參數(shù)資料
型號(hào)： PXB 4330 E
廠商： SIEMENS AG
英文描述： ATM Buffer Manager(異步傳輸模式緩沖器管理器)
中文描述： 緩沖區(qū)管理器自動(dòng)柜員機(jī)（異步傳輸模式緩沖器管理器）
文件頁(yè)數(shù)： 50/61頁(yè)
文件大?。?/td> 551K
代理商： PXB 4330 E
PXB 4330 E
Semiconductor Group
50
Application Note 11.98
1.9
Threshold Determination Methods
1.9.1
One possible procedure for threshold assignment could be to separate the nrt traffic into
two basic classes, high priority VBR and low priority ABR, GFR, UBR. For VBR the MBS
is reserved for each individual queue/connection, to that under no condition an overflow
can occur. The lower priority traffic classes share the remaining buffer space (see
example 2, page 46. The PPD threshold restricting the buffer space for the low priority
traffic classes is dynamically adjusted at each setup or cleardown of a VBR connection.
The VBR queues are limited in size to MBS.
With conservative reservation buffer space may be wasted, as within the VBR
connections no sharing is done. In the following two methods are outlined for more
accurate threshold determination.
Conservative Threshold Determination
1.9.2
Threshold Measurement
The ABM includes the statistical counter LostCellsTrafClass(3:0) (see [
1
], page 47) for
discarded cells per traffic class.
During operation the conservative (relative) threshold values could be decreased
step-by-step by SW until cells are lost due to overflow of the buffer space reserved for
this traffic class (TrafClassMax). Then the threshold is increased again by a safety
margin.
1.9.3
In this section the following assumptions are made:
n queues share a buffer
the output load is <100%, i.e. the sum of guaranteed rates is less than the output rate
each queue receives equal-distributed (Bernoulli-distributed) packets
all packets have identical length
the packet streams are all independent (no correlation)
With these assumptions the shared buffer of the queues can be set equal to a n:1
multiplexor which forwards packets with unlimited speed to an output queue (Figure
12
).
The size of this output queue is identical to the shared buffer space. The cell level
multiplexing results of statistical calculations described in [
2
] and derived in [
7
] are can
be applied to the packets.
These calculations derive the size of the output buffer in the lower part of Figure
12
for
a given cell/ packet loss probability depending on the output load. The output buffer size
is identical to the size of the shared buffer used by the queues (shown in the upper part).
Note that this equivalence only applies to the
size
of the shared buffer, the behavior of
a common output buffer is quite different to per-VC queuing in a shared buffer.
Threshold Dimensioning using statistical Methods
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