T1 ESF CSU ACE User Manual Chapter 1. Introduction
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Loopback Tests
A number of different loopbacks can be invoked locally from the front panel, by T-Watch commands, or
remotely by using special inband codes (AT&T network loop-up and loop-down codes). Additionally, the
loopbacks can be remotely controlled by out-of-band commands using the T1 ESF FDL, or from T-Watch
using a modem connection. A discussion of network loopbacks and DTE interface loopbacks follows.
Network Loopbacks
There are two types of network loopbacks: line loopbacks and payload loopbacks. These are illustrated in
Figure 1-5.
• 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 - Similar to line loopback, except that the framing is extracted from the received data
and then regenerated for the transmitted data.
Figure 1-5. Network Loopback Tests
DTE Interface Loopbacks
Loops all data from the terminal interface (TI) back towards the T1. This loopback may be initiated using
front panel, EIA-232 control port, or T-Watch commands. The DTE (or external test equipment) must
provide any test pattern in order to check the DTE interface. See Figure 1-6.
Figure 1-6. DTE Interface Loopback
