VoIP Quality of Service: VoIP QoS Defined

What is VoIP Quality of Service?

As Canadian businesses have rolled out VoIP Solutions the end-user experience has become more stable as quality goals have been adopted by this industry to assist in the design and operation of great VoIP deployments.  Recently better VoIP engineering and capacity investments have driven quality gains. Still, the VoIP administrator with his/her service partner still needs to work together on some key measurements that enable optimum user satisfaction.

The following reasons can negate satisfaction at even the best quality VoIP Service.

  • WAN Bandwidth allocated per expected voice session
  • Lack of capacity at the ISP where voice may compete with data traffic
  • VoIP Codec selected at the IP PBX
  • Outdated firmware at the LAN, WAN, PBX and phone sets
  • Latency is the delay across the end to end IP path
  • Jitter being the regularity of packet arrival times at the destination
  • Packet Loss – the dropping and subsequent retransmission of sent data

Quality of Service VoIP vs. Data Traffic

The TCP/IP protocol was first built with only data traffic in mind.  It was perfectly acceptable that packets for an email file and attachment arrive at the destination out of order due to routing.  The finished file does not get released until 100% of the file arrives.  For voice traffic packets arriving late or out of order is a catastrophe as conversations will cut out and video will be choppy. For this reason, the educated consumer should be aware of these parameters while selecting VoIP partners.

  • Availability is expressed as a percentage of time the network is available to the users for full service without degradation. The Availability score is often provided by the service provider per site or per network solution and could be a component of a SLA.  Penalties do not apply during scheduled downtime or while troubleshooting time is deferred by the end-user.
  • Latency is expressed in milliseconds (ms). It is the time it takes to send data and VoIP packets from source IP to the final destination.  Generally, round-trip way latency is less than 90 ms across the nation and less than 50 ms within a region of Canada.  It is usually determined by the distance traveled, the number of internet jumps, engineering goals, and capacity management by the ISP.
  • Packet Loss is expressed as a percentage of the total packets that arrive at the destination IP address compared to the total packets sent from the source. Packet Loss is usually less than 1% for best quality VoIP service although up to 3% may be tolerated in some environments without the user detecting an issue.  Loss of packets due to congestion or mismatched components cause re-transmission and the appearance of unacceptable latency.

VoIP QoS Defined

Jitter is the variations of streaming packets arrival times. Voice and Video requires low and steady jitter tolerance for the user to avoid VoIP dead air during speaking or choppy video reception. It is expressed in milliseconds. Generally, a constant score of 30ms or much less will allow jitter buffers at both ends of the VoIP call to normalize traffic for human consumption.

VoIP Quality of Service Tests

Obviously, the Network and VoIP Administrator in every organization needs to be armed with VoIP Quality of Service measurements during normal operations and during times of quality issues. Free tests described below are provided by the Internet Service Provider, the VoIP Provider, a third-party provider, or come from your internal investments in Network Monitoring Tools.  Internet Speed Testing may help isolate if new quality issues are due to a slow network, IP-PBX Issues, LAN issues, or database/data issues that appear to congest or traffic that impacts VoIP traffic.

  • Internet Service Provider Tests. Major Telecom players (Rogers, Bell, Telus, Cogeco) will supply a simple ‘Internet Speed Test’ that usually consisting of Latency (ms), separate Upload and Download speeds in Mbps with Jitter results. The tests will identify your current internet provider based on the endpoint IP address and test point chosen in your region.
  • 3rd Party Testing. The next tier of Regional ISPs and VoIP Providers reviewed in GoneVoIP.ca may also provide VoIP Quality of Service Testing or an ‘Internet Speed Test’ from its public pages or proprietary service portals. Examples are speedtest.net or fast.com
  • Internal Network Monitoring. If the Quality of Service VoIP is of critical importance, the best way to gain the most knowledge about current QoS is to instrument your data and voice infrastructure yourself.  Additionally, you can constantly test the network with traffic generators inserting simulated VoIP calls with test web surfing and data downloading.  These large investments in time and capital and fit only medium and large organizations with critical network mandates.  Free Monitoring software can be found although be aware that you may need full control of the onsite internet routers or also install specific probes in the network.

Managed Network & UC Services. An organizational preference may be to fully outsource the WAN solution and also the VoIP services down to the desktop business phones.  In this case, the Quality of Service VoIP metrics may be supplied you constantly at a service portal, in monthly SLA Reporting, or upon request during joint trouble-shooting.

Quality of Experience – The Mean Opinion Score (MOS)

Perhaps the best way to pinpoint the overall Quality of Service VoIP is to follow the industry’s Quality of Experience (QoE) measurement called the Mean Opinion Score (MOS).  It is a complex arithmetic computation encompassing Jitter, Latency, the bandwidth assigned to each session, and other delays within hardware packet assembly/disassembly. It will attempt to score the quality of the voice reception as subjectively perceived by the human user.  It is a score from 1 to 5 where 5.0 is the highest quality.  A MOS score of 5 is comparable to speaking directly to the other person who is close to you in the same room.  It is unobtainable by current VoIP solutions who are restrained by the individual CODEC limitations used at the IP-PBX and bandwidth assigned.  For comparison, a MOS score of 4 is considered the top end for today’s cell phones.  Less than a 3.6 score is not considered business-grade VoIP quality.

Generally Accepted Industry Mean Opinion Score Ratings (MOS) for VoIP QoE


High-Level Assessment

Granular Assessment


Perfect Quality – Like a face to face conversation

Theoretically not obtainable


Excellent – Business Grade

4.1 & up – very satisfied


Fair to Very Good – Some dissatisfaction

3.5 to 4.0 – most are satisfied


Bad – Annoying quality

2.7 to 3.4 – some dissatisfaction


Unacceptable VoIP or Video quality

1.0 to 2.6 – No satisfaction

Not long ago panels of humans subjectively assigned the VoIP MOS based on actual joint hearing tests. Today the industry injects a synthetic test from routers or switches into the data stream and makes a calculated value based on estimated quality.

VoIP Codecs that use compression techniques will always consume less bandwidth. Conversely, codecs that use compression will lower the voice clarity and introduce some delay. Today G711 codec will top out given near-ideal network conditions at 4.5 MOS. It gives the best quality VoIP Service for more latent connections as the compressed codec’s MOS scores could degrade quickly if latency increases.

Key Takeaways and Action Items

  • Know your baseline performance – Use Free Internet Speed Test to know your latency(s) & jitter. Use that information of a ‘good day’s performance’ to compare a ‘bad day’ of issues.
  • Invest in Network Monitoring – If budgets and time allow set up a Network Management System to constantly test and monitor the Quality of Service your users’ experience. Set threshold alarms and historical reports to managed & prove quality results.
  • Work with your VoIP Provider – Determine if your current network is ready for VoIP traffic or what infrastructure improvements are required to do deliver quality on a large scale
  • Don’t throw bandwidth at every new problem – There may be a miss-configuration or an ISP issue that prevents optimum performance. Keep the firmware updated in the IP-PBX and edge routers if they are in your control.  Re-visit your baseline design assumptions before ordering the ‘quick answer’ of more bandwidth.  You may be masking some hidden faults.
  • Set Clear Demarcations with your Partners – Know where responsibilities start and end with ownership of the overall VoIP Service, and LAN/WAN health. Avoid finger-pointing in times of crisis when trouble-shooting mysterious VoIP quality issues.
  • Consider a fully managed VoIP Solution – In this case, almost all measurements and quality commitments can come from the partner if they contractually agree. Obtain an SLA that includes QoS metrics if your voice system is mission-critical your business goals.

Today the expectation of clear and complete calls are a reality in today’s VoIP industry.  The informed administrator still needs to conduct or consume VoIP Quality of Service Testing to prove results, work with partners, and roll out major VoIP solutions with confidence.