Tuesday, January 25, 2011

PSTN, Voice and data

Voice and Data:
Telephony is the communication of spoken information between two or more participants or devices, by means of carried signals over wires or wireless. Ever since Alexander Graham Bell invented the telephone circuit and first envisioned the public telephone system (PSTN). In today world Consumers and businesses have relied on telephony as a clip of interaction.

In the year 2004, there were almost 200 million traditional, land phone lines in the United States of America. This network connects all these phones together wraps around the world. The main function of the PSTN (Public Switched Telephone Network) is to reliably facilitate telephone conversations at any time of day. The PSTN combines Digital, Analog and electro-mechanical data links that strive to make sure that every time you pick up your phone receiver, you hear a dial-tone, and every time you can make call directly to your friend and family or anyone across the globe whenever you need.
Some key features of PSTN, Voice, PBX and Data

  • The PSTN was developed as a result of premature telephony systems' efforts at building meshed and switched networks.
  • SS7 is the switch-to-switch signaling network using to connect and bill calls placed across the PSTN.
  • POTS (plain old telephone service), is the basic single-line analog voice service from a phone company.
  • Key systems and PBXs provide businesses a method of communicating to others for their business or any other purpose at a lower cost than dealing only with PSTN-provided services.
  • Lines and trunks are links between telephony devices. A line is a link from a switch to a phone. A trunk is a link from a switch to a switch.
  • Voice over IP encompasses a large family of interface technologies, protocols, and standards that enable real-time media applications using IP networks.
  • Traditional telephony isn't scalable like VoIP because it isn't software-driven like VoIP. Its ratio of calling capacity to network infrastructure is fixed, while VoIP's has lots of room for optimization.
  • VoIP is suitable for deployment in homes and businesses, and, thanks to broadband and deregulation, is earning a reputation as a more practical way of delivering telephony services.

With the help of Internet technologies and high-speed data connectivity in the enterprise, a new family of telephony technologies has begun called VoIP. Voice over Internet Protocol, or VoIP, has significant appeal for the enterprise, for service providers, and for end users, because it allows the Internet and commonplace data networks, like those at offices, factories, and campuses, to become carriers for voice calls, video conferencing, and other real-time media applications.

VoIP-savvy organizations are discovering that they can apply the paradigm of distributed, software-based networking to voice applications and enable a new generation of telecommunications features, cost-savings, and productivity enhancements.

VoIP can replace business telephone systems, or it can add value to existing traditional telephony devices. For instance, long-distance connectivity between two offices with traditional telephone systems can often be accomplished with a lower cost per call when VoIP is employed.

VoIP network protocols can serve as a platform for other communication media like text messaging and video conferencing. In fact, you've probably used a flavor of VoIP for such an application by now; they've been popular as an Internet pastime for several years. Yahoo! offers a "party line"-style service that features Voice over IP chat rooms. Apple's iChat and Microsoft's NetMeeting, Google's gtalk applications also offer text, voice, and video calling delivered through VoIP protocols.

Dozens of standards define how Voice over IP works, but little documentation exists on best practices for implementing and maintaining the technology in the enterprise. There's not much introductory instruction for VoIP, so beginners may have a hard time taking their first steps with it. There have been several high-profile implementation failures among large enterprise adopters, and this may be why IP telephony has such an intimidating reputation.

Nonetheless, if it's done right, Voice over IP can transform the cost model of telecommunications by combining the overhead of voice and data expertise and infrastructure. It can also enhance productivity for end users by introducing new features and for telecommunication administrators by centralizing management functions. Voice over IP can decrease the expense of future computer-telephony integration projects, while making it easier to link voice systems with web servers and database applications.

The book Switching to VoIP by Theodore Wallingford will give you practical guidance on switching to Voice over IP from traditional telephony systems. It gives a brief introduction to traditional telecom systems, and correlates their features and fundamentals to those of IP telephony systems, while showing ways of integrating traditional telephony assets into an IP-based voice network. It will describe the standards involved, so you can make educated choices among the large selection of components and vendors. It will also help you conquer some of the most problematic issues that people face when building telephony systems with Voice over IP.

Plain Old Telephone Service
When you pick up a traditional telephone, like the one in your house, the dial-tone you hear is a signal transmitted over the simplest kind of voice circuit: an analog phone line, also called a POTS (plain old telephone service) line. The line is a simple electrical loop. On the other end of the line is the telephone company's switch, or exchange. It is this switch that gives your phone its electrical power, sends the sound signals emitted by its audio transducer, and knows how to handle the numbers you dial on its keypad.

The switch also knows how to provide POTS calling features like call-waiting, a service that allows your call in progress to be placed on hold while you answer another caller who is trying to call your linewhen you tap the receiver's hook, the switch recognizes the electromagnetic signal you created (called a flash), connects your phone line to the call that's waiting on another line on the switch, and places your first caller on hold. Later, when you send the flash signal again, the first caller is re-connected with your line and you can resume the original conversation. On the PSTN, calling features are always provided by the telephone company switchnot by the phone itself.

Despite historically increasing demand for features and gains in efficiency, POTS has been the most popular type of service provided by phone companies for the last several decades. Its core of a copper loop with an electromagnetic telephone receiver on one end and a central office switch on the other end hasn't changed much during that time. POTS' stalwart reputation and simplicity have been key reasons for its longevity.

So why not keep that copper telephone loop forever? POTS works great in low-density scenarios; that is, places with a small number of simultaneous callers, such as a one-family home. POTS can also excel where extensive calling features aren't important or where the telephone is rarely used. But when density and features become more critical, POTS isn't always the best solution.

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