[EFM] Point-to-Point plus Shared Media
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
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.
If one implements the n+1 (OLT) + 2n (ONUs) logical PHY approach, then
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
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
-- Norm Finn