
PXB 4330
Operational Description
Data Sheet
4-56
09.99
4.7
Bandwidth Reservation
Due to the WFQ Scheduler concept of the ABM, the Connection Acceptance Check
(CAC) is very simple:
Check if the Guaranteed Rate of the connection fits within the spare bandwidth of the
Scheduler.
For the definition of the Guaranteed Rate, see
Table 3-1.
Mathematically, the CAC can
be reduced to the following formulas:
For all connections, verify that the Scheduler is not overbooked:
Sum of Guaranteed Rates
≤
Scheduler output rate
(1)
For real-time connections, (CBR, VBR-rt) in equation
(1)
is the only condition required.
For non-real-time connections or connections using the WFQ Multiplexer, additional
conditions must be fulfilled.
VBR, ABR and UBR+ connections must be setup in per-VC queuing configurations, that
is, an empty queue must be found for the connection. The Guaranteed Rate determines
the weight of the queue:
n
i
=
INT
(2)
with
n
i
∈
{1, 2, 3, ...15360} is the WFQ factor for the connection i
(1/n
i
is the weight factor W
i
)
GR
min
the defined constant defining the minimum rate to be guaranteed
Note: n
i
with a maximum value of 15360 due to hardware limitations.
Note: In addition to the hardware related tasks, a key system constant must be pre-
defined: the minimum Guaranteed Rate GR
min
. This parameter provides an
absolute value to the relative weight factors of the WFQ Multiplexers. It is usually
identical for all Schedulers in a system, as the Guaranteed Rate is related to the
service classes - and these are identical for all users, independent of where they
are connected to a network.
INT(x) the integer part of x.
The integer function in equation
(2)
selects the next smaller value of the integer n, that
is to say, the weight factor is higher than required and, thus, the queue is served slightly
faster in order to guarantee the rate.
For UBR connections without any rate guarantee, the following procedure is appropriate
for the WFQ Multiplexer:
Reserve one queue per Scheduler for all UBR connections
GR
m
in
GR
--------------------
2
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