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CHAPTER 3 FUNCTIONAL DESCRIPTION
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3.3.2 Setting the Cell Transmission Rate
(1) VBR, CBR, and UBR cell scheduling
The
μ
PD98405 has 16 VBR shapers that control the transmission rate of a created cell. The operation of
each shaper is based on the dual leaky packet algorithm. The algorithm parameters are set for each
shaper by the user. The user can set the use of a shaper for each channel.
Each of the 16 shapers have a priority. The shaper with highest priority can be used for CBR. The shaper
with lowest priority can be used for UBR.
Of the 16 shapers, one or more can be set as unassigned cell generators. Those shapers set as
unassigned cell generators function as shapers that transmit only unassigned cells at a rate specified by
the user. By using this unassigned cell generator function, all the channels can limit the bandwidth at
which the data will be transmitted.
(2) ABR cell scheduling
The
μ
PD98405 features an ABR scheduler that controls the ABR service transmission rate. It
automatically changes the transmission rate according to information of each received BRM cell.
3.3.3 Supporting Non-AAL-5 Traffic
The
μ
PD98405 has a function for processing packets as raw cells to support traffic other than AAL-5, or for
transmitting or receiving OAM cells. A VC set as a raw cell does not execute AAL-5 processing, such as adding
a trailer, when it is transmitted. Instead, it merely creates cells from the data and transmits those cells. When
such a VC is received, the 53-byte reception cell with a header is stored into system memory, together with 11-
byte indication information (For an explanation of the format, see
Section 5.5.8
).
The header processing performed for each cell and payload processing are executed, using software, by the
host.
The
μ
PD98405 also supports a function for inserting or verifying CRC-10 for non-AAL-5 traffic to mitigate the
processing by the host. When the user enables the insertion of CRC-10 by the transmission VC, the
μ
PD98405
calculates CRC-10 for each cell, and inserts the result of the calculation into the last 10 bits of the payload of
the cell. If verification is always performed and an error is detected as a result, an error occurrence flag is set
as the indication of the raw cell data and this is reported to the host.