Re: [EFM] RE: [EFM-P2MP] Point-to-Point plus Shared Media
David and Roy,
First of all, a disclaimer. I've been working with the group for some time,
and as such, I've listened to many different proposals for bandwidth
allocation on the Ethernet PON. I would like to make it clear that I'm not
associated with any vendor, nor do I have any commitment to a particular
technology, algorithm, or trade secret related to this particular topic.
I've been working with the requirements subgroup, and I'm supporting their
presentation. So please take this as a independent opinion on the technical
merits of the proposal; I'm not going to enter into any debate if it's time
or not to present new ideas, protocols or algorithms to the group.
There are some differences between what I called 'stateless' bandwidth
allocation protocols and 'stateful' protocols. David's proposal belongs
with the second family, as every ONU has to keep track of state information
to synchronize its transmissions.
One potential problem with 'stateful' protocols is that they're prone to
stability problems. If any member of the network lose a single control
message, you're set for trouble. For instance, it can happens if
connectivity is briefly disrupted at some point; it also can happen if a
ONU goes down unnoticed, effectively 'starving' everyone that is waiting
for it's transmission to end. Also, stateful protocols pose some security
concerns; a single forged control message has lasting effects over the
network.
Stateless protocols minimize these problems by making sure that the status
information is short lived, and is frequently refreshed. This helps to make
sure that every ONU will always have a recent copy of the control
information. Any instability is short-lived; at the next control
communication cycle, the information is refreshed. Stateless protocols also
tend to adapt faster to changing conditions; at every control cycle, the
allocation can be fine tuned. So I would suggest that any further
development of this idea to consider making the control message short-lived
- a single round would be fine.
Another potential development would be to use the ranging information to
sort the transmission sequence. By this method, stations closer together
would transmit in sequence, thus increasing the performance of the network.
Simulations can tell us if the performance gain is significant.
Carlos Ribeiro
CTBC Telecom