What Ever Happened to QoS?
(continued)

A similar scale is used in the telephone industry. A high-quality voice call receives a rating of 5, whereas a very low-quality call receives a rating of 1. The official name for this scaling method is the Mean Opinion Score (MOS). 

Figure 1:  Mean Opinion Score (MOS) Scale

Telephone companies have used this scale for years as the determinant for toll-grade calls.  If the MOS rating is above 4.0, then a call is said to be toll grade.  The datacom industry has not used such a rating system because data communication was never intended to be real time.  In data communication, three measurements are used to determine the quality of service. These measurements are dropped packets, jitter, and latency.

When sending data, the basic transport vehicle of IP is not reliable.  Packets can be, lost dropped, or never delivered for several reasons – especially when the network gets busy.  Typically, there is a direct correlation between network utilization and the percentage of dropped packets -- as network utilization increases, so does the percentage of dropped packets.

See a large view of this figure

Figure 2: Dropped Packets vs. Network Utilization

The human ear is a relatively forgiving instrument that can tolerate some percentage of packet drops without noticing a drop in performance or MOS score.  However, when the percentage of dropped packets increases, syllables or whole words are lost from sentences.  When the number of dropped packets starts to exceed 1.5%, there is a perceivable performance change.  The percentage of dropped packets is a major contributor to a poor MOS reading.

Measuring QoS in Data Networks

In addition to packet drops, the other two QoS measurements that must be considered are jitter and latency. Combined these elements are experienced by the end user as a transmission delay.

Latency is the amount of time that it takes a signal to move from point A to point B in a system under test (SUT) with no load conditions.  Figure 3 shows a SUT (System Under Test) in which the signals applied to input A are sent through the SUT to the output B.  The leading edge of A and B are compared and the delta measurement becomes the latency.  Since latency is consistent, test measurements should be repeatable.

Figure 3: Latency Measurements

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