Network Blueprint for IP Telephony

The objective of this chapter is to illustrate scenarios in the area of Cisco voice accounting and performance measurement. It describes the technical background of this topic, which is a combination of device instrumentation features and management applications, such as Cisco CallManager and others.

Today's data networks originated in traditional voice networks. In 1875, Alexander Graham Bell invented the telephone. The first telephone company was founded in 1877 as the Bell Telephone Company. Since then, multitudes of books have been written on the subject of accounting and performance management in traditional voice networks.


For more information on legacy voice systems and SS7, consult the following resources:

  • Signaling System No. 7 (SS7/C7): Protocol, Architecture, and Services (Cisco Press, 2004)

  • Signaling System #7, Second Edition (McGraw-Hill, 1998)

In contrast, this chapter's focus is on voice accounting and performance management in data networks, with an emphasis on IP telephony. In the past, voice traffic was transported over a separate voice infrastructure, and the data network infrastructure was established in parallel so that voice and data traffic did not interfere with each other. Traditional voice accounting and performance functions are standardized within the SS7 system (Common Channel Signaling System No. 7), the global standard for telecommunications, defined by the ITU-T. The success of data networks led to the development of techniques to encapsulate voice traffic in IP packets, and thus Voice over IP (VoIP) was born.

During the initial phase of VoIP, the Public Switched Telephone Network (PSTN) switches remain in the network and take full control over the voice calls. Instead of a dedicated voice trunk between the PSTN switches, gateways to an IP data network now interconnect them by encapsulating voice streams in IP packets. Accounting records and performance statistics are still gathered by the PSTN switches, as defined by SS7. In addition, the gateways provide further details from an IP transport point of view. This scenario is not covered here, because it is only an interim step toward IP telephony.

Consequently, the next step was the development of voice technologies that no longer require PSTN components. Instead, a software switch or IP telephony server delivers the PSTN switch functionality. Alternatively, peer-to-peer applications (such as Skype for voice over the Internet) connect the IP phones without central call control. H.323 (ITU-T) and Session Initiation Protocol (SIP) (IETF) became the standards for voice call signaling and control in an IP network. Relevant data sources for accounting and performance purposes are IP telephony applications as well as voice gateways and network elements. The following sections focus on accounting and performance management for VoIP.

Voice-related performance and accounting data can be collected at multiple places in the network. Figure 15-1 illustrates possible components.

Figure 15-1. Metering Direction and Interface

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A good starting place is the phone; because it initiates the communication, it should be able to meter statistics. In fact, Cisco IP phones gather Real-time Transport Control Protocol (RTCP) report statistics during a call and send these to the Cisco CallManager (CCM) when the call is completed. CCM provides additional reporting capabilities for both performance and accounting purposes. Additional metering sources are IP telephony gateways and gatekeepers. Gateways provide access to the PSTN as well as Private Branch Exchange (PBX) interoperability. They are involved if a call setup targets a destination outside of the local perimeter; therefore, gateways can meter call details. Gatekeepers perform call control and routing in conjunction with address translation and are a good place to meter statistics. Additional sources are network management applications that collect MIB details, Remote Authentication Dial-In User Service (RADIUS) servers, and Syslog servers.

In case of voice transmission, the differentiation between performance and accounting is extremely relevant. Voice accounting is a primary source for traffic engineering or billing applications in service provider networks, and performance statistics are related to voice quality assurance. These two areas of performance and accounting are considered separately here because the metering processes and data representation are different.

Part II: Implementations on the Cisco Devices