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Re: [HSSG] 40G MAC Rate Discussion

I'm returning to an item in the discussion from April 6, where Matt Traverso wrote:

> - I'd like to hear a comment / perspective from the fiber
> manufacturers on the utilization rate of the ribbon fiber strands.
> For a 4x10G MMF approach presumably 8 strands in the 12 ribbon would
> be used 4 for TX and 4 for RX.  For a 10x10G MMF approach it would be
> 2 @12 with 10 @ Tx and 10 @ RX.  What does this do to the cost and
> usage rate metrics of MMF cabling?

Some parts of my response to your inquiry are straight forward and easy to provide without cause for concern.  More in depth response on cost gets tricky and I do not want to cross over the line our chair has reminded us of.  So I have modified it a few times.  As this material developed, it appears that it could be the basis for a presentation.  Hopefully it is acceptable and useful.

For parallel MMF connectivity the cost of the cabling for 4x10G will be half that of 10x10G simply because 12-fiber cabling is used for the former and 24-fiber cabling is used for the latter.  Also the patch panel density (circuits per unit area) will be double for the former compared to the latter.  This may also be the case for active equipment panel density.  They both use the same cabling componentry, but the latter requires twice as much.  

The effect of this difference on the total cost of the system is commensurate with the projected cost difference between the 40G and 100G PMDs, as detailed below.

On April 9 Scott Kip wrote:

> I have heard that the cost of a 5 Gig QSFP are
> comparable to the cost of a single XFP or approximately 10 4-Gig SFPs.
> If the QSFP rolls out in relatively high volumes, the QSFPs are expected
> to approach the cost of 4 SFPs at comparable speeds.

If this relationship is true and from what I know of such things, then the cost to the end user of an installed, tested, warranted, typical-length, 12-fiber, MM structured cabling link, consisting of a cable terminated at patch panels plus array-terminated cords on each end, is in the neighborhood of today's cost of a pair of 20G QSFPs to an OEM (e.g. server, switch, router maker).  In the analysis that follows I'll assume that the future 40G QSFPs are 2x the cost of 20G QSFPs (i.e. 2x XFP cost), and that 100G PMDs are 5x XFP cost (trying to be consistent with jewell_01_0107.pdf where 100G cost = 4x XENPAK cost).  In order to compare cabling and PMD costs at the end user level, a PMD cost adjustment factor to account for integration by the OEM must be applied.  The same factor is applied to all cases.  The tabulations that follow should help make the basic trends over time and between data rate choices clearer.    

Using 2007 XFP cost to the OEM as the monetary unit, the basic end user projected cost comparison is:

data rate       40G  100G
PMD cost       6      15
cable cost       1       2
total cost        7      17

Here cabling cost is ~1/7 (14%) to ~2/17 (12%) of total cost.  40G total cost is 41% of the 100G total cost.

In about three to four years time (assuming both 40G and 100G MM PMDs have the same initial availability date) the cost is:

data rate       40G  100G
PMD cost       3       7.5
cable cost       1       2
total cost        4       9.5

Here cabling cost is ~1/4 (25%) to ~2/9.5 (21%) of total cost.  40G total cost is 42% of the 100G total cost.

If the 40G PMD is available ahead of 100G PMD, then the 40G solution should be relatively lower in cost.

With the caveats of the above assumptions, the conclusion is that the 40G QSFP PMD should offer significant advantage in cost for the total channel relative to the 100G PMD from both transceiver and cabling perspectives.  The relative total cost ratio appears to stay fairly constant, even while the cabling costs become relatively more significant.  This is because the ratio of the cabling costs is similar to that of the corresponding PMD costs.  To illustrate, if dominated completely by PMD cost differences, the ratio would be 40%, while if totally dominated by cabling cost differences, the ratio would be 50%.

If your metric of value is cost per gigabit, the two scenarios practically equate.  But if your metric is absolute cost, then one would not have to pay for more than one needs with a 40G solution.  

To offer another perspective, comparing to a LAGed 10G scenario:

data rate             10G  40G
PMD cost            3        6
cable cost           0.25    1
total cost/ckt       3.25    7
LAG factor          4        1
total cost           13        7

the 40G solution provides a very favorable cost comparison, at a cost that is a bit more than half of the 4xLAG 10G option.

As far as the other part of your question regarding usage rate metrics for the cabling, as you can see from the above analysis, leaving four dark fibers per cable (8 out of 12 active with 40G) instead of 2 (10 out of 12 active with 100G) does not have significant impact, since the user procures the array-terminated cabling on a modulo 12 basis.  Utilization efficiency can make a difference if the modulo-12 cabling supports more than one application at a time.  For example, it makes a cost difference if there are unused fibers for applications where we deploy 2-fiber circuits over a 12-fiber cable.  The 12-fiber cabling gets amortized over one to six circuits depending on utilization.

Both the 40G and 100G scenarios have degraded capacity utilization in the cable compared to 10GbE.  Today a 12-fiber cable supports six 10GbE circuits for a capacity of 60G.  That same cable will support only 40G or 50G capacities for the 40G PMD or 100G PMD respectively.   But in both scenarios there are far fewer patch cords and port interfaces to manage than for 10G links aggregated to match the comparative data rates, and the overall cost for either of the higher-rate solutions should be very favorable compared their LAGed counterparts.

Paul Kolesar
CommScope Inc.
EnterpriseŽ Solutions
1300 East Lookout Drive
Richardson, TX 75082
Phone:  972.792.3155
Fax:      972.792.3111