
XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
PRELIMINARY
REV. P1.1.1
135
Final Notes about the Transmit PLCP Processor
The Transmit PLCP Processor will be disabled, upon
power up or reset. Therefore, a “1” must be written to
this bit in order to enable the PLCP Processor. Selec-
tion of this bit affects both the Transmit PLCP Proces-
sor and the Receive PLCP Processor.
The advantage of selecting the “Direct-Mapped ATM”
option is to result in a more efficient use of the DS3
Bandwidth. This is because in the Direct Mapped ATM
mode, it is not required to include all of the POH bytes
that must be included in PLCP frames.
The Transmit PLCP Processor will inform the external
circuitry that a PLCP frame has been assembled and
transmitted out of the PLCP Processor by pulsing the
TxPFrame output pin ‘high’ during the transmission of
the last trailer nibble.
3.4
Transmit DS3 Framer
3.4.1
Brief Description of the Transmit
DS3 Framer
The Transmit DS3 Framer takes the incoming data, which
can be either PLCP frames from the Transmit PLCP Pro-
cessor or ATM Cells from the Transmit Cell Processor
and maps it into the payload portion of the DS3
frame. The Transmit DS3 Framer supports either the
M13 or C-Bit Parity frame formats. The Transmit DS3
Framer operates at 44.736 MHz and framing is derived
from an input clock signal. The framing overhead bits
are generated and inserted with the DS3 payload bits
to make up the complete DS3 frame. The DS3 frame is
then encoded into either the Unipolar, AMI or B3ZS line
codes. When the Transmit DS3 Framer is operating in
the C-Bit Parity Framing format, it provides an interface
that supports the transmission of path maintenance
data link messages on the outgoing DS3 frames via
the on-chip LAPD Transmitter. The Transmit DS3 Framer
also includes an on-chip Transmit FEAC Processor that
supports the transmission of FEAC (Far End Alarm and
Control) messages over the outgoing DS3 frame. Dif-
ferent transmission conditions like AIS (Alarm Indica-
tion Signal), Idle Condition and the Yellow Alarm can be
generated upon software command. Further, the LOS
(Loss of Signal) condition can be simulated upon
software command.
3.5
T
RANSMIT
E3 F
RAMER
3.5.1
Brief Description of the Tansmit E3
Framer
1
Direct-Mapped ATM Mode:
The PLCP Processor block is bypassed. ATM cells will be directly mapped into
the “outbound” DS3 Frame
T
ABLE
18: T
HE
R
ELATIONSHIP
BETWEEN
B
IT
3
OF
THE
UNI O
PERATING
M
ODE
R
EGISTER
AND
THE
R
ESULTING
“ATM C
ELL
” M
APPING
M
ODE
.
B
IT
3
M
APPING
M
ODE