ATL Telecom OM25 - Manual (Page 19)

19
ATL User Guide
OM25 O Optical M Modem
5
5.2.1.3 LOOP TIMING
In Loop timing mode, the reference clock for the system is provided by the terminal (or network)
connected to the CO.
Fig 11 - Loop timing
The timing mode in the CPE depends on that selected in the CO:
CO Through CPE Through
CO Local CPE Local
CO Loop CPE Local
5.2.2 FRAMING MODES
There are two framing modes - unstructured and structured.
5.2.2.1 UNSTRUCTURED MODE
In unstructured mode, the user data passes transparently through the system. There are no Loss of
Frame Alignment (LOFA), Alarm Indication Remote (AIR) or Alarm Indication Signal (AIS) alarms.
The only alarm relevant to this mode is the Loss of Signal (LOS).
5.2.2.2 STRUCTURED MODE
In structured mode, the modem aligns to the incoming G703 frame from the terminal and extracts
the data from the selected timeslots. It also generates a G703 frame towards the terminal and
inserts the data payload into the selected timeslots.
LOFA is raised if the modem cannot achieve frame lock. Recognition of this alarm complies with
G706.
AIS is raised if there are less than 3 zeros in 512 bits on the G703 interface.
AIR is raised if bit 3 in the Not Frame Alignment Word (NFAW) is set to 1 for 3 consecutive
occurrences.
CRCs may be enabled or disabled. When enabled, errored blocks are indicated back to the terminal
by use of the E bits in timeslot 0. The CRC Fail alarm is active if CRC multiframe alignment cannot
be attained.
G703
CO
Tx
Rx
Terminal
Timing source
Terminal
G703
CPE
Tx
Rx
Timing looped
Timing
looped