VoIP for Dummies 2005

28 Part I: VoIP Basics

What does the NPA-NXX number have to do with VoIP? In Chapter 7, I discuss dedicated networks that have proven to be the highest quality of service (QoS) network type for VoIP networks. All carrier lines for a dedicated VoIP network are priced using the NPA-NXX of each location included in the network.

Following the development of digital services, the corporate sector began demanding more bandwidth from carriers to support their networks. It didn’t take long for carrier companies to develop digital, high-bandwidth transport lines that could meet the diverse needs of the corporate sector.

These newer transport lines would be digital all the way from a customer’s location A to the same customer’s location B, regardless of how many miles were in between. These customer demands led to the development of transport services that multiplied exponentially the amount of available digital bandwidth that could be offered to the corporate sector.

These newer ultrahigh-bandwidth transports were not the same kinds of wires as those in the POTS network. They were usually a thicker-gauge wire or fiber-optic cables. When installed, they connected two or more locations of a customer’s company in a point-to-point fashion versus the circuitswitched method of the PSTN.

These developments contributed to the emergence of private dedicated networks, which in turn ensured that VoIP would be here to stay. (As you find out in later chapters, VoIP becomes a viable option for companies only when used over dedicated networks.) Eventually, on-net VoIP over dedicated networks will replace expensive circuit-switched calling over the PSTN.

The digital services carrier network

The new types of digital lines installed by the carriers began to form a new physical carrier services network. The lines did not cross-connect or intersect with any of the millions of circuit-switched lines that are in place and continue to be installed by the carriers. At the carrier company’s facilities, newer types of fully digital equipment terminated these digital lines.

This new carrier services network was called the digital services carrier network. (It is also known as the digital signal carrier network, or simply as the

DS.) This network used higher-bandwidth digital lines and operated with packet-switched protocols to network computer data. (For more on protocols, which are simply rules for using the network, see Chapter 1.)

Soon thereafter, the DS network was defined based on its fundamental unit of bandwidth, known as the channel. The smallest channel unit provided a bandwidth of 64 Kbps (64 thousand bits per second). This channel was called a DS0, pronounced “D–S–zero.” (Many computer gurus start counting with 0; it’s a binary thing.) DS0 became the base unit of bandwidth from which other dedicated transports were defined. (In Chapter 7, I cover the popular DS standards in detail.)

The DS network also introduced to the telecommunications vocabulary another term that characterizes most of the transport mechanisms that are not part of the older circuit-switched network. Because the lines used by the DS carriers were installed between private customer locations and the public at large could not use or connect to them, DS lines became known as dedicated to the customer leasing them. The entire series of DS standards eventually became known as the dedicated carrier services network.

War Breaks Out Between

Circuits and Packets

The corporate sector’s thirst for leasing dedicated DS lines was unquenchable. Soon a dilemma emerged as to how to distinguish the circuit-switched network and the newer dedicated network, which used packet-switching technology. Since its inception, the circuit-switched network was a public carrier services network. The DS network was being called dedicated, or private, because no one but the customer paying for the DS lines was permitted to use them.

Public versus private a.k.a.

30 Part I: VoIP Basics

It wasn’t a great leap to make the distinction between public and private types of services. At this point, the name public switched telephone network (PSTN) began to be used to characterize the circuit-switched network. Eventually, the PSTN would be referred to by the telecommunications industry as simply the switched network. The DS name stuck with the network that provided private dedicated transport services. Eventually, the DS network would be referred to as simply the dedicated network.

Figure 2-4 illustrates the physically separate PSTN and DS networks.

POTS telephony continues to use circuit-switched protocols that don’t packetize telephony signals. (See Chapter 1 for an explanation of packets.) POTS signals travel from one line to the next line on a given circuit of lines, just like in the fifth-grade science experiment using tin cans and a string. Another good way to understand circuit-switched protocols is to think about a railroad system. Trains must switch tracks along a circuit of tracks based on the destination of the railroad cars traveling over the tracks. The direction of the train is determined by the physical tracks that the train uses. Figure 2-5 illustrates such a circuit-switched train.

VoIP technology has enabled telephony signals to run over dedicated networks using packet-switched protocols. One of the preferred methods of running VoIP in the corporate sector is to use dedicated lines. Instead of being primarily dependent on the PSTN for its telephone service requirements, companies using VoIP protocols can send and receive telephone calls over their private computer networks. Using VoIP, voice signals can be packetized in a manner similar to computer data packets.

VoIP includes the caller and receiver’s network addressing information in the packets sent over the network. If a given circuit on the network is down, VoIP packets can switch to another computer network circuit because the packet is not dependent on the circuit itself for directions. In the previous example, the circuit-switched train is switched solely by the tracks it travels. If the train runs into a broken track, it can’t continue to travel to its destination. VoIP packets can have many alternative routes because the destination address inside the packet tells the network where to route the packet.

Most companies today use packet-switched networks for their computers and separate circuit-switched networks for their voice calls. Figure 2-6 depicts this typical scenario.

32 Part I: VoIP Basics

Typical LAN

Printer

PC

POTS Telephone System

PSTN

Telephone system

Figure 2-6:

Companies typically use nonintegrated

networks for data

and voice.

Telephone

34 Part I: VoIP Basics

To lessen the burden of newer and diverse telephone costs, many companies have acquired their own POTS-based telephone systems. Company-sponsored telephone systems can reduce the monthly bill that consumers and companies pay for telephony services. Four different telephony system models have evolved in the past three decades.

The first model, POTS, has already been described; it is the use of telephony access lines and carrier services over the PSTN through a carrier. The other models are the Centrex, KTS, and PBX models. Each of these are discussed in this sections.

The Centrex model

The second model is the central office exchange service, or Centrex, model. Centrex is physically set up the same as the POTS access line model. Like POTS, Centrex uses the same physical twisted-pair copper lines.

The difference between the POTS and Centrex models lies in how the line is terminated at the carrier company’s facility. Instead of getting switched into the PSTN directly, the Centrex line first goes to the more intelligent main- frame-level telephone system owned and operated by the carrier. From there, the system can provide the customer with many more features not directly available on a plain POTS line. To get these features on a POTS line, the customer typically has to pay for each feature. Centrex provides a bundle of features with little or no added charges.

Because you get more with Centrex, you pay a little more for the line on a per-line per-month basis. Centrex is a good alternative for companies operating out of temporary facilities (such as a lease situation) and for companies that can’t or don’t want to maintain a full-blown telecommunications infrastructure. When you lease a Centrex system, maintenance is usually included, which reduces the need for skilled staff on the company payroll. Figure 2-7 shows how the addition of a Centrex system modifies the model originally shown in Figure 2-1.

The good news is that a VoIP solution exists for the more traditional Centrex situation: VoIP Centrex. You need a computer network in the temporary premises, but that is an expense you can unplug and take with you when you move to your permanent location. With VoIP Centrex, you can start your VoIP network, acquire features galore, and get the maintenance services you need. When you move to your new, permanent location, you simply plug in your network, and you are up and running.

Figure 2-8 shows the addition of VoIP Centrex to the mix.

Person called

PSTN

36 Part I: VoIP Basics

With any Centrex option, you pay more per month in return for avoiding the need to sink costs into your own infrastructure. Also, by using the Centrex host’s facilities, you get a rich feature set with no additional monthly charges. Another benefit is that you can walk away from a Centrex solution anytime you want without penalty. Although some smaller companies keep Centrex forever, most growing companies eventually convert to one of the models described in the following section.

The KTS and PBX models

The other two system models are private telephone systems installed on the company’s premises. Low-volume customers often use a key telephone system, or KTS. High-volume, larger companies often use a private branch exchange, or PBX. These two are a departure from the POTS-line model, where a line is run to each phone on the premises. As such, they are also a departure from the Centrex model, which uses the same type of access line as POTS.

Figure 2-9 illustrates how a typical private telephone system would change the phone mix.

Persons called