| Thread Links | Date Links | ||||
|---|---|---|---|---|---|
| Thread Prev | Thread Next | Thread Index | Date Prev | Date Next | Date Index |
-----Original Message-----I am not clear how the proposed preemption method works.
From: Necdet Uzun [mailto:nuzun@xxxxxxxxxxxxxxxx]
Sent: Wednesday, April 11, 2001 6:22 PM
To: Peng, Harry [SKY:1E11:EXCH]
Cc: Sushil Pandhi; Leon Bruckman; 'davidvja@xxxxxxxxxxx'; stds-802-17@xxxxxxxx
Subject: Re: [RPRWG] Cut through definition?Does a high priority transit packet preempt a low priority add packet?
Can a high priority add packet also preempt a low priority transit packet?
What happens if a previously preempted add packet contends with a same priority packet that was also preempted in an upstream node?
What happens if a previously preempted add packet contents with a same priority previously preempted transit packet that follows a high priority preempting transit packet with a clock cycle gap in between due to clock mismatch?
Do we require a SONET clock to be distributed on the ring?
Is RPR MAC layer one agnostic?Thanks.
Necdet
Harry Peng wrote:
Complexity what complexity:
In the tandem path, if a high priority packet can preempt a low priority packet at
arbitrary boundary then the preempted logic will have to deal with a tandem packet that
is already pre-empted.
This means the fastest pre-emption response time is on internal word size and the pre-empted packet
will have to pad to word boundaries to make live easier.
Furthermore the tandem receiver will have to respond to within one clock cycle as it is the
atomic size. What is the word size for 10G 64 bits 128 bits? What about for 40G or higher.Unless, you are will to have cells. Then why not use ATM.
I agree that we should keep it simple and not Segment packets.
Regards,
Harry
-----Original Message-----
From: Sushil Pandhi [mailto:Sushil.Pandhi@xxxxxxxxxxxxxxx]
Sent: Wednesday, April 11, 2001 10:33 AM
To: Leon Bruckman
Cc: 'davidvja@xxxxxxxxxxx'; stds-802-17@xxxxxxxx
Subject: Re: [RPRWG] Cut through definition?
I agree with Leon that 'pre-emption' will increase the complexity. ATM solves
this by segmenting the message into
smaller size cells and reassembling cells, and using this approach adds a lot
of complexity.If we do not have preemption, and assuming 1522 byte frame just starts
transmission before synchronouus traffic
can be sent, 1522 byte frame at OC-3 rate will take about 82.6 micro-seconds.
If we assume that in the ring, at 32
nodes the same situation arises then we have about 2.6 msec delay because not
doing preemption. So I doubt preemption
will give us much advantage.-Sushil
Leon Bruckman wrote:
> DVJ
> My personal view is that preempting lower traffic in the middle of a packet
> adds complexity that is not really needed. At 1G, the transmission time for
> a 1500 bytes packet is 12 usec, so the worst case for a 256 ring will be 3.1
> msec of added delay because of low packets being transmitted and not
> preeempted. Furthermore, the probability of the worst case is very small. We
> did some simulations with the following assumptions:
> - There is always a low priority packet being transmitted by the node
> - High priority packet may arrive at any time during the low priority packet
> transmission (equal probability)
> Some of the results were presented during the January interim (by Gal Mor).
>
> For a 128 nodes ring operating at 1G the preeemption gain will still be in
> the msec range with very high probability, and this can easily be absorbed
> by the jitter buffers at the receiver.
> Leon
>
> -----Original Message-----
> From: David V. James [mailto:davidvja@xxxxxxxxxxx]
> Sent: Friday, April 06, 2001 4:27 AM
> To: Carey Kloss; Devendra Tripathi
> Cc: stds-802-17@xxxxxxxx
> Subject: RE: [RPRWG] Cut through definition?
>
> All,
>
> Relative to the discussion of cut through, et. al.
> My perception is that a cutthrough node has two
> insertion buffers, for classA (provisioned synchronous)
> and classB (provisioned asynchronous).
>
> The preferred transmit order is as follows:
> a) classA insertion buffer (always)
> b) classA transmit traffic (subject to provisioned rate)
> c) asynchronous traffic.
> The classA insertion buffer only needs to be the size of
> the maximum packets sent by this node, plus (perhaps) some
> extra symbols to deal with hardware decoding latencies.
>
> The classB insertion buffer is to deal with the accumulation
> asynchronous packets that occurs when (worst case) full asynchronous
> is coming in/out and rate-limited synchronous is being transmitted.
> The size of the classB buffer is on the order of several upsteam-link
> delays times rateOfSynchronous/rateOfLink ratio.
>
> Order of the asynchronous traffic (c) depends on the classB
> buffer-filled status, prenegotiated vs. consumed rates, and
> the size of the asynchronous backlog in the client.
>
> The asynchronous transmit buffer is a bit schitzophrenic on its
> behavior. It should be in the client (not the MAC) because that
> allows packets to be reordered/inserted/deleted until the just
> before transmission time. However, the amount of traffic in the
> asynchronous transmit queue may influence the MAC queue-selection
> and throttle-signal assertion properties.
>
> I personally favor allowing cut-through synchronous traffic to
> preempt asynchronous, even in the middle of a packet. That's
> yields the lowest possible jitter, but at some encoding complexity
> costs.
>
> DVJ