
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
XRT74L74
REV. P1.1.1
251
5.3.2.1
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
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
Frame Acquisition Mode Operation
Figure 88). Recall from the discussion in Section
5.1
,
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
valid M-bits. Recall from the discussion in Section
5.1
that each DS3 M-frame consists of three (3) M-
bits that occur in a repeating 010 pattern. The M-bit
search is declared successful if three consecutive M-
frames (or 21 F-frames) are detected correctly. Once
this occurs an M-frame lock is declared, and the Re-
ceive DS3 Framer block will then transition to the In-
Frame state. At this point, the Receive DS3 Framer
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