363-206-285
Circuit Pack Descriptions
Issue 3 June 2001
7-143
General Description of Operation 7
The 27G-U/27G2-U OLIU, for each STS-1, inserts SONET transport overhead
bytes (line and section), and for VT1.5 applications, inserts STS path overhead
bytes and scrambles the resulting signal. Each STS-1 signal drives a separate
laser transmitter to create the two OC-1 outputs.
A received OC-1 signal is converted back to an electrical STS-1. This STS-1 is
descrambled and demultiplexed and transport/path overhead is accessed. Some
of the overhead (for example, section DCC) is passed via serial data links to the
control packs, while other bytes (for example framing, parity check) are processed
on board. The STS-1 signals go through a pointer processor to guarantee STS-1
frame alignment to the local system clock before being routed to the other main
and function unit slots.
If an incoming STS-1 signal from the fiber is to be VT1.5 cross-connected, VT1.5
pointer-processing is performed on this pack and STS-1 path overhead
termination is performed on these signals. All signals entering this pack from
either the backplane or from the optical interface may be routed as STS-1s. If
appropriate, VT1.5 routings may be established between STS-1s using a VT1.5
cross-connection.
Each 27G-U/27G2-U OLIU provides timing signals to and receives timing signals
from the TG circuit packs.
Detailed Description of Operation 7
Figure 7-50 is a block diagram of the 27G-U/27G2-U OLIU.
Transmission Circuitry 7
STS-1 Router and VT1.5 Cross-Connect. 7The STS-1 router is used to select
STS-1s for both the transmit and receive directions. All incoming STS-1s are fed
to the VT1.5 cross-connect. Outgoing STS-1s may be sourced by this VT1.5
cross-connect or sourced directly by an STS-1 input.
Transmit Direction. 7The STS-1 router selects two of the STS-1 signals directly
from the other main and function unit slots or from the VT1.5 cross-connect.
SONET path overhead is then added to STS-1s sourced by the VT1.5 cross-
connect before the signals are sent to the transport overhead processor. The
transport overhead processor takes the two STS-1 signals, adds SONET
transport overhead, then scrambles the signal with a frame synchronous
scrambler. The output from the transport overhead processor is in the SONET
STS-1 format and is used to amplitude modulate the laser transmitter, converting
the electrical signal to an NRZ-encoded SONET compatible OC-1 optical signal.
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