Many telcos have been using Least Cost Routing (LCR) systems for several years now, either in the form of stand-alone applications, or as an adjunct to their interconnect billing systems. These LCR systems have generally been very effective in helping the telcos to push down their interconnect costs, but operators are increasingly finding that to some extent you get what you pay for: cheap interconnect rates often equate to poor call quality.

In theory, modern digital telecommunications networks have the potential to offer near perfect speech quality on every call. However, in real life the performance of a voice telephony call is often degraded from the theoretical ideal because of cost cutting measures taken by some carriers in an attempt to squeeze additional traffic carrying capacity out of their expensive international circuits.

For TDM networks, this might include the use of satellite links and/or Digital Circuit Multiplication Equipment (DCME), which is designed to interleave four or more calls onto a single voice channel. The use of these techniques can often lead to degradation in call quality, manifested by excessive echo and clipped or distorted speech. For VoIP networks, the potential for quality degradation is if anything greater than for traditional TDM networks, with excessive delay or dropped speech being common symptoms of poorly implemented VoIP interconnects.

So telcos need to tread carefully when selecting carriers that offer interconnect rates that seem to be too good to be true. This is where the use of a good Routing Optimisation (RO) system can be of considerable benefit to the commercial routing manager. A fully featured RO system will make recommendations for configuring a telco’s switch routing tables based not only on the selection of low cost carriers, but also by taking into account the quality performance of those carriers.

Sources of quality data

Information about call quality can potentially be gathered from a number of sources, including network probes that monitor the signalling channel (or a sample of the speech channels) on interconnect trunks, test call systems, or from analysis of switch CDRs. Many operators feel unable to justify the considerable cost of the hardware associated with the first two of these options, and hence tend to rely on CDR analysis as their main source of call quality data. Depending on the switch type, and the number of data fields that have been defined in the switch CDR, it is usually possible to extract a significant amount of useful quality data from CDR analysis:

  • Answer Seizure Ratio (ASR) – this is the most commonly encountered ‘quality’ measure used by operators when comparing the respective call quality offered by different carriers. The simple (though not strictly accurate) definition is the proportion of successfully connected calls as a percentage of total call attempts to a particular destination.
  • Network Effectiveness Ratio (NER) – this is similar to ASR, but in principle much more relevant as a measure of network quality, since it measures the percentage of call failures that can be ascribed to network problems, whereas ASR includes call failures that may simply be due to the B-end subscriber not answering. In order to calculate NER, it is necessary for the CDR to include a data field that contains the call clear cause code.
  • Mean Holding Time (MHT)/ Average Duration of Call (ADC) – average call duration can be a useful substitute for more sophisticated call quality measures, based on the assumption that call durations will be lower if the quality of the call is poor. This measure can be derived directly from the call duration information contained within each CDR.
  • Short Call Percentage – this measure is similar in principle to MHT/ADC. Counting the percentage of short duration calls, where the duration is user definable (e.g. <10 seconds), gives an indication of calls that terminate early, possibly due to quality problems.

Quality optimisation

Each RO system will have its own algorithms for including quality criteria in the carrier selection process. Some of the more commonly encountered methodologies include:

  • Threshold cut-off – with this methodology, the quality manager defines a minimum acceptable quality level for each telephony product or service they are offering, and for each call destination. The RO system will compare the actual quality levels being achieved by each carrier, based on analysis of historical CDR data, against the threshold values set up in the system. If a carrier’s call quality to a particular destination falls below the defined threshold level then that carrier will be excluded from the set of carriers that are selected for the final routing table.
  • Price modulation – this technique is more subtle than the yes/no outcome of the threshold method. It involves modifying the price offered by each carrier according to the quality offered by that carrier in comparison to a benchmark level. For quality that exceeds the benchmark level, the effective cost used for ranking carriers in the routing table will be decreased. Conversely, for carriers whose quality is below the benchmark level, their effective cost will be increased in proportion to the difference between the benchmark and the measured quality value. The outcome is that poorly performing carriers are not necessarily excluded from the final routing table, but will be ranked lower than a carrier with a similar price but better quality.
  • Figure of Merit (FoM) – this concept is an elaboration of the basic cost modulation methodology described above that is particularly applicable when it is necessary to take into account a number of different quality measures in the final carrier selection process. The figure of merit is a single measure of carrier call quality on a 1-5 scale that combines several individual quality measures into an overall quality value. The process converts raw quality data into a FoM value, taking into account the subjective impact of each call quality parameter on a caller’s perception of overall call quality. The final FoM value is used to modulate the effective routing rate for each carrier, as described above.


Telcos are increasingly turning to RO systems to manage the quality of their international interconnects. Used intelligently, these systems allow the operator to achieve lower interconnect costs without unduly sacrificing call quality, and allows them to deliver against the quality undertakings that they have made to their customers.