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IBM2520L8767
IBM Processor for ATM Resources
atmrm.chapt03.01
08/27/99
Packet Header
Page 49 of 553
AAL_type #
This field specifies the AAL type to be used with this connection. The following values are decoded by
the hardware:
’0’x = Raw 48-byte mode. When an LCD with this value is encountered, the segmentation logic assem-
bles a cell from the four ATM header bytes in the LCD, a calculated HEC,
and the 48 bytes pointed to by the transmit offset.
’1’x = Raw 52-byte mode. When an LCD with this value is encountered, the segmentation logic assem-
bles a cell from the four ATM header bytes fetched using the transmit offset,
a calculated HEC, and the next 48 bytes following the ATM header.
‘2’x = Raw 53-byte mode. When an LCD with this value is encountered, the segmentation logic assem-
bles a cell directly from the 53 bytes pointed to by the transmit offset.
’3’x = Reserved
’4’x = MPEG blocking with PCR termination. This mode is handled the same as Fixed-size blocking
mode with one additional feature. An MPEG transport stream packet that
contains a PCR causes the current AAL5 frame to be terminated even if the
predefined number of packets has not been assembled into an AAL5 frame.
’5’x = AAL5 mode.
When an LCD with this value is encountered, the segmentation logic assem-
bles a cell in accordance with the AAL5 specification. The four-byte ATM
header is retrieved from the LCD, then the HEC is calculated and appended.
The next 48 bytes of data are fetched from the current segmentation pointer
and the CRC-32 is calculated over all 48 bytes. If fewer than 48 bytes of data
are available, the data is zero-padded to complete the cell. When all data in
the buffer has been exhausted, the last cell will contain the AAL5 trailer. The
CPCS user to user field in the AAL5 trailer depends on the state of the two
bits defined in INTST.
’6’x = Fixed-size blocking. When an LCD with this value is encountered, the segmentation logic assem-
bles a predefined number of packets of a predefined size into an AAL5
packet. The number and size of the packets are defined in two other fields of
the LCD. The length field of the LCD must be initialized with a value that is
an integral multiple of the predefined size and the predefined number of
packets. That is, if the predefined size is x’47’ (188 bytes) and the pro-
grammed number is 2, then the length must be initialized to n
188
2, or 188,
376, 564 etc.
’7’x = MPEG blocking with PCR delay and termination. This mode is handled the same as MPEG
blocking with termination but with one added feature. As well as terminating
an AAL5 frame, a transport stream packet that contains a PCR will be
delayed by the segmentation logic until an internal time base indicates that
the correct time has come for the packet to be delivered to the remote end.
In this mode, the first PCR containing transport stream packet causes the
segmentation logic to derive a PCR adjust value relative to an internal time-
base. All PCR-containing packets processed in the future use this derived
PCR adjust value to determine if the correct amount of time has passed indi-
cating that the packet should be forwarded to it’s destination. If the correct
amount of time has not passed, the last cell of the AAL5 frame is delayed
until the correct time. If the transport stream contains multiple PID values, a
bit in the SEGBF control register can be used to globally enable only match-
ing PID PCR recognition and delaying.
ATM_header #
This field contains the first four bytes of the ATM header.
segmentation_pointer
This field contains a pointer to the next data to be transmitted. In normal operation this field is initialized
by the cell scheduler when a new frame is queued for segmentation.
current_CRC
This field contains the CRC as it is being built.
BIP-16
This 16-bit field contains the Bit Interleaved Parity accumulated over the last block of data. After initial-
ization, this field should only be accessed by the hardware.
ABR Code Variables Definitions
(Page 4 of 7)
Field Name
Field Description