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• Quality of Service or QoS is the quality of a call over a network. It also refers to the ability to prioritize certain types of traffic on an IP network. In the case of VoIP, this typically means prioritizing voice traffic at a higher level than other forms of traffic such as data so that voice traffic will not be delayed or dropped. Most QoS solutions focus on either resource reservation or resource prioritization.
  
  Acceptable Connections:
  Jitter: < 5ms
  Packet Loss: < 0.05%
  MOS Score: > 4.0
  
  Jitter:
  VoIP jitter occurs when voice packets are sent and received with timing variations. Jitter is effectively a variation of packet delay where delays actually impact the quality of the conversation. Think of jitter as variable delays in packet delivery. Participants will notice delays in the conversations impacted by jitter.
  
  Packet loss:
  VoIP packet loss takes place when a large amount of traffic hits the network and causes it to drop packets. It usually manifest itself as dropped conversations or “tinny” sounds. Packet loss should never exceed 1% and most service providers guarantee service levels with .5% or less packet loss. Packet loss of 1% translates into one voice clip or skip every three minutes, while packet loss of .25% translates into one error every 53 minutes.
  Packet loss can cause latency or delays in packet delivery. Physical distance, the number of router hops, encryption, and voice/data conversion all impact latency. Users begin noticing latency as a service level issue when roundtrip latency is greater than 250 milliseconds (ms). The International Telecommunications Union recommends that latency never exceed 150 ms one way (from speaker to listener).
  
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• Mean Opinion Score:
  MOS is expressed in one number, from 1 to 5, 1 being the worst and 5 the best. MOS is quite subjective, as it is based figures that result from what is perceived by people during tests.
  
• Taken in whole numbers, the numbers are quite easy to grade.
  5- Perfect. Like face-to-face conversation or radio reception.
  4- Fair. Imperfections can be perceived, but sound still clear. This is (supposedly) the range for cell phones.
  3- Annoying.
  2- Very annoying. Nearly impossible to communicate.
  1- Impossible to communicate
  
  The values do not need to be whole numbers. Certain thresholds and limits are often expressed in decimal values from this MOS spectrum. For instance, a value of 4.0 to 4.5 is referred to as toll-quality and causes complete satisfaction. Values dropping below 3.5 are termed unacceptable by many users.
  
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• Summary of QoS:
• To effectively manage both, administrators should focus on reducing delay at the network endpoint and prioritizing traffic over the network. Prioritizing VoIP traffic over the network at Layers 2 and 3 also yields latency and jitter elimination. Various solutions are available for VoIP QoS which include a root cause analysis of individual switches, routers and bandwidth restrictions along the network path.
  
  The Most Effective Site QoS Measures:
  1- Appropriate Bandwidth allocation (number of extensions to required Bandwidth)
  2- Firewall/Router processing speed (more expensive/brand name is not always better)
  3- Policy based Network Management via the site Firewall/Router
  4- Allocation of 1 Gig Switch (if required by the number of agent seats)