1 / 14

Resolving a Question about Span Restoration: Do Loopbacks Involve a Capacity Penalty?

This study aims to determine whether loopbacks in span restoration networks actually use excess spare capacity or not. It analyzes the occurrence of loopbacks and their impact on spare capacity, providing insight into the efficiency of loopback elimination strategies.

beadles
Download Presentation

Resolving a Question about Span Restoration: Do Loopbacks Involve a Capacity Penalty?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Resolving a Question about Span Restoration:Do Loopbacks Involve a Capacity Penalty? Wayne D. Grover TRLabs and Dept. of Electrical and Computer Engineering, University of Alberta Matthias Scheffel Munich University of Technology, Institute of Communication Networks (Prof. Eberspaecher)

  2. Motivation / Background • Span restoration (equivalently, pre-planned span protection) is one of the most longstanding options for survivable mesh networking. • Every year or so, a proposal comes forward (or a new patent) having invented span restoration with “loopback elimination.” • It is assumed obvious that such loopback elimination must save a great amount of spare capacity. • To our knowledge, however, that assumption has never been quantitatively tested. • Moreover, a qualitative argument has been published saying why loopback elimination is probably insignificant. • So, the Question is:  Do loopbacks actually use excess spare capacity in a span restorable mesh network, or not !?

  3. Span Restoration Principle • Alternative path segments restore all working channels of the failed span • Local restoration between the end nodes of the failed span • Multiple restoration routes are possible per span • Restoration path segments for different spans can share spare capacity • Mechanisms equivalent to max flow or ksp re-routing

  4. Occurrence of Loopbacks • The end-to-end working path is the same except for the local restoration path segment substituting the working link on the failed span • Working path and restoration segment may thus have common spans (or/and transit nodes) Example: Span restoration view Working paths Span restoration paths +

  5. Occurrence of Loopbacks • Span restoration is not aware of the end-to-end working paths • In this regard, it is like the mesh correspondent to BLSR rings. • The occurrence of loopbacks depends on the assignment of restoration paths to affected working links Example: End to end path view Restoration without loopbacks Restoration with loopbacks • At first glance, the left restoration configuration seems to be more efficient than the right one

  6. The Recurring Idea of “Loopback Elimination” • Nodes along the restoration path may check whether the restoration path and the working path belonging to the affected working link traverse a common span • If so, the node cross-connects the restoration path and the remainder working path • Knowledge about the assignment of restoration paths to working links and the working route requires additional signaling Loopback configuration With loopback elimination

  7. Loopback Elimination • However, the loopback capacity cannot necessarily be released from the network • Optimal span restoration minimizes the spare (and working) resources in the network • Spare capacity used to form loopbacks for one span failure is required in order to recover from other span failures, too • Our hypothesis:What appears as loopback capacity for one failure is almost always required in a non loopback way for at least one other failure scenario. Span failure C-F: 10 capacity units appear to be wasted by the loopback Span failure E-H: 10 capacity units are fully utilized by restoration paths

  8. Forcer-Based Reasoning and Analysis • Forcer concept • In span restoration, each span s with non-zero spare capacity c has a forcer span f whose failure yields restoration paths on s that utilize the entire spare capacity c • An optimal span restoration design usually involves more than one forcer f (co-forcers) for each span s Failure scenario f1 is a forcer of span s if in the optimal design, its restoration requires use of every spare channel on span s

  9. Assessing the Loopback-Revised Spare Capacity • Step 1: Determine the number of protection paths ps,f that traverse span s in case of span failure f for all span pairs (s,f), s≠f • Step 2: Determine the number of working paths ws,f that traverse spans s and f for all span pairs (s,f), s≠f • Step 3: Calculate the number of loopbacks ls,f on span s in case of span failure f by: ls,f = Minimum(ps,f, ws,f) for all span pairs pairs (s,f), s≠f • Step 4: The loopback-revised protection capacity on span s is: cs = Maximum(ps,f1 – ls,f1, ps,f2 - ls,f2, …) over all spans f1, f2, … except span s

  10. Oslo os Glasgow st Stockholm no Norden gl ha Hamburg Copenhagen br Berlin Dublin co Bremen Hanover be se Seattle du hn Hamburg Amsterdam Berlin Warsaw ha am bl Essen wa London lo Ithaca Ann Arbor es Frankfurt Duesseldorf do Brussels br it an fr du Prague Dortmund Strasbourg sa le Paris pa li pi pr bo ur pr co sr Salt Lake City Boulder Pittsburgh Lincoln Princeton Leipzig mu Urbana wa Cologne vi Zurich bu Budapest Munich pa Palo Alto Washington D.C. zu Vienna Frankfurt fr Bordeaux bo ly Lyon mi za be Belgrade Milan at Atlanta Zagreb sn Mannheim ma nu Nuremberg San Diego ba Karlsruhe ka ho Houston ro ma st Barcelona Rome Madrid Stuttgart ul Ulm mu Munich Athens at Test-Case Studies • Optimal integer linear program based design of span restoration by joint optimization of working and protection routing • Calculation of loopback capacity that can be released • Network scenarios: European network US network German network

  11. Results: “US Network” • 44.6% of all restoration paths involve loopbacks • 1.1% spare capacity reduction in the US network

  12. Results: “German network” • 50.2% of all restoration paths involve loopbacks • There is no capacity penalty due to loopbacks

  13. Results: “European network” • 74.1% of all restoration paths involve loopbacks • 0.1% spare capacity reduction is possible by loopback elimination

  14. Concluding Discussion • Span restoration provides alternative path segments at the end nodes of failed working links • Loopbacks arise if the end-to-end working paths and the assigned protection segment have common spans • Results show loopbacks do occur frequently • A Forcer-based analysis was used to calculate the net capacity penalty due to loopbacks • There are none or only extremely small capacity savings when eliminating loopbacks • Slight changes to working capacity values can make the savings strictly vanish as well. • What is the explanation? • As postulated, it does seem that “what is loopback capacity for one failure scenario is almost invariably needed capacity under one or more other failure scenarios.” • The “forcing” failures in the design will need all the spare capacity present in a non-loopbacked way.

More Related