XRT72L56 Datasheet, PDF (169/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
PRELIMINARY
XRT72L56
REV. P1.1.2
The third C-bit (C13 or FEAC) in the first F-frame is
used as the Far End Alarm and Control (FEAC) chan-
nel between the Near-End DS3 terminal and the Re-
mote DS3 terminal. The FEAC channel carries:
• Alarm and Status Information
• Loopback commands to initiate and deactivate DS3
and DS1 loopbacks 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 mes-
sage 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 mes-
sage 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 up-
to 64 distinct messages, of which 43 have been de-
fined in the standards. For a more detailed discus-
sion on the transmission of FEAC Messages, please
see Section 3.2.3.1.
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 (Far-
End Block Error) channel. If the (Local) Receive Sec-
tion (within the Framer IC) detects P-bit parity errors,
CP-bit errors or a framing error on the incoming (re-
ceived) DS3 stream it will inform the Transmit Section
of this fact. The Transmit Section will, in turn, set the
three FEBE bits (within an outgoing DS3 Frame) to
any pattern other than "111" to indicate an error. The
Transmit Section will then transmit this information out
to the Remote Terminal (e.g., the source of the er-
rored-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: A more detailed discussion on the Transmit Sec-
tion's handling of the FEBE bit-fields can be found in Sec-
tion 3.2.4.2.1.9.
The Yellow Alarm or FERF (Far-End Receive Fail-
ure) Indicator
The X-bits are used for sending Yellow Alarms or the
FERF (Far-End Receive Failure) indication. When
the Receive Section (of the XRT72L56), within the
Remote Receiving terminal equipment, cannot identi-
fy valid framing, or detects an AIS pattern in the in-
coming DS3 data-stream, the Framer IC can be con-
figured such that the Transmit Section will send a Yel-
low Alarm or a FERF (Far-End Receive Failure) indi-
cation 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 da-
ta link. The user can access these bit-fields via the
Transmit Overhead Data Input Interface and the Re-
ceive 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. Please see Sections 3.2.3.2 and
3.3.3.2 for more information on the operation and
function of the LAPD Transmitter.
4.2 THE TRANSMIT SECTION OF THE XRT72L56 (DS3
MODE OPERATION)
When the XRT72L56 has been configured to operate
in the DS3 Mode, the Transmit Section of the
XRT72L56 consists of the following functional blocks.
• Transmit Payload Data Input Interface block
• Transmit Overhead Data Input Interface block
• Transmit DS3 Framer block
• Transmit DS3 HDLC Controller block
• Transmit LIU Interface block
Figure 42 presents a simple illustration of the Trans-
mit Section of the XRT72L56 Framer IC.
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