The slot or cell has 53 octets which consist of and ACF field (one
octet), a header (4 octets), and a segment payload (48 octets).
Figure 4 shows the slot format.
Figure 4. QA Slot Format
The ACF field contains the BUSY bit which indicates whether the slot
has any information (BUSY=1) or it does not (BUSY=0). The SL_TYPE
indicates what type of cell the slot is (QA slot --> SL_TYPE=0, PA
slot --> SL_TYPE=1). The PSR field indicates whether the segment in
the previous slot may be cleared (PSR=1) or not (PSR=0). Finally, the
REQUEST field contains three REQ bits, one for each of three possible
priority levels. Each of the REQ_I bits (I=2,1,0) is set to 0 by the
head of the bus (HOB) and may be set to a value between 0 and 2 where
0 is the lowest priority level and 2 is the highest.
The QA segment is divided in a 4 octet header and a 48 octet payload.
The QA segment header contains 4 fields which are inserted by the AU
when it writes the QA segment to an empty slot. The fields are:
- The Virtual Channel Identifier (VCI) which is
20-bits long and helps in identifying to which virtual
channel the segment belongs to.
- The Payload Type which is a 2-bit long field
used to indicate the nature of the data to be transferred.
User data is identified with the PAYLOAD_TYPE=00 and all
other values are referred for future use.
- The Segment Priority which is 2-bits long and
is reserved for future use. It is set to the value 00
- TheSegment Header Check Sequence (HCS) which
is 8-bits long and provides for detection of single bit
errors in the QA segment header. The CRC is calculated
using the following standard generating polynomial:
G(x)=x8+ x2+x+1.
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