Verilink AS2000: The Basics (880-502981-001) Product Manual - Manual (Page 171)

Acronyms and Definitions
Verilink Access System 2000: Basics Glossary-51
X
X.21 International Telegraph and
Telephone Consultative Committee
recommendation relating to
characteristics of interfaces
between data terminal equipment
and data circuit-terminating
equipment for synchronous
operation on a public, circuit-
switched data network (ISDN).
X-bits There are two X-bits (message bits)
per DS3 frame, located in the first
bit position in block 1 of subframe
1 and subframe 2. In any one DS3
frame the two X-bits must be
identical, either both ones or both
zeros. X-bits are used to transmit
“degraded seconds” from the far-
end of the system to the near-end
of the system when the far-end
terminal detects an incoming out-
of-frame or alarm indication signal
(AIS) condition by changing both X-
bits to “0” for one second.