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[ScreenOS] How to troubleshoot poor VoIP quality issues?

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Article ID: KB22633 KB Last Updated: 16 May 2012Version: 1.0
Summary:
This article provides information on how to troubleshoot poor VoIP quality issues.
Symptoms:
  • Voice quality issues could be due to many factors, including insufficient Internet speed, poor ISP service, wiring, viruses, improper voice packet prioritization, and so on.

  • Solving these issues can seem difficult; especially if they are intermittent.

  • When troubleshooting VoIP quality issues, you need to find out, whether or not, the bad quality of the call is caused by the firewall.

  • There are very few reasons, for which the firewall is responsible for poor voice quality.
Cause:

Solution:
When troubleshooting, you have to check the following logs in the firewall:

  • High CPU: If the CPU usage is high, it can drop the packets, which can cause poor quality.

  • Bandwidth: VoIP quality issues can be caused by a lack of available bandwidth. Ensure that the bandwidth provided by the ISP is enough to handle good quality call. The standard G.711 audio codec uses 64 Kbps.

  • UDP packet fragmentation: Check if the received UDP packets are fragmented. If fragmented packets are received, check whether free FCB is available or not, by using the get session frag command. Another case is the firewall fragmenting the traffic and then sending it out.

  • Packet Loss: Packet Loss can occur for a variety of reasons, including link failure, high levels of congestion (which lead to buffer overflow in routers), Random Early Detection (RED), Ethernet problems, and the occasional misrouted packet.

    A bad or noisy Ethernet cable can cause high rates of corrupted packets. This results in high rates of FCS and Alignment errors being reported by network interface cards or analyzers. High rates of frame error can often be caused by Duplex Mismatch. Late collisions are symptomatic of an excessively long Ethernet segment; resulting in too much delay, which impacts the collision detection process.

  • Link failure: This typically appears as a period of consecutive packet loss, which can last for many seconds, followed by a change in delay after the link is re-established. Link failures can be caused by equipment issues (for example, a failed blade in a switch or router, power failure, and so on), a cable being unplugged or cut, a configuration change in the transport network, or potentially a denial of service attack.

    Routers are generally intelligent enough to recognize a link failure and find an alternate route. Link failure will result in significant gaps in received speech. It is unlikely that link failures will occur frequently; however they could potentially last for several seconds.
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