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Chapter 1. Introduction
18 TSU 600 User Manual 61200.076L2-1
Classes of Loopbacks
There are two classes of loopbacks: network interface
and DTE interface.
Network Interface Loopbacks
Network interface loopbacks (see Figure 1-9) affect the
entire T1 data stream. There are two types of network
loopbacks, line loopback and payload loopback.
Line loopback loops all of the received data back
toward the network. The transmitted data is the
identical line code that was received, including any
bipolar violations or framing errors.
Payload loopback is similar to line loopback, except
that the framing is extracted from the received data
and then regenerated for the transmitted data.
TSU 600
DS1
NI CSU
Payload Loopback
Line Loopback
Secondary
Interface
Figure 1-9
Network Loopback Tests
DTE Interface Loopbacks
DTE interface loopbacks affect only that portion of the
data stream programmed to that particular interface
port. The Nx56/64 serial interface offers two loop-
backs, a DTE loopback and a port loopback. See
Figures 1-10 and 1-11.
DTE loopback loops all data from the DTE back
towards the DTE. This loopback occurs just past the
interface circuit allowing a verification of the opera-
tion of the DTE to TSU 600. This loopback may be
initiated by using front panel or T-Watch commands.
The DTE (or external test equipment) must provide
any test pattern in order to check the DTE interface.
See Figure 1-10.