Voice over Internet Protocol (VoIP)

Voice over Internet Protocol (VoIP) has made great strides in recent years, and VoIP networks, public and private, are becoming more and more common. However, IP, by its very nature, is an unreliable networking protocol. In its most basic form, IP makes no delivery, reliability, flow control, or error recovery guarantees. As a result, IP may have changing delays, packet loss, or deliver packets out of order. When attempting to understand and predict the VoIP user experience in pre-deployment planning requires that real world and worst-case network conditions be considered. The PacketStorm family of network emulators addresses these concerns in three ways: product evaluationpre-deployment testing, and troubleshooting.

The above figure illustrates a typical carrier class VoIP system. A traditional phone system and PBX resides on the customer premise, connected to the PSTN with standard analog or T-1 tie lines. However, once the call is delivered to the PSTN, rather than using traditional circuit switching to connect the call to the distant end, the media gateway is responsible for packetizing the voice into RTP data packets, while the signaling gateways and/or softswitches are responsible for translating phone numbers and IP addresses back and forth and routing the call properly. Impairments to the signaling information can cause call set up and tear down problems. Impairments to the RTP voice packets cause voice quality degradation.

Product evaluation: 
When choosing a vendor for VoIP network gear deployment, understanding how that equipment will perform is critical. These devices should undergo vigorous testing and evaluation in a pre-deployment environment. This is achieved by using a PacketStorm system to emulate the conditions in the “network cloud”. Voice gateways, Voice gateway controllers, IP-PBX’s, SIP proxy servers, etc should be tested for the resilience to impaired network conditions. Voice quality can be assessed by inducing dropped packetspacket delays, network jitter, creating duplicate packets, reordered packets, etc, and then observing and measuring the voice quality. Call set up and control devices should also be tested to evaluate their ability to work reliably under these adverse network conditions. Multiple vendors’ equipment should be compared side by side for quality and reliablilty in consistent, repeatable, network test conditions to gauge their performance prior to selection. Negative network conditions should be steadily applied until a product’s ‘breaking point’ is determined. In addition to VoIP specific products, core network devices should also be tested for their QoS capability to ensure that they will be able to prioritize VoIP traffic.

Pre-Deployment testing:
 PacketStorm Communications recommends that VoIP, like any network application, undergoes a pre-deployment testing phase prior to being placed into production. PacketStorm units can be equipped with multiple physical interfaces. Network topologies can be accurately recreated, allowing multiple sites and endpoint routes to be emulated. Individual, or group routes, can be uniquely modeled. Each link can be assigned the specific bandwidth that is used in the real network and independent impairment characteristics. This type of network emulation recreates the characteristics of your unique network topology, observes VoIP performance, and allows for network changes to be tuned for performance before the application is deployed into the production environment.

 PacketStorm units have the capability to capture and save actual network impairment characteristics. The PacketStorm ‘network capture’ feature launches test packets to a remote site and measure the characteristics of that link. These real world impairments are then stored as an impairment set that can be reused in a lab environment. These captured impairments are implemented by impairing traffic on a packet by packet basis so that any real network impairments that may vary due to time or event, are captured and replayed. In addition, the PacketStorm emulator can capture and replay live data to provide generation and impairments while not connected to the production network.

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