XRT7250 Datasheet, PDF (309/463 Pages) Exar Corporation – DS3/E3 FRAMER IC

XRT7250
DS3/E3 FRAMER IC
REV. 1.1.1
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• The Frame Acquisition Mode: In this mode, the
Receive E3 Framer block is trying to acquire syn-
chronization with the incoming E3 frame, or
• The Frame Maintenance Mode: In this mode, the
Receive E3 Framer block is trying to maintain frame
synchronization with the incoming E3 Frames.
Figure 136 presents a State Machine diagram that
depicts the Receive E3 Framer block's E3/ITU-T
G.751 Frame Acquisition/Maintenance Algorithm.
5.3.2.1 The Framing Acquisition Mode
The Receive E3 Framer block is considered to be op-
erating in the Frame Acquisition Mode, if it is operat-
ing in any one of the following states within the E3
Frame Acquisition/Maintenance Algorithm per
Figure 136.
• FAS Pattern Search State
• FAS Pattern Verification State
• OOF Condition State
• LOF Condition State
Each of these Framing Acquisition states, within the
Receive E3 Framer Framing Acquisition/Maintenance
State Machine are discussed below.
The FAS Pattern Search State
When the Receive E3 Framer block is first powered
up, it will be operating in the FAS Pattern Search
state. While the Receive E3 Framer is operating in
this state, it will be performing a bit-by-bit search for
the FAS (Framing Alignment Signal) pattern, of
“1111010000”. Figure 137, which presents an illus-
tration of the E3, ITU-T G.751 Framing Format, indi-
cates that this framing alignment signal will occur at
the beginning of each E3 frame.
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