MPC8280 datasheet(1111/1386 Pages) FREESCALE

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ATM AAL2

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

33-5

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 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 33.3.5.3, “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 33.3.3, “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 33.3.4, “No STF Mode”) enables the transmission of

cells that do not include the STF byte, thus allowing for 48-byte packets.

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