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CHAPTER 5 SAR FUNCTION
162
(9) Cell transmission mode for a shaper in the scan status
A shaper that is in the scan status outputs linked VC cells. Once these cells have been transmitted, the
shaper returns to the non-scan status. The conditions governing whether the shaper returns to the non-
scan status vary with the following two modes. By changing the conditions under which the shaper returns
to the non-scan mode, it is possible to select whether the rate parameter set for the shaper is controlled in
VC units or shaper units.
A different mode can be set for each shaper. The mode is switched by setting the AGM bit of the
scheduler register.
(a) Normal mode (AGM bit = 0)
A shaper in the scan status transmits the cells of all linked VCs in order, one-by-one, then returns to
the non-scan status as soon as all the cells of the linked VCs (LST bit = 1) have been transmitted. In
this mode, the transmission rate set for a shaper is used to control the rate set for each VC. The
area that is occupied by one shaper depends on the number of VCs that is linked to that shaper.
(b) Aggregate mode (AGM bit = 1)
A shaper in the scan status transmits one cell to one of the linked VCs, then returns to the non-scan
status. If that shaper again enters the scan status, it transmits one cell to the next VC, according to
the link list. In this mode, the area for a shaper is controlled based on the set rate parameter.
According to the transmission rate for each VC, the area for a shaper is assigned to the linked VCs.
The area that is occupied by one shaper is not affected by the number of VCs that is linked to that
shaper.
In aggregate mode, the I, M, P, and C parameters are set in the units for the given shaper, rather
than in VC units. That is, the settings for I, M, P, and C are all satisfied by one unit for all the VCs
that are linked to that shaper. Each VC that is linked to a shaper in aggregate mode operates at the
rate that is assigned by those VCs according to the settings of I, M, P, and C. For example, if I/M =
1/4 is set for a shaper in aggregate mode, three VCs are linked to that shaper and each VC transmits
cells at a rate of 1/12.