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-----Original Message-----Ryan,
From: Roy Bynum [mailto:rabynum@xxxxxxxxxxxxxx]
Sent: Sunday, July 15, 2001 6:36 AM
To: Ryan Hirth; 'jc.kuo@xxxxxxxxxxxx'; glen.kramer@xxxxxxxxxxxx; zhangxu72@yahoo.com
Cc: stds-802-3-efm@ieee.org
Subject: RE: [EFM] RE: EPON TDMA
As a service provider, let me wade in here.
I does not matter how the bandwidth is allocated, as long as the interactive bandwidth is a known quantity that the customer can pay for and get what they pay for. The amount of bandwidth that each customer gets is more a function of what they are willing to pay for than what the service provider has available. In many cases it is the security of the data that is more important than the cost. In spite of several years of having VPNs available, of the availability of Frame Relay or ATM, Private Line services, particularly in Metro/Access, has remained very high. Small symmetrical bandwidths at rates well below the full capacity will be well received.
Using provisionable bandwidth assignment by allocating more or fewer time slots per customer is a traditional subscription service enabled by what is called "virtual concatenation". A decent size FIFO to rate adjust between the customers constant stream of traffic and the assigned time slots would be one way of doing that. The additional cost of the FIFO would still be less than the cost of a DSU/CSU in current technology services. The ability to do that variable bandwidth provisioning will be more of a system/upper level application than part of the PHY. That is one reason that those of us working on the OAM part of this have tried to stay away from the "Provisioning" part of the issues.
There is a major difference in the reliability and data stability of Ethernet compared to IP or FR. Even the SAR function of ATM induces a latency variance that does not meet inherent Ethernet quality. In lab testing we have found that Ethernet data stability and reliability are only exceeded by traditional TDM technology. Adding the ability to provision variable numbers of time slots for variable bandwidth provisioning only increases the market penetration that Ethernet access will have.
Thank you,
Roy Bynum
At 05:57 PM 7/13/01 -0700, Ryan Hirth wrote:
Ethernet has always had an inherent form of DBA in the fact it allows a station with traffic to send at up to the line rate or an arbitrated rate less than that. However in a connectionless system there are no service contracts or allocations of that bandwidth, but bandwidth of the media is divided dynamically. SLAs are features which do not belong in the Ethernet MAC layer, however dynamic bandwidth allocation is inherent within Ethernet and that is why Ethernet is so well suited for data traffic.
By creating fixed timeslots in the upstream you are changing the nature of Ethernet. Now the maximum bit rate of one station to burst upstream is limited to its timeslot. I believe according to the AllOptic presentation this would be 25 - 50 Mbps/ station (without DBA). This creates asymmetry which has never been an explicit form of Ethernet.
A new media for Ethernet should present similar characteristics of traditional Ethernets. There is certain level of service which Ethernet has. If you increase the latencies across the media ten fold, is it still Ethernet? The end user will perceive a difference in service.
Ryan Hirth
Terawave Communications
rhirth@xxxxxxxxxxxx
(707)769-6311
-----Original Message-----
From: jc.kuo@xxxxxxxxxxxx [mailto:jc.kuo@xxxxxxxxxxxx]
Sent: Friday, July 13, 2001 4:06 PM
To: glen.kramer@xxxxxxxxxxxx; zhangxu72@xxxxxxxxx
Cc: stds-802-3-efm@ieee.org
Subject: 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.
JC Kuo
jc.kuo@xxxxxxxxxxxx
Alloptic, Inc.
2301 Armstrong St.
Livermore, CA 94550
Phone: (925) 245-7641
Fax: (925) 245-7601
www.alloptic.com
-----Original Message-----
From: glen.kramer@xxxxxxxxxxxx [mailto:glen.kramer@xxxxxxxxxxxx]
Sent: Friday, July 13, 2001 2:55 PM
To: zhangxu72@xxxxxxxxx; glen.kramer@xxxxxxxxxxxx
Cc: stds-802-3-efm@ieee.org
Subject: [EFM] RE: EPON TDMA
Dear Xu,
I think I know what confused you in the presentation as I got several
similar questions.
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).
Thank you,
Glen
-----Original Message-----
From: xu zhang [mailto:zhangxu72@xxxxxxxxx]
Sent: Thursday, July 12, 2001 7:01 PM
To: glen.kramer@xxxxxxxxxxxx
Cc: stds-802-3-efm@ieee.org
Subject: EPON TDMA
hi, glen:
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.
best regards
xu zhang
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