Voice over IP

Voice over IP (VoIP) commonly refers to the communication protocols, technologies, methodologies, and transmission techniques involved in the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. Other terms commonly associated with VoIP are IP telephony, Internet telephony, voice over broadband (VoBB), broadband telephony, and broadband phone.
Internet telephony refers to communications services —voice, fax, SMS, and/or voice-messaging applications— that are transported via the Internet, rather than the public switched telephone network (PSTN). The steps involved in originating a VoIP telephone call are signaling and media channel setup, digitization of the analog voice signal, encoding, packetization, and transmission as Internet Protocol (IP) packets over a packet-switched network. On the receiving side, similar steps (usually in the reverse order) such as reception of the IP packets, decoding of the packets and digital-to-analog conversion reproduce the original voice stream.Even though IP Telephony and VoIP are terms that are used interchangeably, they are actually different; IP telephony has to do with digital telephony systems that use IP protocols for voice communication, while VoIP is actually a subset of IP Telephony. VoIP is a technology used by IP telephony as a means of transporting phone calls.
VoIP systems employ session control protocols to control the set-up and tear-down of calls as well as audio codecs which encode speech allowing transmission over an IP network as digital audio via an audio stream. The choice of codec varies between different implementations of VoIP depending on application requirements and network bandwidth; some implementations rely on narrowband and compressed speech, while others support high fidelity stereo codecs. Some popular codecs include u-law and a-law versions of G.711, G.722 which is a high-fidelity codec marketed as HD Voice by Polycom, a popular open source voice codec known as iLBC, a codec that only uses 8kbps each way called G.729, and many others.
VoIP is available on many smartphones and Internet devices so that users of portable devices that are not phones, may place calls or send SMS text messages over 3G or Wi-Fi
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Virtual private network

A virtual private network (VPN) is a secure network that uses primarily public telecommunication infrastructures, such as the Internet, to provide remote offices or traveling users an access to a central organizational network.
VPNs typically require remote users of the network to be authenticated, and often secure data with firewall and encryption technologies to prevent disclosure of private information to unauthorized parties.
There are two types of VPNs; remote access VPNs and site to site VPNs. Remote access VPNs are for individual users who are not in a fixed location - remote or roaming users like salespeople. Site to site VPNs are for multiple users in a fixed location - like regional offices.
VPNs may serve any network functionality that is found on any network, such as sharing of data and access to network resources, printers, databases, websites, etc. A VPN user typically experiences the central network in a manner that is identical to being connected directly to the central network. VPN technology via the public Internet has replaced the need to requisition and maintain expensive dedicated leased-line telecommunication circuits once typical in wide-area network installations.
Virtual private network technology reduces costs because it does not need physical leased lines to connect remote users to an Intranet.

VPN Connectivity overview

Extranet

An extranet is a computer network that allows controlled access from the outside, for specific business or educational purposes. In a business-to-business context, an extranet can be viewed as an extension of an organization's intranet that is extended to users outside the organization, usually partners, vendors, and suppliers, in isolation from all other Internet users. In contrast, business-to-consumer (B2C) models involve known servers of one or more companies, communicating with previously unknown consumer users. An extranet is similar to a DMZ in that it provides access to needed services for channel partners, without granting access to an organization's entire network.

Intranet

An intranet is a computer network that uses Internet Protocol technology to share information, operational systems, or computing services within an organization. The term is used in contrast to internet, a network between organizations, and instead refers to a network within an organization. Sometimes, the term refers only to the organization's internal website, but may be a more extensive part of the organization's information technology infrastructure, and may be composed of multiple local area networks.
An intranet may host multiple private websites and constitute an important component and focal point of internal communication and collaboration. Any of the well known Internet protocols may be found in an intranet, such as HTTP (web services), SMTP (e-mail), and FTP (file transfer protocol). Internet technologies are often deployed to provide modern interfaces to legacy information systems hosting corporate data.
An intranet can be understood as a private analog of the Internet, or as a private extension of the Internet confined to an organization. The first intranet websites and home pages began to appear in organizations in 1996-1997. Although not officially noted, the term intranet first became common-place among early adopters, such as universities and technology corporations, in 1992.^{[dubious – discuss]}
Intranets are sometimes contrasted to extranets. While intranets are generally restricted to employees of the organization, extranets may also be accessed by customers, suppliers, or other approved parties.^[1] Extranets extend a private network onto the Internet with special provisions for authentication, authorization and accounting (AAA protocol).
In many organizations, intranets are protected from unauthorized external access by means of a network gateway and firewall. For smaller companies, intranets may be created simply by using private IP address ranges, such as 192.168.0.0/16. In these cases, the intranet can only be directly accessed from a computer in the local network; however, companies may provide access to off-site employees by using a virtual private network, or by other access methods, requiring user authentication and encryption.

Web conferencing

Web conferencing refers to a service that allows conferencing events to be shared with remote locations. In general the service is made possible by Internet technologies, particularly on TCP/IP connections. The service allows real-time point-to-point communications as well as multicast communications from one sender to many receivers. It offers information of text-based messages, voice and video chat to be shared simultaneously, across geographically dispersed locations. Applications for web conferencing include meetings, training events, lectures, or short presentations from any computer.

How does the Asynchronous Web work?

To achieve the Asynchronous Web we need to be able to send responses back to the browser spontaneously, but how can this be achieved within the confines of the HTTP protocol? We cannot send a response to a non-existent request, so it is necessary to manipulate the request/response mechanism to achieve the desired effect. The most straight forward way is with a basic polling mechanism. Send requests on a regular basis, and give the system continuous opportunities to update the presentation. This technique, which is illustrated below, is not ideal as there is no ideal polling interval. There is a necessary trade off between timely updates and chattiness of the system. As illustrated, it is possible for multiple events to occur between polls, but it is also possible to have no events occur. In the final analysis, polling is not a truly asynchronous mechanism.

The next option to consider is HTTP streaming, where multiple responses can be sent to a single request, as illustrated below. This is an efficient mechanism, but unfortunately is not ubiquitously acceptable across all proxy/firewall configurations, making it unsuitable for general purpose deployments.

The last option to consider is HTTP long polling, where the request is made in anticipation of a future response, but that response is blocked until some event occurs that triggers its fulfillment. This mechanism, which is illustrated below, is nearly as efficient as streaming and is completely compatible with proxy/firewall configurations as it is indistinguishable from a slow responding server.

So long polling provides a viable mechanism for supporting the Asynchronous Web, and is in fact the mechanism used in industry implementations like Ajax Push and Comet. While the mechanism is relatively simple, the ramifications of holding these blocking requests indefinitely are not. We will now examine these in more detail beginning with the Servlet.

What is the Asynchronous Web, and How is it Revolutionary? By Stephen Maryka 01 Apr 2009 | TheServerSide.com

Legacy web applications are synchronous in nature. The user interacts with the web interface presented in the browser, the browser makes requests back to the server based on that user interaction, and the server responds to those requests with new presentation for the user - fundamentally a synchronous process. This means that the presentation delivered to the user represents a snapshot in time of what is a dynamic system. That snapshot becomes stale in between user interactions and does not necessarily provide an accurate view onto the current state of the system. Even when you bring Ajax techniques into the equation this synchronous process is unchanged. While the use of XmlHttpRequest and Ajax techniques facilitates a more fine-grained interaction model than a full page refresh, the requests are still generated based on user interaction, so the process remains synchronous, and the potential for a stale view onto the system persists.
The Asynchronous Web is fundamentally different, and that difference revolutionizes how web applications behave. In the Asynchronous Web it is possible to deliver spontaneous presentation changes to the user as the state of a dynamic system changes, without the need for the user to interact with the interface. The advantages are obvious as we can now maintain an accurate view onto the system for the user. Examples are numerous, and include any system providing a view onto a dynamic system, such as a stock portfolio, an inventory, or a day timer/calendar. When you have multiple users interacting with the same system, the interactions of one user can spontaneously impact what other users see, thus creating a truly collaborative system - the essence of what Web 2.0 promises. Again, examples are numerous, including a simple chat client, and an eBay bidding system. Ultimately, most systems that humans interact with are collaborative in nature, so the web interface onto those systems should be too.