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By: Aivis Olsteins

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VoIP Routing Part 3: Quality Based Routing

Another important Routing method based on quality characteristics of the Route

Another important routing mode is Quality Based Routing. In the method the route is selected based on the best performance from historic data. In the case of Voice telephony, the quality of an individual call can be derived from one or several parameters such as:

  • ASR (answer to seizure ratio) – percentage of answered calls vs. all call attempts made. Higher ASR is considered an indication of good route. There are many reasons why the calls are not answered, related to the users, for example: user is busy, user did not pick the phone, phone out of coverage etc. These usually make up fixed percentage regardless of which providers route call takes, and may be up to 40% of all calls (ASR would be 60% in this example). In the ideal case the rest of calls will be answered regardless of which route was chosen. However, there are factors which reduce ASR beyond these numbers, like insufficient capacity, temporary network issues, which vary from provider to provider. That makes ASR highly variable between providers. Note, that ASR also depends on the destination dialed.
  • ACD (average call duration) – duration of the call when answered. It is considered an indication of quality: very low ASR values indicate that the conversation was probably unintelligible, and users hung up early. Longer ASR would indicate that voice channel was clear from noise and interruptions, and conversation proceeded to the maximum desired duration.
  • PDD (post dial delay) – secondary quality factor. Indicates time between the moment caller party completed the dialing and moment he/she starts to hear ring-back tone. In real terms, it is silence period until you hear the rings on the other side. Shortest PDD indicates better quality. PDD is not completely independent quality parameter: imagine situation where after very long silence after dialing the number, user gives up waiting for signal and hangs up before other side starts to ring: in this case PDD actually affects ASR, route quality can be assessed from ASR.

The quality parameters all are very client and destination- specific. We can observe different ASR and ACD rates for the same destination for different clients. Example take client1 which is residential caller, and client2 which is outbound sales call center. Many people are reluctant to answer phone calls from unknown callers, so in case with client2, the ASR rate will be lower. Also learning that call is from call center which tries to sell something, many just hang up, therefore also ACD will be lower for client2 than client1.

While calculating quality, several considerations should be taken in account.

First, the data should be recent. Network conditions vary rapidly, and the ASR values from yesterday may not be true today. Therefore system should be able to use quality parameters from the most recent calls, within last minutes or so.

And secondly, it should have sufficient data. If the specific destination receives only few calls per hours, they do not give enough samples to estimate realistic quality characteristics. Therefore a minimum of few hundred recent calls per destination are necessary for quality based routing to function efficiently.

To sum up, the system should have at least two configurable parameters for quality data collection: the maximum age of the records, and minimum number of records to use.

Some systems, especially VoIP, additionally may use more parameters to collect and estimate quality, such as:

  • number of packets received,
  • number of packets sent,
  • number of octets received,
  • number of octets sent.

Low number of packets might indicate problems with voice audio, and large difference in packets/octets received vs sent might indicate problems with one-way audio. On the other hand, these parameters cannot be trusted always: there are cases, especially with automated response systems where audio is one way by definition.

For more straightforward implementation, we would need to agree to one parameter, which would characterize the overall quality of the route. One of the methods used in many systems is to take product of ASR and ACD, since increase of both of these numbers theoretically implies better quality. Therefore Quality Index:


As higher the index, the better is the given route.






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About Author
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My name is Aivis Olsteins and I am owner of DataTechLabs. My experience in Telecoms started in early 1990's and I have worked in multiple technical positions in mobile, messaging and data networks. My expertise lies in telecom networks, database systems, distributed processing and large data analysis. These posts are my attempt to share my knowledge with everyone who might find it useful.

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