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International Journal on Advances in Systems and Measurements, vol 5 no 3 & 4, year 2012, http://www.iariajournals.org/systems_and_measurements/
the electronic bottleneck. In spite of the fast overload of the
passive channel at ONUs, the use of the proposed enhanced
t‒ routing architecture, allows the COs to remain
congestion free. In consequence, the introduction of optically
bypassed channels leads to congestion avoidance, thus the
network can more efficiently support different traffic
requirements while reducing the power consumption.
In a future proof optical access scenario, each CO must
manage much more than 64 ONUs. However, the t‒λ routing
architecture cannot meet this requirement because it presents
a limitation in the number of ONUs managed by the CO.
The t‒λ frame is composed by the same number of
wavelengths as ONUs connected to the CO. Because of the
time division multiplexing, in average each ONU experience
a transmission rate
. The architecture was
proposed to introduce the optical bypass of only one passive
channel per CO [19]. In consequence, for each ONU the
optical bypassed traffic experience in average a transmission
rate of
, while the rest of the traffic experience in
average a transmission rate of . Therefore, as
the number of ONUs ( ) increases, the average transmission
rate of the passive channel perceived by each ONU
decreases. In the case of a CO with 128 ONUs the average
transmission rate of the passive channel perceived by each
ONU will be
In such scenario, the
performance of the t‒λ routing architecture will be very poor,
because there will be one or just few channels associated
with the major destination (up to 80% of offered load
destined to major destination) increasing the bottleneck at the
passive channel.
Hybrid Wavelength Division Multiplexing-Passive
Optical Networks (XDM‒WDM‒PON) seems to be the
solution to tackle the requirements for optical access
networks of the future. The introduction of optical bypass
(on‒the‒fly routed) channels based on the nonuniform access
traffic distribution to achieve a transparent, low latency and
low power consumption Metro‒Access interface could
represent an interesting useful approach to consider in
XDM‒WDM‒PON architectures. Based on our simulation
results, we propose to combine the best of the t‒λ routing
architecture and XDM‒WDM‒PON. A more effective
combination could be by means of hybrid OFDM
‒WDM‒PON or OCDM‒WDM‒PON, in substitution of the
time division multiplexing. Such hybrid WDM‒PON must
be able to provide transparent ONU‒CO connections and
transparent Metro‒Access optical bypass routed paths
(releasing electronic bottleneck) without restrictions to
increase the number of ONUs per CO.
It would be convenient to conduct some additional
experiments introducing inelastic and elastic traffic
differentiation. Using traffic differentiation can assure that
only inelastic traffic will be sent by the optical bypassed
channels, whereas elastic traffic could be sent through
electronically routed channels.
ACKNOWLEDGMENT
This work has been supported by the Applied Networks
and Telematics Group at Simon Bolivar University
(GRETA‒USB), Venezuela, and the Metro Access and Short
Range Systems Group at Technical University of Denmark,
Denmark.
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