Simulate voice networks

-February 01, 2005

Read other articles from this issue: Table of contents, February 2005
Economies of scale, Cover story
Test-system development:
   Do everything first

Vision meets motion
Simulate voice networks


A complete Voice over Internet Protocol (VoIP) network deployed by communications broadband service providers (BSPs) that sell VoIP subscriptions to small and medium-size businesses. The VoIP network consists of a core that houses application software and one or more point-of-presence (POP) interfaces that connect each subscriber office to the public service telephone network (PSTN). The network replaces an end user's private branch exchange (PBX) telephone network.


Simulate traffic on both PSTN and IP networks and perform full functional testing of the IP voice network. Simulated traffic includes voice payloads and signaling for thousands of calls. Test each network component in the presence of packet jitter and packet delays.


  • Empirix PSTN network emulator, IP network emulator, and voice-quality tester.

  • Shunra Software network-emulation software.

  • In-house-built service gateway simulators and terminal simulators.


Voxpath Networks ( manufactures VoIP gateways, phones, and complete networks that replace PBXs in small-to-medium-sized businesses. Businesses that use the equipment subscribe to VoIP phone services from a broadband service provider (BSP).

Each BSP purchases a core that contains an application server, which stores a subscriber's VoIP network applications such as phone numbers and voice mail, and a signaling system 7 (SS7) gateway, which provides signaling between IP networks and the public service telephone network (PSTN).

Each BSP needs one core and at least one point-of-presence (POP) interface. A POP consists of a PSTN gateway, which interfaces a subscriber's IP network to the PSTN. The subscriber's LAN, which includes VoIP, has a gateway that connects it to a POP through the BSP's access network—Ethernet, cable modem, DSL, or dedicated T1 line.

A test bed simulates traditional telephony (PSTN) and IP networks for testing a VoIP network installation.

To test a VoIP network, Voxpath engineers must emulate a PSTN on one side of the core and emulate POPs and a LAN on the other side (see the figure). Led by CTO Alaric Silveira, the engineers test a network's core and POPs as though they were a single device. A PSTN emulator simulates a PSTN Class 5 switch. A voice-quality tester monitors the voice calls and evaluates voice quality. Voice from the PSTN simulator passes through the POP, which converts the voice into IP packets for routing over the IP network. The core handles the SS7 signaling required to set up and tear down each simulated phone call, and the POP handles the voice between the two networks.

To simulate the BSP's networks, Voxpath uses software from Shunra, which introduces delays and packet jitter. The router and phones simulate a subscriber's business. Because it's impractical to test the network with the 40–50 POPs, hundreds of service gateways, and thousands of phones that it might use, Voxpath uses emulators to emulate phones and service gateways.

Engineers begin their tests by manually making calls. If the network passes this functional test, engineers then test the network under load conditions with an Empirix network emulator that generates and terminates up to 15,000 VoIP calls at up to 200 calls/s. An automated test will consist of about 10,000 off-hook/on-hook calls to test the SS7 signaling of the core. The test also exercises the IP gateway and IP phones. Then, the network emulator will generate another 10,000 calls, this time with simulated voice, to test the POP.


The PSTN simulator keeps a log of dropped calls. Using statistics, the engineers look for exceptions to completed calls so they can determine why a call failed. The test bed lets the engineers produce automated tests that record voice quality and keep statistics on network reliability.

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