PXB4219 Datasheet, PDF (55/290 Pages) Infineon Technologies AG – IWE8 Interworking Element for 8 E1/T1 Lines

PXB4219 / PXB4220 / PXB4221
Operational Description
4.3
AAL Segmentation Functions
This function implements the Convergency Sublayer for Structured Data Transfer (SDT)
and Unstructured Data Transfer as well as the Segmentation Sublayer for AAL type 1 as
described in ITU-T recommendation I.363.1 [31]. The structure of AAL1 SAR-PDU is
shown in Chapter 12.
The Octet Receive Processing block is responsible for:
• Segmentation port decorrelation
• Segmentation
• SN/SNP generation
• SDT pointer generation
• RTS value insertion
• Statistics counter event generation
• Write to Segmentation Buffer
The Cell Receive Processing block is responsible for:
• Read cells from Segmentation Buffer
• Padding of partially filled cells
4.3.1 Operation
4.3.1.1 Segmentation Port Decorrelation
In synchronous systems, the microprocessor may activate a number of channels
consecutively, in phase with the segmentation period of a particular channel, causing a
large number of cells to be generated within the same 125 µsec period. This would result
in a large number of cells residing in the Output Queue and increase the Cell Delay
Variation (CDV).
To avoid this, a decorrelation circuit has been implemented in the “Octet Receive
processing” (OR), that inserts a random waiting period between activation of a channel
and start of cell segmentation. This feature can be activated by setting bit “dcor” in the
“AAL Receive Reference Slot” of the channel in RAM1. Otherwise segmentation is
started as soon as the channel has been activated by the microprocessor (field
“channel_mode”)
The decorrelation circuit consists of a free-running 5-bit counter at a frequency of F_CLOCK/
3360 (7.5 KHz if F_CLOCK= 25 MHz) a register containing a random number (bits
“dcor_random_nr”) and a comparator. Each time an octet for this channel is received the
counter is compared with the random value. Only when both values are equal,
segmentation is started.
When using the decorrelation circuit make sure that the random number is written to the
“dcor_random_nr” field of the “AAL Receive Reference Slot” before activating the
channel with “channel_mode”
Data Sheet
55
2002-05-06