Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

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

What is a "reflective" splitter/combiner?
Jack Andresen

"Horne, David M" wrote:
> 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.
> J
> 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
> Austin:
> >
> >
> >  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
> point-
> >     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-to-
> >     point MAC.  All of the ONU's shared medium MACs are on the same
> emulated
> >     shared medium as the OLT's shared media MAC.  The other n ONU MACs
> form
> >     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-to-
> >     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
> requiring
> >     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