XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
PRELIMINARY
REV. P1.0.1
240
frame synchronization with the incoming DS3
Frames.
Figure 88 presents a State Machine diagram that de-
picts the Receive DS3 Framer block’s DS3 Frame Ac-
quisition/Maintenance Algorithm.
4.3.2.1
The Receive DS3 Framer block will be performing
Frame Acquisition operation while it is operating in
any of the following states (per the DS3 Frame Acqui-
sition/Maintenance algorithm State Machine diagram,
as depicted in Figure 88 .)
The F-bit Search state
The M-bit Search state
The P-Bit Check state (optional)
Once the Receive DS3 Framer block enters the In-
Frame state (per Figure 88 ), then it will begin Frame
Maintenance operation.
When the Receive DS3 Framer block is in the frame-
acquisition mode, it will begin to look for valid DS3
frames by first searching for the F-bits in the incoming
Frame Acquisition Mode Operation
DS3 data stream. At this initial point the Receive
DS3 Framer block will be operating in the F-Bit
Search state within the DS3 Frame Acquisition/Main-
tenance algorithm state machine diagram (see
Figure 88 ). Recall from the discussion in Section
4.1
,
that each DS3 F-frame consists of four (4) F-bits that
occur in a repeating 1001 pattern. The Receive DS3
Framer block will attempt to locate this F-bit pattern
by performing five (5) different searches in parallel.
The F-bit search has been declared successful if at
least 10 consecutive F-bits are detected. After the F-
bit match has been declared, the Receive DS3 Fram-
er block will then transition into the M-Bit Search state
within the DS3 Frame Acquisition/Maintenance algo-
rithm (per Figure 88 ). When the Receive DS3 Fram-
er block reaches this state, it will begin searching for
F
IGURE
88. T
HE
S
TATE
M
ACHINE
D
IAGRAM
FOR
THE
R
ECEIVE
DS3 F
RAMER
BLOCK
'
S
F
RAME
A
CQUISITION
/M
AIN
-
TENANCE
A
LGORITHM
F-Bit Search
M-Bit Search
F-Bit Synch
Achieved
In-Frame
RxOOF pin
is Negated.
10 Consecutive F-bits
Correctly Received
Parity Check
(Only if Framing
on Parity is
Selected)
M-bits Correctly
Detected for 3
Consecutive M-Frames
(Framing on Parity is
Selected)
M-bits Correctly
Detected for 3
Consecutive M-Frames
(Framing on Parity is
Not Selected)
OOF Criteria
based upon values
for F-Sync Algo
and M-Sync Algo
Valid Parity
Parity Error in
2 out of 5 frames