363-206-305
Circuit Pack Descriptions
Issue 3 June 2000
7-39
General Description of Operation
7
The T1EXT circuit pack terminates two bidirectional T1 lines complying with
standard T1 signal specifications. The T1EXT circuit pack interfaces to the OLIU
circuit pack at the VT-G rate (6.912 MHz) and to the SYSCTL.
The DS1PM and T1EXT circuit packs are, with respect to transmission,
functionally equivalent. In addition to performing all of the functions of the DS1
circuit pack, the T1EXT circuit pack provides performance monitoring for the DS1
rate transmit signals.
Detailed Description of Operation
7
Transmission Circuitry
7
Transmit Direction.
7
Figure 7-17 provides an overall block diagram of the T1EXT
circuit pack. The transmit direction points toward the VT-G and the receive
direction points toward the DS1s. In the transmit direction, the T1EXT receives
two balanced DS1 bipolar signals. Each of these signals passes through a relay to
a DS1 interface circuit that converts it to unipolar nonreturn to zero (NRZ) format,
recovers its clock, and sends it to the multiplexer (MUX) and framing (framer)
circuits. The MUX circuit converts each DS1 rate input into a 1.728 Mb/s VT1.5
signal and then byte-interleaves the four VT1.5s to create a VT-G signal that it
sends to the OLIU circuit pack, using the frame sync and clock received from the
OLIU.
Receive Direction.
7
The T1EXT receives a VT-G with frame information from the
OLIU and demultiplexes the VT-G into two unique VT1.5 signals. Each VT1.5
passes through circuitry that performs pointer interpretation, removes the VT path
overhead bits, the fixed stuff bits, and the overhead communications channel bits,
and desynchronizes the embedded DS1. The DS1 rate signal is AMI or B8ZS
encoded, preequalized with a selectable line build out (LBO), and then sent as a
balanced signal to Alternate Mark Inversion a DSX-1 that may be located up to
655 feet from the system.
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