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RE: [EFM] RE: EPON TDMA



Title: RE: [EFM] RE: EPON TDMA

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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