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XRT79L71
PRELIMINARY
61
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
To accept both the payload and overhead bits from the other board within the Mid-Network Terminal
Equipment via the System Back-Plane
To extract out the CP-bit values within this DS3 data-stream that it accepts from the System Back-plane and
to re-insert these values into the CP-bit fields, within the outbound DS3 data-stream
To compute the EVEN parity over all payload bits, within a given outbound DS3 frame, and insert this value
into the P-bit positions within the very next outbound DS3 frame
To transmit this DS3 data-stream to the remote terminal equipment.
SOME REQUIREMENTS ASSOCIATED WITH THE MID-NETWORK TERMINAL EQUIPMENT
Once again, the key differences between the P and the CP-bits are listed below.
The values of P-bits are verified and can be recomputed as the DS3 signal passes through a Mid-Network
Terminal equipment which is neither a Source nor a Sink terminal equipment.
The values of the CP-bits as generated by the Source Terminal equipment MUST be preserved as a DS3
frame travels to the Sink Terminal equipment through any number of Mid-Network Terminal equipment.
Processing at the Sink Terminal Equipment
The purpose of the Sink Terminal Equipment is to accept and terminate a DS3 data-stream from the remote
terminal equipment, and to extract out or de-map non-DS3 data, such as ATM cells, PPP Packets, DS1 or E1
signals, etc., from this signal. As the Sink Terminal Equipment receives this DS3 data-stream, it will also do the
following.
Compute and verify the P-bits within each inbound DS3 frame
Compute and verify the CP-bits within each inbound DS3 frame.
Alarm and Signaling-Related Overhead Bits
The DS3 frame includes numerous bit-fields that are used to support the handling of alarms/defects and
signaling information. Each of these bit-fields is defined below.
The AIC (Application Identification Channel) Bit
The AIC bit-field is located in the third C-bit, within F-Frame # 1, as depicted in Figure 14. The purpose of the
AIC bit within the DS3 data-stream is to permit a given Terminal Equipment to determine, though not
conclusively, if it is receiving a DS3 signal that is of the M13/M23 framing format or is of the C-bit Parity framing
format.
For C-bit Parity Applications a given Transmitting DS3 Terminal Equipment will set the AIC bit to "1". However,
for Channelized M13/M23 applications, the AIC bit-position within a given DS3 frame is simply a C-bit that is
either set to "0" or "1" in order to denote stuff opportunities that either were or were not taken whenever this
particular DS3 signal was created by multiplexing the seven (7) lower tributary DS2 signals.
Therefore, if a given DS3 Terminal Equipment receives a DS3 signal, in which the AIC bit is set to "0", then the
Terminal Equipment can definitely conclude that it is receiving a DS3 signal that is of the M13/M23 Framing
format. However, if a given DS3 Terminal Equipment receives a DS3 signal in which the AIC bit is always set
to "1", then the Terminal Equipment can largely assume, but not definitely conclude, that it is receiving a DS3
signal that is of the C-bit Parity Framing format.
For information on how the Receive DS3/E3 Framer block handles the AIC bit, please see
Section 4.3.2.9.Alarm Indication Signal (AIS) Transmission
The Alarm Indication Signal (AIS) pattern is an alarm signal that is inserted into the outbound DS3 data-stream
whenever a service affecting defect or failure condition is detected within the upstream traffic by the Local
Terminal Equipment. Figure 18 presents an illustration in which the transmission of the AIS indicator would be
necessary.