Freescale Semiconductor MPC8260 - Manual (Page 1067)

ATM AAL2
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 32-5
32.3 AAL2 Transmitter
The following sections describe the AAL2 transmitter.
32.3.1 Transmitter Overview
A transmitter cycle starts when the APC schedules an ATM channel number for transmission. The TCT is
fetched and the AAL type of the channel is checked. For AAL2 cells, the transmitter first handles
uncompleted packets from the previous cell of the current CID (partial and split cases) by filling the
beginning of the cell with the remainder of the last packet.
Then, the transmitter performs the priority mechanism (see Section 32.3.2, “Transmit Priority
Mechanism” ) in order to fill the cell with new packets. The priority mechanism determines the order in
which the TX queues are serviced. The transmitter continues to search for ready packets in the TX Queues
until either the cell is successfully filled with packets, or no more packets are ready but the cell is not yet
completed. In the first case the cell is simply sent. In the latter case, the optional Timer_CU (described in
Section 32.3.5.1, “AAL2 Protocol-Specific TCT” ) is examined. If the Timer_CU has expired, the
uncompleted cell is padded with zeros and sent; otherwise, the cell is temporarily stored in external
memory for the CP to attempt to complete it the next time the channel is scheduled.
The TX queues are the data structures that store the CPS packets and SSSAR frames. Each TX queue can
contain different CIDs. Each TX queue is maintained by a Tx queue descriptor (TxQD) that holds the
queue pointer and parameters to manage the queue.
When the transmitter fetches a packet out of an SSSAR TX Queue, it usually takes out of the SSSAR buffer
a number of octets equal to TxQD[Seg_Len] (see
Section 32.3.5.4, “SSSAR Tx Queue Descriptor ). The
channel CID is taken from the BD of the first buffer of the SSSAR frame (see Section 32.3.5.5, “SSSAR
Transmit Buffer Descriptor” ). A CPS UUI = 27 is used for all the in-frame packets until the last packet
from the SSSAR frame is sent. The last packet can optionally contain a per frame, user-defined UUI. After
an SSSAR buffer is completely sent, an optional interrupt event is issued to the host. Also, if an SSSAR
TX queue is empty an optional interrupt event is issued to the host.
In case of CPS TX Queue, the transmitter fetches the packet header out of a buffer descriptor and the
packet payload out of a CPS buffer (see
Section 32.3.5.3, “CPS Buffer Structure” ). The HEC in the packet
header is calculated by the CP or taken from the buffer descriptor based on the user configuration. After a
CPS packet is sent, an optional interrupt event is issued to the host. Also, if the CPS TX queue is empty
an optional interrupt event is issued to the host.
The optional partial filled mode (see Section 32.3.3, “Partial Fill Mode (PFM)” ) limits the number of data
octets per cell. This can be used to ensure that a cell does not contain a split packet or to limit transmission
to one packet per TX cell by setting a low partial fill threshold (PFT).
The no-STF (no start of frame) mode (see Section 32.3.4, “No STF Mode” ) enables the transmission of
cells that do not include the STF byte, thus allowing for 48-byte packets.
32.3.2 Transmit Priority Mechanism
The transmit priority mechanism operates in two modes: