Getting Started

This chapter introduces the concept of converged networking and explains how to find what you're looking for in this guide. This chapter contains the following sections:

Navigating This Guide

This Cisco ICS 7750 Site and Network Design Guide is provided to assist you with the following tasks:

Ensuring that the site where you intend to install the system meets physical requirements such as power, temperature, and cabling. (See "Preparing Your Site.")
Conceptualizing and designing your converged network by completing a five-step design process. (See "Designing Your Converged Network.")
Understanding best practices associated with building and operating secure, scalable, and fault-tolerant IP networks. (See "Data Networking Fundamentals.")
Gathering data about your telephone system and performing the calculations necessary to determine how many "pipes" are required to handle your organization's traffic. (See "Traffic Engineering.")
Verifying that you have completed the tasks necessary for a successful installation and scheduling the installation of the system. (See "Installation Planning and Scheduling.")

Conceptual Overview

Circuit switching is the process of dedicating a pair of wires to each phone call made over the Public Switched Telephone Network (PSTN). Packet switching is the process of sending packets (bundles of specially-formatted data) over a network of interconnected equipment such as hubs and routers. These processes have traditionally been regarded as separate domains.

As the demand for networking bandwidth grows, companies invest more heavily in the infrastructure and equipment necessary to carry a higher volume of traffic. Some companies design entirely new voice-and-data converged (integrated) internetworks. Other companies overhaul their traditional data networks, redesigning them to include infrastructure to support packetized voice transmission.

Note The priority given to voice or data traffic is a key consideration when designing a converged network. Companies that carry data traffic that exceeds voice traffic design their networks principally for data transport. These companies build the ability to carry voice traffic into the design as a secondary requirement. Other companies give preference to voice traffic.

What is a Converged Network?

A converged network integrates the processing of voice and data communications by using the same networking infrastructure (cables, routers, and other hardware). This type of shared communication is referred to as Voice over IP (VoIP) or IP telephony because people (using telephones and voice applications) and computers (sending and receiving data such as e-mail) are communicating with each other using the same protocol—in this case, the Internet Protocol (IP).

The Cisco ICS 7750, which consolidates the functions of a Private Branch Exchange (PBX), voice- and data-capable routers, and Ethernet switches, enables you to send voice packets over a conventional data network rather than route your calls exclusively through the PSTN. Because it is a flexible, widely used protocol, IP enables you to build this type of network on top of or integrate it with your existing data network infrastructure.

What is Required to Set Up a Converged Network?

Whether you are building a converged network from the ground up or seeking to integrate your data network with your existing telephone system, you must consider issues such as the following:

The costs of setting up, operating, and maintaining the necessary equipment
The availability of adequate facilities for the equipment
The issues associated with operating and monitoring an IP network
The need to integrate traditionally separate voice and data domains without disrupting existing telephone service

This guide explores these issues and more so that you can successfully plan for the installation, integration, and operation of the Cisco ICS 7750.

Technical Issues Affecting Voice Traffic

Because of the rhythmic patterns found in speech, voice traffic must be able to traverse the network at a constant rate. As a result, voice traffic passing through a network with congestion or other problems is much more likely to be adversely affected than data traffic passing through the same network.

The following factors can affect voice quality:

Delay is the time it takes for packets to travel between two endpoints. Delay occurs when long pauses in conversation cause the receiver (the person listening) to start to talk before the sender (the person talking) is finished. (For more information about delay, see "Calculating the Voice Delay Budget" section in Chapter 3, "Designing Your Converged Network.")

Note Because of the speed of network links and the limited processing power of many devices, some delay is expected. Users normally accept up to about 150 milliseconds (ms) of delay without noticing problems. You can measure delay by using ping tests at various times of the day with different network traffic loads. If network delay is excessive, reduce it before deploying a network that carries VoIP traffic. (For more information about ping, refer to the Cisco ICS 7750 Administration and Troubleshooting Guide.)

Jitter is a variable-length delay that can cause a conversation to break and become unintelligible. Jitter is not usually a problem with PSTN calls because the bandwidth of calls is fixed. However, in VoIP networks where existing data traffic might be bursty, jitter can occur.

: Cisco voice gateways such as the multiservice route processor (MRP) card have built-in buffering to compensate for some jitter, but if jitter is constant on a network, identify the source and control it before deploying a VoIP network.

Serialization occurs when a router attempts to send both voice and data packets out of an interface (a port). In general, voice packets are very small (80 to 256 bytes), but data packets can be very large (1,500 to 18,000 bytes). On relatively slow links, such as WAN connections, large data packets can take a long time to transmit onto the network. When these large packets are mixed with smaller voice packets, the excessive transmission time can lead to both delay and jitter.
Compression reduces the amount of digital information contained in a particular voice transmission. The quality of compressed VoIP packets has improved and many different compression algorithms are available. The more a digital VoIP packet is compressed, however, the more likely that voice quality will decrease.

: When considering compression, it is important to compare the importance of voice quality to potential bandwidth savings. Because compression optimizes bandwidth, significant monthly cost savings are possible. Traditional voice conversations consume 64 Kb of network bandwidth. When voice traffic is run though a VoIP network, it can be compressed and digitized. This compression can reduce the VoIP packets to as little as 5.3 Kb. However, when the packets go onto the IP network, headers must be added, which can add significant bandwidth to each call (about 40 bytes per packet).

Coding is the process of converting an analog voice signal to its digital counterpart. Two of the most common types of coding are the following:
- Pulse code modulation (PCM), also known as the G.711 standard, is the most common type of coding, where voice traffic is represented as a 64-kbps bit stream. PCM provides no compression and therefore no opportunity for bandwidth savings. It is also the sound quality benchmark against which voice compression schemes are measured.
- Conjugate structure algebraic code-excited linear predictive (CS-ACELP), also known as the G.729a standard, provides eight times the bandwidth savings of PCM. Depending on the type of traffic, using the 8-kbps CSA-CELP can produce cost savings of 40 percent, relative to PCM.

Table 1-1 provides information to help you choose a coding method.

Table 1-1: Coding Method Comparison
Compression Method
MOS¹
Delay (ms)

64 kbps PCM (G.711)

4.4

0.75

8 kbps CSA-CELP (G.729a)

4.1

10

¹MOS = mean opinion score. MOS is a widely used subjective measure of voice quality. MOS scores of 4 to 5 are deemed toll quality (comparable to the voice quality typically delivered over the PSTN).

Table of Contents

Getting Started

Getting Started

Navigating This Guide

Conceptual Overview

What is a Converged Network?

What is Required to Set Up a Converged Network?

Technical Issues Affecting Voice Traffic