Freescale Semiconductor MPC8260 - Manual (Page 1070)

ATM AAL2
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
32-8 Freescale Semiconductor
32.3.4 No STF Mode
The no-STF (no start of frame) mode enables the transmission of 48-byte packets by not including the STF
byte in the CPS PDU. The no-STF mode should always be used with PFT programmed to 47 in order to
prevent split and partial packets. For the AAL2 channels that use this mode, packets must not be larger
than a maximum packet size of 48.
The user activates this mode per ATM channel by doing the following:
1. Setting TCT[NoSTF]=1 and TCT[PFM]=1
2. Establishing a partial fill threshold by programming TCT[PFT]=47
The following figure shows a cell using no-STF mode:
32.3.5 AAL2 Tx Data Structures
The following sections describe the transmit connection tables (TCT) and the structures in which CPS
packets and SSSAR SDUs are stored in memory.
Cell
STF
AAL2 AAL2 AAL2 AAL2
packet 2 packet 3 packet 4 packet 5 Padding header
AAL2
packet 1
Transmit
1. Five packets fit exactly within the PFT limit
Cell
STF
AAL2 AAL2
packet 2 packet 3 Empty header
AAL2
packet 1
PFT 0
PFT
Cell
STF
AAL2
packet 2 Padding header
AAL2
packet 1
Transmit
PFT
2. Because PFT is less than the combined lengths of packet 1, packet 2 and packet 3,
the ATM cell is sent only with packets 1 and 2. (Packet 3 will be sent with the next cell.)
0
0
Remove packet 3
Cell
STF Padding header
AAL2
packet 1
PFT
3. Because the first packet exceeds the PFT value, the CPS-PDU consists only of this packet
(and the unused octets are padded with zeros).
0
Transmit
Cell
AAL2 packet of 48 bytes header
0