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3.0 Functional Description
3.4 Segmentation Coprocessor
Bt8230
ATM Segmentation and Reassembly Controller—SAR
N8230DS1F
3.4.8 Unspecified Bit-Rate Segmentation
Setting the RATE_MODE to UBR in the VCC structure causes a VCC to be seg-
mented from the unspecified bit-rate queue maintained internally by the segmen-
tation coprocessor. This option allows “best effort” service. During each cell slot
that does not have any rate-controlled (VBR) VCCs that attempt to transmit, a
UBR cell is sent. If there is no UBR data to send, an idle cell is transmitted. The
unspecified bit-rate queue contains all UBR VCCs that have data ready to seg-
ment. The VCC at the top of the queue transmits during the first available cell
slot. The VCC is then removed from the top of the queue and placed at the bottom
of the queue. The MAXPND field in the SEG_CTRL register needs to be set such
that the size is greater than the number of simultaneous active UBR connections.
The aggregate UBR transmission rate may be controlled with the Segmenta-
tion UBR Parameter Register [SEG_UBR; 0x30]. If the UBR_GCRA_EN bit of
this register is set, the state of the UBR GCRA is checked before each potential
UBR transmission. If the state of the UBR GCRA does not allow a cell to be sent,
an idle cell is sent instead. The I and L parameters for the UBR GCRA are given
by the SEG_UBR register. The I and L parameters are limited to: I + L is
≤
0x3FFFF.
NOTE:
The UBR rate was erroneously referred to as ABR in previous versions of
this datasheet. All instances of this error have been corrected. However,
Bt8230 register and bit names in software and software documentation still
reflect the erroneous ABR title.
3.4.9 Segmentation Restart
A segmentation VCC may halt because the segmentation data has been
exhausted. The VCC will automatically resume segmentation when more data is
available from the Host or Local Transmit Rings.
3.4.10 OAM, ILMI, and Signaling
OAM, ILMI, and signaling messages are management messages generated by the
host. These messages may be generated from Bt8230 memory instead of host
memory by setting the LOCAL option (VCC_CTRL[14]) in the buffer descriptor.
This may be done on a buffer-by-buffer basis. OAM F5 (Virtual Circuit Level
flow) flows have the same VCI/VPI as normal traffic but a different Payload Type
Indicator (PTI) field. This is accomplished by using the WR_PTI field
(VCC_CTRL[11]) in the buffer descriptor. OAM F4 (Virtual Path Level flow)
cells occur on a specific VCI by using the WR_VCI option (VCC_CTRL[10]) in
the buffer descriptor. The CRC_10 option (VCC_CTRL[4]) in the buffer descrip-
tor can be used for both F4 (Virtual Path Level flow) and F5 (Virtual Circuit Level
flow) cells to generate the required 10-bit CRC. Signaling and ILMI messages
occur on a specific VCI. A separate VCC data structure can be set up to handle
these messages.
OAM PM can be enabled for up to 127 VCCs by setting the PM field in the
VCC structure. For each monitored VCC, a block count and Bit Interleaved Parity
(BIP)-16 is maintained. A forward monitoring OAM PM cell is automatically
inserted at the end of each PM block. A backward reporting cell is generated
whenever the reassembly coprocessor writes a PM entry in the Reassembly/Seg-