Cause of disruption on Swisscom Mobile's NATEL® network has been identified
31 July 2001
Thorough analysis of the cause of the failure has been carried out over the past few days in close co-operation with the system suppliers. A central computer system and a back-up system set up in the same way are responsible for all call signalling on the Swisscom mobile network. The signalling sys-tem manages calls and ensures that mobile customers can be reached at any time and effective switching of calls. Both central systems are set up to enable all traffic to be transferred to the other in the event of failure.
Two software bugs led to failure - tests confirm that error performance functioned
Because commands were processed incorrectly the central system in Lausanne suffered a complete failure at midday on Friday as a result of a software fault. As planned in such cases the redundant system in another locality bore the load for around an hour. However, a second separate software error in this back-up system triggered a chain reaction which significantly reduced the capacity of the system and resulted in complete failure of signalling traffic on the Swiss mobile network in the course of the afternoon.
Analysis carried out in a test environment by Swisscom Mobile´s suppliers confirmed that the sys-tem´s error performance is functioning. The faults in both central systems were repaired in the course of Friday by correcting the software. Because of the huge number of attempted calls by mobile cus-tomers repair work could not be started until 17:00 and normal service could only be resumed in stages which was a time-consuming process. Analysis carried out last weekend confirmed the effec-tiveness of the measures taken.
Despite thorough tests before commissioning individual errors in complex software systems can never be ruled out completely. Swisscom Mobile´s system is successfully deployed worldwide. The faulty software worked perfectly in the weeks leading up to the failure. Swisscom Mobile is consid-ering a compensation claim against the system supplier.
Technical systems guarantee 100% availability
Despite this incident the management at Swisscom Mobile have no doubt that their network is among the most modern in technical terms. Over the past few years the mobile network has guaran-teed availability of around 99.95%. 100% availability cannot be achieved in highly complex technical systems, neither is this guaranteed by Swisscom´s general terms and conditions.
As Swisscom Mobile announced last Friday, calls and SMS messages will be free of charge (national calls and SMS messages on the mobile network, without Business Numbers and Infoservice) for sub-scription customers (Postpaid) on Thursday, 26 July and Friday, 27 July. Subscription customers will also receive a CHF 5 reduction on their July invoice while Natel-easy customers (Prepaid) will receive credit worth CHF 5 in the course of August.
Swisscom Mobile sincerely regrets the incident and wishes to apologise to its 3.3 million customers.