
á
PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
175
bytes that contain information on: Path Overhead
Identification, Bit Interleaved Parity Calculation results,
Far-End Block Error status, and stuffing status. The
generation of PLCP frames can either be synchronized
to an external 8 kHz reference clock or to timing from
the Receive PLCP Processor. PLCP frame generation
can also be asynchronous with respect to any timing
signals. The Transmit PLCP Processor can compute
its “nibble-stuffing” requirements based upon its
configured synchronous timing source (e.g., the ex-
ternal 8 kHz reference clock or Receive PLCP
Timing), arbitrarily controlled via an external pin or by
following a fixed stuffing pattern. Once a PLCP frame
is formed, it is routed to the Transmit DS3 Framer
Block of the UNI for transmission to the “Far End” Ter-
minal. Figure 46 presents a simple illustration of the
Transmit PLCP Processor and the associated exter-
nal pins.
Note:
The user has the option of taking advantage of the
full DS3 payload bandwidth by by-passing the PLCP
Processor altogether. This option will be referred to as
“Direct Mapping” and is discussed in Section 6.3.3.9
6.3.2
Description of the PLCP Frame and the
Path Overhead (POH) Bytes
The Transmit PLCP Processor receives ATM cells
from the Transmit Cell Processor. It then multiplexes
these cells with some overhead (OH) bytes and
frames this composite information into PLCP Frames.
Table 21 presents the byte format of a PLCP Frame.
F
IGURE
46. S
IMPLE
I
LLUSTRATION
OF
THE
T
RANSMIT
PLCP P
ROCESSOR
B
LOCK
Transmit PLCP
Processor
TxPFrame
8kRef
StuffCtl
TxPOHFrame
TxPOH
TxPOHIns
TxPOHClk
From Transmit Cell
Processor
To Transmit DS3
Framer
T
ABLE
21: F
RAME
F
ORMAT
OF
THE
PLCP F
RAME
PLCP F
RAME
2
BYTES
POI 1
BYTE
POH 1
BYTE
PLCP P
AYLOAD
53
BYTES
13–14
NIBBLES
A1
A2
P11
Z6
First ATM Cell
A1
A2
P10
Z5
ATM Cell
A1
A2
P9
Z4
ATM Cell
A1
A2
P8
Z3
ATM Cell
A1
A2
P7
Z2
ATM Cell
A1
A2
P6
Z1
ATM Cell
A1
A2
P5
X
ATM Cell
A1
A2
P4
B1
ATM Cell
A1
A2
P3
G1
ATM Cell
A1
A2
P2
X
ATM Cell
A1
A2
P1
X
ATM Cell
A1
A2
P0
C1
Twelfth ATM Cell
Trailer