Section 1.3. Limits of Traditional Telephony


1.3. Limits of Traditional Telephony

The PSTN's capabilities are largely proportional to its physical connections, because every call must have a circuit, or loop, set up at the beginning of the call and torn down at the end. While the PSTN's switching equipment does a great job of this, some hard limitations are associated with its "circuit-switched" nature.

New features can take a while for the phone company to roll out. It took many years to upgrade central office switches to support features like call-waiting and three-way calling. Even now, some parts of the PSTN still don't support caller ID.

Capacity limits are another engineering challenge on traditional telephony networks. The fidelity of a call's sound reproduction is limited to the available bandwidth between the caller and the recipient, and the maximum number of calls between two offices is limited to the availability of voice circuits that exist between them. The problem posed to the enterprise is one of cost: every PSTN circuit used by the enterprise, be it a POTS line or a T1, adds to its telecommunications expenses.

The telephone companies and phone equipment vendors have made great strides to identify and resolve capacity and cost problems. High-density digital circuits like T1s and T3s have brought the cost of high-density telephony down, and PBX features such as least cost routing (LCR) allow the enterprise to minimize its long-distance calling expenditures. Long-distance calling has become cheaper, and the cost of on-premises PBX equipment and feature-rich business telephones has dropped over time, too.

At one time, telephony features were considered a competitive advantage. As businesses adopted them, they became part of the cost of doing business, and users began seeking a new telephony paradigmone that could inspire big competitive advances again. The question the telecom industry sought to answer was, "Where do we go from here?"

Enterprise telephony innovators began looking to the Internet for the answer. Because of core differences in engineering philosophy and many years of additional discourse on the matter, the Internet is superior in many ways to traditional voice networks.

On the Internet (and IP networks in general), communications protocols are in a constant state of improvement, so more and more features can be delivered while bandwidth efficiency steadily improves and the cost of the network shrinks. On the Internet, capacity is tied closely to the efficiency of software, rather than to the physical capacity of a telephone switch, as with a PBX and the PSTN. As software improves , IP networks grow in capacity, but traditional switches need additional (often expensive) hardware to add capacity.

IP networks have always had another advantage over the traditional public voice network: their software uses standardized hardware components like low-cost PCs. This means that even when hardware upgrades are necessary, they can be procured less expensively. Unlike traditional PBXs, hardware upgrades on IP networks intrinsically improve software productivity and enable more and more capacity. Generally, capacity is easier to scale on IP networks than it is on circuit-switched networks like the PSTN.

While the PSTN is quite reliable, it is far less disaster-proof than IP networks. The Internet Protocol permits redundancy and failover capabilities that are inexpensive and relatively easy to implement. Geographic diversity, a technique used on data networks to circumvent local connectivity interruptions, is very easy for the enterprise to achieve with the Internet, but more difficult on the PSTN. For example, you can connect to two Internet service providers and use the same set of IP addresses with both, thanks to the BGP [*] standard, but it's nearly impossible to use the same set of phone numbers with two telephone companies.

[*] Border Gateway Protocol, RFC 1105.

Because most modern enterprise networks use the same protocols as the Internet, it was only a matter of time before the advantages of those protocols began appealing to designers of voice networks. The result of that appeal is an immense technology family called Voice over IP, or VoIP.

VoIP is loosely defined as using the TCP/IP Protocol suite to facilitate voice conversations, but it's really much more than that. It can be used to replace traditional telephony in the enterprise or in the home or merely to add features to a traditional telephony system. VoIP can also solve connectivity challenges, like linking traditional PBXs at remote sites together, linking private telephone extensions at a single site together like a PBX, or simply aggregating calls among a few analog phones like a key system.

VoIP can be used to facilitate voice communications on many different application substrates, too. It can provide on-demand voice-calling capabilities to users of a customer service web page and allow people to use their personal computers as fully featured business telephones. It can play a role in bridging cell phone and landline systems, too.

VoIP technologies can even run the entire enterprise voice telephone network. The VoIP technology family can even facilitate video streaming, conferencing, whiteboard applications, and instant text messaging, challenging the traditional distinction between data and voice networks.

Of course, VoIP needs a TCP/IP network in order to operate . Until recently, TCP/IP networks were less abundant than were connections to the PSTN. And even when TCP/IP began to rise in popularity, many private networks still weren't connected to the Internet, so the PSTN was always more appealing than the Internet for voice applications. But today, all of that has changed. There are an estimated 36 million permanent, private TCP/IP networks in the United States, and about 30 million of them are linked via high-speed connections to the Internet. [ ]

[ ] FCC statistics, June 2003.

When data networks like the Internet are used to carry real-time media traffic, they are called converged networks. The process of implementing real-time media applications, such as IP telephony, on a data network is called convergence .


Many businesses and campuses consider migrating their telephony applications from traditional circuit-based voice networks (such as the PSTN and PBXs) to VoIP

Reasons to Choose Voice over IP

  • VoIP devices are easier and cheaper to maintain because they leverage the cor-porate data network rather than their own single-purpose voice-only network.

  • VoIP increases the value of the Internet by using it for voice communications.

  • Integrating telephony with computer applications is much easier with VoIP than with traditional voice systems, because in VoIP settings, call management tends to be more open , standardized, and software-driven.

  • VoIP can scale more cheaply than traditional voice systems because it often uses off-the-shelf, frequently updated PC hardware.

  • It allows for more centralized administrative control than a traditional PBX.

  • Managing a VoIP network is accomplished using the same network that carries the voice information itself, unlike the PSTN with its SS7 protocol.

  • Survivability against disasters and network outages is easier to achieve with TCP/IP than with traditional voice systems, due to its basis in software and TCP/IP's remarkable resilience measures and routing protocols.

  • Much of your existing phone equipment can interface with VoIP systems using analog terminal adapters, or ATAs.

  • VoIP's acronym is fun to say ("voyp").


networks (such as the Internet and software-based call management). VoIP's biggest advantages over traditional telephony are scalability and infrastructural cost savings, though easier integration between telephony and computer applications is a big attraction, too.

Enterprise implementers considering VoIP are comparing scalability against that of traditional PBX equipment and discovering that VoIP's basis in software gives it a big advantage. While a small PBX built for 100 telephones is quite costly to scale up to 500 phones, for example, a VoIP call-management solution can usually scale up just as servers and network hardware doand 500 phones is not a particularly tall order for even a moderately equipped VoIP setup.

Cost savings are attracting businesses to VoIP. Since VoIP runs over a data network, it has the same facilities requirements as a data network. Whereas traditional telephony devices such as a PBX normally require separate local area wiring for analog and digitial phone connections, this wiring usually can't be used for local area data networking because it doesn't provide ample resistance to interference and attenuation. With VoIP, the same wiring is used for both data and voice, since the voice is carried within the data network.

The cost of future expansion is almost always less expensive with VoIP than with traditional telephony. Because it can be centrally administered more easily than traditional systems, VoIP allows system expansion, ongoing security enforcement, and back-office call accounting to be cheaper over time.



Switching to VoIP
Switching to VoIP
ISBN: 0596008686
EAN: 2147483647
Year: 2005
Pages: 172

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