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RE: [EFM-P2P][EFM] PMD considerations

Thanks for the framework you provided on fiber PMD. The assessment that fast
Ethernet may be an important speed to consider is worthy of more
consideration. One aspect  about lower rates is the lower dispersion penalty
for longer distance support, greater than 10-12 kilometers while still using
low cost lasers. In North America, operators like SBC have to consider all
alternatives for serving longer loops so we could someday provide ubiquitous
coverage of broadband services.

With regard to multimode fiber placement, the lionshare cost of placing
fiber in new builds or overlays in an outside plant scenario will be the
labor to install.  I envision placing fiber once in an outside plant
scenario that will meet all needs for multiple decades of data rate growth.
Multimode  limitations in rate*distance fundamentally limits a fiber
infrastructure.  I envision only one possible scenario that I would place
multimode fiber in a cable in the outside plant, that would be if single
mode was also in the cable sheath for future growth!  

Perhaps the network you envision multimode application is premises cables or
very short 'drops' of multimode fiber from an active node. As an operator
that does not have existing active node sites close to subscribers, the
passive nature of the PON architecture has some advantages over requiring
building multiple nodes that have to get right of way from local
authorities, commercial power, backup batteries, etc to establish the active
node site. These active node civil works could lead to a cost per sub
exceeding the lower fiber/electronics cost of multimode versus singlemode.


Kent G. McCammon
Access and Video Technologies
SBC Technology Resources, Inc
4698 Willow Road
Pleasanton, CA 94588
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> -----Original Message-----
> From: Ulf Jönsson F (ERA) [mailto:Ulf.F.Jonsson@xxxxxxxxxxxxxxx]
> Sent: Monday, December 17, 2001 11:49 AM
> To: stds-802-3-efm-p2p@xxxxxxxxxxxxxxxxxx;
> stds-802-3-efm@xxxxxxxxxxxxxxxxxx
> Subject: [EFM-P2P][EFM] PMD considerations
> Hi all,
> The following text brings up some considerations regarding 
> the EFM optical PMD from a component perspective. It has been 
> written with great help from our Ericsson internal experts on 
> the optoelectrical component side. 
> For the physical medium, i.e. the O/E-converters and the 
> fiber connecting them, a few aspects may be high-lighted:
> 1. Data rate
> 2. Single or multimode fiber
> 3. Single or dual fiber
> We will discuss these aspects in more detail and will also 
> try to draw a conclusion. Hopefully this will start a 
> discussion on the reflector that may make it easier for us to 
> agree on a (few) baseline proposal(s) in March.
> 1. Data rate
> ------------
> The choice is between 100Mbps and 1000Mbps. Of course one 
> must pay a premium for a tenfold speed increase, throughout 
> the entire system (A more detailed cost analysis will be 
> presented at the January interim). Optimizing an O/E 
> converter design for 100Mbps instead of 1000Mbps means
> * inherent improvement of receiver sensitivity.
> * lowered demands on output optical power (consequence of above).
> * lowered demands on thermal management (both inherent, due 
> to lower speed, and consequence of above)
> * lower crosstalk
> All these factors will facilitate the module design, simplify 
> the assembly and increase the yield, thus substantially lower 
> the costs. The argument for 1000Mbps, that the higher volumes 
> for this product will yield lower cost, neglect the impact of 
> EFM as a cost driving application itself. This application 
> should in itself be enough to create sufficient production 
> volumes. Thus, it does not seem optimal to let the vast 
> majority of connections where 100Mbps is sufficient pay that 
> cost premium, especially as a P2P topology allows for 
> relatively easy individual line upgrades. On the other hand, 
> 1000BASE-X will in a P2P topology be appropriate for premium 
> subscribers and for aggregate traffic higher up in the access 
> network and it will of course be appropriate to use in a P2MP 
> network. Hence, we see a need to include both a 100Mbps PMD 
> and a 1000Mbps PMD in EFM. 
> 2. Singlemode or multimode fiber
> --------------------------------
> As of now, multimode systems are significatly more low-cost 
> than singlemode systems. Though this difference will decrease 
> as the singlemode component volumes increase, a certain 
> difference will always remain, due to the less stringent 
> geometrical tolerances in a multimode system. For those 
> applications where multimode systems are appropriate, there 
> is no need to pay the singlemode premium. What is important 
> is that a large number of connections require singlemode 
> systems, both due to present distance limitations and to 
> future upgradeability.
> 3. Single or dual fiber
> -----------------------
> O/E converters for a single fiber system are inevitably more 
> expensive than those for a dual fiber system, due to the 
> higher complexity. Just as inevitable is the fact that this 
> difference will be more than compensated at very long link 
> lengths. The question is the cross-over distance, and the 
> distribution of potential installations below and beyond this 
> cross-over, respectively. If it is regarded necessary to 
> include both options in the standard, how can that be made 
> with a minimum of effort? Let us examine the implications on 
> the basic parameters.
> 3.1. Power budget
> A dual fiber system can, and should, allow for a wide output 
> power range, in order to achieve high production yields in a 
> low-cost assembly process.
> If wavelength separation is used in the single fiber case, 
> the power specification should be equal for dual and single 
> fiber. The extra attentuation caused by the splitters are 
> hidden inside the converters, and just has to be compensated 
> for by extra laser power and increased internal receiver 
> sensitivity, respectively.
> The single wavelength case is more difficult, due to 
> constraints imposed by the reflection crosstalk. In order not 
> to have completely unrealistic back-reflection demands, the 
> span of the allowed output power must be minimized. 
> Otherwise, the transmitted power from a "low-end" module 
> would drown in the reflected power from a relatively high 
> power module. Assume e.g. a power span of 10dB, a link budget 
> of 10dB and a required SNR of 10dB. This implies a total 
> allowed near-end reflection of below -30dB, which is not 
> easily achieved.
> Thus, if the output power range for dual fiber is e.g. -5dBm 
> to -15dBm, the single fiber version should probably be a part 
> of that, something like -12dBm to -15dBm.
> 3.2. Wavelength
> For dual fiber systems, the operating wavelegth window can, 
> from a component perspective, be selected freely within the 
> SM fiber window 1300-1600mn. A wavelength separated single 
> fiber system of course have strict requirements regarding 
> this matter. For such a system it is also required to have 
> two types of transceivers, for each end of the connection. 
> Depending on the actual implementation of the components for 
> a single wavelength single fiber system, some wavelength 
> restrictions could be needed, as the splitters might have a 
> wavelength dependence. 
> 3.3. Connectors
> For dual fiber, several types of standard connectors should 
> be allowed, e.g. MT-RJ, LC, MU, etc. The requirements on 
> connector performance can be kept low, to reduce costs, since 
> the desired power budget is easily achieved, and there is no 
> back-reflection problem.
> The same should be valid for single fiber WDM systems, even 
> though the power budget is a bit harder to meet in this case. 
> Possibly the connector attenuation must be a bit tighter specified.
> For non-WDM single fiber, the crosstalk problem make low 
> reflection connectors necessary throughout the entire system.
> Conclusion
> ----------
> Eight different P2P relevant configurations, each with its 
> own merits and drawbacks, can be distinguished. These are:
> 100 Mbps MMF dual fiber
> 100 Mbps SMF dual fiber
> 100 Mbps SMF single fiber
> 100 Mbps SMF single fiber WDM
> 1000 Mbps MMF dual fiber
> 1000 Mbps SMF dual fiber
> 1000 Mbps SMF single fiber
> 1000 Mbps SMF single fiber WDM
> of these three already exist as standards within IEEE 802.3, namely
> 100 Mbps MMF dual fiber
> 1000 Mbps MMF dual fiber
> 1000 Mbps SMF dual fiber
> 100 Mbps SMF dual fiber is at present not an Ethernet 
> standard. Still, components exist and are used when needed. 
> ANSI has standardized a PMD for 100Mbps FDDI over SMF (ANSI 
> X3.184-1993). The corresponding FDDI standard for MMF is used 
> as a reference for Ethernet 100BASE-FX.
> The need to incorporate 100 Mbps SMF dual fiber within the 
> Ethernet family is obvious. Since it also seems to be the 
> most appropriate choice for a large number of EFM 
> connections, it should be the first choice for an EFM PMD. 
> This PMD should of course to a large extent be based on the 
> 100BASE-FX, with the physical media specifications optimized 
> for low-cost components with sufficient performance.
> To give a variety of options, it seems reasonable to also 
> incorporate 100Mbps MMF dual fiber as well as 1000Mbps dual 
> fiber in EFM. As already being Ethernet standards, this 
> should be possible without much extra work.
> Single fiber systems are a bit more complicated, since the 
> requirements are more closely connected to the actual 
> implementation, and a PMD are more different from existing 
> standards. One way to go, since the requirements (with 
> wavelength for the WDM solutions as a possible exception) is 
> within the dual fiber specification, only tighter specified, 
> would be to use the dual fiber PMD as a base and have 
> different categories within that. These could be one or two 
> single fiber options, but also extended temperature and 
> extended range dual fiber options. Depending on the progress 
> of the work, the single fiber options can either be tightly 
> defined within the base PMD, or kept rather open for 
> different manufacturer implemenations. The important issue is 
> to let the time-schedule be set by the most straightforward, 
> dual fiber, solution.
> Best regards,
> Ulf Jönsson & Hans Mickelsson
> Ericsson