A growing number of Service Level Agreements (SLA) between customers and Communication Service Providers (CSPs) are being expressed in terms of percentiles. This practice gives CSPs the opportunity to both refine the SLA collaboratively with their customer and also to monetize performance information obtained from their network. It also enables customer to choose from a wider pool of SLAs and adapt them to their specific needs. Some operators even pay penalties if they fail to meet the conditions of the SLA.
What are SLA percentiles and how are they used?
In the network environment, a percentile is a measurable percentage of performance either above or below an agreed level. Percentile measurements are commonly performed over an agreed interval, to ensure they adequately represent the performance of all the packets delivered over that network. A good example is packet delay. Here a SLA might specify that 95% of the packets need to be delivered within 10ms, providing crystal clarity over when a SLA breach occurs.
Calculating percentiles over a measurement interval, however, is resource intensive and, as a result, can prove cost prohibitive. One elegant way to bring the cost down is to obtain percentile measurements using Measurement Bins. A Bin is a kind of counter that increases every time a measured value falls within pre-determined boundaries. In their simplest form, two kinds of Bins can be defined: one for the measurement interval from zero to a threshold value, and a second from this threshold value upwards, infinitely. By just counting the number of values that fall within the first interval or Bin and dividing this value by the total number of values in the measurement interval, one can directly obtain the percentile of measurements that fall within the SLA. Performing this task is, comparably, very efficient, making it easy to verify whether or not the contractual SLA has been honoured.
Extend the Bin concept to multiple Bins, each with their own measurement interval, is both possible and useful, for several reasons. Firstly it makes it possible to measure against multiple SLA thresholds simultaneously. CSPs might want to be able to propose multi-level of SLAs based on the quality of the service or the geographical distance of the network. The use of multiple Bins streamlines the way measurements are done and provides more flexibility in the way the obtained data can be exploited. Secondly, multiple Bins provide better visibility of trends in the network; properly defined Bins provide greater insight into the performance variations across different parts of the network, using different measurement intervals. Usefully, this approach provides solid performance indicators and highlights deviations which might be indicative of future problems. In this way, Bins can also be used as an early warning system that can alert operators to performance degradation prior to an SLA breach occurring.
The deep dive: multiple Bin implementation
EKINOPS implemented the use of Bins in its OneAccess L2 product range to enable more measurement parameters to be expressed in terms of percentiles, beyond minimum, maximum and average values. Here, ten Bins can now be individually configured, including equal-sized Bins based on a minimum and maximum values. Measurement Bins are typically applicable for one-way or two-way delay-related parameters such as Frame Delay (FD), Frame Delay Range (FDR) or Inter-Frame Delay Variation (IFDV). In the figure below you can see how the different measurement Bins are defined as a function of their minimum delay and the next minimum delay of the next measurement Bin. The first Bin starts at 0 delay and the last Bin ends at ꝏ (in practice a timeout which equals approximately the measurement interval divided by the number of measurement samples (N) in the interval) (see figure below).
More details about the use of Measurement Bins can be found in recommendation MEF35.1, describing an Implementation Agreement for Service OAM Performance monitoring and freely available on the public MEF website: www.mef.net.
CSP SLAs: Better than they’ve ever Bin before
CSPs can use all of this new performance information to create a new, premium service for customers, one that provides greater commercial clarity via more detailed visibility into the performance of their contracted services. The end result? A new generation of SLAs that can unlock a new revenue streams for the CSP and also set the tone for a more personalised, equitable and transparent customer relationship.
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