interface is a two-wire interface between the exchange and a network terminating unit, the demarcation point in non-North American networks. The T interface is a serial interface between a computing device and a terminal adapter, the digital equivalent of a modem; the S interface is a four-wire bus. The R interface defines the point between a non-ISDN device and a terminal adapter which provides translation to and from such a device. BRI-ISDN is popular in Europe but is much less common in North America.
It is common in Japan — where it is known as INS64. The other ISDN access available is the Primary Rate Interface, carried over T-carrier with 24 time slots in North America, over E-carrier with 32 channels in most other countries; each channel provides transmission at a 64 kbit/s data rate. With the E1 carrier, the available channels are divided into 30 bearer channels, one data channel, one timing and alarm channel; this scheme is referred to as 30B+2D. In North America, PRI service is delivered via T1 carriers with only one data channel referred to as 23B+D, a total data rate of 1544 kbit/s. Non-Facility Associated Signalling allows two or more PRI circuits to be controlled by a single D channel, sometimes called 23B+D + n*24B. D-channel backup allows for a second D channel in case the primary fails. NFAS is used on a Digital Signal 3. PRI-ISDN is popular throughout the world for connecting private branch exchanges to the public switched telephone network. Though many network professionals use the term ISDN to refer to the lower-bandwidth BRI circuit, in North America BRI is uncommon whilst PRI circuits serving PBXs are commonplace.
The bearer channel is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-channels can be used to carry data, since they are nothing more than digital channels; each one of these channels is known as a DS0. Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines; this has since become less so. X.25 can be carried over the B or D channels of a BRI line, over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale terminals because it eliminates the modem setup, because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines. X.25 was part of an ISDN protocol