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CHAPTER 4 ATM CELL PROCESSOR
Preliminary User’s Manual S14767EJ1V0UM00
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(3) Cell scheduling
ATM Cell Processor uses Scheduling Table, Cell Timer and Tx VC table for scheduling. Before V
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4120A RISC
Processor starts transmitting a packet, it sets the rate information in Tx VC table. ATM Cell Processor calculates cell
transmission interval from the rate information, and put the next transmission time in Scheduling Table. When the Cell
Timer and the next transmission time of certain VC becomes equal, the cell belongs the VC is transmitted.
If the VC is CBR or UBR, only PCR( Peak Cell Rate) is used for scheduling, or if VC is VBR, PCR, SCR( Sustained
Cell Rate) and MBS (Maximum Burst Size ) are used. If the VC is ABR, the V
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4120A RISC Processor has to set all
the ABR parameters ( described in Section. 4.7.6) before packet transmission, and only ACR is used. However, every
time ATM Cell Processor receives RM (Resource Management) cell, it recalculates ACR using ABR parameters and
overwrites the value to ACR field in Tx VC table.
(4) Non AAL-5 traffic support
ATM Cell Processor also handles a cell as a raw cell, in order to support non AAL-5 traffic. For the VC which is set
as raw cell mode, ATM Cell Processor doesn’t execute any AAL-5 dependent operation, such as calculating CRC-32
and adding trailers, but just generate cells from data and transmits them. In receiving mode, ATM Cell Processor
stores cells with header and 11 bytes indication in SDRAM.
ATM Cell Processor has a function of CRC-10 insertion and verification for non AAL-5 traffic. If CRC-10 insertion is
set to enable, ATM Cell Processor calculates CRC-10 for each cell and insert it in the end of payload. ATM Cell
Processor always verifies CRC-10 in receiving cells. If ATM Cell Processor detects an error, it sets error flag in
indication and inform to the V
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4120A RISC Processor.
4.1.3.3 IP flow mapping
ATM Cell Processor supports dynamic mapping between IP address and ATM address. V
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4120A RISC Processor
sets the flow table prior to Tx_Ready command when it sends a packet with new IP address which has not been
registered in this Block yet. Flow table contains the parts of TCP/IP header information. V
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4120A RISC Processor
also reserves the VC table area if it uses new connection and set the parameters in VC table. After that, ATM Cell
Processor automatically recognizes the TCP/IP header from the transmitting packet, and chooses the VC
corresponding to the IP.
4.1.3.4 LLC encapsulation
If LLC encapsulation mode is set in the VC table, ATM Cell Processor adds the LLC header to the top of the IP
packet. In this case, ATM Cell Processor always encapsulates CPCS-PDU as Internet IP PDU. However, if ATM
mode Tx_Ready command is used, ATM Cell Processor does not execute encapsulation.