363-206-295
Circuit Pack Descriptions
Issue 1
December 1997
7-53
Transmission Circuitry
7
Transmit Direction.
7
The 21D/21D-U OLIU circuitry selects the three STS-1
signals from the TSI, adds SONET transport overhead, then byte-interleaves and
scrambles the signal in a frame synchronous scrambler. The output from the
multiplexer is an STS-3 signal that is used to amplitude modulate the LED
transmitter, converting the electrical signal to an NRZ-encoded IS-3 optical signal.
Receive Direction.
7
In the receive direction, the optical receiver converts the light
pulses from an IS-3 optical signal to equivalent electrical pulses. The received
signal may be either NRZ- or RZ-encoded. The output from the optical receiver
goes into a timing recovery device. The device recovers received clock (155.520
MHz) and uses it to retime the received data, providing a retimed STS-3 signal.
The demultiplexer circuit accepts the STS-3 bit stream, frames on the incoming
signal, descrambles it, demultiplexes it into three STS-1 signals, and processes
IS-3 transport overhead. The overhead information is sent to the OHCTL circuit
pack via the intra-shelf control bus and the transport overhead channel interface.
The STS-1 outputs from the demultiplexer are sent to the pointer processor that
performs pointer interpretation and generation on each received STS-1, using the
local timing signals. The output of the pointer processor is three STS-1 signals,
frame synchronous to each other.
Control Circuitry
7
The 21D/21D-U OLIU circuit pack interfaces with the SYSCTL and OHCTL circuit
packs.
The 21D/21D-U OLIU provides maintenance elements for reporting the status of
the circuit pack, status of the incoming STS-1 and IS-3 signals, as well as
inventory information (
CLEI code, date of manufacture, etc.). These maintenance
elements are used by the SYSCTL for fault detection and isolation. Conversely,
the 21D/21D-U OLIU responds to control signals from the SYSCTL, such as LED
operation and protection switching.
The 21D/21D-U OLIU accesses the IS-3 transport overhead and routes it via the
transport overhead channel interface to the OHCTL.
Timing Circuitry
7
Each TSI circuit pack sends timing signals to each 21D/21D-U OLIU circuit pack.
The 21D/21D-U OLIU monitors activity on both timing inputs. It normally selects
the signals from the active TSI as requested by the SYSCTL. If there is a failure of
the active input, the SYSCTL may command the 21D/21D-U OLIU to select the
OLIU input.
