XRT74L73 Datasheet, PDF (341/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
FIGURE 142. THE RECEIVE E3 FRAMER BLOCK AND THE ASSOCIATED PATHS TO THE OTHER FUNCTIONAL BLOCKS
To Receive E3 HDLC
Buffer
Receive Overhead
Data Output Interface
Receive Payload
Data Output Interface
Receive
E3 Framer
Block
From Receive E3
LIU Interface Block
Once the HDB3 (or AMI) encoded data has been de-
coded into a binary data-stream, the Receive E3
Framer block will use portions of this data-stream in
order to synchronize itself to the remote terminal
equipment. At any given time, the Receive E3 Fram-
er block will be operating in one of two modes.
• 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 143 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 143 .
• 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 144 , 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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