Is the Phone Working? - Alternative Telephony SLA

Telephony costs are one of the main targets of cost cutting in many large companies. In this effort, the companies are turning to alternative voice providers, who offer much cheaper calls and more flexible services. But, these new operators are using new technologies and are relatively new on the market, so the buyer should approach the alternative telephony service with care and apply proper Service Level Agreement.

What we are used to?
In a traditional telephony, the voice reliability is taken for granted, and all equipment is designed to offer very high availability. Also, capacity is not an issue, since each incoming circuit to a switch is dedicated, and the switching capacity of the Telco Switch is calculated via well known formulae (Erlang models) to provide switching of all initiated calls.

PSTN availability was measured at 99.99% (maximum of 4 minute outage per month, or a total of 52 minutes outage per year!) in 1993 and that number is closing to 99.994%. Compared to this, classical IP data services are struggling with passing the "two point five nines" (99.5%) which is equivalent to 3.6 hours outage per month or nearly 2 days per year.

For all medium to large businesses (especially in operating a retail business) telephony is a "default" service, one that must ALWAYS work, one that is really taken for granted.

The potential challenges with an alternative voice provider
When a company decides to use the services of an alternative telephony provider several issues may appear. The alternative telephony provider may bypass the ILEC operator (Incumbent Local Exchange Carrier) to minimize costs, and quite often, they may arrive at your premises via a data link to attach to the company's PBX. Once we walk into the realm of data transfer, things get much different:

  1. The data link is terminated on a lower reliability active equipment (usually router or L3 switch) - To mimimize costs, this device will not be of a too high class, and it's hardware reliability will be around 98-99%
  2. The data link can be prone to faults on a physical level - alternative telephony operators are not too big on infrastructure protection and want fast deployment, so it can happen that the operator's cable is strung on power lines, placed in central heating ducts under the city, or in extreme examples, are even illegally dug-in in soft ground areas (parks, recreation tracks, green patches) where they are unmarked and easily fall victims to any other construction or renovation activity.
  3. Data links are by default based on best effort technologies - so IP data packet drops, retransmissions and delays can occur.

All this translates to a whole new ballgame in terms of controlling the services offered by your alternative voice service provider.

Establishing proper criteria for service quality

So in order to properly manage the alternativ voice services, one must define what criteria should be measured.

  1. Keep the good old data SLA - this is to control the overall data link quality, which is easiest to measure
  2. Establish measurement on Established, Failed and Dropped calls - via the router infrastructure connecting you to the alternative telephony provider. This measurement will be enabled through vendor specific router functions, most often through syslog event analysis.
  3. Define the guaranteed volume of simultaneous calls that the provider will deliver - measure the delivered volume of calls in terms of comparing the values of established, failed and dropped calls from point 2.
  4. Define and Apply penalties both on overall link quality (point 1) since it will affect all calls, and on volume of realised calls (points 2 and 3) since they relate to actual ability to use the service as contracted.
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