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CHAPTER 5 SAR FUNCTION
152
P (peak rate)
The
μ
PD98405 usually transmits cells at the average rate set as I/M. If, however, any credits (C)
have been accumulated, cells are transmitted at the peak rate specified with P. The peak rate P is
set for a given VC based on the amount of space between consecutive cells, in units of cells. For
example, if P is set to 3, then an inter-cell space equal to three cells is always inserted. The value
of the peak rate will be 1/(P + 1) = 1/4. Furthermore, regardless of whether any credits have been
accumulated, the setting of P takes priority over that of I/M. For example, if I/M = 1, P = 3, and C =
2 are set (if the peak rate is lower than the average rate), cells are transmitted at the peak rate of
1/4, even during normal transmission.
C (credit)
When credits (C) have been accumulated, the
μ
PD98405 transmits cells at the peak rate. The
maximum number of credits that the shaper can accumulate is set in C (in VC units).
Credits are accumulated according to the following conditions.
Starting from the instant that a shaper is enabled (E = 1), that shaper is scanned and, whenever
the time to transmit a cell is reached, a credit is accumulated, one by one. That is, credits are
accumulated starting from before the T
X
_Ready command is issued, setting the shaper to the
active status. At the start of cell transmission, the accumulated credits (and any in addition to
this) are transmitted at the peak rate.
For a given shaper, whenever the cell transmission timing is reached, but the cell can not be
transmitted, a credit is added. For example, if a low-priority shaper can not transmit a cell
because of a timing conflict with a high-priority shaper, a credit is added to the credits of the low-
priority shaper.
For a shaper for which credits have been accumulated, and which is transmitting those credits
at the peak rate, a credit is added upon reaching the timing at which the cells are to be
transmitted at the average rate. Those cells in excess of the credits set in C are transmitted at
the peak rate.
That is, the maximum number of cells that can be transmitted at the peak rate (maximum burst
size) is determined from the I, M, P, and C parameters. (See
Section 5.4.4 (7)
for details.) The
maximum burst size (MBS) can be determined from the following.
MBS = (1/(P + 1)*C)/(1/(P + 1) - (I/M))
Because the credits set the number of cells to be transmitted at the peak rate, setting C = 1 results
in only one cell being transmitted at the peak rate, while a space that is transmitted at the peak
rate will be at least one cell in size. For example, when one VC is linked to one shaper, setting I/M
= 1/1, P = 0, and C = 1 causes transmission to be performed at a rate of 1/2. That is, when C = 1
is set, there will be occasions when transmission at the specified rate can not be performed. To