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December 2007 | Volume 10/ Number 12
Tech Score

IP Telephony in a “Virtual World” – Part II

In this issue we resume the topic of IP Telephony in a Virtual Environment (VE). The primary benefits of the Virtual telephony application are lower Total Cost of Ownership (TCO), improved service delivery, and better efficiency. The one area we need to probe a little more in detail is the performance characteristics of the application in a virtual environment.

As IT and telecom departments implement IP based PBX solutions, the need to operate in a virtual environment is important. A critical performance concern is maintaining high levels of Quality of Service (QoS) for voice and video. The latency introduced on the disk and network I/O by the hypervisor will have an impact on the QoS. One company that is doing early research in this area in conjunction with a local university is Network Engines (NENG) based in Canton, MA ( We are exploring the Hypervisor Tax and its effects on the disk, network, CPU, and context switching in the Guest OS, stated Kevin Murphy, CTO at Network Engines.

While the Hypervisor [HV] touts little overhead, it's naïve to believe it's not visible. The additional six clicks added to a typical single application call sequence [see Figure 1] will increase the time to fully process the event round trip, said Murphy. Even if the Hypervisor and DOM 0 are only as slow as the native driver, the time is still effectively doubled.

Finally, put all of this on a consolidated system with multiple guest operating systems and there is even more contention for your I/O bandwidth, said Murphy. Remember, virtualization is targeted at those applications that are only using 20% or less of your resources. What they don't tell you is that when applications peak, there is impact to the I/O on the physical system. Bus speeds are fast, memory is abundant, and there are lots of CPU cycles available, but if you want high efficiency, they have to work in concert for mission-critical applications. To date there is no real dynamic policy management for physical resources with a hypervisor. Call it the new QoS....

Early test results running call load testing on an Asterisk-based server shows that the memory/ CPU/NIC intensive process of setting up calls is impacted by a hypervisor with no other resource contention. This supports the theory that while virtualization is here, it's currently themed around the data center and consolidating business applications. The mission critical/high I/O applications like multimedia, unified communications, security, and storage require predictable and manageable performance which is not yet present in the virtual environment.

Another company just beginning to explore the affects of virtualization in the telephony space is Aculab ( Aculab provides VoIP Service Providers and Telecom Equipment Manufacturers (TEMs) their virtualization-ready Prosody S product, which is a comprehensive service deployment platform (SDP). According to Herman Abel, Aculab Product Manager, We have completed functional testing in a virtual environment. We believe that operating in this environment will allow better scheduling of tasks that are not hardware-dependent and will subsequently improve overall performance, but more testing is required.

Final Score

As 2007 draws to a close, we have seen some very significant industry shifts this year that are impacting the telephony market and these will continue into the coming year. The continued rise in server sprawls and overall power consumption is driving the need for virtualization and Software as a Service (SaaS). Small to medium-sized businesses will continue to seek more on-demand voice, video, and data services without committing precious working capital to get it. The demand for on-site, low-cost dedicated telephony boxes will diminish and application developers must focus their attention on performance and scalability in a virtual environment to provide these on-demand services. If the new QoS can be solved, mission critical applications will abound in the virtual world.

Jeff Hudgins is VP of Engineering at Alliance Systems (recently acquired by Network Engines). For more information, visit the company online at

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