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ArchitectureNortel Networks Confidential 3–19
S8000/S8002/S8006 BTS Reference Manual
Frame alignment
Frame alignment is operated by the frame alignment byte occupying the TS0 in
every other frame.
Upon loss of frame alignment the alarm bit is set to 1 and the frame alignment
counter is increased.
The PCM alarms
Each PCM alarm corresponds to the detection of one anomaly type on the PCM
reception.
A seriously erroneous second is a second in which an NOS, SIA, RRA, or LOS
alarm condition occurred or an FE or CRC counter was overrun.
These alarms are indicated by LEDs on the front of the DTI board.
LOS: frame lock loss signal (LFA LED)
Frame lock is considered lost when three consecutive lock signals are received
with errors. This is also the case when bit 2 of TS 0 in frames that contain no
locking signal is received three times in a row with errors.
AIS: Alarm indication signal (AIS LED). Not used in the GSM 1900 frequency
band.
RRA or RAI: Remote Receive Alarm signal or Remote Alarm Indicator (RRA
LED).
NOS: no frame signal (NOS LED)
Upon detecting the beginning of an NOS fault signal, a 64–kbit/s SIA is
generated on each TS of the associated internal PCM link, an RRA is sent on the
external PCM link, and the application associates an NOS error to the errored
second, bringing on the NOS LED.
FE: Frame error (FE LED).
Used in case of 2 Mbit/s external PCM link. Error detection is effected only in
operation without CRC. The application counts the incorrect frame lock words
received, and compares them with the upper and lower thresholds
(programmable).
SKP: hop indication reception. Not used.
CRC: CRC error signal (CRC LED). Not used in the GSM 1900 frequency band.
The quest for CRC multiframe lock is associated with that for frame lock, to
ensure that the frame lock word found does indeed correspond to a single lock
word to which one can lock permanently.
The alarm LEDs on the front of the board are the LFA, AIS, RRA, NOS, FE, SKP,
and CRC. Each alarm event turns on the associated LED for a minimum time
of 200 ms.