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The way to derive a link budget for structured data center cabling is as you articulate below. This works well for optics intended for structured cabling applications, for example MMF SR and SR4 and the new SMF minimum 500m interface.
From: Kolesar, Paul [mailto:PKOLESAR@xxxxxxxxxxxxx]
Scott and Chris,
The crux of the issue is to define a power budget that is useful for data center environments. For single-mode channels in data centers the attenuation of the fiber consumes a minor fraction of the loss needed to properly support useful channel topologies, with a far larger fraction of the loss budget devoted to overcoming connection insertion loss.
There can be a significant difference in what is needed to support 2-fiber channels compared to parallel channels for the same topology. This is because, for pre-terminated structured cabling, there are twice as many connections for 2-fiber channels owing to the deployment of fan-out cassettes that transition from array connections (e.g. MPOs) on the permanent link cabling to multiple 2-fiber appearances (e.g. LCs or SCs) on the front of the patch panel. For parallel solutions these fan-out cassettes are not used because the fan-out function is not needed. Instead, array equipment cords attach directly to array pre-terminated permanent link cabling. Thus parallel solutions deploy only array connections, one at each patch panel appearance, while 2-fiber solutions employ a 2-fiber connection plus an array connection at each patch panel appearance.
For the long data center channels that single-mode systems are expected to support, two-link (or greater) topologies will be very prevalent. A two-link channel will present four patch panel appearances, one on each end of the two links. For 2-fiber transmission systems, that means we need enough power to support four 2-fiber connections plus four array connections, a total of eight connections. For parallel transmission systems, that means we need enough power to support four array connections.
To show an example, I’ll take a simple approach of allocating 0.5 dB per connection. This translates into a need to provide 4 dB of connection loss for 2-fiber systems and 2 dB for parallel systems. Admittedly this isn’t correct because, among other things, 2-fiber connections generally produce lower insertion loss than array connections. But it serves to illustrate that the difference in useful loss budgets between the two systems can be significant, being associated with the loss allocation for four 2-fiber connections.
In prior 802.3 budgets we typically allocated 2 dB for connection loss in single-mode channels. However, in the past the length of fiber was much longer, typically at least 10km. This meant that at least 4 dB of power was devoted to overcome fiber attenuation. For such systems it is possible, and quite typical, to exchange the fiber loss allocation for additional connection loss when the channel is short. So 10km budgets supported the above structured cabling scenario. For P802.3bm we are talking about supporting much shorter channels, so the fiber loss allocation will be much smaller making such trade-off no longer very helpful. But the customer still needs to support channels with at least two links.
The bottom line is that a blanket absolute connection loss allocation is not the best approach. Rather we should be requiring support for a minimum number of patch panels, which I propose to be four for the two-link channel. From this the number of connections will depend on the system, and the connection loss allocation can be appropriated accordingly.
Hopefully this helps draw the disparate views together.
From: Chris Cole [mailto:chris.cole@xxxxxxxxxxx]
For structured links, as explained to us by Paul Kolesar, the number of connections is known, for example 2 for single-link channels and 4 for double-link channels (kolesar_02_0911_NG100GOPTX). This works well when defining MMF link budgets, for example SR or SR4, and also the proposed new 500m SMF standard, which can be viewed as a reach extension of the 100m or 300m MMF application space.
However, that is not how 2km or 10km SMF interfaces are used. These have a variable number of connections and often include loss elements. It is reasonable as a methodology to start out with a nominal number of connections and fiber loss to determine a starting point link budget. However, it is unreasonable to stop there and ignore widespread deployment. So using the methodology outlined below to determine a 2km loss budget gives the wrong answer.
From: Scott Kipp [mailto:skipp@xxxxxxxxxxx]
I suggest that we specify our single-mode links in a similar way that they have been defined in IEEE 802.3-2012.
For 10GBASE-LR, the channel insertion loss is defined as:
Notes below Table 52-14:
c. The channel insertion loss is calculated using the maximum distance specified in Table 52–11 and cabled optical fiber
attenuation of 0.4 dB/km at 1310 nm plus an allocation for connection and splice loss given in 18.104.22.168.
The maximum link distances for single-mode fiber are calculated based on an allocation of 2 dB total
connection and splice loss at 1310 nm for 10GBASE-L, and 2 dB for 30 km total connection and splice loss
at 1550 nm for 10GBASE-E.
Even for 100GBASE-LR4 and 40GBASE-LR4, the connection insertion loss is:
22.214.171.124 Connection insertion loss
The maximum link distance is based on an allocation of 2 dB total connection and splice loss.
So the usual standard for single-mode Ethernet links is 2dB of connection insertion loss and we should continue to specify single-mode links with this connection loss unless there is a good reason to change.
The exception to this way of defining connection insertion loss is 40GBASE-FR. Since 40GBASE-FR was designed to use the same module for VSR2000-3R2 applications as well, the connection loss was increased to 3.0dB and the channel insertion loss was defined as 4dB. This was a reasonable variation from the normal specification methodology to interoperate with other telecom equipment. Since the 100GBASE-nR4 solution does not have this requirement for compatibility with VSR, the 802.3bm task force does not need to carry this costly requirement of a 3.0 dB connection loss and 4.0dB loss budget into 100GBASE-nR4.
The standard even calls out how 40GBASE-FR is defined differently from other Ethernet standards:
89.6.4 Comparison of power budget methodology
This clause uses the budgeting methodology that is used for application VSR2000-3R2 in ITU-T G.693
[Bx1], which is different from the methodology used in other clauses of this standard (e.g., Clause 38,
Clause 52, Clause 86, Clause 87, Clause 88).
For 802.3bm, we should define the channel insertion loss as distance * loss/distance + connection loss.
For a 500m solution, the channel insertion loss would likely be: 0.5km * 0.4dB/km + 2.0dB of connection loss = 2.2dB.
For a 2km solution, the channel insertion loss would likely be: 2 km * 0.4dB/km + 2.0dB of connection loss = 2.8dB.
From: Chris Cole [mailto:chris.cole@xxxxxxxxxxx]
I look forward to hearing the presentations on this today’s SMF Ad Hoc call.
However, having previewed the presentations, I continue to be disappointed that we are still discussing reach as if that was the only important application parameter. For example, shen_01_0113_smf only discusses 2km as if that was sufficient to describe the application. There is no mention of loss budget. Further, by only focusing on reach, the presentation perpetuates the myth that somehow the 10km reach is a niche application, and that we have just discovered 2km as an overlooked sweet spot.
It is well understood that there are few datacenter reaches that are 10km. What is important about widely used 10km interfaces like 10GE-LR, 40GE-LR4, and 100GE-LR4 is their greater than 6dB loss budget. In most applications, the reach is much less than 10km, but the >6dB loss budget is fully utilized.
2km reach has been an important and wide spread application, with both ITU-T and IEEE standardizing on a minimum 4dB loss budget. This means that over the last decade, end users have become accustomed to interfaces labeled 2km supporting a 4dB loss budget, and designed their central offices and datacenters around this. It is not clear why we continue to reinvent the wheel and propose reducing the established 4dB loss budget by fractions of a dB, for example as in vlasov_01_0113_smf to 3.5dB. If we were to deploy such an interface, it will cause problems in existing applications which try to use the new interface and find the supported loss budget less than expected. The new interface will require datacenter link engineering, as opposed to the plug-and-play paradigm that has made Ethernet so successful.
When discussing datacenter interfaces, it will be very helpful to always state both the reach and loss budget, for example 500m/2dB, 2km/4dB, or 10km/6dB, or something else. This way, there will be a clear understanding of what application is being addressed.
From: Anslow, Peter [mailto:panslow@xxxxxxxxx]
As previously announced, there is an SMF Ad Hoc meeting starting at 8:00 am Pacific today Tuesday 8 January.
I have currently received two requests for presentations, so the draft agenda is:
<![if !supportLists]>· <![endif]>IEEE patent policy reminder
<![if !supportLists]>o <![endif]>http://www.ieee802.org/3/patent.html
<![if !supportLists]>· <![endif]>Approval of the draft minutes from 18 December call
<![if !supportLists]>· <![endif]>Presentation
<![if !supportLists]>o <![endif]>100G-BASE-WDM4 optical budget constraints Yurii Vlasov, IBM
<![if !supportLists]>o <![endif]>System vendor perspective to NG100GE SMF interface Tek Ming Shen, Huawei
<![if !supportLists]>· <![endif]>Discussion
Both presentations are now available on the SMF Ad Hoc web page.
Peter Anslow from Ciena has invited you to join a meeting on the Web, using WebEx. Please join the meeting 5-10 minutes early so we may begin on time.
+44-203-4333547 (United Kingdom)
4438636577 (United States)
Pete Anslow | Senior Standards Advisor