ADTRAN DSU III AR User Manual page 148

Appendix G. Acronyms/Abbreviations
SW56 ................. switched 56
sync.................... synchronous
TD ...................... transmit data
TR ...................... data terminal ready
Tx ....................... transmit
UA...................... unavailable
UNI .................... user-to-network interface
VRC ................... vertical redundancy check
WAN.................. wide area network
G-4 DSU IV ESP User Manual 61204011L1-1