Voice over IP (VoIP) for Enterprise Networks: Performance Implications & Traffic Models

Voice over Internet Protocol (VoIP) is a rapidly emerging technology. Currently, the Internet is being wired to support voice and fax traffic. The widespread interest in VoIP is not necessarily the ability of IP to carry voice traffic but the ability to carry voice and fax traffic over data networks. The implications of this are far-reaching. That is, the Internet, for the most part, will eventually replace the PSTN (Public Switched Telephone Network). Furthermore, VoIP will play a major role in e-commerce applications such as IPbased call centers. More importantly, by running intra-company inter-site voice/fax over its IP data network, a company can expect to reduce telephony costs by 70%-80%. In this paper, we discuss VoIP technology and its implications. In addition, we present traffic models to explore the performance implications of VoIP upon enterprise networks. 1 Telephony, be it IP-based or conventional, can be defined as the science of transmitting voice, data, video, or image signals over physical or wireless communication networks. Introduction Voice over Internet Protocol (VoIP) is the transmission of voice and fax traffic in packets over IP-based networks. There are many terms commonly used to describe this technology such as Internet Telephony, IP Telephony, etc. Interestingly, IP Telephony is viewed by some as a simply a means to place “free” telephone calls using the Internet; however, it is much more than that. In this paper, we discuss the importance of modeling VoIP for enterprise networks. We discuss how VoIP can be deployed and its increasing role in enterprise networks. Performance models that characterize VoIP traffic are presented and discussed. The Public Switched Telephone Network The Public Switched Telephone Network (PSTN) has become an “Intelligent Network” (IN). This means that the network has the capability to use real-time database information to route telephone calls. Using information retrieved from data stores, the following lists some of the telephony services that can be performed: ? Toll-free calling Facilitates toll-free telephone services and proper billing. ? Wireless roaming Enables wireless telephony via a series of networks. ? Calling Cards – Enables telephony services and proper billing. ? Local portability – Allows telephone users to change local carriers. ? Call Waiting – Notifies a calling party that another party is calling when the calling party’s line is utilized. ? Caller ID – Enables a caller to be identified. ? Pagers – Supports the abilities for callers to page subscribers of this service IN services are controlled by the Signaling Number Seven (SS7) protocol in the PSTN. SS7 is a layered protocol and its functionality is encapsulated in its software layers. The ISDN user part (ISUP) layer is Figure 1. An Intelligent Network Architecture used for setting up and tearing down phone calls while the transaction control application part (TCAP) layer is used for exchanging real-time database information. Consequently, the ability to support IN services in the PSTN necessitates both ISUP and TCAP support. Figure 1 illustrates the basic architecture of an IN. Signal switching points (SSPs) are telephone switches that initiate and terminate SS7 messages and signal transfer points (STPs) are devices that route SS7 messages within the network. Service control points (SCPs) are database servers that provide real-time data information for IN services. The PSTN is known for its reliability and quality of service (QoS). Voice and data traffic is carried on dedicated connections (switched circuits) and the entire bandwidth is available during a call. As the utilization of the network increases, it is more likely that users will experience busy signals; however, performance will not degrade since bandwidth for a call is always guaranteed. This fundamental aspect of the PSTN is what distinguishes it from IP networks. The architecture in the PSTN is very reliable since the SS7 protocol provides functionality in the event of link and node failures. In these cases, SS7 chooses alternate routes and/or facilitates message re-transmissions to make sure voice and data reach their destinations. Voice over IP A result of the tremendous growth of the World Wide Web (WWW) the Internet Protocol (IP) has become the de facto standard for data networking [BLAC00]. Unlike the circuit-switched technology of the PSTN, IP networks are packet switched and network bandwidth is shared by all users. A consequence of this is that when the network gets more utilized, it is more likely to experience performance degradation. Although PSTN and IP networks are fundamentally different in terms of their architectures, it is possible for the networks to be integrated; that is, exchanging voice and data traffic. Figure 2 depicts one topology to achieve this integration. To see how this works suppose a subscriber connected to an SSP places a long-distance call though the PSTN. Voice traffic is routed though an intermediary toll SSP and there can be many hops involved when routing a call. The SS7 protocol is utilized to reserve voice trunks from one hop to the next and to set up the phone call. VoIP offers an alternative approach. A call is placed to an IP Telephony switch that digitizes and compresses the voice traffic into data packets and then sends this information across the IP network. SSPs on both ends of the call communicate with their respective IP Telephony switches (IPTS) using ISDN connections that include both voice and signaling information. Calls can be initiated or terminated on either the PSTN or the IP network. On IP networks, users send and receive voice using an IP Telephony STP STP SCP SCP SSP SSP SSP SSP SSP SSP