In chapter two of this new blog series of mine I am going to tackle what I consider to be a data center FOLLIE.
Cables (multi-mode or single mode) and Optics (SFPs) go together, hand in glove. Short wave SFPs are always attached to Multi-Mode (MM) cables while Long wave SFPs are always attached to Single Mode (SM) cables. Short wave SFPs will not work with SM cables and Long wave SFPs will not work with MM cables.
Less expensive MM cables are not built to the same, rigid and exact requirements of their more expensive cousins the SM cables. This results in MM cables having a lot more modal dispersion of their light within the cable with the result of a complete corruption of that light signal at some distance down the cable which makes the signal unreadable. So MM cables have distance restrictions based on the speed of the link.
On the other hand, a Brocade switching device using long wave SFPs and SM cables (OS1) can easily transmit signals as far as 25km (15.5 miles) from point-to-point at 8Gbps. The SM core has a smaller diameter and the light traveling through it suffers much less modal dispersion so it provides a much longer distance link than MM can provide.
So at what distance does MM modal dispersion occur? Well, that depends on the signal source (speed) and the type of MM cable used as well as the condition of that cable and all of its connections. For our example we will assume that the MM cable is in top shape and the cable connections are good and tight.
When the physics of light through a FC cable comes into play it dictates that for a specific link rate a valid light signal can only be received at up to a maximum specified distance from the transmitter. For example, a link running at 4Gbps can send a short wave signal across an OM2 MM cable as far as 492 feet (150 meters). When using the better OM3 MM cable, a link running at 4Gbps can send a short wave signal as far as 1,247 feet (380 meters). And if using the latest OM4 MM cable a link running at 4Gbps can send a short wave signal as far as 1,312 feet (400 meters).
When you look at those numbers it is obvious that the type of MM cable helps to determine the longest distance reach that a MM cable has between two connection points.
And this is where the FOLLY is coming into play.
From about 1997 to 2007 customers had a choice of OM1 (62.5 micron, 200 mHz) or OM2 (50 micron, 500 mHz) multi-mode cables. OM1 was a holdover from ESCON and quickly was replaced by OM2. OM2 was developed for 2Gbps FICON and was usually orange in color – although there are no standards for cable coloring. But then 10Gbps came into the market. It actually came out ahead of 4Gbps. And everyone knew that 8Gbps was not too far off on the horizon. So to allow customers to have adequate distance at these new, higher speeds new MM cables (OM3 and then later OM4) were developed.
Another rule of thumb (ROT) that applies to MM cables is that as link speed doubles distance across that cable reduces in half (not quite true but close enough for an ROT). And we can see that distance decreasing as link speed increases with the little table below:
When your eyes travel down the columns it is easy to see that as speed increases the distance that the data (frames) can be sent decreases.
It is also easy to correlate what happens when upgrades are made to your cable farm and newer and better cables carry the data frames. For example, 4Gbps utilizing an OM2 cable (max of 492 feet) has less distance capability than 8Gbps utilizing OM4 cables (max of 623 feet). OM3 and OM4 are superior to OM2 at carrying data frames for longer distances as the link rate increases.
So both speed and quality of cable play a role is distance connectivity across a FC link.
Next up in the technology pipe will be 16Gbps. 16Gbps is already available from Brocade in its switching products but there are no hosts and no storage at those speeds -- yet. And, of course, 32Gbps host/storage is probably only 4 or 5 years away.
The FOLLY that I want to draw your attention to is that with technology getting 2x faster about every 18-24 months, customers are having to either rip and replace MM cables fairly often or they are having to reposition peripheral equipment closer to the server so that the lesser cable distance for higher speed links on the same old MM cables can still be made to work. Lots of effort and lots of waste. And at some point Multi-Mode is just simply not going to get the job done.
I would urge you to consider replacing your old Multi-mode cables with Single Mode cables which are then going to last you for 4-8 or 10 years. Of course there is a cost.
Multi-mode cabling is less expensive than Single mode cabling and short wave SFPs are less expensive than long wave SPFs. But how many times do you have to rip and replace MM cables before the lesser expense at the first rip and replace becomes much more than SM cables at the 3rd or 4th rip and replace. Plus all of the manual effort, project management, and potential risk to your environment at each and every rip and replace.
I really do understand that budgets are tight but I would certainly urge you to make an investment in long wave SPFs and single mode cables and just remove that hassle right out of your life. It is a FOLLY to spend good money time and time again knowing it will have to be ripped out and replaced in only a few years time. Around the world the vast majority of the mainframe customers I am visiting with have decided to go to all long wave SFPs and SM cables in their shops or they are currently considering it. They realize that making this kind of an investment will drive much more value over time with far less risk within their I/O infrastructure in the long term than anything that multi-mode and short wave will allow them to ever do.
OK, I am off of my soapbox now. Thanks for reading my rant.