XRT72L50 Datasheet, PDF (253/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
The A Bit Functioning as the FERF bit-field
If the Receive E3 Framer block (at a Local Terminal) is experiencing problems receiving E3 frame data from a
Remote Terminal (e.g., an LOS, OOF or AIS condition), then it will inform the Remote Terminal Equipment of
this fact by commanding the Local Transmit E3 Framer block to set the A bit-field, within the next outbound E3
frame, to “1”. The Local Transmit E3 Framer block will continue to set the A bit-field (within the subsequent
outbound E3 frames) to “1” until the Receive E3 Framer block no longer experiences problems in receiving the
E3 frame data. If the Remote Terminal Equipment receives a certain number of consecutive E3 frames, with
the A bit-field set to “1”, then the Remote Terminal Equipment will interpret this signaling as an indication of a
Far-End Receive Failure (e.g., a problem with the Local Terminal Equipment).
Conversely, if the Receive E3 Framer block (at a Local Terminal Equipment) is not experiencing any problems
receiving E3 frame data from a Remote Terminal Equipment, then it will also inform the Remote Terminal
Equipment of this fact by commanding the Local Transmit E3 Framer block to set the A bit-field within an
outbound E3 frame (which is destined for the Remote Terminal) to “0”. The Remote Terminal Equipment will
interpret this form of signaling as an indication of a normal operation.
A detailed discussion into the practical use of the A bit-field is presented in Section 5.0.
5.1.1.2 The N Bit
The N bit is typically used to transport PMDL (Path Maintenance Data Link) information, from one terminal to
the next. However, the N bit-field can also be used to transport a proprietary data link, if configured according.
A detailed discussion into the practical use of the N-bit field is presented in Section 5.0.
5.2 The Transmit Section of the XRT72L50 (E3, ITU-T G.751 Mode Operation)
When the XRT72L50 has been configured to operate in the E3, ITU-T G.751 Mode, the Transmit Section of the
XRT72L50 consists of the following functional blocks.
• Transmit Payload Data Input Interface block
• Transmit Overhead Data Input Interface block
• Transmit E3 Framer block
• Transmit HDLC Controller block
• Transmit LIU Interface block
Figure 86 presents a simple illustration of the Transmit Section of the XRT72L50 Framer IC.
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