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RFL Electronics IMUX Ap-Note 17
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ALL ONES.
Format.
Unframed All Ones (UA1) is sometimes referred to as Blue Alarm or Alarm
Indication Signal (AIS). UA1 is defined as a DS1 signal consisting only of
alternating line pulses, that is, containing value 1 in each payload and
framing position. This is different from Framed All Ones, a valid signal that
has specific applications, such as carrier of timing.
Transmitting UA1 affects overhead communication and payload.
Purpose.
UA1 is typically used in conventional systems to signal reception difficulties
to other nodes downstream. Upon detecting receive difficulties, a node may
send UA1 to its neighbor further down the line, continuing in the direction
of the failed signal. This UA1 can be passed on from node to node through
the whole network.
IMUX Implementation.
IMUX Multiplexers take no path-related action upon detecting reception
problems and therefore do not support UA1 generation in order not to affect
traffic on intact links. A Common Module of a Multiplexer will not transmit
UA1, but will report its reception in diagnostic counters.
Similarly, DACS will not send UA1 downstream in order not to affect traffic
on intact links.
When so configured, DACS can reroute traffic from a failed link onto a
backup path. UA1 signaling is used by DACS to assist in path switching,
because it can be detected fairly quic
kly (in particular with Fast Loss of
Frame enabled), much faster than RAI. When path failure is detected by a
DACS port configured to operate in “Handshake” mode, bursts of UA1 are
sent back to the opposite side of DS1 link. This method improves switching
in asymmetrical faults as well as promotes correct recovery sequencing.
Another handshaking method is used to protect a switching network against
multiplexer failure. When a DACS port which is configured as “CM” (i.e.,
facing a Common Module) detects received-path failure, all DACS ports
transmit bursts of UA1. This results in the network switching around and
isolating the failed node.
DACS ports configured to operate in any mode will report UA1 reception in
diagnostic counters.
apnote17 page: 2 of 4 rdw revised 03/26/2002 printed 11/1/2006
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