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2.0 Bt8230 Architecture Overview
2.2 Automated Segmentation Engine
Bt8230
ATM Segmentation and Reassembly Controller—SAR
N8230DS1F
2.2 Automated Segmentation Engine
The Bt8230 can segment up to 64k VCCs simultaneously. It provides full support
of the AAL5 and AAL3/4 protocols as well as a transparent or NULL adaptation
layer, AAL0. The Segmentation Coprocessor block independently segments each
channel and multiplexes the VCCs onto the line with cell level interleaving. For
each cell transmission opportunity, the Traffic Management Table is used to
determine which VCC’s data the Segmentation Coprocessor will send.
Each segmentation channel is specified as a single entry in the Segmentation
VCC Table located in local memory. These VCC Table entries define the negoti-
ated or contracted characteristics of the traffic for that channel. They are initial-
ized by the host either during system initialization or on-the-fly during operation.
An initialized Segmentation VCC Table entry effectively establishes a connection
on which data can be segmented.
The Host initializes the buffer descriptor chain. It then writes to the Host
Transmit Queue. The Segmentation Coprocessor reads the Transmit Queue and
copies its contents into a buffer descriptor in local memory. Next, the coprocessor
reads the buffer descriptor when it is time to process data. It retrieves data from
the buffer indicated by the pointer in the buffer descriptor.
The Segmentation Coprocessor then operates autonomously, formatting the
cells on each channel according to the host-defined Segmentation VCC Table
entries for each channel. The Segmentation Coprocessor formats the ATM cell
header for each cell based on the settings in the Segmentation VCC Table entry
for that VCC. For AAL5 and AAL3/4 traffic, it also generates the PDU-specific
fields in the trailer (and header for AAL 3/4) of the CPCS-PDU and places these
header and/or trailer fields in the appropriate cell for the segmented PDU. The
formatted cells are passed through the Transmit FIFO to the PHY interface for
transmission. For AAL0 traffic, which is intended for client-proprietary use, the
Segmentation Coprocessor segments the SDU to ATM cell payload boundaries
and generates ATM cell headers, but generates no other overhead fields. The user
has per-channel, per-PDU control of 52-Octet (Raw Cell) Mode segmentation,
wherein the Segmentation Coprocessor reads the entire 52-octet ATM cell from
the segmentation buffer and does not generate the ATM headers for the cells.
The Bt8230 reports segmentation status to the host in the Segmentation Status
Queue. The Bt8230 writes a segmentation status queue entry on either PDU
boundaries or buffer boundaries, selectable on a per-VCC basis. PDU boundary
status reporting is called Message Mode, while buffer boundary status reporting
is called Streaming Mode.