XRT79L71
PRELIMINARY
261
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
3.
To support the transmission of Alarms, Status and Data Link Information between the Near-End and the
remote E3 Terminals.
The E3 Overhead bits supporting each of these purposes are further defined below.
Frame Synchronization Bits - The Frame Alignment Signal (FAS) Bits
Each E3, ITU-T G.751 Frame contains a total of 10 bits that support frame synchronization. These 10 bits are
referred to as the FAS (or Framing Alignment Signal). A given Transmitting E3 Terminal Equipemnt will set the
FAS bits to the value "[1, 1, 1, 1, 0, 1, 0, 0, 0, 0]". The Receiving E3 Terminal Equipment will use these FAS
bits in order to acquire and maintain frame synchronization with the incoming E3 data-stream. For more
information on how the Receive DS3/E3 Framer block uses these bit-fields, please see SEE”THE FRAME- Performance Monitoring Bits - The BIP-4 Bits
Each E3 frame can be configured to carry a BIP-4 trailer (e.g., the last four bits of each given E3 frame). These
BIP-4 bits carries the BIP-4 (Bit Interleaved Parity - 4) value of the previous E3 frame for performance
monitoring. As a Transmitting E3 Terminal assembles an E3 frame (prior to transmitting this E3 signal to the
remote terminal equipment) it will (optionally) compute the Bit-Interleave-Parity - 4 (BIP-4) over this entire
outbound E3 frame. The Transmitting E3 Terminal will then insert the resulting BIP-4 value into the BIP-4
nibble position within the very next outbound E3 frame.
As a Receiving E3 Terminal receives a given incoming E3 frame, it will (if configured accordingly) locally
compute its value for the BIP-4 nibble.
Afterwards, this Receiving E3 Terminal will compare its locally-
computed BIP-4 value with the value of the BIP-4 Nibble within the very next incoming E3 frame. If these two
BIP-4 values match, then the Receiving E3 Terminal will presume that the first (of these two) incoming E3
frames received in an error-free manner. If these two BIP-4 values DO NOT match, then the Receiving E3
Terminal will presume that the first (of these two) incoming E3 frames was received in an erred manner.
For information on how the Receive DS3/E3 Framer block handles the BIP-4 nibble within the incoming E3
Alarm and Signaling-Related Overhead Bits
The E3, ITU-T G.751 frames includes two bits that are used to support the handling of alarms/defects and
signaling information. Each of these bit-fields is defined below.
The A (Alarm) Bit
According to the ITU-T G.751 Specification, the A is suppose to function as the FERF/RDI (Far-End Receive
Failure/Remote Defect Indicator) bit. The XRT79L71 also has provisions to permit the user to configure the "A"
bit to function as the FEBE/REI (Far-End Block Error/Remote Error Indicator) bit.
Each of these possible uses of the "A" bit are listed below.
Using the A Bit as the FERF/RDI Bit
According to the ITU-T G.751 Specification, the "A" bit carries the FERF (Far-End Receive Failure)/RDI
(Remote Derfect Indicator) bit within the E3, ITU-T G.751 frame. The purpose of the FERF/RDI bit is to permit
a given E3 Terminal Equipment to transmit a FERF/RDI indicator to the remote terminal equipment. Whenever
a Near-End E3 Terminal equipment declares a servicing-affecting defect condition (e.g., the LOS or the LOF/
OOF defect condition) within its incoming E3 data-stream, then it will inform the remote terminal equipment
(e.g., the source of this defective E3 signal) of this fact by transmitting the FERF/RDI indicator back out to the
Remote Terminal Equipment via the outbound (returning) E3 signal. This Near-End E3 Terminal will indicate
the FERF/RDI condition by setting the "A" bit-field, within each outbound E3 frame to "1" for the duration that
the Receive-related defect condition exists. Conversely, this Near-End E3 Terminal will indicate that it is NOT
transmitting the FERF/RDI indicator by setting the "A" bit-field to "0" for the duration that no Receive-related/
Service-affecting defect conditions exists. This concept of transmitting the FERF/RDI indicator in response to
Figure 114 presents an illustration of a given Near-End Terminal that is exchanging E3 data with a remote
terminal, in an un-erred manner.