Understanding the network.pdf - Back to Home

• Line loopback/line build-out (LBO) facilities—Although the CSU is CPE, it is
also used by the telephone company for line testing and signal equalization.
The CSU can generate a test signal and loop itself to appear as if the circuit
is a functional state to the remote end Data Terminal Equipment (DTE) for
line and bit error rate (BER) testing and for signal loss measurement. It also
provides testing for proper transmission framing format, per channel BER,
and keepalive facilities. Actual loop testing is performed using automated
testing tools or the DSX-1 digital patch panel at the CO. Loop testing can
either be performed in-band with the actual data flow or out-of-band using
the FDL.
• Signal regeneration—The CSU is the last single regenerator on the circuit
path before the DTE.
• Transmission timing and ones density enforcement—T-carrier data
transmissions are synchronous and very sensitive to timing problems. The
circuit clock is maintained (and sometimes generated) by the CSU.
Transmission timing is maintained by counting the binary ones that are
contained in the datastream. In order for proper timing to be maintained, a
certain amount of ones must be transmitted in the datastream. This is known
as ones density (see the AT&T formula discussed previously). Ones density
enforcement is handled by the CSU. If the datastream is not in compliance,
the CSU inserts additional ones into the datastream.
• Basic diagnostics and performance data collection—The CSU provides circuit
status and an alarm indicator. It can also provide ESF statistics, calculate
and collect circuit error statistics, and store them locally for retrieval by the
carrier provider or the user.
A CSU is required for any device that connects to the T-carrier network. For channel
banks and PBXs, this is often a standalone unit. For data transmission applications,
it is incorporated with a data service unit.
The Data Service Unit
The data service unit (DSU) performs two functions: interfacing and diagnostics. It
provides a synchronous or asynchronous (V.35/TIA/EIA RS232) interface to the CPE
data terminal equipment. This interface performs the DTE-signal-to- bipolar-signal
conversion needed to transport digital data signals over the PSTN. This conversion
is accomplished by an asynchronous-to-synchronous converter that is part of DSU's
circuitry. For digital data service (DDS) applications that operate more than
56/64Kbps, the DSU has an integrated CSU, and it converts the unipolar DTE
bitstream from the router or gateway device. The DSU/CSU defines the framing
(AMI/B8ZS) used on the circuit and which T1 channels will be used to transport data
when fractional T1 or frame-based transport is being used for DDS. The DSU also
provides testing and diagnostic facilities such as BER testing, loopback facilities, and