What Is QoS?

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Quality of Service (QoS) is not a new concept. In almost any hotel or restaurant, you will find a Quality of Service survey. Some survey instruments use a rating scale from 1-5, in which 1 equates to very poor service and 5 equates to excellence. This subjective data (soft data) is used to improve Quality of Service and customer satisfaction.

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—see www.ciscoworldmagazine.com/monthly/2000/06/breit_0006.shtml and Figure 8.1).

Figure 8.1: MOS Scale

Telephone companies have used this scale for years as a determinant for toll-grade calls. If the MOS rating for a call exceeds 4.0, then that call is said to be of toll grade. The datacom industry has never used such a rating, because data communication was never intended to operate in real time. In data communications, 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 Figure 8.2).

Figure 8.2: 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 perceptible performance change. The percentage of dropped packets is a major contributor to a poor MOS reading.

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