
XRT7245
DS3 UNI FOR ATM
á
PRELIMINARY
REV. 1.03
222
7.1.2.2.1
The Receive DS3 Framer will be performing “Frame
Acquisition” operation while it is operating in the
following states (per the “DS3 Frame Acquisition/
Maintenance” algorithm State Machine diagram,
as depicted in Figure 68.)
F-bit Search
M-bit Search
Parity Check (optional)
The Framing Acquisition Mode
Once the Receive DS3 Framer enters the “In-Frame”
state (per Figure 60), then it will begin “Frame Main-
tenance” operation.
When the Receive DS3 Framer is in the “frame-
acquisition” mode, it will begin to look for valid DS3
frames by first searching for the F-bits. At this “initial
point” the Receive DS3 Framer will be operating in the
“F-Bit Search” state within the “DS3 Frame Acquisition/
Maintenance” algorithm state machine diagram (see
Figure 68). Recall from the discussion in Section 6.4.2,
that each DS3 F-frame consists of four (4) F-bits that
occur in a repeating “1001” pattern. The Receive
DS3 Framer 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 Framer
will then transition to the “M-Bit Search” state within
the “DS3 Frame Acquisition/Maintenance” algorithm
(per Figure 68). When the Receive DS3 Framer
reaches this state, it will begin searching for valid
M-bits. Recall from the discussion in Section 6.4.2
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
F
IGURE
68. T
HE
S
TATE
M
ACHINE
D
IAGRAM
FOR
THE
R
ECEIVE
DS3 F
RAMER
’
S
“F
RAME
A
CQUISITION
/
M
AINTENANCE
” 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