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CHAPTER 4 INTERFACES
87
Multi-transmit cell transfer is supported by per-VC queuing. The
μ
PD98405 contains a cell buffer which
can contain up to 64 cells, for per-VC queuing. For a VC for which multi-cell transfer is enabled, the
μ
PD98405 allocates a cell buffer area and performs per-VC queuing. When the
μ
PD98405 has
allocated the area, it creates a queue for each VC in the buffer and reads transmit data by performing
multi-cell transfer. When multi-cell transfer is enabled for multiple VCs, it may not be possible to
allocate an area of the size required for a VC. In this case, the
μ
PD98405 reads as much data as the
size of the cell buffer areas that can be allocated. For example, when 5-cell transfer is enabled for VC1,
but only an area which can contain 2 cells can be allocated in the cell buffer, the
μ
PD98405 performs 2-
cell transfer. If no cell buffer area can be allocated, the
μ
PD98405 performs normal 1-cell burst transfer.
As the number of VCs for which multi-cell transfer is enabled increases, the more difficult it becomes to
allocate a cell buffer area, so multi-cell transfer may not be performed for VCs.
A cell buffer area is requested when the target VC is selected by the scheduler and has data to be
transmitted. When data received during multi-cell transfer has been stored into the transmit FIFO, the
cell buffer area is released. When a VC is active, the
μ
PD98405 does not always allocate a cell buffer
area for the VC.
A block diagram of per-VC queuing is shown below.
Figure 4-20. Per-VC Queuing Block Diagram
96 Cell Rx FIFO
10 Cell Tx FIFO
VC#1
VC#1
VC#2
D
64 Cell Buffer for per VC queuing
P
P