Introduction to Voice Technologies

Voice over IP (VoIP) is experiencing explosive growth. Many corporate environments have migrated, are actively migrating, or are researching the process of migrating to VoIP. Some long-distance providers are using VoIP to carry voice traffic, particularly on international calls. Companies, such as Vonage, offer VoIP service as a replacement for traditional telephony service in the home.

Migration is a process that involves gradually phasing out old components and replacing them with new ones. Many terms have been used to describe the technologies and applications for transporting voice in a converged packet network environment. When designing a converged network, it is necessary to clearly define all requirements and understand the various options that are available.

An important first step in designing a converged network is to understand the traditional telephony network and how it interfaces with voice components. You must know, from the start, how legacy voice equipment is connected and its possible migration paths.

The next step toward a good design is being knowledgeable about the components available for VoIP networks. You should be aware of the difference between voice and data flows within the network and the tools for controlling voice calls. Network requirements vary according to the size of the location. Knowing the difference between campus, enterprise, and service provider environments is crucial for choosing the right components and technologies.

This chapter provides an overview of the basic telephony functions and devices, including private branch exchanges (PBXs), switching functions, call signaling, and multiplexing techniques. It also reviews the basic components of the VoIP network and identifies the different requirements in campus, enterprise, and service provider environments. Together, these concepts and techniques provide a solid introduction to the VoIP arena.


Fundamentals of Telephony Networks

In traditional telephony networks, many components and processes are transparent to the customer. As you move from traditional telephony networks to converged voice and data networks, you must manage new components and processes to ensure seamless end-to-end call handling. To maintain acceptable service levels, you need to understand which devices you must now support and the processes that are necessary to ensure end-to-end call functionality.

Basic Components of Telephony Networks

A number of components must be in place for an end-to-end call to succeed. These components are listed here and shown in Figure 1-1:

• Edge devices

• Local loops

• Private or central office (CO) switches

• Trunks

Edge Devices

The two types of edge devices used in a telephony network include:

• Analog telephones Analog telephones are most common in home, small office/home office (SOHO), and small business environments. A direct connection to the public switched telephone network (PSTN) is usually made by using analog telephones. Proprietary analog telephones are occasionally used in conjunction with a PBX. These telephones provide additional functions such as speakerphone, volume control, PBX message-waiting indicator, call on hold, and personalized ringing.

• Digital telephones Digital telephones contain hardware to convert analog voice into a digitized stream. Larger corporate environments with PBXs generally use digital telephones. Digital telephones are typically proprietary, meaning that they work with the PBX or key system of that vendor only.

Local Loops

A local loop is the interface to the telephone company network. Typically, it is a single pair of wires that carry a single conversation. A home or small business may have multiple local loops.


Private or CO Switches

The CO switch terminates the local loop and handles signaling, digit collection, call routing, call setup, and call teardown.

A PBX switch is a privately owned switch located at the customer site. A PBX typically interfaces with other components to provide additional services, such as voice mail.


Trunks

The primary function of a trunk is to provide the path between two switches. There are several common trunk types, as shown in Figure 1-2, including the following:

• Tie trunk A dedicated circuit that connects PBXs directly

• CO trunk A direct connection between a local CO and a PBX

• Interoffice trunk A circuit that connects two local telephone company COs

The telephone installed in your home is considered an edge device because it terminates the service provided by your local telephone company. The local loop is the pair of wires that come to your house and provide residential telephone service. Trunks are the interconnections between telephone switches. They can be between private switches or telephone company switches.