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RE: [EFM] RE: [EFM-P2MP] Point-to-Point plus Shared Media

The "roughly half the cost" issue may be far out-weighed by the cost to support the signaling at the ONU for a reflective design. Is there any additional savings for reflective in labor costs to install as well as in long term maintenance? Any additional benefits for life cycle costs would be interesting to know.

Given an average take rate of 80% population in a PON that has a 16 split capacity nets a 13 ONU scenario. If the cost of a reflective splitter is, for analysis sake, 800 monetary units versus 1600 for a transmissive splitter the savings of 800 monetary units spread over the 13 ONUs nets to be just over 61 units saved per ONU. The additional cost for more transceiver/electronics support will net out to be around 100 monetary units per ONU, which finishes the reflective approach as it does not reduce costs but actually increases them. If a scenario of 32 splits @ 80% take, or 26 installed ONUs, is used the negative impact is far higher.

I agree that the reflective approach may be more LAN-like, however the realm we are now working in is not so LAN-like by definition in many regards, this being only one of them. Therefore, the variance may have to be accepted.
The gain in bandwidth is impressive using reflective, I agree. My concern is with operation and cost. If the reflective design will simply operation and improve the cost, then I believe your point is well made and should be pursued. However, if the overall cost of the PON will increase, then the idea will be unacceptable, especially in FTTH.

One last point goes to compatibility with existing infrastructure, e.g. APON/BPON, and with the proposed FSAN GPON. These designs use a transmissive splitter approach and to look at dis-embedding (better marketing term may be "upgrading") them in the future will require as low a cost impact as possible. This means eliminating any need to replace splitters.


-----Original Message-----
From: Horne, David M [mailto:david.m.horne@xxxxxxxxx]
Sent: Wednesday, November 21, 2001 11:27 AM
To: 'John Pickens';;
Subject: [EFM] RE: [EFM-P2MP] Point-to-Point plus Shared Media

John, have you given any thought to the use of *reflective*
splitter/combiners, as opposed to the transmissive variety that is being
assumed for TDMA PON? It would be much more LAN-like; i.e. more true to
Ethernet operation.

In addition, the reflective splitter/combiner (tree coupler) would be
roughly half the cost of a transmissive coupler, since it has half as many
2x2 sections, with essentially the same loss.

Silicon costs and development time would also be much lower, since the
multiple access design complexity would be far lower (as would the
operational complexity of the overall network). The need for TDMA complexity
essentially disappears, since the reflected signal serves essentially the
role of CSMA in traditional Ethernet. Variable-size frames could be
transmitted without any explicit size reservation, and without any of the
waste associated with fixed slot size.  

As well, because there would be no request/grant protocol or 2-way
transit-time-delay wait time of the distribution fiber, transmission
efficiency is higher.  About 8 full-sized Ethernet frames of additional
capacity can be recovered (between any 2 user transmissions) from the 2-way
transit time of a 10km distribution fiber. This recovered capacity per user
is on par with the *allocated* capacity per user, for TDMA with fixed slots
size that was being discussed.  Not to mention no need for the processing
and scheduling delay for the request/grant at the headend, which recovers
even more of the capacity that is lost to the TDMA protocol overhead. 

Overall, the idea is that changing out one passive component in the outside
plant for another lower-cost passive component with the same signal loss
would allow a high degree of simplification in the design and operation of
PON, and an improvement in transmission efficiency. It would also be more
consistent with traditional Ethernet.

--dave horne

-----Original Message-----
From: John Pickens [mailto:jpickens@xxxxxxxxx]
Sent: Tuesday, November 20, 2001 10:49 AM
To: Norman Finn;;
Subject: Re: [EFM-P2MP] Point-to-Point plus Shared Media

Good clarification.

I would like to study one additional question related to this topic.

How can an operator offer the benefits (in the EPON link segment) of both 
point to point AND point to multipoint to a single endpoint beyond the ONU 
(e.g. personal computer concurrently a. viewing a 20Mbps HDTV video and b. 
engaging in a 400Kbps point to point instant messenger video/audio session) 
and also maintain the link efficiencies gained by point to point.

It is certainly possible to maintain separate networks to the end point - 
separate MAC in ONU, separate 100BT port in the ONU, separate ethernet 
LANs, and separate NICs in the personal computer (even better, separate 
personal computers).  What is less clear is how to converge the networks - 
and configure the networks (PC, LAN, ONU, OLT) so that the "right" traffic 
traverses the "right" path (instant messenger traverses point to point; 
HDTV traverses shared media).

It is also possible to limit the options here and say that an ONU can be 
either shared only or point to point only.  And to say that if 
single-copy-broadcast attribute of the media needs to be accessed, that it 
is acceptable to operate in shared mode (up to 50% reduction in link 
capacity if all ONUs require single-copy-broadcast).

I know there is a contingent within the working group that does not 
consider it a requirement to access the single-copy-broadcast attribute of 
the media, so probably we should poll this question at some point.


At 11:58 AM 11/14/2001 -0800, Norman Finn wrote:

>To clarify my comments at the 802.3 EFM EPON meeting on November 14 in
>  1. Assume an EPON with an OLT and n ONUs.
>  2. In the simplest case, the OLT has n+1 logical MACs.  n of them are
>     to-point MACs, and one of them is a shared medium MAC.  Each ONU has 2
>     logical MACs.  One of them is a shared medium MAC, and one is a
>     point MAC.  All of the ONU's shared medium MACs are on the same
>     shared medium as the OLT's shared media MAC.  The other n ONU MACs
>     point-to-point connections with the corresponding n OLT point-to-point
>     MACs.
>  2. In more advanced configurations, an ONU may have more than one
>     point logical MAC, which means that the OLT must have a corresponding
>     number of point-to-point logical MACs.  There may be more than one
>     emulated shared media, each additional emulated shared medium
>     a logical MAC on each participant, OLT or ONU.  One may even define
>     emulated point-to-point or shared media which connect ONUs only,
>     without a corresponding OLT logical MAC.  It all depends on how far
>     the committee wishes to take the ID/tag fields required to implement
>     the various features.
>  3. In order to emulate a shared medium, (or a point-to-point medium
>     between two ONU logical MACs), the OLT must reflect frames sent by
>     ONUs back downstream, so that the other ONUs can see them.  No such
>     reflection is needed for point-to-point ONU-OLT links.  If a frame
>     is reflected back to the ONU that transmitted it, the ONU absolutely
>     must discard that frame in order to maintain compatibility with
>     existing 802.3 devices, including routers, bridges, and end stations.
>  4. In the absence additional higher-level protocols, beyond the current
>     802.1 bridging protocols, there is not enough information in an
>     Ethernet frame for an OLT or ONU to make filtering decisions that will
>     both 1) filter unwanted data frames from the EPON stream, and 2) pass
>     data frames necessary for proper operation of a bridged network.  This
>     is true for both shared media emulation and point-to-point emulation.
>  5. In order to remedy this difficulty, one may use protocols above the
>     MAC layer.  Such higher layer protocols would allow bridges or other
>     devices to share information about their MAC address databases.
>     Such protocols would be extremely difficult to implement, and would
>     be likely to introduce significant delays in the delivery of frames.
>     Furthermore, no existing standard 802.1 bridge would work on an EPON
>     EFM link without such protocol augmentation.
>  6. Tags carried below the MAC layer solve the problem, as discussed
>     by several presenters.  In their simplest form, a tag on
>     a point-to-point frame identifies the logical MAC which is to
>     receive the frame, and a tag on a shared media frame identifies
>     which logical MAC generated the frame, so that that logical MAC
>     can discard the frame if or when it receives it, again.
>If one implements the n+1 (OLT) + 2n (ONUs) logical PHY approach, then
>one gets:
>  a. The ability to do point-to-point communications without incurring
>     any extraneous waste of bandwidth.
>  b. The ability to do point-to-multipoint transmissions (on the shared
>     media) without waste of bandwidth.
>  c. The ability to connect *existing* bridges with either point-to-
>     point or shared media -- or both.
>  d. Complete compatibility and interoperability with 802.1 and other
>     802.3 media, and interoperability with all existing 802.1 and .3
>     compatible devices, including hubs, bridges, routers, and end
>     stations.
>Additional complexity in the definition and use of the tags buys
>further flexibility in the point-to-point vs. shared media.  It is
>for further study to determine the best balance between complexity
>and flexibility.
>-- Norm Finn