NEC Network Controller uPD98502 - Manual (Page 235)

CHAPTER 4 ATM CELL PROCESSOR
Preliminary User’s Manual S15543EJ1V0UM 235
(3) Cell scheduling
ATM Cell Processor uses Scheduling Table, Cell Timer and Tx VC table for the cell scheduling. Before the
V
R
4120A 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, a cell belongs to 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.
(4) AAL-2 support/OAM support
ATM Cell Processor also supports AAL-2 and OAM F5 function. For the further information, please refer to the
Application notes.
(5) Raw cell 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 the raw cell mode, ATM Cell Processor doesn’t execute any AAL-5 dependent operation, such as calculating CRC-
32 and adding trailers. In receiving mode, the ATM Cell Processor stores received cells with header and 11 bytes
indication in SDRAM.
ATM Cell Processor has a CRC-10 insertion and verification function for non AAL-5 traffic. In the case that CRC-10
insertion is enabled, ATM Cell Processor calculates CRC-10 for each cell and inserts the result in the end of its
payload at transmission side. ATM Cell Processor always verifies CRC-10 in receiving cells. If ATM Cell Processor
detects an error, it sets error flag in indication and informs to the V
R
4120A.
4.1.3.3 LLC encapsulation
When 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 IP PDU. However, if ATM mode
Tx_Ready command is used, ATM Cell Processor does not execute encapsulation.
Figure 4-5. LLC Encapsulation
LLC AAH-AAH-03H
OUI 00H-00H-00H
EtherType
08H-00H
IP PDU
2 bytes
3 bytes
3 bytes