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CHAPTER 5 SAR FUNCTION
164
5.4.5 Transmit Operation
The
μ
PD98405 takes out a transmit segment (48-byte payload data of a cell) from the packet stored in
system memory, adds the AAL-5 trailer or CRC-10 as necessary, and transmits a cell to the PHY layer device.
The
μ
PD98405 features an FIFO of 10 cells. During normal operation, the FIFO stores data received from
system memory, becoming empty once the cell has been transmitted to the PHY layer device.
The transmit machine reads a segment from system memory in the VC sequence as determined by the
scheduler.
The
μ
PD98405 organizes a cell from a segment (48-byte payload data) taken out of system memory and the
information on the cell header stored to the control memory. At this time, dummy data "00H" is inserted into the
HEC field of the cell header.
The GFC, VPI/VCI, PTI, and CLP fields of the cell header are taken out from Word 0 of the VC table and
generated. The first word of the VC table is updated by the
μ
PD98405 by receiving a packet descriptor for each
transmit packet.
In the case of the last cell, the
μ
PD98405 sets "1" for the LSB of the PTI field, while the CLP bit is set in the
CLP mode indicated by the VC table.
As the payload data of the cell, 48 bytes are read from system memory and embedded.
If the current transmit buffer indicates the last buffer, and if the remaining field of the VC table consists of
less than 40 bytes, the cell is the last cell of AAL5-PDU.
If the current cell is the last cell of AAL5-PDU and an 8-byte AAL-5 trailer can be added, the
μ
PD98405 adds
the trailer, padded with an appropriate number of zeros. If the 8 bytes of the trailer cannot be added to the last
cell, a cell containing only information on the trailer, plus padding, is added to the end and transmitted.
For AAL-5 traffic (packet descriptor AAL bit = 1), the transmit machine stores the result of the CRC-32
operation for each segment and the preliminary result of the packet length into the VC table each time it reads a
segment from system memory. Once the last segment of the AAL-5 PDU has been read, the final value of
CRC-32 and the packet length are inserted into the trailer of the AAL-5 PDU, and the contents of the first word
are inserted into the CPCS-UU and CPI fields to generate an AAL-5 trailer.
The transmit machine deactivates the VC once no more valid packet descriptors remain in the transmit
queue. The host detects there being no more valid packet descriptors in the transmit queue by locating a
vacant packet descriptor with V bit = 0 at the end of the list of the valid packet descriptors.