On Fri, Mar 13, 2009 at 12:48:15PM -0300, Ray Burkholder wrote: > I have a carrier which has about 13km to 15km of fibre to their CO. They > run 1310 nm alcatel equipment on their end. I was wanting to use a Cisco > GLC-LH-SM on my end but that is rated for 10km. The GLC-ZX-SM is 1550 nm > 70km device which is incompatible.
I think this has already been answered to death by everyone else on the list, but I'd like to try and summarize a few points into something which can be pointed to on the list archives the next time someone asks this question. 1. All SMF optics have wideband receivers and will happily accept any signal from any other source (typically 1270-1610nm), with absolutely no requirement that it match what the other side is sending. Don't think of a fiber circuit as a single link, think of it as two completely distinct links running in opposite directions which just happen to be parallel to each other for TX and RX. 2. Distances specified on optics mean absolutely nothing, they are there to give you a rough estimation of what you could expect under the worst possible conditions. Optical budget (and at higher speeds, things like dispersion penalties) are pretty much all you care about (ignoring goofy things like reflections which aren't relevent to this discussion). 3. Not all optics perform the same. Think of it like how processors are made, they make a batch and some of them perform better than others. A certain number perform completely outside of spec and have to be thrown away, but even within the "good" batch there will be varying levels of performance and quality. For example, Cisco LX/LH optics have a TX power spec of between -3 to -9.5 dBm, and a RX power spec of between -3 to -20 dBm. The max TX of -3 is so that incase you have two LX optics side by side, the signal isn't so strong that it blinds the other side. The distance spec is calculated assuming the worst possible conditions, a TX over -9.5 dBm, running over old crappy SMF28 fiber. In reality, you're likely to find that most of your LX/LH optics operate much closer to the -3 side than the -9.5 side, and infact you may find some that are hotter than spec. Fixing something thats too hot is easy, you just add an attenuator. You'll find that you can almost always go at least 2dB below your RX budget too, before you actually start taking errors. Think of it like the max capacity sign on an elevator, they actually make the steel cables to hold many times more than that max capacity, "just incase". Also optics tend to cool over time, so what shipped as a +1 might end up as a -1 after a few years, and they don't want your link randomly going out and you complaining. :) 4. Often times, links can be extended simply by changing out one side of the pair with a longer reach optic. When you upgrade one side, not only do you increase the transmitted signal strength (and often switch the signal from 1310 to 1550, which has a lower rate of loss per unit of distance), you're also increasing the receiver sensitivity. The far side can hear you because you're transmitting a stronger signal in a band that has less loss over distance, and you can hear the far side because you have a more sensitive receiver. At the end of the day, almost all your questions (well at least all the simple ones :P) can be answered by a light meter or integrated optical monitoring on newer optics. Anyone who doesn't have a light meter would do well to get themselves over to ebay and spend a couple hundred bucks. <rant> Also, please for the love of god read the instruction manual and learn the difference between "absolute power mode" (i.e. how much signal am I receiving?) and "relative loss mode" (i.e. how much loss does this cable have assuming that there is a known fixed-strength signal generator on the other side?). I'm seriously sick and tired of 95% of supposedly trained remote hands techs telling me I'm transmitting +65dBm. :) </rant> -- Richard A Steenbergen <[email protected]> http://www.e-gerbil.net/ras GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC) _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
