RE: [EFM] RE: EPON TDMA
As PON is just a new media of Ethernet, the overall system will be a base on
"Switched Ethernet" architecture.
Under this architecture, bandwidth shaping and priority queuing will only be
done in the switch fabric. In the MAC and PHY, a mechanism which allow
flexibly assign the data rate may benefit the DBA implementation but DBA
algorithm will not be implemented as part of MAC and PHY layer function.
There is always trade-offs between delay and utilization. Reduce the guard
band and do the packet fragmentation will help the bandwidth utilization,
then the delay can be minimized. EPON is under the umbrella of Ethernet,
keep the Ethernet frame integrity is one of the religions of 802.3 team,
packet fragmentation is not considered as an option for the standard.
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From: glen.kramer@xxxxxxxxxxxx [mailto:glen.kramer@xxxxxxxxxxxx]
Sent: Friday, July 13, 2001 2:55 PM
To: zhangxu72@xxxxxxxxx; glen.kramer@xxxxxxxxxxxx
Subject: [EFM] RE: EPON TDMA
I think I know what confused you in the presentation as I got several
Timeslot is not an analog to a cell. While, from the slide 4 in the
presentation you may conclude that one timeslot is only large enough to hold
one maximum size packet, that is not the case. Timeslot in our example was
125 us, which equals to 15625 byte times. Then you can see that in the
worst case it will have 1518 + 4(VLAN) + 8(preamble)+12(IPG) - 1 = 1541
bytes of unused space at the end of timeslot (assuming there is data to be
sent and no fragmentation). With realistic packet size distribution (like
the one presented by Broadcom), the average unused portion of the timeslot
is only about 570 bytes. That gives channel efficiency of 96%, or
accounting for 8 us guard bands - 90%
DBA is a separate question. While it may be important for an ISP to have
DBA capabilities in their system, I believe it will not be part of the 802.3
standard. But a good solution would provide mechanisms for equipment
vendors to implement DBA. These mechanisms may include, for example, an
ability to assign multiple timeslots to one ONU or to have timeslot of
variable size. Grant/Request approach is trying to achieve the same by
having variable grant size.
Having small timeslots will not solve QOS either. Breaking packet into
fixed small segments allows efficient memory access and a cut-through
operation of a switch where small packets are not blocked behind the long
ones (and it assumes that short packets have higher QOS requirements). In
such a distributed system as EFM is trying to address (distances in excess
of 10 km) the gain of cutting through is negligible comparing to propagation
delay or even the time interval before ONU can transmit in a time-sharing
access mode (be that TDMA or grant/request method).
From: xu zhang [mailto:zhangxu72@xxxxxxxxx]
Sent: Thursday, July 12, 2001 7:01 PM
Subject: EPON TDMA
I had seen your presentation file about EPON TDMA in
PHY, it help me a lot to understand your EPON system.
We had developed the first APON system in china, when
I think of the TDMA of EPON, I think though the uplink
data rate is 1Gbits/s when shared by 16 or 32 users is
still not enough, so the dynamic bandwidth
allocate(DBA) protocal must be a requiremant
especially when take care of the QoS performance. In
DBA protocal, in order to achieve high performance the
time slot need be to small, I think why not we divide
the ethernet packet to 64 byte per solt, it is often
used in ethernet switch when store packet in SRAM.
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