Page 325 - Optical Communications Essentials

Network Management

Network Management 315

18.5. Element Management
This section addresses some of the concepts and functions related to monitor-
ing the overall network performance together with managing the various active
elements in an optical network and checking on their health and status. The
topics covered include BER measurements, wavelength assignment manage-
ment, monitoring the performance of various network elements, fault detection
and recovery techniques, and the implementation of separate wavelengths for
monitoring active devices.

18.5.1. Error monitoring
From the overall system point of view, the bit error rate (BER) is the main per-
formance parameter for any specific lightpath. The BER is calculated by the
receiving equipment after the optical signal has been converted back to an elec-
tric signal.
This BER calculation process is well established in networking equipment
that uses SONET or SDH as the underlying transport protocol. This protocol
contains an inherent set of parity-check bytes that are used for continuously
monitoring the information stream traveling from one SONET/SDH terminal
to another for errors. The ITU-T Recommendation G.709, or Digital Wrapper,
uses the same error-monitoring technique as is employed in SONET/SDH. The
performance metrics that are calculated in these protocols include coding viola-
tions in the incoming bit stream, the number of seconds in which at least one
error occurs, the number of seconds in which multiple errors occur (called sev-
erally errored seconds), and the total number of seconds in which service is not
available.
In local-area networks and other communication environments that use
Gigabit Ethernet (GigE) or Fibre Channel (FC) in place of SONET/SDH as the
underlying transport protocol, the SLA metrics need to be slightly different. As
described in Sec. 16.4, GigE and FC utilize 8B10B encoding in which 8 bits are
encoded into 10 bits. In this scheme only certain groups of bit patterns (called
code groups) are allowed. Thus, the 8B10B decoding process can be used for
error monitoring by detecting invalid code groups. In addition a process called
cyclic redundancy check (CRC) that makes use of a standard preset polynomial
normally is implemented in GigE and FC to check for errors. In this case the
CRC calculation is based on a binary division method involving 32 bits that con-
sist of the data portion of a packet plus a sequence of redundant bits.
Since BER monitoring has been implemented worldwide for many years,
numerous software packages of varying degrees of complexity are available
commercially. Often they are part of a larger network management software
system running on a management terminal. By means of a Java-based graph-
ical user interface (GUI), a network manager using such programs has the abil-
ity to carry out full system monitoring and control via the Internet. The
performance data gathered and calculated by these management packages can
provide an overall graphical view of the network, which typically consists of

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