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[8023-POEP] More DS vs. SS stuff



Guys,

 

I got a phone call from a guy who had questions regarding the reliability issue.  He wishes to remain anonymous, but since this was a topic discussed in Austin, I’ve chosen to reply to the reflector.

 

For those who didn’t attend Austin, here is what happened.  I gave a presentation that examined the pros and cons of SS vs. DS, and part of that was a look at reliability.  (It was mostly a verbal argument and the pdf contains very little of it.)  I concluded that there was no significant reliability advantage of either architecture, and suggested that common sense therefore favors the simpler system, which in my opinion is SS.

 

Hugh disputed my reliability analysis with the following argument:  Hugh claimed that DS is more reliable that SS if the endspan and midspan are in completely separate boxes (which he claims will be the normal case).  He gave two examples to support this claim:

 

  1. Hugh sites the fact that the fans are usually the least reliable component in these systems.  And he pointed out that a fan failure in one box (the endspan for example) would not affect the other box (midspan).

 

  1. The endspan and midspan could be on completely different facility circuits, or even in separate buildings.  Therefore a DS system would be less susceptible to facility power outages.

 

 

After that I pretty much let the argument go, mainly because I was tired and I just wanted to finish my presentation and sit down.

 

While Hugh’s arguments may seem persusive to some, for the record I must disagree for the following reasons:

 

  1. Just because the endspan and midspan are in separate boxes (or even separate buildings) doesn’t mean they offer true redundancy, because they are not equal: If the endspan goes down for any reason, then the whole system is down because there is no longer any data traffic.  Just because the midspan continues to power the PD does not mean the system is still “up”.  If you can’t make a call on your IP phone then the system is down.

 

  1. I don’t buy the fan example.  If a single fan failure can take down an endspan (or midspan) then that is a bug in the design of that box, and does not reflect a weakness in the PoE architecture.  Suppose you had a power supply that kept failing because a cap didn’t have a large enough ripple current rating; would you fix this by adding a second identical power supply in parallel?  Of course not, you’d fix this design bug by using a better cap.  Network equipment makers know very well about the failure rates of fans, so they design their boxes with multiple fans, carefully arranged such that the air flow pattern doesn’t change radically if one fan dies.  (Assuming of course that they know what they’re doing.  I can provide a text book reference if anyone needs it.)

 

  1. I don’t buy the separate buildings example for two reasons:

 

    1. Who is actually going to route all their cat-5 cables from the endspan in Building A to the midspan in Building B, and then fan out from there to all the PDs?  I think we can all agree that the endspan and midspan will almost always be in the same closet, if not the same rack.  Although they could be on separate circuit breakers, but not if they’re running off the same UPS.

    2. I think we all know that most PoE systems use UPS backup.  The main reason why many customers buy PoE equipment is to keep their IP phones alive during a blackout so that people can call 911.  The whole facility power issue seems moot when a UPS is used.

 

 

I suppose we could argue about this forever.  (What if you had the endspan and midspan on separate UPS and separate facility breakers?)  But it would be hair splitting.  I still don’t see any clear logical reason why DS is more reliable than SS, or vise versa.  Therefore, I stand by my original conclusion: Neither DS nor SS offers any significant reliability advantages, and we’re probably better off just selecting the least complex system.

 

Of course all this is just about the reliability aspect of the SS vs. DS debate, and doesn’t include any of the other pros or cons.  We have to consider everything.

 

Steve