[EFM] 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:
>
>ENDPOINTS: LOGICAL MACS AND MEDIA
>
> 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.
>
>ACCOMPLISHING THE EMULATION:
>
> 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.
>
>NET RESULT:
>
>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