Cell Relay Archive

["Saso Stojanovski" <sasos@nortelnetworks.com>]

hi dan,




> I'm wondering about the simultaneous application of EPD and PPD. Is
> there a benefit to using both of these discard policies at once? If I

normally, PPD is always used in an ATM switch, with or without EPD.

this is because a non-PPD cell buffer is much worse than a simple FIFO
packet buffer, for obvious reasons.

while PPD is mandatory, EPD is optional, and, usually, very useful.


> understand correctly, then PPD would still catch the tail-end of
> frames that "snuck past" the EPD threshold. Ie. the beginning of the
> AAL5 frame was transmitted when the queue was just under the EPD
> threshold.

if the thresholds are adjusted properly, this kind of situation should
not arrise very often.



> Also, I'm wondering about how EPD threshold values are generally
> chosen. For systems that implement per-vc queueing, is it wiser to
> pick a percentage of the buffer size (what numbers are typical?), or
> subtract the expected AAL5 PDU size from the buffer size.

concerning some buffer management algorithms, you may have a look in the
atm forum tm-4.01 specification.
there is some iinformational stuff in the appendices concerning the GFR
service category (WFBA, DFBA, double-EPD schemes, or their derivatives).

as for the threshold values, i doubt there are firm agreed-on
guidelines.

in several papers i have read on this topic, the PPD threshold was set
as high as 90-95% of the buffer size, whereas the EPD threshold was set
lower than 50%.

you may also take a look on RED buffer management in the internet. the
"EPD" threshold there is usually set quite low, but the dropping
decision is taken on the average rather than instantaneous buffer
occupancy.

cheers,
    v
--saso