Verilink XEL XSP-100 SHARK IAP (9SA-USRS-9.0R1.02) Product Manual - Manual (Page 194)

Chapter 8- Maintenance/Diagnostics/Monitoring
Chapter 8-10 XEL P/N & Release: 9SA-USRS-9.0R1.02 SSHARK™ IAD User's Guide
Remote and Payload Loopbacks loop the selected T1 back to the network. The
Remote loopback loops the T1 data stream back to the network before the T1
module's Framer. The Payload Loopback loops the T1 data stream back to the
network after the T1 module's Framer.
The Framer Loopback and the Local Loopback loop the T1 data stream back to
the cross connect module within the SHARK™ IAD itself. The Framer
Loopback allows the T1 data stream to go through framer. Then the data stream
is looped back to the T1 module's framer towards the cross connect. The Local
loopback loops the T1 data stream back to the cross connect module after it goes
through the LIU's Jitter attenuation circuitry.