Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index


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£©£¨
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:
> 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
> -----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:
> 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:
> 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
> __________________________________________________
> Do You Yahoo!?
> Get personalized email addresses from Yahoo! Mail

Do You Yahoo!?
Get personalized email addresses from Yahoo! Mail