
MOTOROLA
Chapter 35. ATM Overview
35-9
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
UTOPIA Operation
UTOPIA interface is higher than that of serial ATM which additionally implements the
transmission convergence (TC) layer.
The following sections describe the transmit and receive mechanisms for the UTOPIA
interface. The expanded cell option is also discussed. Detailed information about UTOPIA
35.7.1 UTOPIA Transmit Overview
The UTOPIA transmit process begins with the ATM pace control unit (APC). The APC
schedules the ATM trafc using a scheduling table and a user-congured APC timer (CPM
general-purpose timer 4) that denes the maximum transmit bit rate (bandwidth). The APC
maintains the trafc parameters for each channel and divides the total bandwidth among the
active channels. It can provide CBR, VBR and UBR trafc services. ABR can also be
supported through application software manipulation of APC parameters. See
Chapter 39,the APC.
With each tick of the APC timer, the APC prepares the channel(s) in the current time slot
for transmission by inserting the channel number(s) into the transmit queue. When the PHY
asserts the transmit cell available (TxClav) signal, the transmitter takes the next channel
number from the transmit queue. The transmitter uses the channel number to nd the
channel’s transmit connection table (TCT).
For AAL5, the transmitter then copies 48 bytes (or up to 65 bytes for channels congured
with expanded cells) from the external buffer, performs CRC32, copies the cell header from
the cell header entry of the TCT, and sends the complete cell through the UTOPIA
interface. For the last cell of an AAL5 frame, the transmitter appends the trailer of the
common part conversion sublayer-protocol data unit (CPCS-PDU) to the user frame. It
pads as required, appends the length (calculated during the frame transmit), and copies the
CPCS-UU and CPI from the TxBD. The transmitter also sets the PTI[1] bit in the header.
An interrupt can be optionally generated to declare the end of the transmit frame.
For AAL0, the transmitter simply copies the cell (except the HEC) prepared by the user
from the channel’s buffer and sends it through the UTOPIA interface. The ATM controller
can optionally generate CRC10 on the cell payload and place the result at the end of the
payload (CRC10 eld). This feature is used to support OAM CRC10; refer to the ITU
specication I.610 for additional details.
If, however, the current active channel’s buffer is not ready, the transmit process ends and
no cell is sent to the PHY. The PHY is responsible for generating an idle cell in an empty
cell slot. An empty cell slot will continue to be generated each time the APC schedules this
channel in the transmit queue until either a buffer is made ready or a TRANSMIT DEACTIVATE