
2.0 Architecture Overview
CN8236
2.3 Automated Segmentation Engine
ATM ServiceSAR Plus with xBR Traffic Management
2-12
Mindspeed Technologies
28236-DSH-001-A
2.3 Automated Segmentation Engine
The CN8236 can segment up to 64 K VCCs simultaneously. 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 xBR Traffic Manager tells the segmentation coprocessor which
VCC to send.
The CN8236 provides full support of the AAL5 and AAL3/4 protocols and a
transparent or NULL adaptation layer, AAL0.
Each segmentation channel is specified as a single entry in the segmentation
VCC table located in SAR-shared memory. A VCC specifies a single VC or VP
in the ATM network. These VCC table entries define the negotiated or contracted
characteristics of the traffic for that channel, and are initialized 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.
NOTE:
ABR VCCs occupy two table entries.
The host submits data for segmentation by first linking buffer descriptors that
point to the buffers containing the PDU to be transmitted, and then submitting
that chained message to the SAR by writing to one of 32 independent circular
transmit queues.
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 formatting functions include the following:
The segmentation coprocessor formats the ATM cell header for each cell,
based on the settings in the segmentation VCC table entry for that VCC.
The segmentation coprocessor also generates the CPCS-PDU header and
trailer fields in the first and last cell of the segmented PDU.
For AAL5 traffic, the SEG coprocessor also generates the PDU-specific
fields in the trailer of the CPCS-PDU, and places these in the last cell (the
End of Message [EOM] cell) for the PDU.
Each AAL3/4 cell carries 44 octets of payload and four octets (in five
fields) of header and trailer information. The SAR performs the formatting
steps necessary to create AAL3/4 cells.
AAL0 is intended for client-proprietary use. For AAL0, the segmentation
coprocessor segments the Service Data Unit (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 Raw Cell Mode
segmentation, where 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 formatted cells are passed through the transmit FIFO buffer to the
PHY interface for transmission.