Error and Alarm
monitoring (SDH alarms)
Large
numbers of alarm and error messages are an integral part of SDH networks. In
SDH, these are referred to as defects
and anomalies, respectively. They are
coupled to network sections and the corresponding overhead information. The advantage
of this detailed information is illustrated as
follows:
Complete
failure of a circuit results, for example, in a LOS alarm (loss of signal) in
the receiving network element. This
alarm triggers a complete chain of subsequent messages in the form of AIS
(alarm indication signals; see figure below .
The transmitting side is informed
of the failure by the return of an RDI alarm(remote
defect indication). The alarm messages are transmitted in fixed bytes in the
SOH or POH. For example, byte G1 is used for the HP-RDI alarm.
If
the received signal contains bit errors, the sensor indicates BIP errors. Since
this is not the same as a complete
failure of the circuit, the alarm here is referred to as ananomaly that is
indicated back in the direction of transmission.
The return message is called a
REI (remote error indication). Table below is
a list of all possible defects and anomalies and the corresponding bytes
and their meanings.
One of the weak
points of SDH is its inflexibility and coarse divisions of bandwidth channels:
for a 5 Mb/s service a 2 Mb/s (VC-12) channel is not enough while a 34 Mb/s
channel (VC-3) means low utilization. Virtual concatenation, makes it possible
to allocate several VC-n’s into a group (VCG) of bundled channels giving 2 Mb/s
granularity. The term “virtual” comes from the fact that the transport network
(ADMs/DXCs) need not know about or identify the different channels in the VCG,
which only is defined at the end-nodes. Dynamic channel re-sizing end-to-end
can be done using Link Capacity Adjustment Scheme (LCAS) signaling .
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