6. Wavelength Multiplexing 143
such as from an outband signal in the data network. The alternative to
signaled data transmission is data switching using header information.
This is somewhat different from a typical ESCON or gigabit Ethernet
switch, which performs optical to electrical conversion, reads the data
header, sets the switch accordingly, and then reconverts the data to the
optical domain for transmission (sometimes retiming the signal to remove
jitter in the process). An example of outband switching networks has been
demonstrated [8]; in the approach, a data packet is sent simultaneously
with a header that contains routing information. The headers are carried
along with the data, but out of band, using subcarrier multiplexing or
different wavelengths. At switch nodes inside the network, the optical
signal is samples just prior to entering a short fiber delay line. While most
of the signal is being delayed, a fast packet-processing engine determines
the correct state of the switching fabric, based on the incoming header
and a local forwarding table. The switch fabric is commanded to enter a
net state just before the packet exists the delay line and enters the switch.
This method provides the lower possible latency for packet transmission
and removes the task of pre-calculating the arrival time of burst mode
data packets.
6.4 Protection and Restoration
Backup fiber protection or restoration refers to the multiplexer’s ability to
support a secondary fiber path for redundancy in case of a fiber break or
equipment failure. The most desirable is the so-called “1 + 1” SONET-
type protection switching, the standard used by the telecom industry, in
which the data is transmitted along both the primary and the backup
paths simultaneously, and the data switches from the primary to the
backup path within 50 ms (this is the SONET industry standard for voice
communications; while it has been commonly adopted by protocol inde-
pendent WDM devices, the effects of switching time on the attached
equipment depends on the application, as in the previous discussion of
latency effects). This is also known as an optical unidirectional protec-
tion switched ring (OUPSR); there are of course many variations, such
as a bi-directional protection switched ring (BPSR). There are different
meanings for protection, depending on whether the fiber itself or the
fiber and electronics are redundant. In general, a fully protected system
includes dual redundant cards and electrical paths for the data within
the multiplexer, so that traffic is switched from the primary path to the

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