NEC Network Controller uPD98502 - Manual (Page 270)

CHAPTER 4 ATM CELL PROCESSOR
270 Preliminary User’s Manual S15543EJ1V0UM
4.8.2.6 LLC encapsulation
If LLC encapsulation is indicated in Tx VC table, ATM Cell Processor adds the LLC header to the top of the IP
packet. ATM Cell Processor always encapsulates CPCS-PDU as Internet IP PDU.
Figure 4-31. LLC Encapsulation Format
LLC AAH-AAH-03H
OUI 00H-00H-00H
EtherType
08H-00H
IP PDU
2 bytes
3 bytes
3 bytes
4.8.3 Receiving function
Receiving data structure is described in Section 4.5.2 Rx pool structure.
4.8.3.1 Receiving procedure
(a) Setting up a receive pool
Before receiving a packet, V R 4120A prepares the receive pool to store receive cell data and sets the
information about the pool in the pool descriptor in Work RAM.
(b) Opening the receive channel
When V
R
4120A is going to open a new connection, V
R
4120A issues Open_Channel command. If
Open_Channel command is issued, ATM Cell Processor assigns a VC Table block in Work RAM and reports
its start address to V R 4120A using a command indication. V R 4120A sets the assigned block as a VC table.
(c) Setting the receive VC table
V R 4120A sets the 16-word assigned blocks in Work RAM as the receive VC table for each VC.
(d) Setting up the Rx lookup table
The VPI/VCI of the receive cell are set in the Rx lookup table.
(e) Receiving the first cell of the packet
Upon ATM Cell Processor receiving a cell, it checks if the VPI/VCI of the received cell has been registered in
Rx lookup table. If not registered, the cell is discarded. If registered, ATM Cell Processor gets the VC number
from Rx lookup table. If it is a first cell of a packet, ATM Cell Processor assigns the new buffer directory. The
VC is added to the T1 timer list.
(f) Receiving a cell
ATM Cell Processor transfers the received cell data from SAR FIFO to system memory using Scatter/Gather
DMA. In the same way as in transmission, the CRC-32 value, calculated for each received cell data, is stored
into the "CRC-32" field of the VC table. The “Current Count of Bytes” and “Remaining words in current buffer”
fields, in the VC table, are updated.
(g) Receiving the last cell
<1> The trailer information is checked for errors.
<2> A receive indication is stored into the mailbox and an interruption is issued.
(h) Reading the receive indication
V R 4120A reads the receive indication, updates the read pointer in the mailbox, and extract received data from
the pool.