Ryan,
>> I
likewise disagree with your definition of DBA
These were not my definitions. Below are the definitions given in
Peter Dyson’s dictionary of networking:
Dynamic Bandwidth Allocation:
A method of bandwidth
allocation that subdivides the available bandwidth between multiple
applications almost instantaneously, to provide each application with just the
amount of bandwidth that it currently needs.
Statistical
Multiplexing:
Abbreviated stat
mux. In communications, a method of sharing a transmission channel by using
statistical techniques to allocate resources. A statistical multiplexer can analyze traffic density and
dynamically switch to a different channel pattern to speed up the transmission.
At the receiving end, the different signals are merged back into individual
streams.
If that is not
enough, below are few excerpts from a white paper on your company’s web site:
“Dynamic
bandwidth allocation (DBA) is the ability to quickly reapportion bandwidth on
the PON based on current traffic requirements. The granularity of allocation
can range from the coarse (per ONT) to the very fine (quality-of-service
[QoS]-based priority-level queues or even individual flows on each ONT). The
method of apportioning bandwidth can be determined by policy and subject to the
individual subscriber's service-level agreement (SLA) or contract with the
provider.
Strictly
speaking, if the PON is considered in its entirety, it is a limited resource
with fixed maximum bandwidth of either 155 Mbits/sec or 622 Mbits/sec. As long
as the total "instantaneous" bandwidth needs of all ONTs on the PON
do not exceed capacity, the PON can function as a delivery vehicle without
significant data accumulation in the ONT buffers. In essence that utilizes the
notion of statistical multiplexing (stat-muxing) of data traffic. Stat-muxing
relies on the fact that the bursty data traffic of user applications leads to
time-varying peaks and valleys in the data rates required. Stat-muxing exploits
the high likelihood that the peaks of the various applications' data rates will
not be concurrent. It allows the PON to be over-provisioned with more ONTs (and
hence, more customers).
DBA harnesses
stat-muxing by apportioning part of the instantaneously available bandwidth to
particular ONTs or to specific classes (implemented as QoS queues) within each
ONT, as when necessary in accordance with the SLA. From the point of view of an
ONT (or a queue within), the connecting "pipe" always has exactly
enough bandwidth to match the data rate at low PON utilization levels. When the
PON traffic load is high, DBA ensures the guarantees of the SLA first,
redistributing the remaining bandwidth in a predetermined manner.”
For me the
fundamental difference is that DBA is application aware. It relies on some specific mechanisms
in classifying the traffic and making its decision on how to allocate
bandwidth. Those mechanisms, as
Terawave’s paper mentions above, are based on policing, SLA, and contracts with
the provider, and is QOS-aware. In
that sense DBA was never part of Ethernet! I am not arguing that it should not be considered, I’m just
opposing your claim that “Ethernet has always had an inherent form of
DBA…”. What Ethernet always
had is statistical multiplexing.
It relies on the fact that a station’s burst rate considerably exceeds
its average transmission rate. No
notion of application or priority here.
If DBA and stat muxing are the same things, then the phrase “DBA
harnesses stat-muxing…” wouldn’t make sense, would it? Statistical nature of stat muxing means
that guarantees on bandwidth are non-enforsable, and the available bandwidth
changes dynamically not only based on user’s needs, but also based on behavior
of other users. I believe
providers won’t like it too much.
Even though
Ethernet has always had stat muxing (before 10 Gig Ethernet anyway), this does
not automatically make it a requirement for EFM. What is on the other end of ONU’s port is what pays
providers money. Would providers
want to give excess bandwidth for free? I don’t think so. If your burst does not fit into a pipe
buy a bigger pipe!
Glen
-----Original
Message-----
From: Ryan Hirth
[mailto:RHirth@xxxxxxxxxxxx]
Sent: Sunday, July 15, 2001 3:44
PM
To: 'glen.kramer@xxxxxxxxxxxx';
zhangxu72@xxxxxxxxx; Ryan Hirth
Cc: stds-802-3-efm@ieee.org
Subject: RE: [EFM] RE: EPON TDMA
Glenn,
I likewise disagree with your definition of DBA. This
definition sounds more like one for GFR. DBA is simply allocating
bandwidth to those who need it when they need it. Without DBA a system
must hold its data until its bandwidth becomes available (even though available
bandwidth may exist on the media). This delay can significantly affect
the performance of traffic such as IP.
I compare EPON to Ethernet because it is
"Ethernet Passive Optical Network" and this is a group of
802.3. I am open to changes from traditional Ethernet, however I believe
that we agreed to "... minimal augmentation of the MAC operation,
...". For this reason I think that all proposals should be compared
to that of Ethernet for behavior and performance at the MAC/PHY layer. I
am not opposed the OAM group studying SLAs, QOS, ... but that is at a higher
layer, perhaps in the OAM group. I believe that a 16 split PON should be
compared to a 16 port Ethernet device, and our simulations and modeling of the
different approaches should use this as a baseline. (I'm not limiting
this to GigE, it could be a channelized 10Mbit, or ...) In this fashion,
the end user should be able to replace a P2P with P2MP network and have minimal
perceived differences.
Ryan Hirth
Terawave Communications
rhirth@xxxxxxxxxxxx
(707)769-6311
-----Original Message-----
From: glen.kramer@xxxxxxxxxxxx [mailto:glen.kramer@xxxxxxxxxxxx]
Sent: Friday, July 13, 2001 11:33 PM
To: zhangxu72@xxxxxxxxx; RHirth@xxxxxxxxxxxx
Cc: stds-802-3-efm@ieee.org
Subject: RE: [EFM] RE: EPON TDMA
These are comments for both Xu's and
Ryan's postings.
First let's not mix stat muxing and
dynamic bandwidth allocation. These are
different concepts.
DBA is a method allowing
"just-in-time" bandwidth allocation to an
application that requires it. As an example, consider a
network carrying
voice and data. In the absence of voice traffic all the
bandwidth is given
to data traffic. When new voice call arrives "some
mechanisms" will reduce
the bandwidth available to data traffic and will allocate it to
voice
traffic. This bandwidth will be guaranteed to voice traffic
in a sense that
each voice packet won't need to struggle to get its share of the
bandwidth.
When voice call completes, the same "mechanisms" will
return the bandwidth
back to data traffic.
Statistical multiplexing is a way of
statistically allocating channel
bandwidth, i.e., stealing chunks of bandwidth when other users
(node) failed
to do so. "Statistical" nature means that
bandwidth available to a user
will converge to some fixed value only when averaged over long
observation
time. But there is no way to predict how much bandwidth will
be available
to a node in any given short interval of time.
Ethernet (specifically CSMA/CD) uses
statistical multiplexing. DBA, on the
other hand, was never part of Ethernet. But when we think of
Ethernet in
the First Mile, we realize that this is whole new world for the
Ethernet,
where it has never gone before. Suddenly stat muxing in its
current form
(CSMA/CD) becomes very harmful due to its statistical
nature. Yes, we want
to utilize bandwidth efficiently, but most importantly - we need
to provide
SLAs to users. CSMA/CD is a non-deterministic service:
packets may collide
some number of times and then be discarded. DBA in this new world
becomes
important, as we want to be able to deliver all services: voice,
video,
data, etc.
How this could be solved in EFM?
Let's first consider P2P solution as I see
it. In P2P deployment a very smart switch will be located in
CO. This
switch will monitor traffic for each user in terms of both volume
and
application. As the uplink bandwidth is clearly a limited
resource, the
switch will make an arbitration decision of which packets to drop
in terms
of both keeping the user within its pipe and maintaining some sort
of DBA
within each pipe. We hope the switch will be
SLA-aware. Of course, it will
be proprietary to each vendor how switch fabric will be
implemented. It is
higher level, above MAC and PHY, and the standard is not concerned
with it.
The point is that both decision of how to keep user within its
pipe and
execution of this decision are done in the CO.
Now consider P2MP. In the same way
as in P2P, higher layers in OLT will
make a decision how to keep user within its SLA. The only
difference is
that execution of this decision and ensuring DBA within user's
pipe are
delegated to an ONU. And if in P2P the switch may decide to give
entire
uplink bandwidth to one ONU, so in P2MP, the OLT may do so by
giving all
timeslots to one ONU, or just by making it one large
timeslot. Of course,
real implementation is a bit more complicated: changed ONU state
needs to be
propagated to OLT. This may be done through OAM communication
channels,
proactively of otherwise, and except increased delay has no side
effects.
Letting PHY be timeslot-aware is just a mechanism for ONU to
execute the
OLT's decision. OLT may choose to modify timeslot
assignments or size as
often as it deems feasible. Specific values of timeslot, frequency
of
updates, and algorithm used to make such decisions are all outside
the scope
of the project.
I readily agree with Xu's comment that we
need a model to analyze. Once EFM
graduates into a work group and technical work begins, I think we
will
proceed by building a simulation model for various approaches.
On a general note, I would like to suggest
to group members to refrain from
comments like "Ethernet never did that..." or
"Ethernet traditionally does
that...". Ethernet traditionally supported CSMA/CD, and
in 802.3ae it
doesn't anymore. And it never was used in WAN and now it
is. Ethernet
never had OAM, and now it will. Without fair amount of
"heresy" in each new
project Ethernet would never become ubiquitous protocol as it is
now. We
have PAR and objectives to govern our direction. Tradition and
religion is
not one of them.
Thank you,
Glen
-----Original Message-----
From: xu zhang [mailto:zhangxu72@xxxxxxxxx]
Sent: Friday, July 13, 2001 7:28 PM
To: Ryan Hirth
Cc: stds-802-3-efm@ieee.org
Subject: RE: [EFM] RE: EPON TDMA
I agree with Hirth's opinion, in order to
keep the
statistic multiplexing nature of ethernet, the DBA
is needed.
in a large time solt. such as 125us, if the ONU has
large traffic, the time solt may be not enough, if the
ONU has little traffic, the bandwidth utilization will
be reduced a lot. In a fixed size time solt, the DBA
is easy to implement, but in order to achieve high
bandwidth utilization the time solt need to be small,
when using variable size time solt, the DBA is hard to
implement, but it can keep statistic multiplexing
nature of ethernet and at the same time achieve high
bandwidth utilization.
I think whether the frame will be
segmented of not
segmented, how long the time solt will be,
the DBA or SBA(static bandwidth allocate£(c)£¬
using variable size time slot or fixed size time slot,
we need a model to calculate.
--- Ryan Hirth <RHirth@xxxxxxxxxxxx>
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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