Motorola PowerQUICC II MPC8280 Series Reference Manual page 1173

— The UUI field is stored for the host into the RxBD after receiving the whole
SSSAR frame.
• SSSAR Transmitter
— Segmentation of SSSAR SDUs from SSSAR TX queue into CPS packets
— An SSSAR TX queue may contain several CIDs
— Performs all the above mentioned CPS transmitter functions
— A programmable segment length (Seg_Len) is copied from the SSSAR SDU into
the CPS packet payload, except for when at the end of the frame or buffer.
— UUI mode available. When UUI mode is enabled, the SSSAR UUI is copied
from the byte following the last byte of the frame.
33.3 AAL2 Transmitter
The following sections describe the AAL2 transmitter.
33.3.1 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 33.3.2, "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 33.3.5.1,
"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 33.3.5.4,
"SSSAR Tx Queue Descriptor). The channel CID is taken from the BD of the first buffer
of the SSSAR frame (see Section 33.3.5.5, "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
MOTOROLA
Freescale Semiconductor, Inc.
Chapter 33. ATM AAL2
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AAL2 Transmitter
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