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Sources of Failure in the Public Switched Telephone Network
BY RAGHU SUNDEEP.T EE 585: Fault Tolerant Raghu Sundeep: Sources Of Failure In PSTN
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DESCRIPTION Like all telephone switching networks, the PSTN performs a fairly simple task: It connects point A with point B. The PSTN contains thousands of switches Switches include redundant hardware and extensive self-checking and recovery software The PSTN is a large, complex, distributed system with strong dependability guarantees. Raghu Sundeep: Sources Of Failure In PSTN
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A Layout Of PSTN DLSU (Digital Local Switching Unit)
DLSU (Digital Local Switching Unit) DLE (Digital Local Exchange) DCCE (Digital Cell Centre Exchange) DMSU's (Digital Main Switching Unit) DLSU (Digital Local Switching Unit) DLE (Digital Local Exchange) DCCE (Digital Cell Centre Exchange) DMSU's (Digital Main Switching Unit) Raghu Sundeep: Sources Of Failure In PSTN
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General Phone Connection
The -48V DC power supply is derived from the mains and is backed up by a massive set of batteries to ensure power is available for the PSTN should the mains fail at the exchange Master Socket has three components - the Spark Gap (SG) which protects your phone should the exchange develop a problem and produce a line pair voltage greater than about 90V. It also protects the wiring from your phone to the exchange should your phone develop a fault and try to inject mains voltage (for those who have mains powered phones) down the line pair Raghu Sundeep: Sources Of Failure In PSTN
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Failure Analysis of the PSTN: 2000 Approach
Find the areas that are failing then try to fix/address the problems Use PSTN as a case study for ROC: Large, widely used, networked system Highly reliable infrastructure Provides an upper limit for reliable computer service BEST CASE Raghu Sundeep: Sources Of Failure In PSTN
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Collecting Failure Data
Target System: US Public Switched Telephone Network (PSTN) Detailed telephone service failure data available from the Federal Communications Commission (FCC) Telephone Disruption reports: company name, duration, time, cause, and event disruption Required by law for outages affecting 30,000 people or lasting at least 30 minutes Raghu Sundeep: Sources Of Failure In PSTN
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Causes of Failure Human Error Acts of Nature Hardware Failure
Software Failure Call overloads Vandalism Raghu Sundeep: Sources Of Failure In PSTN
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Categorizing the Failures
Human Error Company workers Includes Contractors and Vendors External Acts of Nature Fire Rain Lightning Winds Floods Raghu Sundeep: Sources Of Failure In PSTN
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Categorizing the Failures
Hardware Failure Network component failure Cable, power outage Software Failure corrupt/incorrect communication software Call Overloads Over network capacity Vandalism Intentional harm to telephone network equipment Raghu Sundeep: Sources Of Failure In PSTN
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Categorization Challenges
Outages may have multiple causes Terminology Root Cause - cause behind the outage Direct Cause - immediate trigger i.e. Root Cause – latent error in software Direct Cause – Maintenance error (human) Raghu Sundeep: Sources Of Failure In PSTN
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Outages Breakdown by Number:
Human Error accounts for 55% of the outages for 2000 Human-company Human-external Hardware Failure Software Failure Overload Vandalism Acts of Nature Total: 202 outages Raghu Sundeep: Sources Of Failure In PSTN *Vandalism accounts for < 1%
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Outage Breakdown by Number (Nature Factored Out)
Human Error accounts for 59% of all Outages Human-company Human-external Hardware Failure Software Failure Overload Vandalism Raghu Sundeep: Sources Of Failure In PSTN Total: 187 outages
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What could humans possibly do wrong?
Cut incorrect cables Upgrade software incorrectly Incorrectly repair hardware Follow instructions incorrectly Fail to read documentation Do things out of order Raghu Sundeep: Sources Of Failure In PSTN
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Measuring Availability
Number of Outages Only measures the number of outages. Does not include the duration of the outages. There’s more important information than simply the number of outages. Outage Duration Customers Affected Blocked Calls Raghu Sundeep: Sources Of Failure In PSTN
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A Second Metric Customer Minutes
Outage duration in Minutes * Customers affected Captures collective customer experience Assumes all affected customers or lines attempted to make a call Raghu Sundeep: Sources Of Failure In PSTN
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Number of Outages vs. Customer Minutes customer minutes/year
Humans = 54% Total: about 95 Million customer minutes/year Raghu Sundeep: Sources Of Failure In PSTN Total: 187 outages
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Summary: Humans were the greatest cause of failure.
Humans caused most of the outages “Traditional Computing concentrates on tolerating hardware and operating system faults, ignoring faults by human operators…” (David Patterson, 2001) Raghu Sundeep: Sources Of Failure In PSTN
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Trends in Customer Minutes
Cause Trend 2000 Human Error: Company 98 131 Human Error: external 100 125 Hardware 49 60 Software 15 155 Overload 314 2 Vandalism 5 Raghu Sundeep: Sources Of Failure In PSTN
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Future Work: Directly apply data to the ROC project
Could the ROC techniques have avoided these outages? Further categorize the data More specific categories within each general category Telephone Company Geographic location Breakdown Human error further Vendors, contractors, technicians, outsiders… Include more years of outages for further comparison Raghu Sundeep: Sources Of Failure In PSTN
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REFERENCES From IEEE Computer, Vol. 30, No. 4 (April, 1997).
Kühn, DR: Sources of Failure in the Public Switched Telephone Network, IEEE Computer (April 1997) DATA available from FCC Raghu Sundeep: Sources Of Failure In PSTN
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