XRT72L50 Datasheet, PDF (146/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
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The third C-bit (C13 or FEAC) in the first F-frame is used as the FEAC channel between the Near-End DS3
terminal and the Remote DS3 terminal. The FEAC channel carries:
• Alarm and Status Information
• Loop-back commands to initiate and deactivate DS3 and DS1 loop-backs at the distant terminals.
The FEAC message consists of a six (6) bit code word of the form [d5, d4, d3, d2, d1 d0]. This message is
encapsulated with 10 framing bits to form a 16 bit FEAC Message, as illustrated below. The FEAC signals are
encoded into repeating 16 bit message of the form:
0 d5 d4 d3 d2 d1 d0 0
1
1
1
1
1
1
1
1
Since each DS3 frame carries only one FEAC bit, 16 DS3 frames are required to deliver 1 complete FEAC
message. The six bits labeled "dx" can represent upto 64 distinct messages, of which 43 have been defined in
the standards.
NOTE: For more information on the transmission of FEAC Messages, please see Section 4.2.3.1.
FEBE - Far End Block Error (Only available for the C-bit Parity Frame Format)
F-Frame # 4 consists of 3 bit fields for the FEBE channel. If the Local Receive Section detects P-bit parity
errors, CP-bit errors or a framing error on the incoming (received) DS3 stream it informs the Transmit Section.
The Transmit Section then sets the three FEBE bits within an outgoing DS3 Frame to any pattern other than
"111" to indicate an error and then transmits this information out to the Remote Terminal (e.g., the source of the
errored-data). The FEBE bits in the outbound DS3 frames are set to "111" only if both of the following
conditions are true:
• The Receive DS3 Framer has detected no M-bit or F-bit framing errors, and
• No P-Bit parity errors have been detected.
• No CP-Bit errors have been detected.
NOTE: For more information on the Transmit Section's handling of the FEBE bit-fields, see Section 4.2.4.2.1.9.
The Yellow Alarm or FERF (Far-End Receive Failure) Indicator
The X-bits are used for sending Yellow Alarms or the FERF indication. When the Receive Section of the
XRT72L50 within the Remote Receiving terminal equipment, cannot identify valid framing or detects an AIS
pattern in the incoming DS3 data-stream, the Framer can be configured such that the Transmit Section will
send a Yellow Alarm or a FERF indication to the Remote Terminal by setting both of the X-bits to zero in the
outbound (returning) DS3 path. The X-bits are set to "1" during non-alarm conditions.
4.1.4 The Data Link Related Overhead Bits
UDL: User Data Link (C-bit Parity Frame Format Only)
These bit-fields are not used by the framer and are set to "1" by default. However, these bits may be used for
the transmission of data via a proprietary data link. These bit-fields can be accessed via the Transmit
Overhead Data Input Interface and the Receive Overhead Data Output Interface blocks.
DL: Path Maintenance Data Link (C-bit Parity Frame Format Only)
The LAPD transceiver block uses these bit-fields for the transmission and reception of path maintenance data
link (PMDL) messages via ITU-T Q.921 (LAP-D) Message frames.
NOTE: For more information on the LAPD Transmitter, see Section 4.2.3.2 and Section 4.3.3.2 .
4.2 The Transmit Section of the XRT72L50 (DS3 Mode Operation)
When the XRT72L50 has been configured to operate in the DS3 Mode, the Transmit Section of the XRT72L50
consists of the following functional blocks (Figure 30).
• Transmit Payload Data Input Interface block
• Transmit Overhead Data Input Interface block
• Transmit DS3 Framer block
• Transmit DS3 HDLC Controller block
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