WDM: NA, part V
As these technologies advanced, the number
of wavelengths that could be supported within the
1550 nm transmission band increased. In 1994 and
1995, Pirelli and Nortel supplied four-channel
systems, and IBM introduced a 20-wavelength
system; 1996 saw the introduction of Ciena's
16-channel system, with other firms subsequently
rising to the same channel count. More recently,
32- and 40-channel systems have been announced
by several firms for 1998 availability. Advances in
WDM are now combined with the advancing pace of
time-division multiplexing (TDM). The capacity of
each WDM channel is rising, slowly, from 2.5 Gbs
to 10 Gbs. The result is that the realizable
transmission capacity of a single fiber will have
soared from 5.0 Gbs in early 1995 (two wavelengths
each carrying 2.5 Gbs) to 100 Gbs or more in 1998.
This is shown in Fig. 1, which plots the
maximum realizable transmission capacity of
single-mode fiber using commercially available
WDM and TDM telecommunications systems.
Why Use WDM?
It is not sufficient that the new technology
offer a vision of future capacity; it must offer
short-term economic advantages. In practice, there
are three basic alternatives that carriers can
consider in adding capacity to their networks: adding
more fiber; increasing the baud rate of transmission;
and now, dense WDM in combination with TDM.
The selection of which mechanism (or combination)
to use will be complex, but always based on
economic considerations.
Because fiber deployment is both costly and
immensely time-consuming, carriers typically want
to do this just once. In practice, all long-distance
networks have added considerable amounts of fiber
to their original backbones, but the fact remains that
this alternative is one that lacks the appeal of using
TDM or WDM to expand the capacity of an
installed network of fibers.
The "traditional" path to upgrading the
capacity of transmission networks has been to
increase the baud rate of transmission. Over the
past 15 years or so, bit rates in use in long distance
networks have been increasing at a relatively
steady pace: a fourfold rise in capacity every four
years. Transmission systems operating at 2.5 Gbs
(OC-48 in SONET parlance) have been on the
market since 1991 and have become the basic
building block of the networks, implying a need for
their successors in 1995. There were, however,
serious problems in moving to 10 Gbs (OC-192) as
the next stage of network evolution: |
