Chapter 7. Voice Transport Design

This chapter discusses how to design the transport of voice within a network, and includes the following sections:

• Making the Business Case

• What Is Voice Transport?

• Quality of Service

• VoIP Components

• IP Telephony Components

• Voice Coding and Compression Techniques

• Bandwidth Requirements

• IP Telephony Design

• Voice Security

This chapter describes how voice can be transported along with other data within a network.

We first introduce why it would be advantageous to include voice in your network design. The mechanics of voice transport, including digitization, packetization, and call processing are explained. Quality of service (QoS) for voice is reviewed. The components required in a Voice over IP (VoIP) network and those in an IP telephony network are described. The standards for how voice calls are coded and compressed are introduced, and the bandwidth requirements for voice traffic are explored. IP telephony design scenarios are examined. We conclude with an introduction to security considerations for voice.

Note

Appendix B, "Network Fundamentals," includes material that we assume you understand before reading the rest of the book. Thus, we encourage you to review any of the material in Appendix B that you are not familiar with before reading the rest of this chapter.

While those in the networking world have been hearing about Voice over IP for quite a while now, over the past couple of years (at least within North America), the mainstream press has been introducing the term VoIP to the ordinary consumer. Although the residential market is currently small, it is growing quickly, as prices are slashed for VoIP packages that include such features as unlimited calls within the continent, low-cost international calling, voice mail, selection of your own area code, call forwarding, and call display.

An enterprise might also want to implement VoIP, in other words, to use its data network to also carry its voice traffic. Referring to the discussion in Chapter 1, "Network Design," a new technology shouldn't be implemented just because it can be, but rather because it meets the network's requirements. The goal for converging an organization's voice and data networks is to positively "impact key initiatives, business processes and business results."^[1] This can be accomplished in many ways, including the following:

• Direct cost savings, which is typically the initial reason that a company considers converging its voice and data networks. These savings include the following items:

  - Toll bypass Long-distance voice calls go over the data network instead of the public switched telephone network (PSTN), thus saving the associated charges. For example, calls between branch offices and headquarters can go on the same network as data. Calls to customers can go as far as possible over the converged network, and then onto the PSTN for the last segment.
  
  

  - Having only one network to implement, maintain, manage, and operate Cost reductions can include those for equipment purchases, equipment space, maintenance contracts, and so forth.
  
  

  - Reduced add/move/change costs Employees can take their extension number with them when they move offices, and new employees can be added easily.
  
  

  - Reduced cabling costs Because voice travels over the data network, no additional cabling is required.
  
  

  - Possible reduced capital expenditures costs For example, the company might not have to buy a new private branch exchange (PBX) switch to manage the corporate phone system.
  
  

• Increased end-user productivity, because more applications are available through the Internet or from an IP phone. Two examples are a video telephony application that allows video conference calls between any locations, and unified messaging that allows employees to pick up voice-mail messages on the Internet, thus avoiding costly long-distance charges when they are traveling.

• Revenue-generation opportunities, which can also be a competitive advantage. For example, a call center could be set up to take sales orders and provide customer service.

• Improved customer service and response, because more information can be made available to customers themselves or to the customer-service agents.

• Improved employee empowerment and satisfaction, for example, by providing employees that do not currently have a workstation access to the Internet and applicable applications through an IP phone.