mt90520 Zarlink Semiconductor, mt90520 Datasheet - Page 80

• Manufacturer Part Number: mt90520
• Description: 8-port Primary Rate Circuit Emulation Aal1 Sar
• Manufacturer: Zarlink Semiconductor

As in the UDT RX_SAR, the SDT RX_SAR is responsible for compensating for sequence number error conditions.
Up to 6 dummy cells may be inserted into the various SDT Reassembly Circular Buffers corresponding to the VC in
question.
The SDT RX_SAR compares the value of the SDT RX_SAR’s write pointer and the TDM module’s read pointer,
prior to the transfer of each cell to the SDT Reassembly Circular Buffers. The result of the comparison determines
whether a slip-error needs to be reported in the MIB statistics, and may also result in an adjustment of the write
pointer’s location before a cell is written to the SDT Reassembly Circular Buffers (see “SDT Overflow and Underrun
Detection” on page 84).
Pointer Processing
Within SDT-formatted cells, pointers are used to indicate the location of the first channel of data within an AAL1
structure.
A pointer can be received only from an even-numbered cell which is identified as a pointer-type cell because its CSI
bit is set. Pointers are received in this manner for all SDT VCs, except for non-CAS cells carrying one channel. In
that special case, no pointers should be received.
Once an even-numbered cell with a set CSI bit is received, the pointer byte of the cell is processed. First, some
error-checking procedures are performed:
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If the above error conditions are not met, the pointer byte is considered to be valid and is processed as such by the
SDT RX_SAR:
the pointer byte is examined for even-parity errors. If there is a parity error, the contents of the pointer byte
are ignored, the Pointer Parity Errors MIB statistic field in the SDT Reassembly Control Structure is
incremented, and the pointer-processing routine is exited.
if no parity error is found, the 7 least-significant bits of the pointer byte (the 8-bit pointer byte, less the parity
bit) are then examined to see if they exceed the maximum allowable “valid” pointer value of 93d (5Dh). If
either of the following cases is true, the pointer processing routine is exited:
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if the pointer byte has a value of 127d (7Fh), it is a “dummy” pointer (a valid pointer value); this pointer
value is ignored by the SDT RX_SAR.
if the pointer byte has a value exceeding 93d and does not equal 127d, it is an illegal pointer value;
therefore, the pointer value is ignored and a Pointer_Out_of_Range error is detected, causing the Po
status bit of the SDT Reassembly Control Structure for the VC to be set.
the pointer byte is automatically considered to be correct (based on the fact that there is a lot of
protection on the AAL1 byte, so bytes should not be received marked erroneously as pointers).
Therefore, the “golden” received pointer value is always used to determine where the SDT RX_SAR
should currently be operating within a VC’s AAL1 structure. Regardless of where the SDT RX_SAR is
pointing in the AAL1 structure, it is adjusted to point to the location identified by the received pointer byte.
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if the received pointer matches the SDT RX_SAR’s internal location controller, all is well and no errors
are reported.
if the internal location controller doesn’t match the received pointer, a framing/data misalignment error
has occurred:
- if the received pointer is equal to 0, the pointer misalignment is corrected immediately;
therefore, the Pointer Reframes statistic field in the SDT Reassembly Control Structure
is incremented.
- if the received pointer is not equal to 0, the misalignment between the internal
controller and the received pointer is not resolved immediately:
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MT90520
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Data Sheet