Fiber Optic Essentials

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