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2.0 Architecture Overview
2.4 Automated Reassembly Engine
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
The user can, on a per-channel basis, establish Raw Cell mode reassembly. In
this mode the Header Error Check (HEC) octet is deleted to align the 53-octet cell
to 32-bit boundaries, and the Rsm coprocessor reassembles the entire 52-octet
ATM cell into the reassembly buffer.
The RS8234 provides the user with generous per-channel control of the reas-
sembly process, including:
Assignment of priorities for reassembly buffer return processing.
Cell filtering on inactive channels.
Mechanisms to establish per-VCC firewalling by allocating buffer credits
on a per-channel basis. This limits the possibility of one VCC consuming
all of the memory resources.
Per-VCC activation and control of a background hardware time-out func-
tion where the user selects one of eight programmable time-out periods.
The background function then automatically detects partially reassembled
PDUs and reports this status to the host so that these buffers can be recov-
ered and re-allocated.
Per-VCC monitoring of the length of the reassembled PDU, with status
reporting if the length exceeds a set maximum length for that channel.
The RS8234 implements an early packet discard feature to enable discarding
of complete or partial CPCS-PDUs based upon service discard attributes or error
conditions. The early packet discard function halts reassembly of the CPCS-PDU
marked for discard until the next Beginning of Message (BOM) cell and/or the
error condition has cleared. The SAR writes a status queue entry with the appro-
priate status flags set which indicate the reason for the discard. This function can
be enabled for the following conditions:
Frame Relay discard based on the frame’s Discard Eligibility (DE) setting
and the channel exceeding a user-defined priority threshold.
CLP packet discard based on the received cell’s CLP bit setting and
exceeding channel priority threshold.
LANE-LECID packet discard on ELAN channels which implements echo
suppression on multicast data frames.
Early packet discard on AAL5 channels when the reassembled PDU length
exceeds the user-defined maximum PDU length for that VCC.
Early packet discard on channels encountering a free buffer queue empty
(underflow) condition (meaning there are no available buffers in the free
buffer queue that channel is assigned to).
Early packet discard on PDUs when a DMA Incoming FIFO Full condition
occurs.
Early packet discard on channels encountering a reassembly status queue
full (overflow) condition.
Early packet discard on AAL3/4 channels with these MIB errors: ST_ERR
(Segment Type error), SN_ERR (Sequence Number error), and LI_ERR
(SAR-PDU Length error).
The system designer can set the reassembly status reporting for any channel to
either Message Mode or Streaming Mode. In Message Mode, a status entry is
written only when the last buffer in a message completes reassembly. In Stream-
ing Mode, a status entry is written for each buffer as it completes reassembly.