RE: [EFM] RE: EPON TDMA
Roy,
I
agree with some of your points, however they seem to be more oriented towards
business usage than residential. EFM is oriented towards both, not just high
margin business services. For example, in DOCSIS, it is true that the CPE
equipment (a DOCIS Cable Modem) has much higher interface speeds than what the
user gets in committed bandwidth. In fact the number of users on a single 30Mbps
DOCSIS downstream pipe goes into the thousands. Yes, this is a low margin
business but this is made up for (as usual) by high volume. DOCSIS modems cost
about $100 and are on the verge of going to $50. This is true in spite of the
fact that they have interfaces running at much higher than the committed
bandwidth. DBA, at least in this case, is critical to allowing this sharing and
getting a high perceived bandwidth at orders of magnitude higher than what a SBA
scheme would provide.
It is
not unreasonable to ask that any MAC/PHY approach show how DBA may be
accomplished.
Hal
Ryan,
I hate burst your bubble. DBA is
has historically been a "boon doggle" for all services except the lowest
margin, best effort IP services. The service providers have had the
ability to provide DBA in the SONET and other TDM environments for
years. The problem is that customers have been unwilling to pay for the
equipment on their end that it takes to make use of DBA. In the IP
market, the customer still has to purchase equipment that has much higher
interface speeds than what he is paying for in committed bandwidth. His
instantaneous burst traffic is subject to major data loss, without warning,
causing his applications to do retransmissions and suffer performance
issues. This, in turn, burdens the service and data communications
infrastructure with data duplication and additional service inefficiencies.
DBA is more of a vendor pushed technology than something that has actually
worked out well.
Besides, EFM has not taken on the role of defining the
technology to do service provisioning. DBA is a provisionable service,
not a PHY level function. I believe that DBA is actually out of scope of
EFM.
Thank you,
Roy Bynum
At 05:39 PM 7/15/01 -0700, Ryan
Hirth wrote:
Roy,
I'm not opposed
to allowing these "small symmetrical" divisions of bandwidth to customers
within a SLA. However I do not believe they should be imposed by the
MAC/PHY layer.
DBA is a function of how the traffic is scheduled in the MAC/PHY
layer within the limits imposed by a SLA (if one exists). There should
not be any large FIFOs or significant cost differences in implementing
DBA.
DBA does
matter to you (the service provider) and your customers. DBA will
improve the performance your network allowing your customers to utilize more
of the bandwidth that they paid for. Data traffic is bursty by nature
and bandwidth allocation should follow that profile for best
performance.
Ryan Hirth
Terawave Communications
rhirth@xxxxxxxxxxxx
(707)769-6311
- -----Original Message-----
- From: Roy Bynum [mailto:rabynum@xxxxxxxxxxxxxx]
- Sent: Sunday, July 15, 2001 6:36 AM
- To: Ryan Hirth; 'jc.kuo@xxxxxxxxxxxx'; glen.kramer@xxxxxxxxxxxx;
zhangxu72@xxxxxxxxx
- Cc: stds-802-3-efm@ieee.org
- Subject: RE: [EFM] RE: EPON TDMA
- Ryan,
- 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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