VoIP terminology
This section describes common VoIP terminology. If you are new to VoIP networking or just need more information about a certain term, this section may help.
Table 31. VoIP terminology
R-factor and MOS
These are overall quality ratings, based on the E-model, which take into account network conditions, equipment ratings, and other variables to come up with an objective Quality score.
The R-factor is a scale from 0-100; MOS ranges from 1 to 5. In both cases, higher readings indicate better quality.
The R-factor is more useful for live, real-time assessment of what users are experiencing. Unavoidable degradation means that 93.2 is the highest reading you will see on an actual VoIP call; scores below 80 typically result in dissatisfied users.
The MOS (Mean Opinion Score) measures how a user would assess quality, from 1 (poor) to 5 (excellent). Although it is also useful as a real-time measure of VoIP health, it is especially useful for pre-deployment tests where you compare the MOS scores of call data from both ends of various connections to identify and resolve bottlenecks. Scores of 3.5 or less typically result in dissatisfied users.
Using computer networks to transmit and reproduce sound requires a steady, predictable stream of packets to arrive at the receiving devices. Jitter is the variability in arrival time, too much of which can degrade call quality.
Understanding jitter can help you improve overall call quality by adjusting jitter buffers or providing more bandwidth through QoS prioritization or other mechanisms.
Bursts and Gaps of packet loss
Packets can get dropped for many reasons on a network, some more serious than others. For example, a temporary spike in bandwidth utilization causing a few packets to get dropped is usually not a problem, as VoIP equipment is designed to fill in the missing data. Following long-standing conventions of telephony periods where packet loss is minimal are called gaps.
In contrast, burst periods (i.e., periods when a high percentage of packets are being lost) usually does degrade call quality, and may point to more serious problems.
Density refers to the rate of packet loss during bursts and gaps.
Understanding the density and duration of bursts and gaps can help you quickly respond to (and sometimes prevent) voice degradation on the VoIP network. For example, an extremely high burst density (20% or more) coupled with extended burst duration times (more than a second or two) can suggest problems with hardware either failing or being completely overwhelmed by traffic.
Gap densities climbing over time, coupled with low-density, short-duration “burstiness” can mean the VoIP network is attempting to service too many calls given the available bandwidth.
Also called a Type of Service (ToS) or Precedence, the QoS bit is part of the TCP header that certain routers and switches recognize so they can prioritize traffic according to what particular kinds of applications require. VoIP typically requires the highest level of priority.
Incorrectly set QoS can lead to contention of VoIP and other data on a network.
Contention can cause VoIP jitter and packet loss, leading to poor voice quality and dissatisfied users.
Codec is an abbreviation for Coder/Decoder, referring to the algorithm used to convert the analog voice signal into packets on the network, and back again.
Different codecs use different sampling rates to implement different levels of compression. Lower sampling rates can compromise call quality, although sometimes a lower sampling rate can reduce contention and prevent worse degradation. Here are some of the more common codecs and their sampling rates:
G.711: 64kbps (no compression)
G.729: 8kbps
G.723: 6.3kbps, 5.3kbps
The E-Model is based on conditions, equipment, and expectations at your site. Some of the values (Send and Receive Loudness Ratings, for example) are functions of your phone specifications. Others (Room Noise levels, for example), are functions of ambient conditions at your site.
To fine-tune the E-model to exactly match the conditions at any particular site is an involved process that requires test equipment and a thorough understanding of the E-model. See the ITU G.107 specification for a more detailed discussion of the E-model.
R-Factor and Mean Opinion Scores (MOS) are derived from the E-Model. The E-Model is based on ITU-T Recommendation G.107, and takes into account the many types of network impairments that can affect voice quality. VoIP, unlike some other network applications, is quite sensitive to packet delay and jitter.
To prevent network conditions from affecting voice quality, many VoIP deployments use Quality of Service (QoS) prioritization so that VoIP traffic is favored over other less delay-sensitive traffic. Even with QoS, a saturated network can mean poor voice quality, which will be reflected in lower R-factor and Mean Opinion Score (MOS) readings. In most deployments, users will start to complain about quality when MOS scores fall below 3.5 and R-Factor scores fall below 80 or so.