Unavailability Analysis of Long-Haul Networks

Transcription

1 Unavailability Analysis of Long-Haul Networks Michael To and Philippe Neusy IEEE Journal on Selected Areas in Communications, 1994 Presented by Arnon Tonmukayakul

2 Outline Introduction and objectives Unavailability analysis of a short system Unavailability analysis of a long-haul system Conclusion

3 Fiber systems are widely deployed in the long-haul networks Unavailability of these networks is being seen as unacceptable Causes of unavailability: 1. Power supply 2. Maintenance activity 3. Equipment failures 4. Fiber cuts Introduction

4 Objectives Analyze a number of architectural choices and their meeting of a set of availability objective Definitions: Unavailability = Probability that the channel is out of service at any given time (% of down time or minute per year) down time down time + available time 100% down time down time + available time min/yr

5 Choices of Network Architecture Architectures Descriptions When networks fail Level of protection 1. Networks with diversity Traffic is divided and carried on different routes. Only a portion of traffic is affected Low 2. Networks with restoration capability Traffic is restored via alternate route in case of failure Traffic is reroutes within finite restoration time Medium 3. Survivable networks Use various selfhealing schemes No traffic is affected (restorable scenarios) High

6 Analysis of a Short System 50 miles bidirectional DS3 channel between source and destination with one regenerator

7 1) Equipment Failures in a Channel with No Equipment Protection U channel = 4 U T/R

8 2) Equipment Failures in a Channel with 1:1 Equipment Protection U channel = U working channel U protection channel

9 3) Equipment Failures of a Channel with 1:N Equipment Protection Failure Scenarios 1) Both working channel Y and protection channel are unavailable 2) Working channel Y and one of other working channel are unavailable

10 3) Equipment Failures of a Channel with 1:N Equipment Protection U channel = U scenario1 + U scenario2

11 4) Equipment Failures of a Channel with Digital Cross-Connect (DCS) Based Restoration Failure Scenarios 1) Soft failures - Primary channel fails, recovered by automatic restoration through DCS - Network is unavailable during restoration process (2 minutes) 2) Hard failures - Failures occurs on both primary and alternative channels - Network is unavailable during overlapped repair times of both channels

12 4) Equipment Failures of a Channel with Digital Cross-Connect (DCS) Based Restoration 4.1) Soft Failure Analysis Failure of the primary channel Automatic restoration takes 2 minutes U SoftF ailure = failures/yr 2 min = 1.1 min/yr Unavailability is highly dependent on restoration time

13 4) Equipment Failures of a Channel with Digital Cross-Connect (DCS) Based Restoration 4.2) Hard Failure Analysis MTTR Alternate ch. 2 hrs service outage Critical Period = 4 hrs MTTR Primary ch. 2 hrs service outage 4 hrs. = MTTR(Primary ch) + MTTR(Alternate ch) MTTR Alternate ch. 2 hrs Failure on the alternate channel during critical period

14 4) Equipment Failures of a Channel with Digital Cross-Connect (DCS) Based Restoration # of hard failures = # failures of primary channel # failures of alternate channel in the critical period = = failures/year Determine the outage time MT T R primary MT T R alternate MT T R primary + MT T R alternate

15 4) Equipment Failures of a Channel with Digital Cross-Connect (DCS) Based Restoration MT T R primary MT T R alternate MT T R primary + MT T R alternate = 1 hour outage time = 1 hour + 2 minutes = 62 minutes U HardF ailure = failures/yr 62 min = min/yr U channel = U SoftF ailure + U HardF ailure = = min/yr Unavailability is highly dependent on restoration time

16 Analysis of a long-haul network Reference Model LATA: Local Access and Transport Area

17 1) Network with Diversity with No Protection

18 2) Network with Diversity with 1:N Protection

19 3) Network with Restoration Capability (DCS) double failures in the same section single failures: less impact but higher rate of occurrence

20 4) Survivable Networks 4.1) Dual Fed Systems Bottom channel is selected when failure on the top channel is detected. No need for DCS restoration for single failures.

21 4.1) Survivable Networks: Dual Fed Systems double failures in the same section double failures in the different sections

22 4.2) Survivable Networks: 1+1 Systems with Diverse Routing

23 4.2) Survivable Networks: 1+1 Systems with Diverse Routing

24 4.3) Bidirectional Line Switched Ring (BLSR)

25 4.3) Bidirectional Line Switched Ring (BLSR) Analysis of 2 and 4- Fiber Rings working span reverse span Failure Scenarios 4.3.1) Two equipments failures 4.3.2) Two cable cuts 4.3.3) One cable cut and one equipment failure

26 4.3) Bidirectional Line Switched Ring (BLSR) Analysis of 2 and 4- Fiber Rings 4.3.1) Two equipment failures (working span and reverse span) 4-Fiber ring : no outage (span switching) 2-Fiber ring : outage

27 4.3) Bidirectional Line Switched Ring (BLSR) Analysis of 2 and 4- Fiber Rings 4.3.2) Two cable cuts 4-Fiber ring: outage 2-Fiber ring: outage 4.3.3) One cable cut and one equipment failure 4-Fiber ring: no outage (span switch) 2-Fiber ring: outage

28 4.3) Bidirectional Line Switched Ring (BLSR) Analysis of 2 and 4- Fiber Rings 4.3.3) One cable cut and one equipment failure 2-Fiber ring: outage (8 working spans U C )(8 reverse spans U LinkF ailure ) 4-Fiber ring: outage (8 working spans U C )(30 spans U LinkF ailure )

29 4.3) Bidirectional Line Switched Ring (BLSR)

30 Conclusion Comparison of different architectures from an unavailability point of view Network with diversity cannot meet the proposed objective Network with restoration capability barely meet the objective Digital cross connect based: unavailability is highly dependent on automatic restoration time Ring provide better survivability than 1+1 systems because there are 2 interconnection between rings 4-Fiber ring can have lower availability than 2-Fiber ring since the unavailability due to the combination of cable cut and equipment failure can dominate survivability of two equipment failures